case study on contamination of water

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Flint Water Crisis: Everything You Need to Know

After officials repeatedly dismissed claims that Flint’s water was making people sick, residents took action. Here’s how the lead contamination crisis unfolded—and what we can learn from it.

Fearful of using the tap water to wash their food, Flint residents Melissa and Adam Mays prepare meals with bottled water.

Brittany Greeson

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A story of environmental injustice and bad decision-making that has yet to be fully resolved , the water crisis in Flint, Michigan, began on April 25, 2014, when the city switched its drinking water supply from Detroit’s system to the Flint River in a cost-saving move. Inadequate treatment and testing of the water resulted in a series of major water quality and health issues for Flint residents—issues that were chronically ignored, overlooked, and discounted by government officials even as complaints mounted that the foul-smelling, discolored, and off-tasting water piped into Flint homes for 18 months was causing skin rashes, hair loss, and itchy skin. 

The Michigan Civil Rights Commission, a state-established body, concluded that the poor governmental response to the Flint crisis was a “result of systemic racism.”

Later studies would  reveal that the contaminated water was also contributing to a doubling—and in some cases, tripling—of the incidence of  elevated blood lead levels in the city’s children , imperiling the health of its youngest generation. It was ultimately the determined, relentless efforts of the  Flint community —with the support of doctors, scientists, journalists, and citizen activists—that shined a light on the city’s severe mismanagement of its drinking water. It forced a reckoning over how such a scandal could have been allowed to happen.

Flint water crisis summary

Flint water crisis update, why is lead-contaminated water bad, beyond flint.

Long before the crisis garnered national headlines, the city of Flint was eminently familiar with water woes. For more than a century, the Flint River, which flows through the heart of town, has served as an unofficial waste disposal site for treated and untreated refuse from the many local industries that have sprouted along its shores, from carriage and car factories to meatpacking plants and lumber and paper mills. The waterway has also received raw sewage from the city’s waste treatment plant, agricultural and urban runoff, and toxics from leaching landfills. 

Not surprisingly, the Flint River is rumored to have caught fire—twice.

As the industries along the river’s shores evolved, so, too, did the city’s economy. In the mid-20th century, Flint—the birthplace of General Motors—was the flourishing home to nearly 200,000 people, many employed by the booming automobile industry. 

But the 1980s put the brakes on that period of prosperity, as rising oil prices and auto imports resulted in shuttered auto plants and laid-off workers, many of whom eventually relocated. The city found itself in a precipitous decline: Flint’s population plummeted to just 100,000 people, a majority of whom are Black, and about one-third of its residents live below the poverty line. Nearly one in six of the city’s homes had been abandoned.

This was the lay of the land in 2011, when Flint, cash-strapped and shouldering a $25 million deficit, fell under state control. Michigan Governor Rick Snyder appointed an emergency manager (basically an unelected official chosen to set local policy) to oversee and cut city costs. 

This precipitated the tragic decision in 2013 to end the city’s five-decade practice of piping treated water for its residents from Detroit in favor of a cheaper alternative: temporarily pumping water from the Flint River until a new water pipeline from Lake Huron could be built. Although the river water was highly corrosive, Flint officials failed to treat it properly, and lead leached out from aging pipes into thousands of homes.

Five-month-old Dakota Erler of Flint gets blood drawn to have her lead levels tested at Carriage Town Ministries in 2016.

Lead levels in Flint water

Soon after the city began supplying residents with Flint River water in April 2014, residents started complaining that the water from their taps looked, smelled, and tasted foul. Despite protests by residents lugging jugs of discolored water, officials maintained that the water was safe. 

A study conducted the following year by researchers at Virginia Tech revealed the problem: Water samples collected from 252 homes through a resident-organized effort indicated citywide lead levels had spiked, with  nearly 17 percent of samples registering above the federal action level of 15 parts per billion (ppb), the level at which corrective action must be taken. More than 40 percent measured above 5 ppb of lead, which the researchers considered an indication of a “very serious” problem.

Even more alarming were findings reported in September 2015 by Flint pediatrician Mona Hanna-Attisha: The incidence of elevated blood-lead levels in children citywide had nearly doubled since 2014—and nearly tripled in certain neighborhoods. As Hanna-Attisha noted, “Lead is one of the most damning things you can do to a child in their entire life-course trajectory.” In Flint, nearly 9,000 children were supplied lead-contaminated water for 18 months.

More problems with Flint water

Flint’s water supply was plagued by more than lead. The city’s switch from Detroit water to the Flint River coincided with an outbreak of Legionnaires’ disease (a severe form of pneumonia) that killed 12 and sickened at least 87 people between June 2014 and October 2015. The third-largest outbreak of Legionnaires’ disease recorded in U.S. history—as well as the discovery in 2014 of fecal coliform bacteria in city water—was  likely a result of the city’s failure to maintain sufficient chlorine in its water mains to disinfect the water. 

Ironically, the city’s corrective measure—adding more chlorine without addressing other underlying issues—created a  new problem : elevated levels of  total trihalomethanes (TTHM), cancer-causing chemicals that are by-products of the chlorination of water.

Flint residents go to court

One of the few bright spots of the Flint water crisis was the response of everyday citizens who, faced with the failure of city, state, and federal agencies to protect them, united to force the government to do its job. 

On the heels of the release of test results in the fall of 2015 showing elevated lead levels in Flint’s water—and its children— NRDC joined with local residents and other groups to petition the U.S. Environmental Protection Agency (EPA) to launch an immediate emergency federal response to the disaster. The EPA failed to act, which only spurred residents on.

In early 2016, a coalition of citizens and groups—including Flint resident Melissa Mays, the local group Concerned Pastors for Social Action, NRDC, and the ACLU of Michigan— sued the city and state officials in order to secure safe drinking water for Flint residents. Among the demands of the suit: the proper testing and treatment of water for lead and the replacement of all the city’s lead pipes. 

In March 2016, the coalition took additional action to address an urgent need,  filing a motion to ensure that all residents—including children, the elderly, and others unable to reach the city’s free water distribution centers—would have access to safe drinking water through a bottled water delivery service or a robust filter installation and maintenance program.

Those efforts paid off. In November 2016, a  federal judge sided with Flint residents and ordered that the government provide every home in Flint with either a properly installed and maintained faucet filter or door-to-door delivery of bottled water. 

A more momentous win came the following March with a major settlement requiring the city to replace the city’s thousands of lead pipes with funding from the state, and guaranteeing further funding for comprehensive tap water testing, a faucet filter installation and education program, free bottled water through the following summer, and continued health programs to help residents deal with the residual effects of Flint’s tainted water.

But the work of Flint residents and their advocates isn’t finished yet. Ensuring that the provisions of the 2017 settlement are met is an ongoing task. Indeed, members of the lawsuit are  still in court to ensure that the city properly manages its lead service line replacement program.

Melissa Mays and other Flint residents address the media after the House Committee on Oversight and Government Reform hearing to examine the Flint water situation in 2016.

Molly Riley/Associated Press

Does Flint have safe water yet?

Governor Snyder seemed to signal the all clear in April 2018 when he announced that the city would stop providing bottled water to residents. While the situation has improved, with lead levels remaining below the federal action level for the past seven years, the city has failed to meet its court-ordered deadlines to check the service line material at all eligible homes and replace the lead service lines it finds. 

This means potentially hundreds of Flint residents are still getting their water from lead pipes. And the federal action level for lead is not a health-based number; it is merely an administrative trigger for remediation by the water utility. The EPA and other health authorities agree that there is no safe level of lead in water, so the continuing presence of lead pipes at hundreds of Flint homes remains a concern, particularly in light of their cumulative lead exposure over many years. Indeed,  in 2024 , the EPA proposed reducing the federal action level for lead from 15 ppb to 10 ppb and mandating the replacement of all lead service lines in the United States within 10 years. 

Flint’s program to replace the thousands of lead and galvanized-steel service lines that connect city water mains to local homes began in March 2016. The program was initially scheduled to be completed within three years but as of April 2024, 10 years since the city of Flint set off the water crisis, the work of identifying and replacing lead service lines remains unfinished. Nearly 2,000 homes also still require repairs for property damage caused by the lead pipe replacement program. Meanwhile, the city’s population has declined by nearly 20,000 people since the crisis began.

The slow pace of progress has drawn NRDC and local residents back to court—multiple times—to demand that Flint comply with its obligations.  Recently , a federal court found the city in contempt of a February 2023 order to reach certain milestones in its lead pipe replacement program.

Flint water crisis charges

In early 2016, Michigan Attorney General Bill Schuette announced an independent review to “determine what, if any, Michigan laws were violated” during Flint’s drinking water disaster. This mission to criminally prosecute those responsible for causing or contributing to the crisis was continued by Attorney General Dana Nessel upon taking office in 2019. 

In 2021, nine people were charged by the attorney general’s office, including Governor Snyder; Nick Lyon, director of Michigan’s Department of Health and Human Services; and Dr. Eden Wells, the state’s chief medical executive. 

But in October 2023, after  facing legal setbacks , the attorney general’s office announced an end to the criminal prosecutions. While Flint residents have been successful in some civil lawsuits, including one that was settled for  $626 million in 2023, none of the individuals in power have faced criminal penalties for their actions.

Resident Lorenzo Lee Avery Jr. stands outside of Flint City Hall during a Flint Lives Matter event in 2016 while the city’s water crisis left residents dependent on bottled water.

Easy to melt and malleable,  lead is a heavy metal that has been used by people for millennia. The Romans added it to makeup, cookware, and pipes. 

Yet, then as now, lead exposure was linked to serious health impacts—including madness and death. Modern science shows that even low levels of lead can impair the brain development of fetuses, infants, and young children. The damage can reverberate for a lifetime, reducing IQ and physical growth and contributing to anemia, hearing impairment, cardiovascular disease, and behavioral problems. Large doses of lead exposure in adults have been linked to high blood pressure, heart and kidney disease, and reduced fertility.

Pure lead pipes, solders, and fittings were banned from U.S. water systems in 1986 (it was only in 2014 that allowable lead levels in plumbing and fixtures dropped to 0.25 percent), and national regulations for lead testing and treatment of public water supplies were established in 1991 with the Lead and Copper Rule. While action by the water utility is required once the level of lead in public water supplies reaches 15 ppb (as measured at the 90th percentile of samples collected), the EPA acknowledges that “there is no safe level of exposure to lead.” 

Independent tests conducted in fall 2015  revealed that nearly 17 percent of samples from hundreds of Flint homes measured above the 15 ppb federal lead action level, with several samples registering above 100 ppb.

Safe water is a human right that should not be determined by where you live or what you look like. But Flint serves as a reminder that safe water isn’t a guarantee. Far more than pipes are corroded during a water crisis like this one. City, state, and federal missteps can also destroy residents’ trust in government agencies.

One NRDC analysis found that thousands of community water systems have violated federal drinking water laws, including the Lead and Copper Rule, which provides safeguards against lead. Meanwhile, there are many  water contaminants that aren’t even monitored or federally regulated, such as perchlorate (a component of rocket fuel) and  PFOA/PFOS/PFAS (chemical cousins of Teflon).

To protect our water supplies, it is crucial that we upgrade our nationwide water infrastructure, prioritizing the replacement of millions of lead pipes, which are  found across every state . After years of public advocacy, federal laws like the Bipartisan Infrastructure Law are finally infusing this work with desperately needed funds.

Strengthening existing government protections is also critical. Later this year, the Biden administration is expected to publish a new Lead and Copper Rule, including a requirement that water utilities replace their lead service lines within 10 years. But NRDC and other allies in the fight for clean drinking water are keeping a close eye on  much-needed improvements to the final rule . 

If you are  concerned about your own drinking water , take a look at your water utility’s annual water quality report (also called a consumer confidence report), which is usually posted online and is required to disclose if contaminants have been found in your water. If contaminants have reached dangerous levels, the water supplier is required to send customers public notification. 

The EPA’s Safe Drinking Water Information System also maintains information about public water systems and their violations. You can go one step farther by having your water tested, either by your water supplier (which may provide this service for free) or by a certified lab.

If you discover your water is contaminated, one option is to use NSF-certified water filters that are designed to eliminate specific contaminants. It is most important, though, that you notify your water utility. If necessary, you can also contact your elected officials, your state’s drinking water program, or the EPA’s Safe Drinking Water Hotline (800-426-4791).

What happened to the people of Flint should never have happened. Yet seven years after the city agreed to clean up its act—and after six legal motions to enforce that agreement—it is still not honoring its commitments to the community nor the court. The residents of Flint and their partners, including NRDC, will not quit until the job is done.

This story was originally published on November 8, 2018, and has been updated with new information and links.

This NRDC.org story is available for online republication by news media outlets or nonprofits under these conditions: The writer(s) must be credited with a byline; you must note prominently that the story was originally published by NRDC.org and link to the original; the story cannot be edited (beyond simple things such as grammar); you can’t resell the story in any form or grant republishing rights to other outlets; you can’t republish our material wholesale or automatically—you need to select stories individually; you can’t republish the photos or graphics on our site without specific permission; you should drop us a note to let us know when you’ve used one of our stories.

Millions of Americans drink tap water served by toxic lead pipes.

Tell the epa we need safe drinking water.

There is no safe level of lead exposure. But millions of old lead pipes contaminate drinking water in homes in every state across the country. We need the EPA to do its part to replace lead pipes equitably and quickly.

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October 21, 2021

The Clean Water Case of the Century

The nation's highest court sided with clean water advocates in a decades-long legal dispute involving a wastewater treatment plant, its pollution discharges, and a partially dead coral reef in Hawaiʻi.

“This decision is a huge victory for clean water,” said David Henkin , the Earthjustice attorney who argued the case before the U.S. Supreme Court.

Our Clients Hawaiʻi Wildlife Fund, Sierra Club-Maui Group, Surfrider Foundation, West Maui Preservation Association Read the Supreme Court Decision

In 2020, the U.S. Supreme Court issued its decision solidifying the Clean Water Act’s place as one of the nation’s most effective environmental laws.

The following year, in the first application of the Supreme Court's test, the Hawai‘i district court reaffirmed protections for the nation's waters.

What started as a local water pollution case could have had disastrous repercussions for clean water across the United States.

What did the U.S. Supreme Court decide?

The U.S. Supreme Court's decision leaves in place vital protections for the nation’s oceans, rivers, and lakes.

The court found that point source discharges to navigable waters through groundwater are regulated under the Clean Water Act. In its decision on County of Maui v. Hawai ʻ i Wildlife Fund , the court held that the Clean Water Act “require[s] a permit if the addition of the pollutants through groundwater is the functional equivalent of a direct discharge from the point source into navigable waters.”

In other words, the Clean Water Act prohibits unpermitted discharge of pollution “into navigable waters, or when the discharge reaches the same result through roughly similar means.”

In doing so, the Court rejected the Trump administration’s polluter-friendly position in the clearest of terms: “We do not see how Congress could have intended to create such a large and obvious loophole in one of the key regulatory innovations of the Clean Water Act.”

The opinion was written by Justice Breyer with a vote of 6-3; with Chief Justice Roberts joining the opinion, along with Ginsburg, Sotomayor, Kagan, and Kavanaugh. (Learn about what happens next with this case, following the Supreme Court's decision.)

Abigail Dillen, President of Earthjustice, explains what happened and what the ruling means:

It’s stunning to think how close we came to a world where industries could just point their pipes straight down into groundwater to dispense of their pollution indirectly into clean water without repercussion. — Abbie Dillen (@AbbieDillen) April 23, 2020

What happened during oral arguments at the U.S. Supreme Court?

Earthjustice attorney David Henkin presented oral arguments in November, before the nine Justices of the U.S. Supreme Court in County of Maui v. Hawaiʻi Wildlife Fund

The Justices posed tough questions to both sides. (Read the transcript.) A summary of the hearing:

• The County’s interpretation of the Clean Water Act is that a “point source” (such as a pipe) must be the thing that delivers pollution for it to be regulated. That, once in groundwater or not straight from the “point source,” the Clean Water Act does not regulate that discharge.
• A few Justices feared the County’s position would create a roadmap for polluters to evade regulation. Justice Breyer asked: What if we end the pipe five feet from the ocean?
• Justice Kagan doubled down, saying nobody would get a permit if they could cut the pipe a few feet short.
• But the Justices also asked Earthjustice attorney Henkin: What should be a definition for a limit to what is regulated? Would this interpretation mean homeowners' septic tanks that leach through groundwater to a river need to get permits under the Clean Water Act (or face stiff fines).
• Henkin explained that pollution that is “traceable” and a “proximate cause” would be regulated and that for three decades, U.S. EPA had gone with that interpretation without millions of homeowners on the hook for pollution from their septic tanks. ( More on the back and forth over the Justice’s questions on limits. )
• The County also deflected responsibility back to the states, saying state groundwater permitting, grant programs, etc., are sufficient to regulate that flows from a point source (such as Maui County’s wastewater wells) through groundwater and to a protected body of water.
• Justice Sotomayor interjected that that’s a problem because it presumes the state will regulate that pollution. Justice Kagan also stated that this case isn’t about relying on state backstops.
• The attorney for the U.S. Government made an analogy about spiking a punch with whiskey . Henkin deftly turned it back around.

The attorney for the U.S. Government made an analogy about spiking a punch with whiskey. Henkin deftly turned it back around. @Earthjustice ’s Sam Sankar explains here. pic.twitter.com/XZrQIjAQM3 — Earthjustice (@Earthjustice) November 6, 2019

• Justices generally seemed to reject the County's extreme position that only pollution direct from a point source is regulated, so pollution sprayed through air or that travels over ground would also be free of Clean Water Act regs. But they seemed unsure how far to go.
• At the end, Justice Sotomayor brought it home asking: What current regulations exist that stop the county from polluting the ocean? It’s definitely happening, and Maui County says the Clean Water Act shouldn't stop them — so are they just going to get away with it? What is being done to stop this?

. @Earthjustice attorney David Henkin gives his final thoughts on defending America's clean water in front of the Supreme Court, likely the biggest day of his career. #CleanWaterActIntact pic.twitter.com/b6k9I8nflc — Earthjustice (@Earthjustice) November 6, 2019

What’s County of Maui v. Hawai‘i Wildlife Fund about?

At its most basic level, this case was about whether a wastewater treatment facility in Maui is violating the Clean Water Act by polluting the ocean indirectly through groundwater.

Since the 1980s, Maui’s Lahaina wastewater treatment facility has been discharging millions of gallons daily of treated sewage into groundwater that reaches the waters off Kahekili Beach, a favorite local snorkeling spot. Depending on local geological conditions, groundwater, which is any water that exists beneath the land’s surface , can flow into major waterways like rivers, streams, and, in the Maui case, the ocean.

In 2012, after years of complaints from the community and unsuccessful negotiations with county officials over the destruction the pollution has caused to the reef and marine life, Earthjustice sued Maui County on behalf of four Maui community groups — Hawaiʻi Wildlife Fund , Sierra Club-Maui Group , Surfrider Foundation , and West Maui Preservation Association .

What is the legal history of this case, before it reached the Supreme Court?

Prior to the U.S. Supreme Court's Apr. 23 decision, two courts ruled in favor of Earthjustice and its clients. In 2016, the U.S. Environmental Protection Agency also agreed with the courts that Maui County was acting illegally.

The county doesn’t dispute that its wastewater pollution reaches the ocean.

Instead, it argued that the discharge of pollution from the facility’s wells does not require Clean Water Act permits because the pollutants do not flow directly into the Pacific Ocean, but indirectly through groundwater. Both the district court and the Ninth Circuit appeals court rejected the county’s claims.

“At bottom, this case is about preventing the county from doing indirectly that which it cannot do directly,” the Ninth Circuit ruled in 2018.

The District of Hawaiʻi court added in its 2014 ruling on the same issue that: “[Maui County’s claim] would, of course, make a mockery of [the Clean Water Act’s regulatory scheme] if [the] authority to control pollution was limited to the bed of the navigable stream itself. The tributaries which join to form the river could then be used as open sewers as far as federal regulation was concerned. No less can be said for groundwater flowing directly into the ocean.”

But Maui County wasn’t giving up. In Feb. 2019, it successfully petitioned the United States Supreme Court to hear the case, an act which now endangers clean water protections writ large.

On Sept. 20, 2019, the Maui County Council voted to settle County of Maui v. Hawaiʻi Wildlife Fund , a decision intended to avoid a standoff at the U.S. Supreme Court that could jeopardize clean water across the United States. But the County of Maui had to officially submit the paperwork to settle the case.

Why does this case matter beyond Maui?

If the Supreme Court had sided with Maui County and overturned the Ninth Circuit’s ruling , it would have allowed industry to freely pollute U.S. waters as long as the pollution isn’t directly discharged into a water source.

Over the past four decades, the U.S. EPA and states across the country have used their Clean Water Act authority to prevent a variety of industries — including wastewater treatment facilities, chemical plants, concentrated animal feeding operations, mines, and oil and gas waste-treatment facilities — from contaminating the nation’s waters via groundwater.

Industry groups are closely watching this case, and the list of groups that have filed amicus briefs to the county’s claims is a who’s who of polluters.

A Supreme Court decision reversing the Ninth Circuit’s ruling would have blown a hole in the Clean Water Act. It would have essentially allowed groundwater to “launder” pollution, allowing polluters to evade responsibility even if their waste contaminates clean water. This was the perverse logic underlying Maui County’s claim that it doesn’t need a permit as long as its pollution runs through the groundwater before reaching the ocean.

Earthjustice attorney David Henkin finds this contention “absurd.”

“According to Maui County, a polluter can avoid the law by taking a pipeline that discharges waste directly into the ocean and cutting it ten feet short of the shoreline,” Henkin said.

Instead of discharging waste directly into the ocean, the polluter is discharging waste onto the beach that then makes its way into the ocean.

“At the end of the day, the water is still polluted,” says Henkin. “And, under the county’s twisted logic, the polluter would get off scot-free.”

Who is on the county’s side?

The list of groups that support Maui County’s efforts to gut the Clean Water Act include Kinder Morgan , Energy Transfer Partners (the company behind the Dakota Access Pipeline ), the U.S. Chamber of Commerce, American Fuel & Petrochemical Manufacturers, National Mining Association, and industrial agricultural business organizations.

The U.S. EPA under the Trump administration has also done an about-face to side with these industries. In April, the agency reversed four decades of agency guidance that the Clean Water Act does regulate discharges of pollution that reach our nation’s waters through groundwater.

4,600 miles due east of Maui, gasoline is flowing into Browns Creek, South Carolina, via contaminated soil and groundwater. Kinder Morgan says that isn’t the corporation’s problem. Justice for the community could hinge on the outcome of this Supreme Court case. Read the story of the small town of Belton, Anderson County.

Who is on the side of clean water?

Eleven different groups that include former U.S. EPA administrators and officials from multiple administrations, 13 states, two counties facing similar pollution, a Native American tribe, craft brewers , law professors, aquatic scientists and scientific societies, and clean water advocates filed briefs in support of Earthjustice and its Maui community clients.

“As the amicus briefs vividly illustrate, this case pits those who are committed to the protection of life-giving, clean water against the Trump administration and polluting industries that want free rein to use groundwater as a sewer to dump their waste and toxic discharges into our nation’s lakes, rivers, and oceans,” Earthjustice attorney David Henkin says.

What happened after the Supreme Court decision?

The case went back to the Ninth Circuit, which then sent it back to the district court.

The next step in the case was for the lower court to decide whether Maui’s discharges meet the new test established by the Supreme Court: whether the sewage plant discharges to the groundwater, through which the sewage migrates inevitably and inexorably to the ocean a quarter mile away, are the functional of direct discharges to the ocean.

On Oct. 20, 2021, the Hawai‘i district court did just that — reaffirming protections for the nation's waters in the first application of the Supreme Court's Maui test.

The court denied the County of Maui’s request to reconsider the court’s Jul. 26, 2021, decision that the county must get a Clean Water Act permit for injection wells at the Lahaina Wastewater Reclamation Facility in West Maui.

“As the first court to apply the Supreme Court’s test, the court sent a strong message of hope to communities seeking to protect their oceans, rivers, and lakes from polluters like Maui County that are fouling those life-giving waters by using groundwater as a sewer,” said Earthjustice attorney David Henkin.

Why does this case matter to me?

Maui County’s argument was not only absurd, it was extremely dangerous. If the Supreme Court had ruled in the county’s favor, it would have jeopardized clean water across the country.

If you care about clean water, then you should care about this case.

At Earthjustice, we’re a nonprofit in the business of building a better future for our planet.

Which is why your support is so crucial.

We stand alongside hundreds of public-interest clients at the frontlines of the fight for a better today and tomorrow. Case by case, our lawyers face off against deep-pocketed interests — and we win.

Our lawyers measure success in clean air, clean water, and safeguards for communities across the country.

Every one of our clients gets top-tier legal representation, free of charge. We can’t keep fighting for our planet without your help . Whether you give $5 or $500, this will be the best investment you make today.

Established in 1988, Earthjustice's Mid-Pacific regional office in Honolulu focuses on environmental and community health issues, including ensuring water is a public trust and achieving a cleaner energy future.

Mid-Pacific Office

Established in 1988, Earthjustice's Mid-Pacific Office, located in Honolulu, Hawaiʻi, works on a broad range of environmental and community health issues, including to ensure water is a public trust and to achieve a cleaner energy future.

The legal case: Lahaina Injection Well

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Sources and Consequences of Groundwater Contamination

• Published: 02 January 2021
• Volume 80 , pages 1–10, ( 2021 )

Cite this article

• Peiyue Li   ORCID: orcid.org/0000-0001-8771-3369 1 , 2 ,
• D. Karunanidhi 3 ,
• T. Subramani 4 &
• K. Srinivasamoorthy 5  

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Groundwater contamination is a global problem that has a significant impact on human health and ecological services. Studies reported in this special issue focus on contaminants in groundwater of geogenic and anthropogenic origin distributed over a wide geographic range, with contributions from researchers studying groundwater contamination in India, China, Pakistan, Turkey, Ethiopia, and Nigeria. Thus, this special issue reports on the latest research conducted in the eastern hemisphere on the sources and scale of groundwater contamination and the consequences for human health and the environment, as well as technologies for removing selected contaminants from groundwater. In this article, the state of the science on groundwater contamination is reviewed, and the papers published in this special issue are summarized in terms of their contributions to the literature. Finally, some key issues for advancing research on groundwater contamination are proposed.

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• Environmental Chemistry

Avoid common mistakes on your manuscript.

Groundwater is a major source of fresh water for the global population and is used for domestic, agricultural, and industrial uses. Approximately one third of the global population depends on groundwater for drinking water (International Association of Hydrogeologists 2020 ). Groundwater is a particularly important resource in arid and semi-arid regions where surface water and precipitation are limited (Li et al. 2017a ). Securing a safe and renewable supply of groundwater for drinking is one of the crucial drivers of sustainable development for a nation. However, urbanization, agricultural practices, industrial activities, and climate change all pose significant threats to groundwater quality. Contaminants, such as toxic metals, hydrocarbons, trace organic contaminants, pesticides, nanoparticles, microplastics, and other emerging contaminants, are a threat to human health, ecological services, and sustainable socioeconomic development (Li 2020 ; Li and Wu 2019 ).

Over the past three decades, chemical contamination is a common theme reported in groundwater studies. While groundwater contamination is a great challenge to human populations, this subject also presents a great opportunity for researchers to better understand how our subsurface aquifers have evolved and for decision makers to grasp how we can protect both the quality and quantity of these resources. Fresh water aquifers are one of the most important sections of the Critical Zone (CZ), which extends from the top of the vegetation canopy down to the bottom of the aquifer (Lin 2010 ). As part of the global effort to understand the functions, structures, and processes within the CZ, a range of investigations have been performed that contribute to our knowledge of the circulation and evolution of groundwater (Sawyer et al. 2016 ; Goldhaber et al. 2014 ).

Many of the contaminants in groundwater are of geogenic origin as a result of dissolution of the natural mineral deposits within the Earth’s crust (Basu et al. 2014 ; Pandey et al. 2016 ; Subba Rao et al. 2020 ; He et al. 2020a ). However, due to rapid expansion of the global population, urbanization, industrialization, agricultural production, and the economy, we now are faced with the challenge of the negative impacts of contaminants of anthropogenic origin. The countries most affected by these global changes are those that are going through rapid economic development, with many of them located in the eastern hemisphere (Clement and Meunie 2010 ; Hayashi et al. 2013 ; Lam et al. 2015 ). Thus, it is appropriate that this special issue entitled, “The fate and consequences of groundwater contamination” focuses on studies of the unique challenges related to contaminants of both anthropogenic and geogenic origin in groundwater in several countries in the eastern hemisphere, including China, India, Turkey, Bangladesh, Ethiopia, and Nigeria. Figure  1 illustrates the countries where the research was conducted and the classes of chemical contaminants reported in the articles in this special issue.

Eastern hemisphere, showing the countries where the groundwater research was conducted and the classes of contaminants studied in the articles published in this special issue

The range of topics included in articles in this special issue includes: (1) Latest methods for detecting and tracking the movement of groundwater contaminants; (2) Novel techniques for assessing risks to human populations consuming contaminated groundwater; (3) Effects of groundwater contamination on the abiotic environment, such as soil, sediments, and surface water; and (4) Case studies and remedial actions to control groundwater contamination from natural and anthropogenic sources. The co-editors of this special issue anticipate that these articles will facilitate an understanding of the origins and extent of groundwater contamination and its consequences and will provide examples of approaches that can be taken for remediation of groundwater contamination and protection of groundwater quality.

Major Contaminants

Groundwater contamination is defined as the addition of undesirable substances to groundwater caused by human activities (Government of Canada 2017 ). This can be caused by chemicals, road salt, bacteria, viruses, medications, fertilizers, and fuel. However, groundwater contamination differs from contamination of surface water in that it is invisible and recovery of the resource is difficult at the current level of technology (MacDonald and Kavanaugh 1994 ). Contaminants in groundwater are usually colorless and odorless. In addition, the negative impacts of contaminated groundwater on human health are chronic and are very difficult to detect (Chakraborti et al. 2015 ). Once contaminated, remediation is challenging and costly, because groundwater is located in subsurface geological strata and residence times are long (Wang et al. 2020 ; Su et al. 2020 ). The natural purification processes for contaminated groundwater can take decades or even hundreds of years, even if the source of contamination is cut off (Tatti et al. 2019 ).

The numbers of classes of contaminants detected in groundwater are increasing rapidly, but they can be broadly classified into three major types: chemical contaminants, biological contaminants, and radioactive contaminants. These contaminants can come from natural and anthropogenic sources (Elumalai et al. 2020 ). The natural sources of groundwater contamination include seawater, brackish water, surface waters with poor quality, and mineral deposits. These natural sources may become serious sources of contamination if human activities upset the natural environmental balance, such as depletion of aquifers leading to saltwater intrusion, acid mine drainage as a result of exploitation of mineral resources, and leaching of hazardous chemicals as a result of excessive irrigation (Su et al. 2020 ; Wu et al. 2015 ; Li et al. 2016 , 2018 ).

Nitrogen contaminants, such as nitrate, nitrite, and ammonia nitrogen, are prevalent inorganic contaminants. Nitrate is predominantly from anthropogenic sources, including agriculture (i.e., fertilizers, manure) and domestic wastewater (Hansen et al. 2017 ; He and Wu 2019 ; He et al. 2019 ; Karunanidhi et al. 2019 ; Li et al. 2019a ; Serio et al. 2018 ; Zhang et al. 2018 ). Groundwater nitrate contamination has been widely reported from regions all over the world. Other common inorganic contaminants found in groundwater include anions and oxyanions, such as F − , SO 4 2− , and Cl − , and major cations, such as Ca 2+ and Mg 2+ . Total dissolved solids (TDS), which refers to the total amount of inorganic and organic ligands in water, also may be elevated in groundwater. These contaminants are usually of natural origin, but human activities also can elevate levels in groundwater (Adimalla and Wu 2019 ).

Toxic metals and metalloids are a risk factor for the health of both human populations and for the natural environment. Chemical elements widely detected in groundwater include metals, such as zinc (Zn), lead (Pb), mercury (Hg), chromium (Cr), and cadmium (Cd), and metalloids, such as selenium (Se) and arsenic (As). Exposures at high concentrations can lead to severe poisoning, although some of these elements are essential micronutrients at lower doses (Hashim et al. 2011 ). For example, exposure to hexavalent chromium (Cr 6+ ) can increase the risk of cancer (He and Li 2020 ). Arsenic is ranked as a Group 1 human carcinogen by the US Environmental Protection Agency (EPA) and the International Agency for Research on Cancer (IARC), and As 3+ can react with sulfhydryl (–SH) groups of proteins and enzymes to upset cellular functions and eventually cause cell death (Abbas et al. 2018 ; Rebelo and Caldas 2016 ). Toxic metals in the environment are persistent and subject to moderate bioaccumulation when they enter the food chain (He and Li 2020 ; Hashim et al. 2011 ).

Organic contaminants have been widely detected in drinking water, and many of these compounds are regarded as human carcinogens or endocrine disrupting chemicals. In groundwater, more than 200 organic contaminants have been detected, and this number is still increasing (Lesser et al. 2018 ; Jurado et al. 2012 ; Lapworth et al. 2012 ; Sorensen et al. 2015 ). Some organic contaminants are biodegradable, while some are persistent. The biodegradable organic contaminants originate mainly from domestic sewage and industrial wastewater. Many of these organic substances are naturally produced from carbohydrates, proteins, fats, and oils and can be transformed into stable inorganic substances by microorganisms. They have no direct toxic effects on living beings but can reduce the dissolved oxygen levels in groundwater. Common organic contaminants include hydrocarbons, halogenated compounds, plasticizers, pesticides, pharmaceuticals, and personal care products and natural estrogens, among others (Lapworth et al. 2015 ; Meffe and Bustamante 2014 ). Many of the halogenated compounds (e.g., chlorinated, brominated, fluorinated) are stable in the environment and can be accumulated and enriched in organisms, causing harmful effects in organisms from higher trophic levels, including humans (Gwenzi and Chaukura 2018 ; Schulze et al. 2019 ). The persistent organic contaminants are mainly compounds used for agriculture, industrial processes, and protection of human health (Lapworth et al. 2015 ). Because these compounds degrade very slowly or even not at all, they may permanently threaten the quality of groundwater for drinking purposes (Schulze et al. 2019 ).

Radioactive contaminants in groundwater can originate from geological deposits of radionuclides but also can originate from anthropogenic sources, such as wastes from nuclear power plants, nuclear weapons testing, and improper disposal of medical radioisotopes (Dahlgaard et al. 2004 ; Lytle et al. 2014 ; Huang et al. 2012 ). Radioactive substances can enter the human body through a variety of routes, including drinking water. However, radioactive contaminants have been rarely detected in groundwater at levels that are a threat to human health.

Biological contaminants include algae and microbial organisms, such as bacteria, viruses, and protozoa. For microbial contaminants, more than 400 kinds of bacteria have been identified in human and animal feces, and more than 100 kinds of viruses have been recognized (Shen and Gao 1995 ). Some of these microbial organisms originate from natural sources, but some include microscopic organisms that co-exist with natural algal species and compete for available resources (Flemming and Wuertz 2019 ; Lam et al. 2018 ). Drinking water contaminated by microbial contaminants can result in many human diseases, including serious diarrheal diseases, such as typhoid and cholera. Currently, the COVID-19 virus has resulted in pandemic affecting every corner of the world. This coronavirus is primarily transmitted from person-to-person through respiratory droplets (Centers for Disease Control and Prevention 2020 ). However, water contaminated by this virus also can threaten human health (Bhowmick et al. 2020 ; Lokhandwala and Gautam 2020 ). Algal contamination is very common in surface waters, such as lakes and reservoirs due to eutrophication, but algae are rarely found at a high biomass in groundwater.

Consequences of Groundwater Contamination

Groundwater contamination can impact human health, environmental quality, and socioeconomic development. For example, many studies have shown that high levels of fluoride, nitrate, metals, and persistent organic pollutants are a health risk for human populations (Wu et al. 2020 ). This is especially critical for infants and children who are more susceptible to the effects of these contaminants than adults (He et al. 2020b ; Wu and Sun 2016 ; Karunanidhi et al. 2020 ; Mthembu et al. 2020 ; Ji et al. 2020 ; Subba Rao et al. 2020 ; Zhou et al. 2020 ). For example, “blue baby syndrome,” also known as infant methemoglobinemia, is caused by excessive nitrate concentrations in the drinking water used to make baby formulas. Human health also can be affected by the groundwater contamination through effects on the food production system. Irrigation with groundwater contaminated by heavy metals and wastewater containing persistent contaminants can result in the accumulation of toxic elements in cereals and vegetables, causing health risks to humans (Jenifer and Jha 2018 ; Yuan et al. 2019 ; Njuguna et al. 2019 ).

Groundwater contamination also can negatively affect the quality of lands and forests. Contaminated groundwater can lead to soil contamination and degradation of land quality. For example, in many agricultural areas in arid regions, high groundwater salinity is one of the major factors influencing soil salinization (Wu et al. 2014 ). The soluble salts and other contaminants, such as toxic metals, can accumulate in the root zone, affecting vegetation growth. Groundwater contaminants also can be transported by surface water-groundwater interactions, leading to deterioration of surface water quality (Teng et al. 2018 ).

Sustainable economic development requires a balance between the rate of renewal of natural resources and human demand (Li et al. 2017b ). Freshwater is probably the most valuable of the natural resources. However, chronic groundwater contamination may reduce the availability of freshwater, breaking the balance between water supply and demand and leading to socioeconomic crises and even wars. Water shortages induced by contamination may become a factor causing conflicts among citizens in the future (Schillinger et al. 2020 ), possibly delaying the socioeconomic development of a nation. Groundwater contamination is not only an environmental issue but also a social issue, demanding collaboration between both natural scientists and social scientists.

Articles in the Special Issue

Nineteen papers are included in this special issue. The topics of these papers cover a range of contamination issues, including the sources of geogenic and anthropogenic contamination, seasonal cycles in contamination, human health risks, and remediation technologies. Figure  2 illustrates a word cloud generated using the words in the titles and abstracts of the articles in this special issue, showing the most frequently used terms. The word cloud shows that the most frequently used technical terms in the articles are water, risk, metals, nitrate, fluoride, polycyclic aromatic hydrocarbons (PAHs), health, limits, and values. These terms reflect the main topics of the articles, which cover the assessment of the concentrations of trace metals, fluoride, nitrate, PAHs, and other organic contaminants in groundwater and the associated risks to the health of human populations. Some more minor terms, such as geogenic, source, removal, statistical, EWQI, and mobility, indicate that some articles focus on evaluating the sources of groundwater contamination, approaches to groundwater quality assessment, and contaminant remediation techniques. The main contributions of each article in this special issue are summarized below.

Word cloud generated using the words in the titles and abstracts of articles in this special issue

Toxic metals are persistent contaminants and can be bioaccumulated in human tissues via food chain (He and Li 2020 ). In this special issue, six articles focused on the assessing trace metal pollution in groundwater. Çiner et al. ( 2021 ) used multivariate statistical analysis to identify the sources of trace elements in groundwater, including Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, and Ba, and assessed the health risks from arsenic contamination in a region of south-central Turkey. Their research results indicate that the carcinogenic risks from exposure to arsenic to both adults and children were higher than the guideline limit, and the geogenic processes are the main cause of trace element contamination in groundwater in this region. Chandrasekar et al. ( 2021 ) also identified geogenic metal contamination in their article focused on the source, geochemical mobility, and health risks from trace metals in groundwater in a Cretaceous-Tertiary (K/T) contact region of India. However, Raja et al. ( 2021 ) concluded that industrial activities and leaching from municipal dumpsites were the main sources of the metal pollution in the groundwater in the industrialized township (Taluk) of Virudhunagar in India.

In addition to contamination of groundwater, trace elements can be transported via groundwater into surface waters and into oceans. In the article by Prakash et al. ( 2021 ), estimates were made of the submarine groundwater discharge and associated trace element fluxes from an urban estuary region to the marine environment in the Bay of Bengal in India. This study revealed that submarine groundwater discharge is an important factor contributing to the fluxes to the sea of dissolved trace elements.

Finding efficient and cost-effective technologies for removal of trace elements from groundwater is crucial for the sustainable management of water resources. Zhao et al. ( 2021 ) studied Cd removal from water using a novel low-temperature roasting technique associated with alkali to synthesize a high-performance adsorbent from coal fly ash. Dutta et al. ( 2021 ) proposed to use electrocoagulation with iron electrodes as a treatment technology for arsenic removal from groundwater, and a pilot scale filtration unit was used to remove ferric hydroxide flocs produced during the process.

Fluoride is of value in trace amounts for promoting dental health, but this anion is toxic when present in high concentrations in water and food (Adimalla and Li 2019 ; Li et al. 2014 , 2019b ; Marghade et al. 2020 ). In this special issue, two articles specifically address fluoride occurrence, distribution, and health risks. The article by Haji et al. ( 2021 ) describes a study of groundwater quality and human health risks from fluoride contamination in a region within the southern Main Ethiopian Rift. Keesari et al. ( 2021 ) used the empirical cumulative density function to estimate the health risks from consuming fluoride contaminated groundwater in northeastern parts of Rajasthan in India. These authors also produced a fluorosis risk map to aid decision makers in taking necessary remedial measures to improve the groundwater quality.

Organic pollutants, including polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), are common contaminants of anthropogenic origin in groundwater that could cause serious health problems. In this special issue, two articles focused on these organic pollutants. The article by Ololade et al. ( 2021 ) reported an investigation into PAHs and PCBs in groundwater near selected waste dumpsites located in two southwestern states in Nigeria. They found that the more water-soluble, low molecular weight-PAHs accounted for more than 61% of the total PAHs detected across all locations, but surprisingly the more highly chlorinated hexa-PCBs dominated the congener profiles. In another paper in this issue by Ambade et al. ( 2021 ), the occurrence, distribution, health risk, and composition of 16 priority PAHs were investigated in drinking water from southern Jharkhand in the eastern part of India. These authors found that lower and middle molecular weight PAHs were dominant in groundwater from the study area, but the levels are currently below concentrations that are a carcinogenic risk.

Studies of radioactive elements in groundwater often are neglected, but these radionuclides can be a hazard to human health. Adithya et al. ( 2021 ) conducted a study in Tamil Nadu state in southern India to measure the levels of radon (Rn) in groundwater and quantify the health risks. Their study showed that the Rn is released into groundwater from granitic and gneissic rocks within uranium-enriched lithological zones. However, the Rn levels determined in Bequerels per litre were lower than the guideline limit and the groundwater does not pose health risks to consumers.

In this special issue, Adimalla and Qian ( 2021 ) conducted a study on the spatial distribution and potential health risks from nitrate pollution in groundwater in southern India. The article revealed high nitrate levels in groundwater, at concentrations up to 130 mg/L. Both adults and children were judged to face health risks from consumption of nitrate in drinking water, but children were identified as more susceptible to the effects of groundwater nitrate pollution. The paper by Karunanidhi et al. ( 2021 ) describes the improvements in groundwater quality that occurred in an industrialized region of southeastern India between January and June of 2020. These improvements included reduced nitrate contamination, which may have been due to reduced transport of nitrate into groundwater before the monsoon period, but also could have been due to the decline in industrial and agricultural activity in the region during the lockdown in India that began in March 2020 in response to the first wave of the COVID-19 pandemic. In this study, fluoride concentrations of geogenic origin also were lower in groundwater before the monsoon.

Understanding the seasonal and spatial variations in groundwater quality is essential for the protection of human health and to maintain the crop yields. Subba Rao et al. ( 2021 ) used multiple approaches to identify the seasonal variations in groundwater quality and revealed that the groundwater quality for drinking and irrigation purposes was lower in the post-monsoon period relative to the pre-monsoon period. The deterioration of groundwater quality in the post-monsoon period was attributed to contaminant transport occurring through groundwater recharge but also was influenced by topographical factors and human activities.

Understanding the hydrogeochemical processes affecting groundwater chemistry is the basis for effective management of groundwater resources. Ren et al. ( 2021 ) adopted statistical approaches and multivariate statistical analysis techniques to understand the hydrogeochemical processes affecting groundwater in the central part of the Guanzhong Basin, China. The main contribution of this article is that it could help local decision makers to make water management decisions in the densely populated river basin by providing them with useful groundwater management options.

There are four articles in this special issue that focus specifically on methods to assess groundwater quality and humfluoride and associated arsenicosis and fluoan health risks. Shukla and Saxena ( 2021 ) assessed the groundwater quality and health risk in the rural parts of Raebareli district in northern India. Wang et al. ( 2021 ) identified the hydrochemical characteristics of groundwater and assessed health risk to consumers in a part of the Ordos basin in China. Adimalla ( 2021 ) applied two indices: the entropy weighted water quality index (EWQI), and the pollution index of groundwater (PIG) to assess the suitability of groundwater for drinking purpose in the Telangana state in southeastern India. Khan et al. ( 2021 ) assessed the drinking water quality and potential health impacts by considering physicochemical parameters, as well as bacteriological contamination of groundwater in Bajaur, Pakistan.

Collectively, these articles contribute to the literature on scientific developments in the field of groundwater contamination. The case studies presented in these articles are useful for policy makers and the public to understand the current water quality status in these regions. In particular, these articles provide a window into the groundwater contamination issues that are affecting low- and middle-income countries and countries with emerging economies in the eastern hemisphere. Researchers from Europe, North America, and other high-income countries often do not grasp the extent of groundwater contamination from geogenic and anthropogenic sources in these regions and do not realize that many human populations have no choice but to consume the contaminated drinking water.

The Way Ahead

Groundwater contamination is now a global problem and the resolution of these problems requires close collaboration among researchers in universities and government agencies, industries, and decision makers from all levels of government. To solve the groundwater contamination problems, international collaboration is needed. This is particularly true in countries with developing economies where financial resources and access to advanced technologies are not readily available. Special focus should be given to the following aspects of research and training:

Groundwater contamination issues in different countries should be addressed with a range of measures, techniques, and policies. Although groundwater contamination is a global problem, its nature and influencing factors are different between countries, climatic regions, and geological features. It may not be optimal to adopt remediation approaches that are successful in other countries or regions. For example, nitrate pollution is caused by fertilizer and manure applications in some agricultural regions (Zhang et al. 2018 ) but also may be caused by pollution by industrial and domestic wastewater in other areas, or even by explosives used in mineral exploration (Li et al. 2018 ). It may be necessary to use different approaches to mitigate different types of nitrate pollution. Even in instances where fertilizer application is the common cause of nitrate pollution in a tropical and a temperate region, the remediation approaches could be different, as climate factors and soil characteristics will have a great influence on the mechanisms and extent of contaminant transport.

With the rapid technological development, many novel techniques have been developed to study groundwater contamination, including geophysical and geoinformatics techniques. Geographical information systems (GIS) and remote sensing (Ahmed et al. 2020 ; Al-Abadi et al. 2020 ; Alshayef et al. 2019 ; Kannan et al. 2019 ) have accelerated the development of groundwater science. In the future, artificial intelligence, “big data” analysis, drone surveys, and molecular and stable isotope analysis technologies will be more widely available for applications in groundwater research. Groundwater scientists need to adopt and apply these new technologies for the study of groundwater contamination.

Governments, particularly in countries with developing economies need to invest in and encourage research and training in groundwater science. In many regions, human populations have no alternative but to consume groundwater that is contaminated with chemical or biological agents, potentially causing wide ranging health effects. Investment is needed to determine the extent of this contamination and how to remediate the impacts on human health, or to find alternate sources of drinking water.

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Acknowledgements

Editing a successful special issue is not easy. The Guest Editors must ensure that the topic is of importance and of broad interest so that there are an adequate number of contributors willing to submit their manuscripts. They must also make sure that the peer review process is efficient and effective, while maintaining the high quality of the papers. All of these cannot be fulfilled without the support of the Editor in Chief. So, we are extremely grateful for Prof. Chris Metcalfe’s guidance and support for this special issue. We are also sincerely thankful to the reviewers who provided constructive comments that are essential for maintaining the high quality of the special issue. Last but not the least, the authors whose manuscripts were included and those whose manuscripts were rejected are acknowledged for their interest in contributing to the special issue. The special issue was edited in a situation in which the COVID-19 struck in nearly every corner of the world. We are impressed by the dedication of doctors who fought and/or are fighting against the coronavirus. Prof. Peiyue Li is grateful for the financial support granted by the National Natural Science Foundation of China (41761144059 and 42072286), the Fundamental Research Funds for the Central Universities of CHD (300102299301), the Fok Ying Tong Education Foundation (161098), and the Ten Thousand Talents Program (W03070125), which allow him to carry out various investigations. The year 2021 is the 70th anniversary of Chang’an University. Congratulations!

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Li, P., Karunanidhi, D., Subramani, T. et al. Sources and Consequences of Groundwater Contamination. Arch Environ Contam Toxicol 80 , 1–10 (2021). https://doi.org/10.1007/s00244-020-00805-z

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DOI : https://doi.org/10.1007/s00244-020-00805-z

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Case Study: Iowa Cities Struggle to Keep Farm Pollution Out of Tap Water

Almost three-fourths of the Raccoon River’s watershed – 1.7 million acres – is planted with corn, soybeans and other crops, treated each year with millions of pounds of fertilizer and other chemicals [1] It is also home to 2.3 million hogs and 16 million chickens and turkeys, [2]  whose manure is applied to millions of acres annually. [3]  At the southern tip of the watershed is the city of Des Moines, where each day the Des Moines Water Works supplies water drawn from the river to just over half a million people. 

Figure 1: The 3,625-square-mile Raccoon River watershed drains 1.7 million acres of cropland.

Source: EWG

Commercial fertilizers and manures contain a chemical called nitrate, a form of nitrogen, which gets into the river when rain washes it off fields. It can be fatal to babies who ingest too much of the chemical in tap water and it has also been linked to cancer in adults.

Between the spring of 2014 and the fall of 2015, the average nitrate level in untreated Raccoon River water was 11.12 parts per million, or ppm. The Environmental Protection Agency’s legal limit for nitrate in drinking water is 10 ppm. This standard was set 25 years ago to protect infants against so-called blue baby syndrome and has not been reviewed since. But recent studies by the National Cancer Institute have found that drinking water with just 5 ppm of nitrate increases the risk of colon, kidney, ovarian and bladder cancers. As such, the EWG Standard for nitrate is 5 ppm.

To keep average nitrate levels below the legal limit – although not below the level linked to an increased risk of cancer – the Des Moines Water Works treated the polluted river water with sodium chloride through a process called ion exchange . In 2014 and 2015, nitrate in treated, or “finished,” water from the utility averaged 5.16 ppm. During that period, average levels met the standard, but nitrate levels in individual samples of finished water went up and down, ranging from 0.08 ppm to 9.21 ppm.

Eighty miles downstream from Des Moines, the city of Ottumwa faced the same challenge of keeping nitrate in the water it served to its citizens below the legal limit.

Ottumwa, with a population of about 25,000, does not have a nitrate treatment system, but relies on blending Raccoon River water from other sources to bring down the level of the contaminant. In 2014 and 2015, nitrate levels in Ottumwa’s finished water averaged 6.42 ppm – again, below the legal limit but above the increased cancer risk level. In January 2015 the nitrate level was 9.7 ppm, perilously close to the legal limit.

Real-time water quality monitoring by the U.S. Geological Survey shows a strong correlation between seasonal averages of nitrogen in the river and nitrate levels in drinking water for Des Moines and Ottumwa (see Figure 2). When nitrate in the river spikes, the utilities manage to keep drinking water below the legal limit, but contamination consistently exceeds the increased cancer risk level.

Communities across Iowa face similar problems.

In 2014 and 2015, nitrate levels for tap water in 45 Iowa public water systems averaged at or above 5 ppm. All but three of those systems draw from groundwater under the surface, while the rest, like those of Des Moines and Ottumwa, depend on the river. According to data from the Iowa Department of Natural Resources’ Source Water Protection Program, 30 of the 45 systems drew from wells classified as highly susceptible to contamination. [4]

Private well water is also plagued by high levels of nitrate. A 2016 study by Iowa Watch, a nonprofit news organization, estimated that 288,000 Iowans rely on water from private wells. The study looked at nitrate levels in 28 wells throughout rural southwest Iowa in May and June of that year. They found nitrate levels as high as 168 ppm, with 11 wells at or above 45 ppm.

In 2016 the Iowa Department of Public Health tested more than 1,700 private wells for nitrate. It found that 19 percent were at or above the legal limit of 10 ppm. [5]  This was up slightly from 2015, but down significantly from 2014, when 29 percent of the more than 5,000 wells tested had nitrate levels at or above the legal limit.   

Nitrate not the only problem

Nitrate is not the only threat to drinking water polluted by agricultural runoff.

When it rains, the runoff from poorly protected farm fields carries phosphorous fertilizer and organic matter like manure, mud and crop residues into streams. Phosphorous triggers blooms of algae, which multiply the amount of organic matter in the stream.

To protect people from fecal bacteria or pathogens, utilities must disinfect the water with chlorine or other chemicals. But those chemicals react with algae and other organic matter in the water to produce disinfection byproducts. The byproducts, called trihalomethanes, or TTHMs, carry long-term health hazards.

Drinking tap water contaminated with TTHMs increases the  risk of developing bladder cancer  in humans. In animal studies, TTHMs are also associated with liver, kidney and intestinal tumors. Studies suggest that TTHMs increase the risk of problems during pregnancy as well, including miscarriage, cardiovascular defects, neural tube defects and low birth weight.

The EPA has set a legal limit of 80 parts per billion, or ppb, for TTHMs in drinking water. The limit was based on the technical feasibility of removing TTHMs from drinking water after disinfection and did not consider long-term toxicity. In 2010,  California state scientists estimated that exposure to 0.8 ppm  of TTHMs – 100 times lower than the federal legal limit – would pose a one-in-a-million lifetime risk of cancer.

EWG’s Tap Water Database , which collects test results from almost 50,000 utilities nationwide,  shows that in 2014 and 2015, 33 water systems in Iowa had average nitrate levels at or above 75 percent of the legal limit for TTHM, or at 60 ppb. Three of those systems are in the Raccoon River watershed.  

Who is responsible for cleaning the water?

Pollution of source water from farm runoff puts utilities between a rock and a hard place. They don’t control what happens in the watersheds or above the underground aquifers from which they draw drinking water. But utilities and their customers bear the cost of cleaning contaminants out of the water and adhering to federal regulations.

In 2015, the Des Moines Water Works brought a lawsuit against three upstream drainage districts within the Raccoon River watershed for nitrate pollution. They also sought to require the districts to obtain permits similar to those required under the Clean Water Act for industrial facilities and other so-called point source polluters. In 2017, the Iowa Supreme Court threw out the lawsuit on the grounds that the drainage districts were powerless to control farm runoff.

Iowa and some other farm states have conducted studies of how farmers can manage their lands to keep more nitrogen, phosphorus and other chemicals the soil, rather than running off into rivers, lakes and streams. Some states have codified these strategies and practices into reduction goals, but those rely on voluntary practices, for which farmers can receive taxpayer-funded assistance grants. But a look at two of those practices in the Raccoon River watershed show that voluntary programs are not enough.

Cover crops

Cover crops are grasses or other plants seeded to cover fields after the commercial crop has been harvested. They are remarkably effective at preventing mud, fertilizers and farm chemicals from running off of farm fields when it rains. Their roots also capture and hold nitrate in the soil that a commercial crop didn’t use, and keep it from flowing into streams or ditches or seeping into groundwater.

Capturing this unused nitrate is critically important in fields that have been underlain with pipes to drain water from the soil, a practice that improves crop yields. The pipes are buried a few feet below the surface, and send water out from below the surface into a ditch or stream. Miles and miles of these drainage pipes are buried beneath fields in the Raccoon River watershed (see Figure 3 below). Water from these pipes, which the Department of Agriculture estimates drain more than half of Iowa’s cropland, is the main source of nitrate polluting Des Moines’ drinking water.

Figure 3. Example of a drainage network beneath cropland.

EWG used satellite imagery to locate fields protected with cover crops between 2009 and 2010, and between 2015 and 2016. The good news is in that period the amount of cover crops planted to protect the Raccoon River grew from 7,000 acres to nearly 24,000 acres. The bad news is those 24,000 acres are less than 2.5 percent of the amount of cover crops needed to clean up the watershed. Iowa’s Nutrient Reduction Strategy recommends that statewide, 60 percent of Iowa’s fields should be protected with cover crops every year. That would be more than 1.02 million acres in the Raccoon watershed – 43 times more than were protected in 2016.

Federal programs could do more to encourage planting of cover crops. Some progress is being made: In 2015, there were 163 contracts through the Environmental Quality Incentives Program to support planting of cover crops, up from just three in 2009. In counties within the Raccoon River watershed, support for cover crops through the Conservation Stewardship Program grew more slowly, from 26 in 2012 to 30 in 2015.

But that’s far from enough. A recent EWG mapping project shows that at current levels of spending, it would take 40 to 75 years before enough cover crops are planted to protect 60 percent of Iowa’s cropland.

Riparian buffers

Riparian buffers are strips of grass or trees planted between crops and streams. When it rains, the buffers filter mud, fertilizer, manure and other pollutants out of water running off fields. Buffers also strengthen stream banks that otherwise may collapse and foul streams with mud and other pollutants.

Riparian buffers are critically important to controlling phosphorus runoff, which can spark the growth of harmful algal blooms in streams, rivers and lakes. Algal blooms, along with mud and manure, are a major source of the organic matter that triggers TTHMs and other disinfection byproducts contaminating drinking water.

EWG used high-resolution satellite imagery provided by Planet – a private earth observation company – to check for buffers along over 2,500 miles of waterways bordered by cropland in the Raccoon River watershed. Our investigation compared acres of buffers within 100 and 50 feet of stream banks from 2010 to 2011 and 2015 to 2016.

We found a net loss, as some landowners added 803 acres of buffers within 100 feet of stream banks, but other landowners destroyed 1,070 acres of buffers. Losses also overwhelmed gains within 50 feet of stream banks. And almost one-fifth of the waterways had no protective buffers at all. Most of those streams are intermittent, flowing only after rains, but are still a major source of pollution.

Data from the federal Conservation Reserve Program confirmed our findings. In Raccoon River watershed counties, between 2009 and 2014, there was a net loss of 376 riparian buffer acres enrolled in the CRP.

See riparian buffer losses and gains on an interactive map.

Time to act

Time is running out for the millions of Americans who depend on water flowing through or under intensively farmed land. Exposure to high levels of nitrate and TTHM are putting their health at risk and increasing the cost of clean water. The cost of adding treatment systems to remove nitrate can be crippling for small communities.

Utilities are doing their best to deliver water within the legal limits, but it is an ever-increasing challenge as pollution of source water goes unchecked. EWG's Tap Water Database shows that from 2014 to 2015, water in more than 1,700 public water systems, serving over 6.7 million people, was contaminated with nitrate at or over an average of 5 ppm, the increased cancer risk level. More than 460 systems, serving more than 500,000 people, had average nitrate levels at or above 7.5 ppm.

Government programs that encourage farmers to act have an important role to play, and focusing these programs on getting the right pollution prevention practices in the right places would make them more effective. But decades of experience show that voluntary programs alone aren't enough.

Landowners who voluntarily adopt a protective practice can voluntarily take it out. Funding for these programs is inadequate to address the scope and scale of the problem. And it’s not fair to ask people already paying utilities to treat their water to also pay to keep contaminants out of the water in the first place.

It’s time for states to enact a basic standard of care – a set of common-sense pollution prevention practices that farmers and landowners should be expected to maintain as part of the responsibilities that go hand-in-hand with the rights of land ownership.

States should tighten existing standards or enact new standards to ensure fertilizers and manure are applied when, where and in the right amount to prevent pollution. Minnesota now requires 50 feet of protective vegetation between cropland and public waterways. More states should follow suit. Landowners should be expected to take simple and well-understood measures to prevent gully erosion that delivers mud, fertilizer, manure and farm chemicals to waterways.

The basic standard of care should be tailored to reflect differences between farming systems and local watersheds, or unique threats to water quality. But a basic set of standards must be in place in every county to create a solid foundation on which a far more effective suite of voluntary government programs can be built.

Beyond state action, reauthorization of the federal farm bill in 2018 is a remarkable opportunity to jump-start progress. 

But it is more than fair to expect farmers and landowners to expand their efforts to protect the environment in return for the generous farm and insurance subsidies they receive. According to the Congressional Budget Office, in 2016 alone, those subsidies totaled $14.5 billion, with a projection of another $64.3 billion in spending over the next five years.

The conservation compact between farmers and taxpayers in the 1985 Farm Bill sparked dramatic progress in cutting runoff from the most vulnerable cropland and saving wetlands. More than 30 years later, it’s time for Congress to require all subsidized growers and all the cropland they farm to meet conservation standards to cut polluted runoff.

To remain eligible for farm program benefits and crop insurance premium subsidies, farmers and landowners should take steps on all annually planted cropland earning subsidies to:

• Achieve a rate of soil erosion no greater than the soil loss tolerance level on all annually planted cropland;
• Prevent ephemeral gully erosion; and
• Establish and maintain a minimum of 50 feet of perennial vegetation between annually tilled cropland and intermittent or perennial waterways.

Many farmers are likely already doing everything needed to meet their obligations under a new and stronger conservation compact. Those who aren’t should have five years to get their plans in place and another five years to fully apply the plans on their farms.

Donate today and join the fight to protect our environmental health.

The U.S. Geological Survey national hydrography dataset, the Iowa Department of Natural Resources channelized stream layer, and the Iowa Flood Center top-of-bank data were used to establish a footprint for surface water. Those data were subset to remove all public lands, forest, residential area and any land tracts with less than 10 percent area in cultivated land.

The U.S. Department of Agriculture Farm Service Agency Common Land Units, or CLU, were used to calculate the percent of agricultural area by summarizing pixels of corn and soybeans taken from the USDA’s Cropland Data Layer.

The buffer universe was created by using a 100-foot and 50-foot distance from all agricultural waterways so long as it intersected with CLU classified as agriculture.

For buffer detection, vegetation was detected from the Normalized Difference Vegetative Index, or NDVI, calculated from 5-meter resolution Rapid Eye imagery from Planet. All vegetation was detected between mid-to-late May 2009 and 2010 and mid-to-late May 2015 and 2016. The grass universe was used to calibrate the existence and longevity of buffers within the buffer universe.

Landsat 5 TM and Landsat 8 OLI were used to detect the emergence and vigor of grass using derived NDVI to capture the field level persistence and vigor during the spring and fall. All cover crop acres were subset to areas of corn and soybeans using the USDA’s Cropland Data Layer.

1 U.S. Department of Agriculture, National Agricultural Statistics Service, Cropland Data Layer. Accessed June 1, 2017. USDA-NASS, Washington, D.C. Available at nassgeodata.gmu.edu/CropScape/

2 Iowa Department of Natural Resources, Confinement Feeding Operations Registered with the Iowa DNR. 2016. Accessed June 1, 2017. Iowa DNR, Des Moines, Iowa.  Available at programs.iowadnr.gov/nrgislibx/

3 U.S. Department of Agriculture, 2012 Census of Agriculture, Table 41. Fertilizers and Chemicals Applied: 2012 and 2007. Available at www.agcensus.usda.gov/Publications/2012/Full_Report/Volume_1,_Chapter_2_US_State_Level/st99_2_041_041.pdf

4 Iowa Department of Natural Resources, Source Water Protection Wells. 2015. Accessed June 1, 2017. Iowa DNR, Des Moines, Iowa. Available at www.iowadnr.gov/Environmental-Protection/Water-Quality/Source-Water-Protection

5 Iowa Department of Public Health. Iowa Public Health Tracking Portal. Nitrate Measures. Accessed June 1, 2017. Iowa DNR, Des Moines, Iowa. Available at pht.idph.state.ia.us/Dashboards/Dashboards/Private%20Drinking%20Water/Nitrate%20Measures.aspx

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The wildfires that burned across Maui, Hawaii, in August 2023 became the deadliest conflagration in the United States in more than a century. While the harm to homes and tourism drew the most attention, agriculture was also heavily affected across the island, and the harm did not stop once the flames were out.

In some cases, fires smoldered underground for weeks. Water systems were destroyed, and some were contaminated in ways scientists are only beginning to understand.

As an environmental engineer , I work with communities affected by wildfires and other disasters. I also led a team of university and public works professionals to assist in Maui’s response to the fires.

In a new study based on that effort, my team worked with the Hawaii Department of Agriculture to assess damaged water systems, including water pipes, wells and pumps that are essential for livestock and crops. It was the first study of its kind to examine wildfire damage to agriculture water systems.

The results show the types of damage that can occur when a fire burns through property, and they offer a warning to agricultural regions elsewhere. With the U.S. averaging over 60,000 wildfires and 7.2 million acres burned each year, it is clear that wildfires have become a whole-of-society problem .

Contaminated water infrastructure poses risks

Wildfires often knock out power, which can disable water pumps that farmers and ranchers rely on. They can also damage pipes in ways that can release toxic chemicals and have long-lasting effects.

Recent municipal water system studies by my team and others have shown that water sources and even the pipes and tanks can become unsafe to use . Studies in fire-swept areas have found levels of volatile organic compounds, or VOCs, such as benzene, a carcinogen, above hazardous waste limits . Exposure to this water can cause immediate harm to people.

When water pumps stop working or components are destroyed, municipal water systems lose pressure. When that happens, VOCs can enter from heated or burning plastics , structures and vegetation.

An insidious challenge is that VOCs penetrate plastic water lines , gaskets and tanks like water going into a sponge. Even after bad water is flushed out, chemicals can leach from the plastic and make the water unsafe for weeks to months . Damaged components have to be replaced.

In the wake of the Maui fires, however, there was no immediate guidance on how farmers and ranchers should inspect and test their water systems.

Learning from Maui’s experience

Farms and ranches had many plastic water system components. On one ranch, fire destroyed more than nine miles of plastic water pipe. Much of the pipe ran above ground alongside fencing, which also burned.

Plastic irrigation systems were destroyed. Numerous other components melted, were leaking or lacked water. The loss of power sometimes prevented water pumps from keeping the pipes full of water.

While wells can become contaminated and well casings can burn , the wells themselves were not contaminated. This was mostly because the wells were set back from combustible materials and because firefighters and property staff helped to protect them.

Debris and particles from smoke, however, did enter animal troughs, buckets and waterers. These items had to be drained and cleaned for the safety of the animals. Water systems were repeatedly flushed with clean water after the fire, and VOC testing of the water supplies did not find lingering contamination.

Lots of questions still to answer

There are still many unanswered questions. Since there was no VOC testing procedure for agricultural water systems before the fires, there is no data to show the frequency and severity of this kind of contamination.

Not all municipal water systems that suffer fires become contaminated. Contamination is related to differences in the sites, systems and the fires themselves.

There is also no data on the degree to which this wildfire-contaminated water would harm animals and crops. Would animals avoid the water and become dehydrated? Can crops become contaminated? Will exposure affect the meat of livestock? Many of these unanswered questions will require the expertise of veterinary medicine and crop and soil scientists.

What are the solutions?

One thing that was clear is that farmers and ranchers lack adequate guidance to prevent wildfire-caused pollution of their water systems. Some practical lessons learned can help these community members bounce back:

Defensible space should be established by keeping equipment 30 feet away from combustible materials. Burying plastic components 3 feet underground helps protect them from fire.

Similar to municipal water systems after a fire, damaged agriculture water system components should be isolated . Pipes and tanks should be rapidly refilled and extensively flushed with water to help remove potential contamination.

Water delivery devices, including troughs, buckets and tire waterers, should be drained and cleaned. When contamination is a concern, chemical water testing should be conducted. In some cases, components will have to be replaced.

A 2024 survey of California farmers shows that the top three resources “relied on and wished for during wildfire” were generators, water pumps and water storage tanks. These items would help prevent water system pressure loss and contamination.

Who can help?

Wildfire risk to farms and ranches can be reduced. State and federal agriculture departments and insurance companies can provide financial assistance. Technical assistance is available from universities .

Lessening the impact of wildfires and expediting recovery can help farms and ranches do yeoman’s work to support health and the economy.

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Detection of contaminants in water supply: A review on state-of-the-art monitoring technologies and their applications

Syahidah nurani zulkifli.

a Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia

Herlina Abdul Rahim

Woei-jye lau.

b Advanced Membrane Technology Research Centre (AMTEC), Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia

Water monitoring technologies are widely used for contaminants detection in wide variety of water ecology applications such as water treatment plant and water distribution system. A tremendous amount of research has been conducted over the past decades to develop robust and efficient techniques of contaminants detection with minimum operating cost and energy. Recent developments in spectroscopic techniques and biosensor approach have improved the detection sensitivities, quantitatively and qualitatively. The availability of in-situ measurements and multiple detection analyses has expanded the water monitoring applications in various advanced techniques including successful establishment in hand-held sensing devices which improves portability in real-time basis for the detection of contaminant, such as microorganisms, pesticides, heavy metal ions, inorganic and organic components. This paper intends to review the developments in water quality monitoring technologies for the detection of biological and chemical contaminants in accordance with instrumental limitations. Particularly, this review focuses on the most recently developed techniques for water contaminant detection applications. Several recommendations and prospective views on the developments in water quality assessments will also be included.

List of Acronyms

1. introduction.

Waste production from agriculture, industrial sewage, animal and human activities are affecting the boundaries between clean water and wastewater, causing the reduction in the fresh water supply for human. Water ecology provides services such as food production, nutrient cycling, habitat provision, flood regulation, water purification and soil formation [1] . Biological and chemical contaminants in tap and drinking water, initiate the evolution of contagious diseases [2] . Therefore, fast and sensitive detection techniques are crucial to ensure safe and clean water supply. Unsafe water supply affects human health, causing contagious diseases such as hepatitis, influenza, SARS, pneumonia, gastric ulcers and pulmonary disease [3] . There are numerous non-biological contaminants existed in the water supply and some of the examples are silica, sodium, sulphur, ammonia and chlorine [4] . Other hazardous substance of heavy metals such as cadmium (Cd), lead (Pb), arsenic (As), mercury (Hg) and nickel (Ni) are also found in water supply [5] . These non-biological contaminants are among the commonly detected pollutants in urban areas that constitute a wide array of human activities.

The preservation of water quality has been regulated since the introduction of directive 91/271/EEC , which requires accurate treatment process targeting on organic contaminants, nitrogen and phosphorus [6] . In addition to these contaminants, other concern on the water quality includes the existence of microbiological contaminants in tap and drinking water at point of consumption. Derivation of pathogenic activity in water supply poses serious threats not only to human but also the entire water ecosystem. Pathogenic microorganisms can be categorised into bacteria (e.g., Salmonella typhi , Vibrio cholera and Shigella ), viruses (e.g., Poliovirus ) and protozoa (e.g., Giardia lambia and Cryptosporidium ). These types of microorganisms have been periodically detected in the water samples of river, groundwater and drinking water [7] , [8] . Hence, minimizing the exposure of deadly diseases is important by providing early warning detections on the presence of pathogens [9] . According to World Health Organization (WHO), the most commonly found microorganisms in the drinking water sources are Cryptosporidium, Legionella, Pseudomonas , Giardia and E. coli [10] , [11] , [12] , [13] . A list of possible water supply contaminants that is based on standard guideline is summarized in Table 1 .

List of the most commonly found contaminants in water supply [9] , [10] , [11] , [12] , [13] .

Recently, analytical technologies in water monitoring have taken a variety of directions. There are several water monitoring techniques, including conventional instrumental analysis (laboratory-based analysis), sensor placement approach, model-based event detection, microfluidic devices, spectroscopic approach and biosensors. Selecting suitable detection technique(s) is strongly dependent on the purpose of intended detection analysis, whether it requires quantitative, qualitative or hybrid measurement. Biological and chemical sensors have been in great demand for use in water monitoring technology, and they appear to be feasible for device integration and commercialization.

Previously, the detection of water contaminants has often been conducted manually in water laboratory facilities [14] . At the laboratory level, analyses are usually carried out by skilful personnel using high-end and cutting-edge technologies. Conventionally, multiple fermentation tube technique [15] , [16] , filtration method [17] , [18] , DNA amplification [19] , fluorescence in-situ hybridization (FISH) techniques [20] , [21] , capillary electrophoresis [22] , [23] , field-flow fractionation [24] , [25] , chromatography [26] , [27] and mass spectrometry [28] , [29] are the commonly used instruments to detect contaminants in the water samples. The potential benefits of laboratory-based analytical methods have been recognized for a long time, but studies have shown that they are not very efficient for on-site monitoring applications. With the technological advancement in the analytical chemistry, new techniques are developed through the introduction of advanced spectroscopy [30] , model-based event detection [31] , water quality sensors [32] , [33] , microfluidics [34] , [35] , [36] and biosensors [37] , [38] , [39] . Recently, wireless sensor network has been adapted in various detection techniques for portability. The evolution of water contaminant detection techniques is illustrated in Fig. 1 .

Evolution of contaminant detection techniques in water analysis application.

Contaminant detection analysis is gaining importance in the water monitoring and environmental applications. An initial growth in water quality sensor fabrication and optimal sensor placement for event detection has been introduced over the past several years [40] , [41] , [42] , [43] . The synthesis of the aligned water sensors, also known as sensor placement approach, could improve the device detection performance by increasing the detection sensitivity [44] , [45] . Deploying sensors in water distribution system allow the measurements of temperature, pH, turbidity, conductivity, oxidation reduction potential (ORP), UV-254, nitrate-nitrogen and phosphate both on-line and off-line. These water quality parameters are important to the treated water could meet the limit of detection (LOD) set by the United State Environmental Protection Agency (USEPA) [46] . However, the installation of numerous sensors along the distribution system seems not very practical owing to high installation cost [47] .

Determining the presence of contaminants in water requires accurate and fast response detection techniques. Intentional sabotage events such as water contamination in Japan [48] , the incidence of mesophilic Aeromonas within a public drinking water supply in Scotland [49] and occurrence of Aeromonas spp . in tap water in Turkey [50] triggered awareness on the need to have high-accuracy sensors installed along the water distribution system (WDS). Since then, obtaining optimal sensor placement has been widely explored for the security of WDS by utilizing model-based event detection technique. Optimal sensor placements, detection likelihood, expected contaminant concentrations and affected populations could be predetermined using detection algorithm by obtaining signals from conventional water quality sensors.

A surrogate approach for contamination detection is also suggested by the EPA to determine irregularities of water quality parameters obtained from sensing mechanism for an early warning of possible presence of contaminants [51] , [52] . Obtaining an early detection system using multiple sensor data station on-site is more beneficial compared to the data from a single site [53] . Most of the water quality parameters are used as primary indicators for contamination events in WDS, which are obtained from an online database, such as CANARY Database [54] , [55] , [56] , [57] . The main objective of the event detection method is to: (1) identify the possibility of event occurred, (2) characterize the event into subgroup (e.g., spatial area, time duration and severity level), and (3) detect contamination as accurately and early as possible. Achieving optimal time responses are superior for early indications of potential contaminants in WDS [58] . To minimize the spread of outbreaks, rapid and sensitive detection of pathogens is important.

As many different types of contaminants could present in WDS, case-to-case approach is necessary for accurate qualitative and quantitative analysis. To tackle the issues, handheld detection devices such as microfluidics sensors, miniaturized biosensors as well as portable spectroscopy are widely considered. Nowadays, lab-on-a-chip platforms that require microscopic amount of fluids (10 −9 –10 −18  l) are achievable for sampling, filtration, pre-concentration, separation and detection of biomolecules or analytes. Microfluidic composes channels with dimension up to hundreds of micrometers (μm). The integration of microfluidic sensors constitutes a multi-disciplinary theorems for sensing [59] . This kind of sensors is widely used in biomedical, science, genomics, forensics and environmental studies and for immunology or biochemistry applications [60] .

The most frequently used vibrational spectroscopy instruments in water monitoring technology are Infrared (IR) and Raman spectrometers. Vibrational spectroscopy is based on the correspondence of radiation absorptions to a discrete energy level, which is generated from the stretching or bending of atom molecule vibrations at frequencies of 1012–1014 Hz. In recent years, the integration of biosensor and spectroscopic techniques with digital microfluidic (DMF) devices is widely explored for the use in contaminant detections. The popularity of such integrated technique can be reflected by the number of technical papers published in the literature [61] , [62] , [63] , [64] . However, two major concerns regarding the DMF device are the complete elimination of analytes from sensing surface of DMF and disassemble of target analytes from the biosensor receptors for every sensing cycle [65] .

This paper intends to provide an overview on the developments of water monitoring technologies for both biological and non-biological contaminants determination. As detection mechanism varies with water contaminants, extensive review on analysis and data mining is also provided. This review emphasizes the current trend in water monitoring technologies and compares their performances with conventionally used methods. Significant limitations and drawbacks of the techniques are also discussed and recommendations are provided for future development in the water quality monitoring. Nevertheless, some analytical methods such as photo-acoustic, ultrasonic and microwave spectroscopy are not included in this review as they are very limited in applications.

2. Discontinuous (sample-based) methods

2.1. biological contaminants.

Microbiological parameter in tap and drinking water focuses on the detection of various pathogens using indicator organisms. Transmission route disease, also known as faecal-oral route, occurs when pathogens in faecal particles are transmitted into oral cavity of another host. According to the European Commission Drinking Water Directive, indicator bacteria E. coli and Intestinal enterococci should not be present in 100 mL of water volume [66] . The proliferation of water monitoring technology has prompted awareness over the safety of microorganism contaminants to human health and environment. Despite the advancement of the other techniques, the conventional culture-based methods are still in-use for the detection of microbiological parameters in water [67] .

2.1.1. Multiple tube fermentation (MTF) technique

Multiple tube fermentation (MTF) technique is one of the standard laboratory methods that can be used to detect microbiological activity in water samples. The MTF technique is executed in a three-stage procedure which is known as presumptive stage, confirmed stage and completed test [68] . Presumptive stage consists of a series of tube incubation process resulting in the formation of gas indicating positive presumptive test. Enumeration procedure for each bacterium sample is executed using suitable broth medium in the presumptive phase [69] . The inoculation process of testing tube samples should be performed instantly after any gas formation occurs. This procedure is known as confirmation stage.

According to EPA’s Standard Methods 9131 for Total Coliform: Multiple Tube Fermentation Technique [70] , completed test is finalized by gas formation and the presence of bacteria in the culture colonies. Detection of coliforms is often carried out by using MTF technique [71] . The concentration of bacteria in water samples is evaluated by examining a series of tubes containing suitable selective culture medium and dilutions of water sample. Formation of turbidity will occur accordingly due to the microbial growth and the results are expressed in terms of statistical estimation of the mean, known as the most probable number (MPN).

The presence of coliforms bacteria, known as ‘indicator’ organisms, is identified based on the MTF technique with an A-1 medium for a MPN test procedure described in the Standard Methods for the Examination of Water and Wastewater [72] . Using this technique, various types of coliform bacteria such as E. coli , Enterococci , Salmonella and Bacillus could be easily detected in different water samples [73] , [74] , [75] , [76] . On the other hand, MTF technique was also found to be effective in the process of yeast isolation and identification of E. coli , Enterococcus spp. and P. Aeruginosa densities [77] . The result showed the positive correspondence between yeast densities and counts of standard indicator, suggesting that yeast may also be considered as an organism indicators of sewage contamination.

MTF method was also used to determine E. coli in the water samples and high detection rate (100%) could be achieved compared to only 75.5% shown by a membrane filter (MF) method [78] . The presence of faecal contaminants in the water samples is confirmed based on turbidity measurements, which are reflected by the increase of water temperature and decrease of dissolved oxygen (DO) content [79] . Reduction in DO content may reduce the survival rate of coliform bacteria in aquatic environments [80] . However, determining DO measurement in surface level water samples is considered irrelevant due to high production of oxygen on its surrounding that leads an indirect proportional relationship between turbidity, DO and faecal coliform [81] .

2.1.2. Membrane filtration (MF) method

The MF technique is recognized by the USEPA and UNEP/WHO as a method for detecting biological contaminants of potable water. This technique is capable of isolating and eliminating discrete microbiology colonies in relatively large number of sample volume compared to the MTF technique [82] , [83] , [84] . Similar to the MTF technique, MF method is generally used for major indicator organisms [85] . In the past, both methods have been conducted at laboratory level, but due to the advancement of portable technologies, MF and MTF method are now able to be executed for on-site applications.

Incubation of MF on solid and/or liquid selective media at appropriate temperature allows the growth and development of organism cultures providing a direct count of total coliforms colonies. The choices of temperatures depend on the type of bacteria indicator and the selective media. For instance, P. aeruginosa , E. faecalis and Penicillium were able to be detected using cellulose nitrate membrane filters which after being incubated for 48 h at 44 °C on the solid and liquid selective media [86] . The MF technique has also been used for retaining virus, known as F-RNA coliphages via incubation using mixed cellulose nitrate and acetate membrane filters [87] .

Work conducted by Grabow et al. [88] indicated that theoretical efficiencies of 100% are achievable with procedures governing to direct plague assays on 100 mL samples and the presence-absence test on 500 mL samples. In addition, positively-charged filter media have been widely used for recovering bacterial viruses and phage. However, poor detection of viruses and phage was experienced following poor absorption and inactivation caused by extreme pH level exposures [88] , [89] . In general, microfiltration and ultrafiltration membranes are preferable to be used for the filtration purpose due to their appropriate range of surface pore sizes that could retain microorganism effectively [90] . Previous work has shown that the removal of bacterial contaminants in water could be carried out using microfiltration-based method by observing the in-vitro nitric oxide production and binding response of Limulus amebocyte lysate (LAL) assays [91] .

Instead of using single type of membrane filter, simultaneous use of microfiltration and ultrafiltration membrane filters were attempted in recent years. Xiong et al. [92] has established the first quantitative assessments in the water samples using integrated method and reported the total organic carbon (TOC), total suspended solid (TSS) and turbidity of the samples to be 95–630 mg/L, 180–1300 mg/L and 150-1900 NTU, respectively. The integrated method has also been used to reduce peroxidase activity in red beet stalks in order to maintain natural pigment stability [93] . Results showed that it achieved more than 99.5% reduction of peroxidase activity and 99.9% reduction in turbidity.

Previous work showed the applications of membrane bioreactor (MBR) process based on aerobic fermentation method [94] . MBR combines the membrane filtration process with reactor that involves biological reactions. The efficiency to monitor microbial population in water sample depends on the attachment of bacteria onto inert material inducing high biomass [95] , [96] . Integration of bioprocess and MF method provides a better efficiency in the removal of bacteria as, demonstrated by Adan et al. [97] in which a derivation of photo catalysis-microfiltration hybrid system was used to remove E. coli from water source.

MF, selective medium broth and culture plate methods have become the standard (ISO 16654:2001) to monitor the presence of E . coli and other pathogens in water [98] . However, the drawbacks of employing these methods are time-consuming, laborious and low sensitivity in detecting contaminants at low concentrations [99] . Limitation of selective culture or immunological methods due to lack of consistent differentiation in phenotypic traits may also affect detection accuracies [100] . The efficacy of various membranes was elaborated by Snyder et al. [101] in which MF is capable of reducing concentration of contaminants with specific properties. It was reported that the degree of contaminant removal was highly dependable on the characteristic of the membrane and the molecular properties of the targeted analytes. Microfiltration and ultrafiltration membranes showed the least value of contaminant removal whereas reverse osmosis membranes are capable of removing almost all investigated contaminants [101] . Fig. 2 illustrates a process that combined ultrafiltration and reverse osmosis as advanced treatment process for wastewater. The findings indicated that reverse osmosis membrane could remove almost all targeted contaminants, achieving values below the method reporting limits (MRL).

Process flow diagram of a submerged Zenon ZeeWeed™ 1000 (ZW1000) ultrafiltration unit integrated with a multi-pass reverse osmosis unit (Synder et al. [101] ).

2.1.3. DNA/RNA amplification

DNA amplification is used to detect molecular biology by amplifying a single copy or a few copies of targeted DNA molecules to produce specific DNA particles in vitro . Polymerase chain reaction (PCR) invented by Kary B. Mullis was the first DNA amplification method designed to study particular DNA molecules [102] . Traditionally, replicating DNA sequence requires days or weeks to complete. But, with amplification of DNA sequence using PCR, it only takes several hours [103] . Because of its high detection sensitivity and level of amplification, PCR is capable of replicating miniscule amount of DNA sequences and is extremely useful in commercial uses, including genetic identification, forensics, quality control industrial applications and in vitro diagnostics.

In general, PCR amplification reaction constitutes three major elements: (1) a thermo-stable DNA polymerase, (2) a mixture of deoxynucleotide triphosphates (dNTPs) and (3) two oligonucleotide primers [104] , [105] . One cycle amplification denotes a series of temperature and time, hence amplifies the amount of targeted DNA sequence after reaction takes place. There are three steps in PCR protocols, i.e., denaturation at 93–95 °C for 1 min followed by 45 s annealing at 50–55 °C and 1–2 min elongation at 70–75 °C [106] . Modification on the standard PCR protocol could also be performed using different techniques, e.g., a multiplex PCR protocol for detection of E. coli , Campylobacter spp. and Salmonella spp. in both drinking and surface water [107] , [108] , [109] .

Fig. 3 shows the PCR amplification product developed using oligonucleotide primers Rfb and SLT-I for the purpose of detecting E. coli O157 and E. coli virulence gene SLT-I in drinking water. Optimization of PCR protocol was tested with E. coli O157:H7 strain. Another example is conventional hot-start PCR technique for the detection of Actinobacillus actinomycetemcomitans by heating the reaction components under DNA melting temperature before mixing with polymerase to reduce non-specific priming amplification [110] , [111] . A touchdown PCR amplification was then established to identify the presence of bacteria in aquatic samples by gradually decreasing the primer annealing temperature in later cycles [112] , [113] , [114] . The analysis of bacterial water contaminant was performed using a universal conserved bacterial 16S rDNA sequence, which are specifically used for amplification of 16S rDNA fragments with GC-clamp-EUB f933 and EUB r1387 primers ( Table 2 ).

A typical PCR amplification product optimized using E. coli O157:H7 strain. (a) Lane 1: 1 kb DNA ladder; Lane 2: 292 bp O157 gene amplified with Rfb F and R primers. (b) Lane 1: 50 bp DNA ladder; Lane 2: 210 bp SLT-I gene amplified with SLT-I F and R primers (Imtiaz et al. [109] ).

Primer sequences and positions [114] .

Although the PCR method has a high detection successful rate, it is still associated with several limitations that include low sensitivity to certain classes of contaminants and reduction of amplification efficiencies in the case where inhibitors are in present in water samples [115] . Over the past few decades, there has been a diversity of newly developed technologies to overcome these limitations. Some of the examples are quantitative real-time PCR assays (qPCR), reverse transcription real-time PCR (RT-qPCR) protocol, loop-mediated isothermal amplification (LAMP) technologies, strand displacement amplification (SDA), ligase chain reaction (LCR), rolling circle amplification (RCA), helicase-dependant DNA amplification (HDA) and the most recently developed random amplified polymorphic DNA analysis (RAPD) [116] , [117] .

The qPCR automates both amplification and detection in quantitative measures. The simplified quantification is obtained through quantification cycles (Cqs) which are determined by fluorescence threshold or maximum second derivative [118] . Exponential phase in qPCR technique can be continuously observed for 30–50 Cqs and can be used to estimate the initial number of targeted DNA. The use of a qPCR assay to positively detect E. coli O157:H7 strains in drinking water was carried out using molecular beacons (MBs) oligonucleotide probes [119] , InstaGene™ matrix from Bio-Rad specially formulated 6% w/v Chelex resin [120] , minor groove binding (MGB) probes with 6-FAM (6-carboxyfluorescein) [121] and propidium monoazide (PMA-based) qPCR assay [122] . Quantitative PCR assay provides the possibility of quantitative analysis for E. coli target by using formulated structural quantification curve as shown in Fig. 4 . Such measure reduce the false positive results during analysis.

(a) Sensitivity of real-time PCR assay consist of ten-fold serial dilutions of DNA template isolated from E. coli JM109 strain ATCC 43985 and (b) Linear curve for real-time PCR assay with wide range of initial target concentrations (from 10 2 to 10 7 CFU mL) (Sandhya et al. [119] ).

There have been several commercially designed real-time PCR assays for the detection of pathogens (e.g., F. tularensis, B. anthracis and Y. pestis ) with high detection sensitivity and diversity of pathogen detection capabilities [123] , [124] . The qPCR techniques have been found useful for the detection of Naegleria sp . by referring to melting curve analysis SYTO9 and qPCR TaqMan assay [125] , [126] , [127] . Melting curve analysis is beneficial for the manipulation key conditions, including temperature interval and time delay before data are collected for each step. For example, melting curve that provides three informative peaks within temperature range of 79–86 °C ( Fig. 5 ) can be used to distinguish species based on the positions and height of the peaks obtained.

Melting curve analysis of the 5.8S rDNA/ITS product of seven Naegleria species: (a) N. fowleri , (b) N. lovaniensis , (c) N. italic , (d) N. australiensis , (e) N. gruberi , (f) N. byersi, (g) N. carteri and (h) Willaertia magna (Robinson et al. [125] ).

Another extended version of standard PCR method is the RT-qPCR, which is a useful technique to identify specific messenger RNA (mRNA) as well as DNA from any type of living microorganism cells, either qualitative or quantitative measures [128] , [129] , [130] . This technique evolved tremendously after the introduction of hybridization on target DNA sequence using an oligonucleotide probe. The RT-qPCR technique involves the hybridization of oligonucleotide primer to produce a complementary DNA (CDNA). This process of deoxyribonuclease I (DNase I) is used to eliminate contaminated DNA that triggers false positive results. The application of RT-qPCR assay approach has been used in detecting pathogens such as mRNA in E. coli cells [131] , family of Filoviridae viruses and RNA transcription from Ebola viruses [132] , cereulide-producing Bacillus cereus [133] , RNA molecules of Salmonella [134] and rotaviruses and coronavirus in feces contaminations [135] . The reverse transcription PCR (RT-PCR) techniques proven to provide high efficiencies by amplifying both DNA and RNA sequence. Conventional PCR methods meanwhile only amplifies DNA. As reported by Wang et al. [136] , RT-PCR has high detection sensitivity on bacterial quantity (as low as one bacterium) compared to those of PCR-based techniques.

Although PCR-based techniques could show significantly higher positive detection rate, performing accurate thermal cycling and utilization of sophisticated instrumentation (e.g., fluorescence measurement) require higher throughput and longer time [137] , [138] , [139] , [140] , [141] , [142] , [143] . Therefore, an alternative to PCR is isothermal-based amplification methods. This method can be carried out without undergoing repeated thermal denaturation procedure and does not require sophisticated instruments [144] . Typically, loop-mediated isothermal amplification (LAMP) mechanism comprises two pairs of primers (inner and outer) and are dependable to strand displacement synthesis of DNA polymerase to produce loops amplifications [145] . LAMP has been widely used for diagnosis of biological specimens and is commercially available for environmental monitoring applications [146] , [147] , [148] . It has high sensitivity and rapid detection capability as well as greater ability to quantify several bacteria [149] , [150] . A pilot study was conducted to detect Staphylococcus aureus , E. coli , Pseudomonas aeruginosa , Klebsiella pneumonia , Stenotrophomonas maltophilia , Streptococcus pneumonia , and Acinetobacter baumannii using quantitative LAMP (qLAMP) with better identification (P < 0.001) than that of traditional culture-based method [151] . The functionality of loop primers designed for LAMP assays improved the detection specificities and sensitivities by several magnitudes [152] . This was proven by Sotiriadou and Karanis [153] , by employing LAMP assays approach for the evaluation of Toxoplasma in water samples based on amplification of B1 and TqOWP Toxoplasma genes with 100% success detection rate. Separately, a gene amplification using hydroxyl naphthol blue could successfully detect Naegleria floweri within 90 min reaction time with a Kappa coefficient of 0.855 [154] .

Based on the previous discussion, one can realise that LAMP method is less expensive to perform as it involves no real-time thermal cycler. Furthermore, it has greater sensitivity and can be potentially deployed for on-site water contaminant detection. Gallas-Lindemann et al. [155] reported that Giardia spp. and Cryptosporidium spp. could be detected using LAMP assays with 42.7% and 43.6% detection rate, respectively compared to 33.5% and 41.9% shown by the conventional nested-PCR. Besides, the LAMP technique offers 100% detection sensitivity with LOD of 50 fg/mL compared to the conventional PCR method [156] . Integrating the extension method with the standard PCR could provide faster, less false positive indicators, better compatibility for detection of multiple pathogens [157] , [158] , [159] . Moreoevr, LAMP was comparable to the qPCR method for surveillance of Dehalococcoides spp. in groundwater using six LAMP primers designed for each three RDase genes [160] .

Enteric viruses generally yield between 105 and 1011 virus particles per gram of individual stool [161] , [162] . There is no direct relation between the occurrence of bacteria and enteric viruses, hence suggesting the need to separately evaluate the presence of viruses in water supply. Culture-based method is not the preferable approach for evaluating enteric viruses as it requires higher analysis cost and longer analysis period. It is also found to have complexity related to the permissive system of some non-cultivable viruses in vitro [163] .

According to Kim et al. [164] , molecular detection done by qPCR and qRT-PCR methods could overcome the issues regarding the sensitivity and analysis time. Huang et al. [165] and Jiang et al. [166] also agreed that in comparison to the conventional nested PCR approach, the qPCR method offers better efficiency (>95%) in quantifying enteric adenovirus serotype in environmental waters. However, no method is completely perfect by taking into account the principle and procedure of each method. For instance, the detection of pathogenic viruses is obtained in a disinfection procedure, but this method is not suitable for the detection of coliform bacteria due to low concentration of bacteria indicator [167] . The generation of DNA-based amplification method has evolved due to demands in producing combined method of detection with higher specificity and rapidity. Probe based real time loop mediated isothermal amplification (RT LAMP) assay was then introduced for the quantification of Salmonella invasion gene (InVA), aiming to achieve significantly higher sensitivity [168] .

2.1.4. Fluorescence in situ hybridization (FISH)

In situ hybridization is a technique that enables the detection, identification, localization and enumeration of microorganisms. Cellular component and targeted analytes can be visualized via fluorescence probe based on fluorescence in situ hybridization (FISH) technique. The FISH technique has been used in a wide variety of research fields such as cytogenetic, microbiology and genetic diagnostics applications [169] . There are a few important factors such as probe design and fluorophore selections that need to take into consideration before execution of FISH experiments. Commonly used probe is 15–30 nucleotides long that is labelled with fluoorophore of 3′ or 5′ at each end. These probing designs are specifically used for the detection of microorganism and mRNAs as shown in Fig. 6 . One of the earliest application to embrace the usage of FISH technique was microbial ecology. Similar to DNA amplification method, the most commonly used DNA probe is 16S rRNA sequences for the detection of bacteria in living tissues as well as in aquatic environment samples. In recent years, the FISH technique has been applied in microbiological monitoring field. However, FISH has low fluorescence signals which limits the detection factor to the specified microbial community [170] . To address this issue, a multi-labelled FISH technique is recommended so as simultaneous detection of microbial groups could be achieved by improving the fluorescence signals [171] .

mRNA localization of cyp6CM1 and ABC transporter genes in midgusts of the whitefly Bermisia tabaci using FISH, (a) bright field of a B. tabaci midgut, (b) FISH fr mRNA localization on this midgut showing cyp6CM1 gene expression mainly in the filter chamber, (c) bright field of a B. tabaci midgut and (d) FISH for mRNA localization on this midgut showing an ABC transporter gene expression mainly in the filter chamber. (Definition – am: ascending midgut; dm: discending midgut; ca: caeca; fc: filter chamber and hg: hindgut) (Kliot et al. [169] ).

Previous study reported the detection and quantification of β-Proteobacteria and Cytophaga-Flavobacterium cluster in an urban river after 3–7 days of formation [172] . A similar technique was used to detect various members of Cytophaga-Falvobacterium cluster, classes of proteobacteria and members of Planctomycetales in an aquatic environment, ranging counts of 50% cells detection [173] .

A FISH-probe using Bacillus subtilis 16 s rRNA has been also reported with the aim of distinguishing targeted nucleotides between 465 and 483 genes [174] . However, the results showed that FISH method was not able to identify strains, i.e., B. altitudinis, B. cereus, B. gibsonii, B. pimulus and B. megaterium . The development of FISH technique over the past several years has increased its usage in determining various types of MRNA and DNA molecules [175] . Fluorescent nanomaterials, also known by quantum dots (QDs), have been introduced to improve fluorescent-brightness, photochemical cohesion and coherence emission spectra [176] , [177] , [178] . The QD-FISH method offers the ability to detect specific target genes. For instance, synthesis of biotin-streptavidin deoxyuridine triphosphate (dUTP) labelled DNA probes via PCR using dNTP mixture enable the detection of Ectromelia virus (ECTV), a member of the Poxviridae family [179] , resulting a genome detection of 80% after 36 h of post-infection with significant visual of red fluorescence. The detection of green micro-algae, U. prolifera using FISH was also carried out targeting the 5S rDNA of U. polifera, Ulva linza and Ulva flexuosa molecular genes using 5S-1 and 5S-2 probes [180] . Six species of green algae were also tested, however only U. prolifera could be labelled by both specific probes. Because of the complex structure of bio-analytes, direct detection and quantification of single-cell bacteria using FISH are rather difficult. Therefore, an additional combination of flax desegregation protocol with quantitative FISH technique is recommended [181] .

2.2. Non-biological contaminants

Generally, there are two categories of non-biological water contaminants. Chemical contaminants consist of elements or compound, such as volatile organic chemicals, disinfection by-products and synthetic organic chemicals which can be occurred naturally or man-made. Whereas, radiological contaminants are from an unstable atom that emits radiation, for instance plutonium and uranium [182] . Another water contaminants that are starting to raise awareness are engineered nanoparticles/nanomaterial. Example of them are metallic nanoparticles (e.g., Ag, Au, and Fe), oxides (e.g., CeO 2 , TiO 2 and ZnO) and quantum dots (e.g., ZnS). Although nanomaterials are beneficial for many industrial applications, the release of them may unintentionally promote hazardous occupations to environment and posses health risk to humans.

According to the EPA’s Chemical Contaminants – CCL 4 that was recently drafted, it is found that non-biological contaminants are possibly present in tap and drinking water [183] . The majority of organic water contaminants are from industrial activities, environmental degradation, agricultural run-off and naturally-occurring elements. Whereas, an inorganic water contaminants are the derivation of natural minerals resulted from erosions and runoff. With respect to the analysis of non-biological contaminants, chemical parameters such as pH, hardness, temperature, dissolved organic nitrogen, total organic carbon (TOC) and chemical oxygen demand (COD) are always considered. According to the WHO Guidelines for Drinking Water Quality (Fourth Edition), a derivation of the tolerable daily intake (TDI) should be taken into account when involving drinking water municipal [184] .

2.2.1. Capillary electrophoresis (CE)

Capillary electrophoresis (CE) is a separation technique used to analyzes molecular polarity and atomic radius based on ions electrophoretic mobility. The movements of analytes through electrolyte solutions are directly proportional to the applied voltage, where high electric field leads to faster mobility. CE has the ability to perform separation in capillaries with diameters in mm and in micro to nano-fluidic channels. The CE technique is often related as capillary zone electrophoresis (CZE), however, there are other CE-based methods which include capillary gel electrophoresis (CGE), capillary isoelectric focusing (CIEF) and micellar electrokinetic chromatography (MEKC) [185] . There have been several studies related to the determination of NH 4+ , Na + , K + , Mg 2+ and Ca 2+ ions in environmental samples using the CE detection method [185] .

The determination of existing unions (nucleotides, metal-ethylenediaminetetraacetic acid, haloacetics, etc.) in aquatic environments is achievable by analyzing their electrophoretic mobility within detection wavelength of 350/20 nm. Researcher have proposed a method consisting 50-μm straight capillaries with baseline noise modification to determine existing unions [186] . This method was found to be useful for the screening analysis of anions in liquid samples. Anions separation was executed simultaneously using a highly alkaline pH condition to attract a negative charge, triggering migration towards anode as shown in Fig. 7 . Because of this, the existing anions in aquatic environment can then be analysed based on their electrophoretic mobility within selective wavelength [187] . CE technique has also been reported in peptide analysis, qualitatively and quantitatively [188] .

A typical electropherogram of a 43-component (7 inorganic anions; 5 organic acids; 16 amino acids; 15 carbohydrates) anion standard mixture (Soga et al. [187] ).

Analysis on trace chloroanilines in water samples was developed by Pan et al. [189] using CE technique and the method could achieve LOD between 0.01 and 0.1 ng/L for eight aniline compounds within 25 min of detection. Under the optimum conditions, the enrichment factors were obtained within the range of 51–239 and exhibited linear calibration over three orders of magnitude (r > 0.998). Water contaminated by herbicide species is contagious to human health and potentially reachable to toxic levels. The identification and quantification of herbicides can be obtained using an extended CE method coupled with low voltage eigenmode expansions (EME) modelling technique [190] . The preconcentration and detection of environmental pollutants, such as 2,4-dichlorophenoxyacetic acid (2,4-D), 4-(2,4-dichlorophenoxy) butanoic acid (2,4-DB), and 3,6-dichloro-2-methoxybenzoic acid in water samples were executed using a Box-Behnken design (BBD) and response surface methodology (RSM) related to extraction efficiency. Because of this, herbicides could be detected using a novel MEKC method [191] . The composition of 25 mM borate, 15 mM phosphate, 40 mM sodium dodeclysulfate (SDS) and 3% (v/v) of 1-propanol at pH 6.5 was used as an optimum buffer. A successful LOD ranging from 0.02 up to 0.04 ng/g and LOQ of 0.1 ng/g was reported within the optimized conditions.

A similar method with the combination of online sweeping preconcentration in MEKC method was developed for the detection of five triazine herbicides in water samples [192] . However, under optimized condition, the LOD was slightly different with value shown in a broader range (0.05–0.10 ng/mL) in comparison to the conventional MEKC method as presented in Fig. 8 . Due to vast usage of animal-based fertilizers in agriculture, the contamination of water with estrogenic compounds cannot be prevented. These estrogenic compounds were found present in mineral and wastewater samples with an alarming rate. The adjoint detection method proposed by D’Orazio et al. [193] using ammonium perfluorooctanoate (APFO) – based MEKC was effective to detect 12 estrogenic analytes with LOD ranging from 0.04 to 1.10 μg/L.

Comparison of the electropherograms obtained by (A) conventional MEKC method (sampling: 1.0 μg/mL of the triazine herbicides in BGS, direct injection at 0.5 psi for 5 s), (B) the sweeping-MEKC method (sampling: 0.5 μg/mL in 50 mmol/L H3PO4 (pH 2.5), direct injection at 0.5 psi for 120 s) and (C) the combination of DLLME with the sweeping-MEKC method (sampling: starting from 5.0 mL of 10 ng/mL water sample for DLLME). Peak identifications: 1: prometon; 2: simetryn; 3: propazine; 4: atrazine; 5: simazine; u: unidentified peaks (Li et al. [192] ).

Since contaminants in water can exist in nano-scale measures, application of nanoparticles together with CE techniques has been presented in order to achieve a safe and sustainable water supply. Sensitivity of analytes detection could be improved using electrophoretic mobility integrated near the inlet capillary. Whether the analytes bind specifically to the sensitive capillary, deployments of several arrays of these capillaries are required for simultaneous analysis. Because of the complexity of equipment arrangements, its industrial implementation is still ambiguous.

The commonly used techniques to distinguish nanoparticles are based on either gel electrophoresis or capillary electrophoresis [194] , [195] , [196] . Detection of engineered nanoparticles (ENPs) such as bioconjugated quantum dots, have been demonstrated using polyacrylamide gel electrophoresis (PAGE). However, due to small pore size of polyacrylamide (PA) gels (<10 nm), the separation method is not practical. Hence, Hanauer et al. [197] introduced the separation techniques for nanoparticles with applications of agarose gel electrophoresis (AGE), with pore size of agrose gel ranging between 10 and 100 nm. The work presented a derivation of silver and gold nanoparticles using polylethylene glycols that, acted as electrophoretic mobility controller. However, findings turned out to be unsatisfactory for gold nanoparticles separation at common CE conditions in comparison to the ICP-MS and UV detection methods [198] .

Bioconjugated quantum dots [199] , [200] and protein-nanoparticle interactions [201] have also been found to be able to distinguish environmental samples using CE. The detection of nanoparticles continued to be favourable in metal and metal oxide nanoparticles separation by using various inorganic buffers as electrolytes [194] , [195] , [196] . Using CE sodium dodecyl sulphate, various nanoparticles, such as Au, AU, Pt and Pd, were able to be detected with resolution as low as 5 nm [202] , [203] . The CE technique is getting more and more popular and a number of publications on the modification and integration of various CE-based detection methods could be found in literature. These advanced methods intend to overcome some limitations of CE instrument, such as unsymmetrical peak identification [204] , poor mobility time repeatability [205] , low separation resolution [206] and limited injection efficiency ranging only from 10 −3 to 10 −7  μL [207] .

2.2.2. Gas/Liquid chromatography-Mass spectrometry (MS)

Mass spectrometry (MS) is an analytical tool used to measure molecular mass of targeted sample. Mass spectrometer used for environmental analysis is commonly coupled with a separation method such as gas chromatography and liquid chromatography [208] . Recently, there have been combinational methods for determination of non-biological contaminants in water samples. For instance, Albishri et al. [209] used a UV-based reversed phase liquid chromatography with integration of liquid phase micro extraction for the determination of five organophosphorus pesticides with concentration of 0.01–0.1 ng/mL in tap, well and lake water samples. The derivation of pesticides in water samples has been detected by using a novel approach of sensitive ultrasound-assisted temperature-controlled ionic liquid (IL) diquid −phase microextraction combined with reversed-phase liquid chromatography. Five organophosphorus pesticides were investigated by varying the IL type, IL volume, ionic strength, sonication time, heating/cooling temperature, centrifugal time and speed. In comparison to the conventional liquid chromatography technique, the proposed method improved the extraction efficiency up to 98%. A selected group of pesticides in tap and drinking water was also distinguished using liquid chromatography in which pesticide dimethoate, carbaryl, simazine, atrazine, ametryne, tebuthiuron, diuron and linuron were perfectly isolated ( Fig. 9 ) [210] . A series of gas chromatography (GC) with a nitrogen-phosphorus detector (NPD) has also been reported with the capability of detecting trace amount of eight different type of pesticides presented in drinking water [211] . Despite providing high detection sensitivity ( Fig. 10 ), GC analysis is considered unsuitable for non-volatile and high molar mass compound such as pesticides [210] .

Chromatogram obtained for the separation of pesticide standards using liquid chromatography: (1) carbendazim, (2) dimethoate, (3) simazine, (4) tebruthiuron, (5) carbaryl, (6) atrazine, (7) diuron, (8) ametryne adnd (9) linuron (Queiroz et al. [210] ).

Chromatogram obtained for separation of pesticides using mixed standard solution (gas chromatograph) (Qian et al. [211] ).

Quantifications of nicotine in tap water and wastewater at trace levels were performed using a novel gas chromatography mass spectrometry (GC–MS) with liquid–liquid extraction process and a satisfactory LOD of 2.6 ng/mL was reported [212] . A study on detection of benzene, toluene, ethylbenzene and xylenes in the water sample was conducted by Franendez et al. [213] using the magnetic solid-phase extraction (SPE) method prior to GC–MS technique. The experiments showed LOD of 0.3 μg/L for benzene and 3 μg/L for other compounds.

Disinfection by-products (DBPs) are very likely to be found in drinking water and are strongly linked to cancer [214] , [215] . To date, profound usage of GC–MS for the determination of DBPs is due to the wide range of available mass spectral library databases [216] . Unfortunately, GS-MS has limited detection sensitivity, which can only detect compounds with low molecular weight (< 800 g/mol) [217] . Hence, a new technique that combined multiple solid phase extraction (SPE), dual-column liquid chromatography high resolution-LCMS and precursor ion elimination (PIE) was proposed by Richardson et al. [218] . Verstraeten et al. [219] and Erickson et al. [220] also employed such technique to analyse public water samples that contained hormones, pharmaceutical personal care products (PPCPs), polyfluoroalkyl substances and herbicides. Barnes et al. [221] validated the occurrence of pharmaceutical contaminants using LC–MS and reported that the concentrations of sulfamethoxazole and carbamazepine that exceeded 0.1 ng/L were recorded in 9 wells while another 5 wells showed 0.07 ng/L concengration. The determination of pharmaceutical contaminants could also be found in work of Llorca et al. [222] in which LC–MS was used to detect 33 analytes. Other works on the detection of sulfamethoxazole and carbamazepine in drinking water and groundwater can be found elsewhere [221] , [222] , [223] , [224] .

Separately, perfluorinated chemicals (PFCs) are a large chemical compound that are used in wide variety of heavy industries, such as aerospace, automotive, buildings and construction, due to its ability to reduce friction [225] . Unlike other chemical excess, PFCs are frequently released into the aquatic environment due to the massive usage in industrial activities and food productions. As previously reported, LC–MS technique could be used to analyze the content of water samples containing perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) with detection limit as low 10 ng/L [226] .

Field-flow fractionation (FFF) technique with higher analytes sensitivity and selectivity is a family of analytical separation technique used to extract detailed information on chemical composition, functionality and molecular architecture. It was initially introduced by Calvin Giddings to separate macromolecules and colloids [227] . The working principle of FFF is due to the use of external field that is applied perpendicularly to the direction of phase flow within a capillary to derive analytes separation. In comparison to traditional chromatography approach, FFF is beneficial to those of analytes detection techniques in terms of effective separation components, minimum shear degradation, ultra high resolution, adjustable separation ability and mild operation condition which allows fragile analytes analysis [228] , [229] , [230] . Measurements of colloidal phosphorus in natural waters were demonstrated using an asymmetric FFF technique integrated with high resolution of ICP-MS and membrane filtration [231] . This separation technique comprises two categories, which are centrifugal force-based sedimentation (SdFFF) and perpendicular flow-based (FlFFF) that is mainly used for discrimination of engineered nanomaterial (ENMs) in aquatic environments. High density particles, such as metallic nanoparticles with a relatively large size were detected by SdFFF [232] , [233] , [234] owing to their ability to achieve higher resolution during separations.

The use of analytical techniques to detect and quantify ENPs in environment is limited due to the complex matrices of samples and extremely low concentrations of nanoparticles. A novel approach based on coupling hydrodynamic chromatography (HDC) and FFF has been proposed to separate polystyrene, silver and gold nanoparticles from environment samples [235] . Measurement of hydrodynamic radii of nanoparticles and retention time was conducted using calibration curves and an exceptional polynomial fit from on-line detectors (DLS, SLS) with size ranging between 20 and 80 nm (R 2  = 0.98) could be obtained ( Fig. 11 (a)). Comparison of nanoparticle hydrodynamic radii was further made using manufacturer’s off-line detectors (DLS, AUC, SP-ICP-MS) with detection of particle radius of 20.3 ± 0.6 nm at 24.7 min as shown in Fig. 11 (b).

(a) Calibration curve based on the polystrene (black points), gold (red point) and silver (pink points) standards and (b) Partial chromatogram for river water sample spiked with 4 μg/L of nAg following separation by HDC and detection using SP-ICP-MS (Proulx et al. [235] ). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

By combining methods for differentiation of nanomaterial characterization, the possibility of assessing silver nanoparticles in water samples is achievable. As reported by Antonio et al. [236] , the proposed combined technique (asymmetric flow field-flow fractionation, ICP-MS and UV–vis) enable the agglomeration process of silver nanoparticles in artificial seawater. Several works have been carried out to investigate the stability and detection of nanoparticles, including Ag, Au, Se, TiO 2 , and ZnO in natural systems [237] , [238] , [239] . The existence of nanoparticles in the aqueous environments must be taken into account since the main factors causing the derivation are due to surrounding effects, such as temperature, light, oxygenation and total surface area [240] , [241] . In addition, separation techniques related to inorganic engineered nanomaterial are currently expanding with the aim of achieving nano domain analysis.

A method of coupling ICP-MS with FFF technique was used by Lyven et al. [242] to differentiate iron- and carbon-based colloidal carriers based on the particle size difference. Peak deconvolution analysis was used to quantify and estimate the distribution between organic carbon- and iron-rich colloids and the results indicated the consistencies of chemical properties from two carrier colloids [243] . Although chemical chromatography (HPLC, GC, GC/MS) could offer identification and quantification of analytes, it is only able to detect specific contaminants [244] . Besides, it often involves multi-stage protocols and suffers a number of biases, such as loss of absorption due to the high reactivity [245] . Nonetheless, it must be pointed out that the chemical chromatography is still the preferable method for chemical identification.

3. In-line sensor-based monitoring

Continuous monitoring for microbiological contaminants, especially for chemical contaminants, is a challenging task due to the presence of variety of contaminants at low concentrations [246] . As discussed in the previous sections, standard/sample-based laboratory methods for detection of various water contaminants are often based on the discontinuous approach (off-line analysis). Hence, a sensor-based detection methods such as sensor placement approach (SPA), microfluidics, spectroscopic techniques and biosensors have been tremendously evolved over the last decade.

3.1. Sensor placements approach (water quality sensors)

In contrast to conventional analytical methods, deployment of multiple sensor station in the distribution system is an alternative approach to detect contaminants in a simultaneous manner. In recent years, multi-parametric sensor arrays have emerged to be less cost-oriented and user-friendly to monitor quality of water ecology systems [247] . The performance of multiple sensor stations was evaluated by Jeffry et al. [248] using real-time contaminants drinking water and the sensor was reported to be able to detect the existence of aldicarb, glyphosate, colchicines and nicotine in water samples. The data analysis was conducted based on a combination of both distance-based statistics and contaminant transitions.

An alternative low-cost sensor network was developed by Lambrou et al. [249] using multiple electrochemical optical sensors to detect E. coli and As in real-time distribution system. The system comprised of six different types of water quality parameter sensors that were able to determine water flow, temperature, conductivity, pH, ORP and turbidity simultaneously as shown in Fig. 12 . As water distribution system is increasingly polluted with low concentration hazardous chemical, there is an urgent need for rapid detection.

System architecture sensor placement approach comprising three main subsystems: PIC32 MCU based board used for central measurements, a central node for data transmission via internet, charts and email/message alerts and water quality sensors installation (Lambrou et al. [249] ).

Similar method was also reported in the work of Che et al. [250] , but it was found that EC and UV-254 sensor failed to detect contaminants, owing to the possible hidden responses and fluctuations from water source. Instead of using single sensor researchers were preferable to determine water quality based on multiple sensor placement approach [251] , [252] , [253] . Since obtaining optimal sensor station requires certain expertise, Berry et al. [254] and Tratchman [255] introduced a complex optimization tools, TEVA-SPOT and PipelineNet that employed EPANET to provide guidance for simulation in water distribution systems. The sensor placement method for water contaminants detection however is relatively complicated [256] , [257] .

Researchers have different views regarding the use of multiple water quality sensor approach. Hypothetically, contaminants could present at any point within a specified time period along the water distribution system. Some contamination events may not be detected because of inaccurate sensor placements and low detection sensitivity [258] , [259] . Ever since the development of water monitoring technologies, detecting the presence of contaminants in the water supply has become an extremely complex task. Arrays of sensor platforms were implemented to identify unique contaminations according to the sensor capabilities [260] . A study conducted by Ostfeld et al. [261] evaluated the performances of different sensor placements by experimenting with 126 sensor node stations and 168 pipes, which were subjected to a simulation period of 48–96 h per step. Inaccurate analysis might occur as a result of long transmission delays and slow response times in capturing data using sensors [262] , [263] , [264] . In addition, sensor placement approach that requires major in-pipe water distribution system alterations could increase cost [265] . Although sensor placements are among the most analyzed research area, obtaining a ‘perfect sensor’ when any concentration of contaminants is in contact with the sensor leads to an immediate response, which is considered to be a complete uncertainty.

3.2. Microfluidic sensors

Micro-scale technologies have been previously used for detection of non-biological contaminants such as pesticides [266] , [267] , phosphate [268] , Hg in water [269] , ammonium ion [270] and As ions [271] . In addition, this technology has shown tremendous LOD improvements of biological contaminants. Previous work has successfully identified E. coli O157 and Salmonella typhimurium using microfluidic reactor with volume of 29–84 nL [272] . Another example of E. coli identification that established by Schwartz and Bercovici [273] involved the integration of high concentration labelled antimicrobial peptides (AMPs) within microfluidic channel, aiming to achieve limit of detection as low as 105 cfu/mL and yield 4 bacteria in 2 min. The fabrication of micro-scale sensors can be found in the work of Jiang et al. [274] in which the customized sensors were employed to determine the bacteria concentration in drinking water. Having to adopt the principle of electrical impedance spectroscopy method, the design of a low cost and sensitive bacteria sensor was successfully developed ( Fig. 13 ), aiming to achieve pre-concentration microfluidic-based with LOD of 10 bacterial cells per mL. As shown, wireless system integration is based on Bluetooth receiver via Android cellphone (HTC ONE X), a microcontroller and impedance converter network analyser (AD 5933). The fabrication of microfluidic sensors however varies depending on field of applications.

Wireless mobile phone bacteria sensing system, (a) syringe injection of water sample into sensor package, (b) EIS bacteria sensor package, (c) schematic diagram of smartphone sensing app and wireless bacteria sensor and (d) schematic diagram of wireless sensing system (Jiang et al. [274] ).

A miniature microfluidic using long-period fibre grating (LPEG) was designed by Wang [275] and used to measure chloride ion concentration in water samples. The fabricated unit was found to achieve excellent correlation (R 2  = 0.975) with light intensity transmitted at 1550 nm. The result showed that the miniature microfluidic could detect chloride ion at concentration of 5.0 × 10 −6  mW/mg/L within 2400 mg/L limit of detection. The design and actual setup of LPFG-based microfluidic chip is shown in Fig. 14 . Optical-based microfluidic platforms were also found useful to measure various types of chemical and biomolecules at different concentrations [276] , [277] , [278] .

(a) Schematic diagram of experimental setup for LPFG-based microfluidic chip system, (b) Actual setup of LPFG-based microfluidic system, (c) Microfluidic chip and (d) 3D illustration of the structure and fluidic operation (Wang [275] ).

Digital microfluidic (DMF) enables the precise control of droplets dispensations on a microliter (10 −6  L) to picoliter (10 −12  L) scale for liquid volumes of the fabricated micro-device. Recently, the studies based on nucleic acid amplification and detection assays using DMF technology could be found in several work [279] , [280] , [281] . The implementation of chip-based nucleic acid assays have led to a significant increase in microfluidic sensor production for the purification and extraction of nucleic acid samples. According to Kaler et al. [282] , DMF method is beneficial for proteomics and nucleic acid-based bio-diagnostics application via liquid handling technology, allowing execution of pre-treatments and analysis process on a single device.

A droplet-based sensor embedded on an electro wetting-on-dielectric (EWOD) microfluidics system was also developed by Zengerle et al. [283] by integrating SU-8 polymer micro resonator layered on top of an EWOD plate system. This system only required a single droplet of less than 100 nL of a liquid sample to trigger the sensing process. This trial was the first demonstration of a EWOD-based micro resonator-sensing system with full droplet movement capability. Because of its low power consumption coupled with extremely small sample volume and small data sets, microfluidic sensing platforms are chosen for portable point-on-care (POC) diagnostic devices. In contrast to the low potential of the DMF system for large deployments of chemical and biological micro-reactor applications, an intelligent digital microfluidic system with fuzzy-enhanced feedback for multi-droplet manipulation has been developed by Gao et al. [284] . This pilot DMF prototype aimed to (i) improve complicated image signal processing by using the ability to profile different droplet hydrodynamics, (ii) preserve up to 21% of the charging time using fuzzy-enhanced controllability to enhance the DMF chip’s lifetime and (iii) employ automation of multi-droplet routing countermeasure decisions in real-time. The DMF module was made of the following three operation layers: a chip holder, control electronics and a field-programmable gate array (FPGA) board. Volume growth of droplets enabled the execution of sensing module responses that were assembled between two adjacent electrodes. Samples of DI water, phosphate buffered saline (PBS) and 1% bovine serum albumin (BSA) in PBS were injected via a syringe pump into a 4 × 11 mm hole with a constant flow rate of 5 μL/min. The analysis was then carried out using a 0.1 mol/L concentration of Na 2 CO 3 , PBS, CaCl 2 and FeCl 3 .

Attempts have been made to use an identical approach to detect E. coli in drinking water [285] , nutritional biomarkers [286] , prostate specific antigen (PSA) [287] and A. acidoterrestris lysates in milk, juice and water [288] . The abilities of the microfluidic analytical platform to detect water contaminant at a very low concentration and minimum reduction of particle size could promote the usage to distinguish nanoparticles characterization in water samples. A single microfluidic channel has the ability to detect nanoparticles as small as 220 nm [289] . The proposed method utilized microfluidic resistive pulse sensor and was integrated with a submicron sensing gate and two detecting channels using differential amplifier. Detection of CdS electrochemical quantum dots nanoparticles in water sample was able to be detected using integration of hybrid polydimethylsiloxane-polycarbonate microfluidic chip with screen printed electrodes [290] . Under optimized condition, the fabricated microfluidic chip was able to detect CdS QDs with concentration of 50–8000 ng/mL, whilst having LOD of 0.0009 μA/(ng/mL). For more details regarding the detection and quantification of inorganic nanomaterial using microfluidic chip, one can refer to the relevant review article [291] .

There is a huge potential associated with microfluidic sensor fabrication and the introduction of a new cost-effective measure is forecasted to increase in response to commercialization demands [292] . Previously, the issue on chemical contaminant analysis using microfluidic platform was raised due to lack of ability to conduct concurrent analysis [293] . However, many studies have developed a method combining microfluidic and microarrays technologies, enabling multiplex detection of contaminants [294] , [295] , [296] . The pre-treatment of samples however is vital when utilizing chip-based microfluidic sensors. It must also be noted that this additional step may cause the overall process and operation system more complex [297] , [298] , [299] , [300] , [301] .

3.3. Spectroscopic techniques

In principle, spectroscopic technique employs a light electromagnetic radiation source to interact with matter, and requires a specific probe (depends on the features of a sample) to analyze chemical or biological components. The spectra obtained from different spectroscopic techniques provide an understanding of the properties associated with light electromagnetic radiation and its interaction with matter. Nowadays, there are many types of spectroscopic techniques available for utilization. These include impedance sensing, light emission, vibrational, Raman and surface-enhanced Raman spectroscopy.

3.3.1. Impedance sensing approach

Electrical impedance spectroscopy (EIS) and dielectric impedance spectroscopy (DIS) are the types of impedance sensing technique. Both spectroscopies have been productively used for the bio-detection of targets, such as bacteria and biomarkers. However, EIS is most likely the preferable method for bio-sensing detection applications [302] . It is correlated with microfluidic sensing systems, which implies the integration of electrodes (a working electrode, reference electrode, and counter electrode) that can be either conventional or screen-printed electrodes. Multi-layers of screen-printed electrodes are implemented on flat substrate surfaces. EIS has been widely used in fields, such as medicine, water quality analysis and environmental engineering [303] , [304] , [305] , [306] , [307] .

Previously, the development of impedance screen-printed electrodes has been explored by Zhang et al. [308] to monitor 2,4,6-trinitrotoluene (TNT) in water using a bio-sensing platform ( Fig. 15 ). The integrated system developed with an alternative current (AC) impedance of approximately 20 kHz consisted of an AD5933 impedance analyzer chip, an Arduino microcontroller and a smartphone-based platform. The detection limit concentration is as low as 10 −6  M TNT-specific impedance properties. Initially, the TNT was purposely attached to the peptide that was bonded to the electrode surface. This was to prevent electron transmission and allow electrode interface impedance. Signals was then transmitted to a smartphone app on real-time basis. The detection of TNT steadily increased at low frequency ranging between 10 and 30 kHz. However, the optimum frequency-dependent impedance measurement for TNT detection was reported to be 20 kHz [308] .

Smartphone-based impedance monitoring system principle and design for TNT detection, (a) Binding of biorecognition elements (peptides) and TNT analytes on the surface of the electrodes, (b) Schematic of screen-printed electrodes, containing working electrode, counter electrode and reference electrode, (c) Basic diagram of hand-held smartphone-based system, (d) Impedance monitoring device consist of expansion and arduino board and (e) Welcome window of the App in smartphone for TNT measurements (Zhang et al. [308] ).

On the other hand, Ghaffari et al. [309] developed a low-cost wireless multi-sensor to detect nitrate fertilizer in water sample using DIS platform. The measurements were monitored and controlled via a wireless network system. The DIS platform was developed by assembling commercial microelectronics components that included a multiplexer, a dielectric spectroscopy analyzer, a digital signal controller and a ZigBee transceiver. Similar to this approach, another impedance-based microelectronic sensor, known as a Real-Time Cell Electronic Sensor (RT-CES) was also developed by Xing et al. [310] for dynamic monitoring of cyto-toxicants in the water supply. Measurements of cells, including the cell number, viability, morphology and adherence, were carried out using an electrical impedance-based sensor. The system consisted of a circle-on-line microelectrode array that was specially designed to cover almost 80% of the bottom of the sensor area. These microelectrode arrays were then assembled on a glass slide separating 16-layered wells with 9 mm spacing between each one. The ability of the sensor was to select wells for measurement automatically and to conduct testing continuously.

3.3.2. Light emission/luminescence spectroscopy

The principle underlying light emission or luminescence spectroscopy is the high-energy level absorbance of molecular matter that emits energy as light. The excitation of a high-temperature energy source induces light emitted from matter, which is known as an optical emission. Classifications of light emission spectroscopy include absorbance, reflectance, chemoluminescence, luminescence and light scattering signal and optical-based spectroscopy [311] . Of the reported luminescence platforms, several characteristics have been associated with light emission spectroscopy. For instance, reagent-mediated activity in the form of a light emission medium was employed as an auxiliary reagent to detect and identify contaminants [312] . These emerging techniques have been extensively discussed as a potential tool to monitor water quality in-situ [313] .

Several studies have acknowledged the potential use of fluorescence spectroscopy in detecting dissolved organic matter (DOM), which can be used as an alternative for standard water quality parameters [314] , [315] , [316] . In addition, the fluorescence spectroscopy approach has also been reported useful to quantify the structural composition of DOM in water samples collected from eight different urban rivers [317] . In this work, the quality of river samples was analyzed using multivariable analysis by segregating the structural composition of DOM that was collected from the west side of Shenyang City, China River. River pollution is increasing at an alarming rate because of high concentration of phosphorus and ammonia nitrogen, released from industrial and domestic sewage [317] . Measurements of ammonia nitrogen (NH 4- N), nitrate nitrogen (NO 3- N) and DOC were carried out via the fluorescence-based water quality analyzer known as the YSI 600 multi-probe. Such analyses however required respective reagent to perform.

As the characteristics of lakes and rivers are influenced by climate changes, on-going industrial wastewater release and anthropogenic activities, in situ continuous detection and quantification of river water quality are thus urgently needed. A portable chromophoric dissolved organic matter fluorescence sensor (FDOM) is available in the market and can be used to monitor the conditions of different water environments such as wetlands, watersheds and tidal marshes. FDOM can be considered for the detection of DOC concentrations as well as other biogeochemical compositions [318] , [319] , [320] . Niu et al. [321] developed potential applications for real-time water monitoring using FDOM, by taking 218 water samples from Lake Taihu, China as target samples. The CDOM concentration of the lake water was determined using an in-situ CDOM fluorescence tool invented by TRIOS GmbH, Germany, with emission wavelengths of 370 and 460 nm.

Apart from the fluorescence-sensing platform, a great deal of the current methods is based on the liquid-based microelectrode glow discharges developed by Wilson and Gianchandani [322] . To improve the detection limit, the liquid electrode spectral emission chip (Led-SpEC) was used to trace Na and Pb concentrations in water samples with detection limit of <10 mg/L and 5 mg/L, respectively. Similar to the glowing detection techniques, a patented submersible spectrofluorometer for the real-time sensing of water quality was developed by Puiu et al. [323] . The submersible spectrofluorometer was designed to extend the fluorescence measurements into an LED excitation spectrum interval between 200 nm and 1100 nm, enabling the instrument to detect chlorophyll-a concentrations as low as 0.2 μg/L. A comparison of the temperature, turbidity, water Raman scattering and fluorescence emissions parameters was also conducted to validate the design.

Similarly, measurements of the DOM concentrations in water were also extracted by using a dipping-based deposition method [324] . A water sensor for global oxygen detection was invented, and it is generally known as anodized-aluminium pressure-sensitive paint (AA-PSP). AA-PSP was anodized in sulphuric acid followed by the dipping of the anodized aluminium coated model into a luminophore solution. It was placed at the bottom of a water tank, and a xenon lamp was excited through a 400 nm band-pass filter. Fourteen-bit CCD cameras were used to capture luminescent images through an optical fibre located 90 mm from the AS-PSP. The work found that AA-PSP was capable of detecting oxygen with a sensitivity of 4.0%/mg/L at a temperature of −2.8%/°C.

Light emission due to reagent-mediated activations, such as glucose, benzalkonium chloride and chromium (VI), has also been introduced over the years [325] . This approach is similar to the one used by Sharma et al. [326] to evaluate As and E. coli contamination in the India River via bioluminescent bio-reporter. For more details about the light emission spectroscopy via an optical fibre sensor platform, one can refer to the work conducted by Ibanez et al. [327] and Chong et al. [328] .

In relation to water quality monitoring, the detection of E. coli and B. subtilis in drinking water by employing quartz tubes as optical light guidance tools with UVC-light emitting diodes (LEDs) has been developed by Gross et al. [329] . The experimental setup consisted of two containers, a soda-lime AR glass and a 100 cm quartz glass, filled with 9 mL of E. coli and B. subtilis . These samples were compared in relation to their disinfection rates with and without the total reflection of UVC radiation for time interval of 10, 40 and 90 s. The determination of E. coli was also discussed in the work of Miyajima et al. [330] in which a fibre-optic fluoroimmunoassay system was employed to monitor the fluorescence dynamics. In addition to the optical spectroscopy approach, a multi-wavelength based optical density sensor for monitoring microalgae growth in real-time has also been established by Jia et al. [331] . In comparison with the previous studies, the sensing system constituted a laser diode module as a light source, photodiodes, a controller circuit, a flow cell and temperature controller housing for the sensor platform. The detection of the microalgae concentration was identified via optical density measurements at wavelength of 650, 685 and 780 nm.

Although emission, optical and luminescence methods are among the most commonly used water contaminant detection technologies, there are associated with several drawback. One notable drawback is the sensitivity of light emission spectroscopy to changing temperatures. Because of this, it requires expert guidance during monitoring process [332] . Furthermore, as the techniques are sensitive to illumination, extensive care must be taken when they are in-use.

3.3.3. IR, MIR and NIR

Several studies have been conducted to identify chemical/biological compositions, monitor reaction progress and study hydrogen bonding using Infrared (IR) spectra measurements based on three different spectra, i.e., the near-infrared (NIR), mid-infrared (MIR) and far-infrared (FIR). The IR spectroscopy is a noninvasive technique, known as a ‘ green ’ analytical method due to its reagent-free approach [333] . Instead of analysing measurements based on a singular mean or maximum value, compact data sets provided by employing vibrational spectroscopy can be easily interpreted, resulting in a continuous detection of contaminants and increasing the chance to overlook high contamination [334] .

The commercial MIR spectrometer has been expanding for the use in environmental monitoring assessments due to its dense spectral information and high intensity of its spectral peaks [335] . Previously, MIR spectrometer was reported for oil and grease determinations using tetrachloroethylene extraction [335] . A 100 mg/mL oil and grease solution, that contained octanoic acid and isooctane, was used as the target sample. Absorbance measurements were analysed using a PerkinElmer ® Spectrum™ 400 FT-IR spectrometer in MIR mode.

A comparison between NIR and MIR reflectance spectroscopy for measuring soil fertility parameters has been reported by Reeves et al. [336] using IR spectra method. The combination of these techniques with chemo-metric analysis led to good calibration for the detection of organic carbon, total nitrogen (TN) and soil texture. However, it appeared that MIR tended to provide better calibration than that of the NIR region. These precise new approaches for the acquisition of soil data correlate to various degrees of MIR abilities. Despite the high cost in comparison with NIR and UV sensors, the advantages of using MIR sensors outweigh its disadvantages to end users.

A simultaneous quantification analysis of xylene, benzene and toluene isomers in water has been successfully performed with detection limits reported to be 20 ng/L, 45 ng/L and 80 ng/L, respectively [337] . This study developed an evanescent field spectroscopy that correlates with ATR crystal or MIR transparent optical fibres that serves as an optical transducer. Approximately 18 min of operational time response was needed to determine the solute concentrations in water samples. In contrast to the MIR measurements, the in-situ monitoring of water parameter composition by NIR spectroscopy is increasingly favourable among scientists worldwide. This approach includes the detection of nucleation and polymorphic transformations using different types of sensor arrays [338] . A novel approach to chemo-metric analysis was introduced by applying a composite sensor array (CSA) to obtain better information and to detect various crystallization mechanisms. A comparison between CSA-based techniques using Raman, NIR, FBRM, PVM and thermocouple probes was made to determine the optimal robust detection of matter. Having a clear view of nucleation and polymorphic transformation was found to be very informative in relation to Raman and NIR spectra relative to other probe. However, the ability to detect nucleation, crystal growth and polymorphic transformation by NIR was limited due to presence of water in the system [338] .

A novel and simplified optoelectronic system was designed on the basis of an NIR technique at wavelength acquisitions of 630, 690, 750 and 850 nm using a LED as a light source for evaluating fruits and vegetables [339] . Validation was performed on dye solutions resulting in the system’s ability to discriminate among the reflectance rate’s low limit levels, which were in the range of 2–4%. The tendency to reduce the cost and size of instrument analysis has led to the development of an LED device as a source of narrow bands that are able to excite NIR radiation. As NIR can be used to measure translucent packaging material, there have been many deployments of NIR instruments in raw material quality control such as development of non-invasive detection of hydrogen peroxide and its concentration in a drinking bottle [340] .

3.3.4. Raman in comparison to surface-enhanced Raman spectroscopy

The principle of Raman spectroscopy correlates the excitation of atoms or molecules to a higher energy state using monochromatic light radiation. When an atom at a higher energy level returns to the ground state, energy is dispersed by Rayleigh scattering, which results in the frequency shifting of atoms known as the Raman effect. The basic Raman experimental setup consists of an angle configuration (90° and 180°), a wavelength selector, a filter, a mirror and an excitation source.

In developing advanced technology for analyte detection, an approach using Raman spectroscopy in microfluidics (MRS) was reported in the work of Ashok and Dholakia [295] . The Raman spectroscopy detection was assembled using an on-chip fibre with a polydimethylsiloxane (PDMS)-based microfluidic platform. The setup of a PDSM-based chip via soft lithography as presented in Fig. 16 involved an excitation probe, fluidic channels, a collection probe and an inlet and outlet. Device validation was conducted using a urea solution with a 200 mW laser at a 785 nm excitation wavelength with an acquisition time of 5 s. The minimum detection limit is 140 mM.

(a) Depiction of integrated microfluidic SERS device under LabRam Raman spectrometer. The inset shows an SEM image of the silver-PDMS nanocomposite at approximately 90 K magnification, (b) Schematic illustration of alligator teeth-shaped microfluidic channel. The confluent streams of silver colloids and trace analytes are effectively mixed in the channel through the triangular structures and (c) Schematic diagram of the integrated microfluidic chip and the biomolecular Raman imaging system. (Ashok and Dholaki [295] ).

An investigation of dissolved sulphate ions (SO 4 2− ) and methane (CH 4 ) in pure water was performed using Raman spectroscopy based on two new detection approaches, namely the liquid core optical fibre (LCOF) for SO 4 2− and an enrichment process for CH 4 [341] . Both methods employed Raman instrumental measurements, which primarily consisted of 0.3W laser power, a detachable dichroic mirror and a Raman optical fibre probe. An LCOF-based Raman signal for SO 4 2− was captured, and the analysis showed that the intensity was 10 times greater than that of conventional Raman setup. The extraction of CCI 4 for the detection of CH 4 indicated the location of the Raman peak at 2907 cm −1 with a methane concentration of less than 1.14 mmol/L.

The use of Raman spectroscopy for in-line water quality monitoring is also found in large-scale deployments due to its superiorities with respect to ease of use, portability, compactness and sensitivity to water environment [342] . Application areas such as environmental analysis of organic and inorganic samples [343] , tissue imaging [344] and liquid sample evaluations [345] have employed the Raman scattering spectroscopy technique to a tremendous extent. The non-destructive Raman approach has a fast operation time which enables detection of pesticides not only in the water samples but also food products. Raman spectroscopy is well recognized as a powerful method for the on-site evaluation and determination of chemical-biological molecular compositions [346] . However, one of the major challenges in bio-detection is the ability to instantly diagnose variety of low concentrated toxin contaminants. Hence, a cell-based Raman spectroscopy biosensor was introduced by Ioan [347] to differentiate the biochemical changes that occurred in cells with a large range of toxic agents. The target samples were nucleic acids, proteins, lipids and carbohydrates. An example of Raman spectra results is shown in Fig. 17 (a) in which different target samples were found in a living cell. More importantly, the cell-based biosensor Raman spectroscopy was able to quantify and qualify between live and dead cells as shown in Fig. 17 (b).

(a) Typical Raman spectra of a living cell and the main biopolymers components found in cells and (b) Comparison between Raman spectra of living and dead cells (Ioan [347] ).

A lower limit of detection is achievable by using the SERS technique in comparison with the standard level of LOD contaminants in water as set by the EPA [342] . Because of the high-intensity signal used in SERS, this technique is among the most useful tools for environmental science, electrochemistry and analytical chemistry/biology applications. The application of SERS technique is to discriminate antibacterial properties such as tranquilizer (phenothiazine), diclofenac sodium and diclofenac sodium β-cyclodextrin complex and non-natural β-amino acids [348] . In comparison with the Raman spectra signal analysis, the SERS spectra provided tremendous measurement information about the molecular structures and absorbance behaviours. A similar approach used for bacterial classification and discrimination was also reported in the work of Wu et al. [349] in which vancomycin-functionalized silver nanorod array (VAN AgNR) was considered. This analysis was conducted through the isolation of 27 different bacteria from 12 species. Measurements were obtained from an NIR diode laser excitation source at 785 nm, and underwent chemo-metric analysis using a combination of PCA and HCA approaches. The results showed that the use of VAN AgNR substrates tended to generate more SERS spectra, making the bacteria differentiation more accurately.

Although vibrational spectrometer measurement seems to be a promising technique for water monitoring technology, poor applicability to continuous on-line monitoring because of high water interferences must be addressed [350] . It has a tendency to operate efficiently because of its low light absorbance in water, but spectrum analysis presents a hurdle owing to the overtones and overlapping absorption bands [351] . For these reasons, the spectral bands become weaker.

3.4. Biosensors

Biosensors have been increasingly utilised in recent years in the field of pathogen detections. This kind of sensors has the ability to measure molecular signals by applying specific bio-recognition elements, such as enzymes, whole cells, antibodies and nucleic acids, which are integrated with electrical interfaces via transducer platforms to obtain measurable signals. Because of the high sensitivity and no requirement of sample pre-concentration step, biosensors offer a faster operational time in comparison to other conventional techniques [352] . An environmental pollution can be caused by both human activities and industrial discharge. A biology approach was reported to be useful in detecting heavy metal ions and bacterial compositions in water source [323] , [353] . Examples of genes that act as bio-receptors and have been previously determined by whole-cell biosensors are genetically modified mutant Pseudomonas sp. Dmpr , Pgp protein-based resistance genes and mer resistance genes [354] , [355] , [356] , [357] .

The efficiency of whole-cell biosensors to monitor the presence of water contaminants is dependent on bio-receptors, transducers and immobilization techniques [358] . For instance, study demonstrated by Kuncova et al. [359] using Pseudomonas putida strain TVA8 bio reporter was able to detect benzene, toluene, ethyl benzene and xylene in water samples with concentration in the range of 0.5–120 mg/L. Kubisch et al. [360] evaluated the robustness of different cell lines by detecting cytotoxic substances in wastewater using whole-cell biosensors via eukaryotic cell lines. The detection of selected target samples (i.e., NiCl 2 , CuSO 4 , nicotine and acetaminophen) was carried out on the basis of the cellular effects of each substance, which involved the pH, O 2 and the impedance of water. HT-29, canine hepatocytes, HepG2, L6 and NHDF cell lines were used to test the acidification rate of target samples, whereas V79 cells were used to obtain the respiration rate of the target samples. These cell-lines were assembled onto six different channels of a silicon surface biosensor chip. Nevertheless, the study was only aimed to identify the presence of contaminants. Quantifying single substances was not performed.

Nanosilver is known to be the most commonly used engineered nanomaterial for water treatments, thus it is likely that Ag + ions will be released to the environments [361] . Previous works have reported that such nanomaterial can be detected using label-free sensitivity biosensor approach [362] , [363] . In order to meet the need for reliable and sensitive methodology for the detection of nanoparticles in aqueous samples, a low-cost and portable detection assay was established to determine the reactivity and characterization of selected nanoparticles (Ag, Au, CeO 2 , SiO 2 and VO 2 ) with particle size ranging between 5 and 400 nm [364] . As a leading approach for detection and exploration of nanoparticles, whole-cell biosensors was developed by integrating golTSB genes from Salmonella enterica serovar typhimurium to induce Au (I/III) complexes as illustrated in Fig. 18 [365] . The quantification of gold nanoparticle complexes with concentration as low as 0.1 μM was able to be identified. The fabricated biosensor was also used to identify other metal ions, including Ag (I), Cu (II), Fe (III), Ni (II), Co (II), Zn and Pb (II). This contradicted a previous study where the response of golB genes only increased in response to Au (III)-complexes but not other metal ions [366] .

(a) The golTSB regulon regulated by Au ions in Salmonella enterica serovar typhimurium . A synthetic golTSB regulon was made by fusing a promoter-less lacZ reporter gene downstream of the golB open reading frame as a transcriptional fusion, (b) golTSB:lacZ transcriptional fusion was introduced as a single copy into the chromosome of E. coli , (c) A single clone was taken for testing and incubated overnight used to inoculate new media, then metals were added for incubation process (16 h), (d) Cells were permeabilized for access to the β-galactosidase produced by lacZ gene in the presence of Au and (e) The permeabilized cells were transferred to the electrochemical cell (Zammit et al. [365] ).

Another genetically engineered gene system found in the literature is yeast/mammalian cell line which was used to study galactosidase and luciferase activity in water sources [367] . However, it must be noted that engineered microbial biosensors do not provide complete quantification, but rather on semi-quantitative analysis [368] . Bioassays based on sulphur-oxidizing bacteria (SOB) reactor have also been used for the toxicity identification of Cr (III) and Cr (IV) in water samples [369] . The results indicated that significant increase in the slope of electrical conductivity could be obtained when SOB DNA was exposed to Cr (III) which may be caused by increment of salt concentration and exposures of unstable reactor conditions. On the other hand, a trace amount of atrazine in ground water supply was able to be identified using an integration of printed circuit board chip nanoporous alumina membrane label-free bioassays with electrochemical impedance spectroscopy [370] .

Bioassays adjacent to sophisticated biotechnology instrumentations demonstrate a fast response with a relatively simple method for identification of various water contaminants [371] . An inline water analyzer adjacent to whole-cell biosensor was established to carry out surveillance of water network using reporter gene of bacterial luciferase lux operon ( luxCDABE ) driven by E. coli promoter P rpoD [372] . In accordance with non-biological contaminants, a portable gold screen printed electrodes amperometric biosensor was developed by Salvador et al. [373] for the detection of Irgarol 1051 in water samples. The immunoassays reagents (As87- and 4e-BSA-based) used in this study were also found elsewhere [374] , [375] . Antibody peroxide (AntiIgG-HRP) was used for the binding reaction between target analytes and 4e-BSA competitor. In the presence of analytes, stable signals were able to achieve within 10 s upon initial acquisition. Separately, Belkhamssa et al. [376] designed a biosensor and used it to detect alkylphenol in water environment. The analytes detection was observed through an immunoreaction of 4-nonylphenol and the accuracy of developed biosensor was validated with enzyme-linked immunosorbent assay (ELISA). The outcomes are very promising with reproducibility of 0.56 ± 0.08%, repeatability of 0.5 ± 0.2% and LOD for nonylphenol as low as 5 μg/L.

Cytotoxic substances present in tap water could also be detected using bioluminescent E. coli bio-reporter strain TV1061 via integration of specific heat-shock grpe promoter with luxCDABE reporter operon [377] . The bioassays and microbial biosensors employed for the toxicity assessment involved Chlorella sp., Chlorella vulgaris, Monoraphidium sp., Scenedesmus subspicatus and Brachionus calyciflorus sp . [378] . Due to presence of countless toxic cyano-bacteria in water, Weller [379] made an attempt to study their existence using biosensors. Cyano-bacteria produces algal toxins in fresh water which are hazardous to aquatic ecosystem and human health. In order to reduce the risk of a possible breakdown of toxic cyanobacterial in drinking water, a multi-barrier approach, comprising prevention, source control, detection optimization and monitoring was recommended [380] .

On the other hand, a corresponding approach using an ammonia-oxidizing bacterium (AOB)-based nitrosomonas europaea biosensor has been designed by Zhang et al. [381] to determine allylthiourea and thioacetamide concentrations in water by measuring the ammonium oxidation rates. The results showed 0.17 μM and 0.46 μM for allylthiourea and thioacetamide , respectively. Another enzymatic-based (2-phospho- l -ascorbic acid trisodium salt) biosensor made of screen-printed carbon electrodes with modified gold nanoparticles was used to detect the tungsten ions present in tap water, purified laboratory water and bottled drinking water [382] . More information about the use of membrane-based biosensors for pathogen could be found elsewhere [383] , [384] , [385] , [386] .

Since E. coli is the frequently found contaminant in drinking water, a rapid and sensitive assays for bacterial identification is required. Rapid detection of E. coli was developed by Hassan et al. [387] using 4-methylumbelliferyl-β- d -glucuronide (MUG) substrates. The quantitative results was obtained due to the yielding of a fluorogenic 4-methylumbelliferone (4-MU) product via substrates hydrolization. Bacterial such as Klebsiella, Salmonella, Enterobacter and Bacillus , which were used for validating the MUG substrate specificity could result in significant fluorescence signals.

3.5. Wireless sensor network and remote sensing applications

Online monitoring is usually defined as a real-time measurements for sampling and analysis, providing larger data frequency in comparison to the conventional sample-based method. Online monitoring is more flexible and can be conducted in remote locations with faster response. The design of an online monitoring instrumentation strongly depends on the desired identification of water parameters. Table 3 summarizes the online water quality monitoring parameters for each category.

Parameters of online water quality monitoring.

Constructing an online monitoring detection system using wireless sensor network (WSN) could offer several advantages such as simultaneous data measurements, higher detection accuracy and sensitivity, sufficient data sets and easy monitoring assessments. Furthermore, WSN which requires low power consumption results in lower operating cost [388] . There have been several applications of WSN in water monitoring [389] , [390] . For instance, a WSN-based online monitoring system that consisted of data monitoring nodes, base station and monitoring centre was developed for the water quality assessment on the artificial lake at Hangzhou Dianzi University, China [391] . With this monitoring system, water quality parameters such as pH, dissolved oxygen, EC and temperature could be easily transmitted to a remote monitoring centre for further analyses via GPRS network. The measurement was automatically carried out every h generating sufficient data for monitoring purpose. On the other hand, Wu et al. [392] designed a self-powered mobile sensor for real-time contaminant detection in water distribution pipelines aiming to detect pH level, water hardness (Ca 2+ , Mg 2+ and HCO 3− ) and disinfectant-related ions (NH 4+ and CI − ). The mobile sensor operated within a 2.76 inch diameter of spherical-shaped shell consisting of potentiometric electrochemical-based multi-analyte biochip, microfluidics, electronics controller and energy harvesting system for power supply.

Furthermore, a low cost, miniaturized and sensitive microelectronic wireless nitrate sensor network was established for quantification of nitrate concentration in water environments [393] . The conceptual design of sensor network consisted of sensor interface (input and output parameter interface), a low-power processor and wireless communication named ‘Imote2′. To obtain wireless communication, electrochemical potentiostat was needed to be miniaturized and portable. The results showed that the microsensor was able to detect nitrate concentration in water samples with LOD between 25 and 83 ng/L. However, the implementation of such sensor network on field is still at the early stage of development.

Nitrate concentration in water samples was also identified using a similar approach based on a dielectric impedance sensor (DIS) node on ZigBee mesh communication [309] . The system was constructed to perform a continuous detection within a frequency between 5 and 100 kHz under 250 mW. According to the author, this was the first development of wireless platform via AD5933 touchscreen device and chemical sensor. The detection of waterborne disease-causing bacteria in water sources were carried out by Kim and Myung [394] using an enzyme substrate assay method. The colorimetric properties were monitored via Wi-Fi connection through a web-based user interface.

The use of WSN often involves multiple sensors to improve system stability and fault tolerance [395] . However, managing continuous long-range communication networks is a challenging task due to constant requirements in power supply. Energy harvesting system has been recommended to manage wireless-based sensor power supply, however, most energy harvesting systems rely on solar cells [396] . Several self-powering mobile sensors were found to be used in water distribution pipelines [397] , [398] . They are operated via rotational miniaturized motor, hydraulic energy and thermal energy in water-air-temperature gradient and kinetic energy in water pressure.

4. Algorithmic model-based event detection

Generally, there are two methods used to conduct detection algorithm. Initially, the model-based event detection method involves a signal-to-noise principles using laboratory and sensor test-loop evaluation. Indication of contamination events is derived from the chemical changes in background water quality signals which are responsive to integration of event detection technique [399] , [400] . However, it must be pointed out that the variation of the background water quality in the experimented systems would not be exactly the same as the variation of actual WDS [401] . Meanwhile, the second method used for event detection is based on signal processing and data-driven technique. Many studies have focused on the development of data-driven estimation model detection algorithm such as statistical, pattern-based recognition, machine learning approach, and image processing to detect contaminants based on real-time water quality measurements [402] , [403] , [404] , [405] . Contamination event detection in WDS has become a challenging research topic, in accordance with improved water system analysis.

At present, a wide variety of event detection approaches, including statistical, machine learning and optimization methods have been used. However, challenges in utilizing this methodology are the merging of single alarms that could be triggered by each quality indicator and the false detection alarms [406] . Because of this reason, an event detection model-based approach known as integrated logit detection (ILD) was proposed, which is an extended statistically based fusing process of dynamic threshold method (DTM) [406] . These two event detection models generate algorithmic evaluations to explore the most effective training phase performance for identifying contaminants using Receiver Operating Characteristic (ROC) curve, which represents the trade-off between false and true positive for probability threshold as shown in Fig. 19 . The ROC curve demonstrated that the higher true positive rate of ILD than that of rate of DTM. The observation resulted in high probability threshold of 0.9 and low probability threshold of 0.5 for ILD and DTM, respectively.

ROC curve comparing the performance of the two methods (ILD and DTM) on low type events (Housh and Ostfeld [406] ).

A similar methodology was used to study the effectiveness of two different event detection models of multivariate classification techniques. Commonly used sequence analysis of classifying events are an un-supervised minimum volume ellipsoid (MVE) and a supervised support vector machine (SVM) [53] . In terms of formulation and framework unity, the MVE model reduces the complexity of the algorithmic analysis in predicting contamination events relative to the SVM model. In addition, the Gaussian distribution data that were used for the MVE model approach could contribute high accuracy of 17% in separating modular boundaries. On the other hand, anomaly-based water contamination detection methods that include Artificial Intelligence (AI), have converged over the last decade. The classification of water quality measurements into anomalous categories often employs an artificial neural network (ANN) and a support vector machine (SVM) [407] , [408] , [409] .

In order to improve accuracy in data event modelling, combined method that included integration of data analysis from all sensors, hydraulic model networks and single spatial warning systems was introduced [44] . Several studies have made targeted improvements in event detection decision-making by extending a single-sensor event detection model to a spatial multiple sensor with an on-line approach. It has been previously reported that the conventional water quality sensors integrated with a real-time method based on the Mahalanobis distance approach was highly dependent on feature vector of each contaminant [55] .

Another type of model-based event detection approach is by using Monte-Carlo simulation. Such approach has been used to detect chlorine at various sensing locations along water distribution system [410] , [411] . The event detection model has a wide variety of optional algorithms based on the quality parameter analysis and their accuracies in determining different contaminants. Execution on the basis of multi-sensor fusion can also be achieved by deploying an extended Dempster-Shafer method [56] . Research on the prediction of future water quality parameters in the absence of automated on-line water quality sensors using an autoregressive model was also investigated to compare the performance of various event detection models. Several studies on chlorine concentration measurements in water have been reported using a Radial-Basis Function network [56] , [411] . Nonetheless, there are contradictory views on its efficiencies.

A web-based tool LOAD ESTimator (LOADEST) reported in the work of Park et al. [412] , [413] was developed to estimate the pollutant load by integrating stream-flow watershed data measurements and water quality data as the model inputs via server web access. A model-based event detection using a fuzzy comprehensive genetic algorithm was introduced by Wen et al. [57] to measure the toxicity of seawater samples with high levels of spatial variation, oil contamination, silicate and heavy metals (Zn and Pb). In short, assumptions of ideal and realistic sensor placements are essential for high accuracy in event detections [414] . The genetic algorithm has become the preferable method used by many researchers for water system design optimization techniques [415] , [416] . Although there are challenges associated with a wide variety of optimization problems, each water quality factor can be weighted carefully using additional logic simulation methods. Efforts to deploy the contamination event detection and surrogate approach has been made as alternative ways to overcome drawbacks regarding conventional laboratory-based analysis and the SPA method. A variety of techniques for water quality event detection have been well-developed, including the statistical, heuristic, machine learning and optimization methods used to analyze contaminant changes and the possibility of contamination [417] . Unlike other available methods, developing a model-based detection scheme involves intensive computational algorithm [406] . In addition, the requirements for calibrations and fabrications would further increase the complexity of the overall system [411] . Predictions and assumptions are the primary variables when utilizing an event detection model-based approach. The contamination evaluation process is complex due to high variability in environmental conditions [418] .

5. Future recommendations

Current global challenges caused by climate changes, urbanization and industrialization have prompted the need of safe, clean and readily treatable water resources. The production of high-quality water is becoming more challenging because of alignments in the detection limit concentration that correlates with the WHO and EPA water quality parameter standards. Owing to the fact that one in nine people around the world does not have access to clean water supplies, innovative water contamination detection technologies must be able to (1) achieve a fast response early warning detection, (2) improve water treatment efficiency, (3) minimize risk of harmful contaminant exposure, (4) quantify and identify the types of contaminants and (5) continuously detect unwanted contaminants simultaneously.

A comparison on the pros and cons of the state-of-the-art water monitoring technologies is summarized in Table 4 . The overall monitoring and detection system must acquire accurate data to minimize statistical methods, increase spatial hybridity (with a combination of quantitative and qualitative measurements), reduce operation time to evaluate the presence of contaminants and reduce the project cost. However, it is rather difficult to continuously monitor real-time water contaminants, especially when they reach the point of end-user.

Comparison between water contamination detection methods.

Although the achievements that have been made in the conventional analytical techniques are remarkable, the use of agents as receptors and transducers to capture contaminants could negatively affect raw data measurements to a certain extent [419] . More research efforts is still needed to develop efficient yet cost effective water quality monitoring systems. In general, the analyses done by the conventional instruments are not only labour intensive [420] but also relatively expensive [421] . These instruments in most of the cases are only capable to yield small data sets [422] , [423] . Major challenges that limit commercialization are instrumental complexity and large data mining capabilities.

Even though WSN and remote sensing technologies have been adopted in current water monitoring systems, many of them are lack of hybrid analysis and are not user friendly for continuous detection/monitoring [424] , [425] , [426] . Those wireless sensing detection devices are mainly focused on the deployments in water network distribution rather than the point of water consumption, which is end-user water supply. Hence, the opportunities to develop water monitoring tool kits with a graphical user-interface (GUI) that is user-friendly, easy to operate and re-usable are on demand. As micro-scale device is more sensitive to micro-organisms than macro-scale device, it is more ideal to overcome these challenges. Nano-scale sensing device meanwhile has received a great deal of attention in recent years owing to its extremely low detection limits for contaminant concentrations [427] , [428] .

Furthermore, it is highly recommended to incorporate two or more sensing devices, such as hybrid of microfluidic-based or biosensor-based platform with a spectroscopic detection system to enhance detection sensitivity and accuracy. This approach combines potential in-situ water monitoring technologies, such as NIR-Raman spectroscopy and NIR-FTIR-SERS techniques, which are particularly suitable for water analysis because of its simplicity, high speed detection response, strong light absorption in water and very low LOD [224] . It is quite certain that this innovative approach could play an important role in meeting the ever-increasing demand for water quality assessment.

In large deployments of an on-line monitoring system, it is crucial to minimize the complexity of the overall system to prevent data transmission interruptions, which might result in data losses. In view of this, highly reliable hand-held devices and novel user-friendly toolkits are crucial during water monitoring process. In assessing water quality using contaminant detection technologies, potential contaminants in potable water should also be considered by conducting intensive risk management, risk assessments and risk research to minimize the hazardous contaminants that are present in tap and drinking water.

Acknowledgments

The authors would like to express gratitude to the Ministry of Higher Education (MOHE) of Malaysia and Universiti Teknologi Malaysia (UTM) for supporting this research financially. A sincere appreciation to Dr. Zulkifli Mohamed Hashim, an expertise in physical chemistry from Institute of Nuclear Malaysia, for critically reviewing the manuscript and intensive discussions. The staff of University Laboratory Management Unit (UPMU), UTM deserves thanks for their generous assistance in providing useful information related to analytical instruments.

Biographies

Syahidah Nurani Zulkifli received her B. Eng. degree (Honours) in Electrical Engineering (Electronics) from Universiti Teknologi Malaysia (UTM), Skudai, Malaysia and M. Sc. in Innovation Engineering Design from Universiti Putra Malaysia (UPM), in 2012 and 2014, respectively. Currently, she is a PhD candidate in Control and Instrumentations, related to monitoring system for analytical chemistry applications at Universiti Teknologi Malaysia (UTM). Throughout her study and research, she has her interest in monitoring and control system, sensor technology and software engineering. Previously, she has worked with Malaysian Nuclear Agency for industrial training and was exposed to various chemical analytical techniques include Raman, X-ray Diffraction, NIR, GCMS and ICPMS.

Assoc. Prof. Ir. Dr. Herlina Abdul Rahim is an Associate Professor at Faculty of Electrical Engineering, Universiti Teknologi Malaysia. She received her BEng and MSc in Electrical Engineering (Control and Instrumentation) from Universiti Teknologi Malaysia in year 1998 and 2000, respectively. She received her PhD in Electrical Engineering from Universiti Teknologi MARA (UiTM) in year 2009. At present, she is actively involved in R&D and has filed 33 IPR including patent fillings and copyrights. Her research and teaching interest are in the field of sensor technology, artificial intelligent system, and analytical chemical instrumentation. Most of her project involves in NIR, MIR, Raman spectroscopy and SERS. She has been exposed in various analysis of chemical/biological compositions.

Dr. Lau Woei Jye is a senior lecturer senior lecturer at Faculty of Chemical and Energy Engineering and a research fellow at Advanced Membrane Technology Research Centre (AMTEC), UTM. He was an assistant professor at Universiti Tunku Abdul Rahman (UTAR), Kuala Lumpur. He obtained his Bachelor of Engineering in Chemical-Gas Engineering (2006) and Doctor of Philosophy (PhD) in Chemical Engineering (2009) from Universiti Teknologi Malaysia (UTM), Malaysia. Dr Lau has a very strong research interest in the field of water and wastewater treatment processes using membrane-based technology. As at May 2017, he has published over 95 scientific papers, 10 reviews and 7 book chapters with total citation of 2206 (Google Scholar) and 1672 (Scopus). He is the author of the book entitled Nanofiltration Membranes: Synthesis, Characterization and Applications published by CRC Press in December 2016. He has also written articles on the subject of water separation and purfication and published in newspapers and magazines at both national and international level.

Ground Water Vulnerability Assessment: Predicting Relative Contamination Potential Under Conditions of Uncertainty (1993)

Chapter: 5 case studies, 5 case studies, introduction.

This chapter presents six case studies of uses of different methods to assess ground water vulnerability to contamination. These case examples demonstrate the wide range of applications for which ground water vulnerability assessments are being conducted in the United States. While each application presented here is directed toward the broad goal of protecting ground water, each is unique in its particular management requirements. The intended use of the assessment, the types of data available, the scale of the assessments, the required resolution, the physical setting, and institutional factors all led to very different vulnerability assessment approaches. In only one of the cases presented here, Hawaii, are attempts made to quantify the uncertainty associated with the assessment results.

Introduction

Ground water contamination became an important political and environmental issue in Iowa in the mid-1980s. Research reports, news headlines, and public debates noted the increasing incidence of contaminants in rural and urban well waters. The Iowa Ground water Protection Strategy (Hoyer et al. 1987) indicated that levels of nitrate in both private and municipal

wells were increasing. More than 25 percent of the state's population was served by water with concentrations of nitrate above 22 milligrams per liter (as NO 3 ). Similar increases were noted in detections of pesticides in public water supplies; about 27 percent of the population was periodically consuming low concentrations of pesticides in their drinking water. The situation in private wells which tend to be shallower than public wells may have been even worse.

Defining the Question

Most prominent among the sources of ground water contamination were fertilizers and pesticides used in agriculture. Other sources included urban use of lawn chemicals, industrial discharges, and landfills. The pathways of ground water contamination were disputed. Some interests argued that contamination occurs only when a natural or human generated condition, such as sinkholes or agricultural drainage wells, provides preferential flow to underground aquifers, resulting in local contamination. Others suggested that chemicals applied routinely to large areas infiltrate through the vadose zone, leading to widespread aquifer contamination.

Mandate, Selection, and Implementation

In response to growing public concern, the state legislature passed the Iowa Ground water Protection Act in 1987. This landmark statute established the policy that further contamination should be prevented to the "maximum extent practical" and directed state agencies to launch multiyear programs of research and education to characterize the problem and identify potential solutions.

The act mandated that the Iowa Department of Natural Resources (DNR) assess the vulnerability of the state's ground water resources to contamination. In 1991, DNR released Ground water Vulnerability Regions of Iowa , a map developed specifically to depict the intrinsic susceptibility of ground water resources to contamination by surface or near-surface activities. This assessment had three very limited purposes: (1) to describe the physical setting of ground water resources in the state, (2) to educate policy makers and the public about the potential for ground water contamination, and (3) to provide guidance for planning and assigning priorities to ground water protection efforts in the state.

Unlike other vulnerability assessments, the one in Iowa took account of factors that affect both ground water recharge and well development. Ground water recharge involves issues related to aquifer contamination; well development involves issues related to contamination of water supplies in areas where sources other than bedrock aquifers are used for drinking water. This

approach considers jointly the potential impacts of contamination on the water resource in aquifers and on the users of ground water sources.

The basic principle of the Iowa vulnerability assessment involves the travel time of water from the land surface to a well or an aquifer. When the time is relatively short (days to decades), vulnerability is considered high. If recharge occurs over relatively long periods (centuries to millennia), vulnerability is low. Travel times were determined by evaluating existing contaminants and using various radiometric dating techniques. The large reliance on travel time in the Iowa assessment likely results in underestimation of the potential for eventual contamination of the aquifer over time.

The most important factor used in the assessment was thickness of overlying materials which provide natural protection to a well or an aquifer. Other factors considered included type of aquifer, natural water quality in an aquifer, patterns of well location and construction, and documented occurrences of well contamination. The resulting vulnerability map ( Plate 1 ) delineates regions having similar combinations of physical characteristics that affect ground water recharge and well development. Qualitative ratings are assigned to the contamination potential for aquifers and wells for various types and locations of water sources. For example, the contamination potential for wells in alluvial aquifers is considered high, while the potential for contamination of a variable bedrock aquifer protected by moderate drift or shale is considered low.

Although more sophisticated approaches were investigated for use in the assessment, ultimately no complex process models of contaminant transport were used and no distinction was made among Iowa's different soil types. The DNR staff suggested that since the soil cover in most of the state is such a small part of the overall aquifer or well cover, processes that take place in those first few inches are relatively similar and, therefore, insignificant in terms of relative susceptibilities to ground water contamination. The results of the vulnerability assessment followed directly from the method's assumptions and underlying principles. In general, the thicker the overlay of clayey glacial drift or shale, the less susceptible are wells or aquifers to contamination. Where overlying materials are thin or sandy, aquifer and well susceptibilities increase. Vulnerability is also greater in areas where sinkholes or agricultural drainage wells allow surface and tile water to bypass natural protective layers of soil and rapidly recharge bedrock aquifers.

Basic data on geologic patterns in the state were extrapolated to determine the potential for contamination. These data were supplemented by databases on water contamination (including the Statewide Rural Well-Water Survey conducted in 1989-1990) and by research insights into the transport, distribution, and fate of contaminants in ground water. Some of the simplest data needed for the assessment were unavailable. Depth-to-bedrock information had never been developed, so surface and bedrock topographic

maps were revised and integrated to create a new statewide depth-to-bedrock map. In addition, information from throughout the state was compiled to produce the first statewide alluvial aquifer map. All new maps were checked against available well-log data, topographic maps, outcrop records, and soil survey reports to assure the greatest confidence in this information.

While the DNR was working on the assessment, it was also asked to integrate various types of natural resource data into a new computerized geographic information system (GIS). This coincident activity became a significant contributor to the assessment project. The GIS permitted easier construction of the vulnerability map and clearer display of spatial information. Further, counties or regions in the state can use the DNR geographic data and the GIS to explore additional vulnerability parameters and examine particular areas more closely to the extent that the resolution of the data permits.

The Iowa vulnerability map was designed to provide general guidance in planning and ranking activities for preventing contamination of aquifers and wells. It is not intended to answer site-specific questions, cannot predict contaminant concentrations, and does not even rank the different areas of the state by risk of contamination. Each of these additional uses would require specific assessments of vulnerability to different activities, contaminants, and risk. The map is simply a way to communicate qualitative susceptibility to contamination from the surface, based on the depth and type of cover, natural quality of the aquifer, well location and construction, and presence of special features that may alter the transport of contaminants.

Iowa's vulnerability map is viewed as an intermediate product in an ongoing process of learning more about the natural ground water system and the effects of surface and near-surface activities on that system. New maps will contain some of the basic data generated by the vulnerability study. New research and data collection will aim to identify ground water sources not included in the analysis (e.g., buried channel aquifers and the "salt and pepper sands" of western Iowa). Further analyses of existing and new well water quality data will be used to clarify relationships between aquifer depth and ground water contamination. As new information is obtained, databases and the GIS will be updated. Over time, new vulnerability maps may be produced to reflect new data or improved knowledge of environmental processes.

The Cape Cod sand and gravel aquifer is the U.S. Environmental Protection Agency (EPA) designated sole source of drinking water for Barnstable County, Massachusetts (ca. 400 square miles, winter population 186,605 in 1990, summer population ca. 500,000) as well as the source of fresh water for numerous kettle hole ponds and marine embayments. During the past 20 years, a period of intense development of open land accompanied by well-reported ground water contamination incidents, Cape Cod has been the site of intensive efforts in ground water management and analysis by many organizations, including the Association for the Preservation of Cape Cod, the U.S. Geological Survey, the Massachusetts Department of Environmental Protection (formerly the Department of Environmental Quality Engineering), EPA, and the Cape Cod Commission (formerly the Cape Cod Planning and Economic Development Commission). An earlier NRC publication, Ground Water Quality Protection: State and Local Strategies (1986) summarizes the Cape Cod ground water protection program.

The Area Wide Water Quality Management Plan for Cape Cod (CCPEDC 1978a, b), prepared in response to section 208 of the federal Clean Water Act, established a management strategy for the Cape Cod aquifer. The plan emphasized wellhead protection of public water supplies, limited use of public sewage collection systems and treatment facilities, and continued general reliance on on-site septic systems, and relied on density controls for regulation of nitrate concentrations in public drinking water supplies. The water quality management planning program began an effort to delineate the zones of contribution (often called contributing areas) for public wells on Cape Cod that has become increasingly sophisticated over the years. The effort has grown to address a range of ground water resources and ground water dependent resources beyond the wellhead protection area, including fresh and marine surface waters, impaired areas, and water quality improvement areas (CCC 1991). Plate 2 depicts the water resources classifications for Cape Cod.

Selection and Implementation of Approaches

The first effort to delineate the contributing area to a public water supply well on Cape Cod came in 1976 as part of the initial background studies for the Draft Area Wide Water Quality Management Plan for Cape

Cod (CCPEDC 1978a). This effort used a simple mass balance ratio of a well's pumping volume to an equal volume average annual recharge evenly spread over a circular area. This approach, which neglects any hydrogeologic characteristics of the aquifer, results in a number of circles of varying radii that are centered at the wells.

The most significant milestone in advancing aquifer protection was the completion of a regional, 10 foot contour interval, water table map of the county by the USGS (LeBlanc and Guswa 1977). By the time that the Draft and Final Area Wide Water Quality Management Plans were published (CCPEDC 1978a, b), an updated method for delineating zones of contribution, using the regional water table map, had been developed. This method used the same mass balance approach to characterize a circle, but also extended the zone area by 150 percent of the circle's radius in the upgradient direction. In addition, a water quality watch area extending upgradient from the zone to the ground water divide was recommended. Although this approach used the regional water table map for information on ground water flow direction, it still neglected the aquifer's hydrogeologic parameters.

In 1981, the USGS published a digital model of the aquifer that included regional estimates of transmissivity (Guswa and LeBlanc 1981). In 1982, the CCPEDC used a simple analytical hydraulic model to describe downgradient and lateral capture limits of a well in a uniform flow field (Horsley 1983). The input parameters required for this model included hydraulic gradient data from the regional water table map and transmissivity data from the USGS digital model. The downgradient and lateral control points were determined using this method, but the area of the zone was again determined by the mass balance method. Use of the combined hydraulic and mass balance method resulted in elliptical zones of contribution that did not extend upgradient to the ground water divide. This combined approach attempted to address three-dimensional ground water flow beneath a partially penetrating pumping well in a simple manner.

At about the same time, the Massachusetts Department of Environmental Protection started the Aquifer Lands Acquisition (ALA) Program to protect land within zones of contribution that would be delineated by detailed site-specific studies. Because simple models could not address three-dimensional flow and for several other reasons, the ALA program adopted a policy that wellhead protection areas or Zone IIs (DEP-WS 1991) should be extended upgradient all the way to a ground water divide. Under this program, wells would be pump tested for site-specific aquifer parameters and more detailed water table mapping would often be required. In many cases, the capture area has been delineated by the same simple hydraulic analytical model but the zone has been extended to the divide. This method has resulted in some 1989 zones that are 3,000 feet wide and extend 4.5

miles upgradient, still without a satisfactory representation of three-dimensional flow to the well.

Most recently the USGS (Barlow 1993) has completed a detailed subregional, particle-tracking three-dimensional ground water flow model that shows the complex nature of ground water flow to wells. This approach has shown that earlier methods, in general, overestimate the area of zones of contribution (see Figure 5.1 ).

In 1988, the public agencies named above completed the Cape Cod Aquifer Management Project (CCAMP), a resource-based ground water protection study that used two towns, Barnstable and Eastham, to represent the more and less urbanized parts of Cape Cod. Among the CCAMP products were a GIS-based assessment of potential for contamination as a result of permissible land use changes in the Barnstable zones of contribution (Olimpio et al. 1991) and a ground water vulnerability assessment by Heath (1988) using DRASTIC for the same area. Olimpio et al. characterized land uses by ranking potential contaminant sources without regard to differences in vulnerability within the zones. Heath's DRASTIC analysis of the same area, shown in Figure 5.2 , delineated two distinct zones of vulnerability based on hydrogeologic setting. The Sandwich Moraine setting, with deposits of silt, sand and gravel, and depths to ground water ranging from 0 to more than 125 feet, had DRASTIC values of 140 to 185; the Barnstable Outwash Plain, with permeable sand and fine gravel deposits with beds of silt and clay and depths to ground water of less than 50 feet, yielded values of 185 to 210. The DRASTIC scores and relative contributions of the factors are shown in Tables 5.1 and 5.2 . Heath concluded that similar areas of Cape Cod would produce similar moderate to high vulnerability DRASTIC scores. The CCAMP project also addressed the potential for contamination of public water supply wells from new land uses allowable under existing zoning for the same area. The results of that effort are shown in Plate 4 .

In summary, circle zones were used initially when the hydrogeologic nature of the aquifer or of hydraulic flow to wells was little understood. The zones improved with an understanding of ground water flow and aquifer characteristics, but in recognition of the limitations of regional data, grossly conservative assumptions came into use. Currently, a truer delineation of a zone of contribution can be prepared for a given scenario using sophisticated models and highly detailed aquifer characterization. However, the area of a given zone still is highly dependent on the initial assumptions that dictate how much and in what circumstances a well is pumped. In the absence of ability to specify such conditions, conservative assumptions,

FIGURE 5.1 Contributing areas of wells and ponds in the complex flow system determined by using the three-dimensional model with 1987 average daily pumping rates. (Barlow 1993)

such as maximum prolonged pumping, prevail, and, therefore, conservatively large zones of contribution continue to be used for wellhead protection.

The ground water management experience of Cape Cod has resulted in a better understanding of the resource and the complexity of the aquifer

FIGURE 5.2 DRASTIC contours for Zone 1, Barnstable-Yarmouth, Massachusetts.

system, as well as the development of a more ambitious agenda for resource protection. Beginning with goals of protection of existing public water supplies, management interests have grown to include the protection of private wells, potential public supplies, fresh water ponds, and marine embayments. Public concerns over ground water quality have remained high and were a major factor in the creation of the Cape Cod Commission by the Massachusetts legislature. The commission is a land use planning and regulatory agency with broad authority over development projects and the ability to create special resource management areas. The net result of 20 years of effort by many individuals and agencies is the application of

TABLE 5.1 Ranges, Rating, and Weights for DRASTIC Study of Barnstable Outwash Plain Setting (NOTE: gpd/ft 2 = gallons per day per square foot) (Heath 1988)

TABLE 5.2 Ranges, Rating, and Weights for DRASTIC Study of Sandwich Moraine Setting (NOTE: gpd/ft 2 = gallons per day per square foot) (Heath 1988)

higher protection standards to broader areas of the Cape Cod aquifer. With some exceptions for already impaired areas, a differentiated resource protection approach in the vulnerable aquifer setting of Cape Cod has resulted in a program that approaches universal ground water protection.

Florida has 13 million residents and is the fourth most populous state (U.S. Bureau of the Census 1991). Like several other sunbelt states, Florida's population is growing steadily, at about 1,000 persons per day, and is estimated to reach 17 million by the year 2000. Tourism is the biggest industry in Florida, attracting nearly 40 million visitors each year. Ground water is the source of drinking water for about 95 percent of Florida's population; total withdrawals amount to about 1.5 billion gallons per day. An additional 3 billion gallons of ground water per day are pumped to meet the needs of agriculture—a $5 billion per year industry, second only to tourism in the state. Of the 50 states, Florida ranks eighth in withdrawal of fresh ground water for all purposes, second for public supply, first for rural domestic and livestock use, third for industrial/commercial use, and ninth for irrigation withdrawals.

Most areas in Florida have abundant ground water of good quality, but the major aquifers are vulnerable to contamination from a variety of land use activities. Overpumping of ground water to meet the growing demands of the urban centers, which accounts for about 80 percent of the state's population, contributes to salt water intrusion in coastal areas. This overpumping is considered the most significant problem for degradation of ground water quality in the state. Other major sources of ground water contaminants include: (1) pesticides and fertilizers (about 2 million tons/year) used in agriculture, (2) about 2 million on-site septic tanks, (3) more than 20,000 recharge wells used for disposing of stormwater, treated domestic wastewater, and cooling water, (4) nearly 6,000 surface impoundments, averaging one per 30 square kilometers, and (5) phosphate mining activities that are estimated to disturb about 3,000 hectares each year.

The Hydrogeologic Setting

The entire state is in the Coastal Plain physiographic province, which has generally low relief. Much of the state is underlain by the Floridan aquifer system, largely a limestone and dolomite aquifer that is found in both confined and unconfined conditions. The Floridan is overlain through most of the state by an intermediate aquifer system, consisting of predominantly clays and sands, and a surficial aquifer system, consisting of predominantly sands, limestone, and dolomite. The Floridan is one of the most productive aquifers in the world and is the most important source of drinking water for Florida residents. The Biscayne, an unconfined, shallow, limestone aquifer located in southeast Florida, is the most intensively used

aquifer and the sole source of drinking water for nearly 3 million residents in the Miami-Palm Beach coastal area. Other surficial aquifers in southern Florida and in the western panhandle region also serve as sources of ground water.

Aquifers in Florida are overlain by layers of sand, clay, marl, and limestone whose thickness may vary considerably. For example, the thickness of layers above the Floridan aquifer range from a few meters in parts of west-central and northern Florida to several hundred meters in south-central Florida and in the extreme western panhandle of the state. Four major groups of soils (designated as soil orders under the U.S. Soil Taxonomy) occur extensively in Florida. Soils in the western highlands are dominated by well-drained sandy and loamy soils and by sandy soils with loamy subsoils; these are classified as Ultisols and Entisols. In the central ridge of the Florida peninsula, are found deep, well-drained, sandy soils (Entisols) as well as sandy soils underlain by loamy subsoils or phosphatic limestone (Alfisols and Ultisols). Poorly drained sandy soils with organic-rich and clay-rich subsoils, classified as Spodosols, occur in the Florida flatwoods. Organic-rich muck soils (Histosols) underlain by muck or limestone are found primarily in an area extending south of Lake Okeechobee.

Rainfall is the primary source of ground water in Florida. Annual rainfall in the state ranges from 100 to 160 cm/year, averaging 125 cm/year, with considerable spatial (both local and regional) and seasonal variations in rainfall amounts and patterns. Evapotranspiration (ET) represents the largest loss of water; ET ranges from about 70 to 130 cm/year, accounting for between 50 and 100 percent of the average annual rainfall. Surface runoff and ground water discharge to streams averages about 30 cm/year. Annual recharge to surficial aquifers ranges from near zero in perennially wet, lowland areas to as much as 50 cm/year in well-drained areas; however, only a fraction of this water recharges the underlying Floridan aquifer. Estimates of recharge to the Floridan aquifer vary from less than 3 cm/year to more than 25 cm/year, depending on such factors as weather patterns (e.g., rainfall-ET balance), depth to water table, soil permeability, land use, and local hydrogeology.

Permeable soils, high net recharge rates, intensively managed irrigated agriculture, and growing demands from urban population centers all pose considerable threat of ground water contamination. Thus, protection of this valuable natural resource while not placing unreasonable constraints on agricultural production and urban development is the central focus of environmental regulation and growth management in Florida.

Along with California, Florida has played a leading role in the United

States in development and enforcement of state regulations for environmental protection. Detection in 1983 of aldicarb and ethylene dibromide, two nematocides used widely in Florida's citrus groves, crystallized the growing concerns over ground water contamination and the need to protect this vital natural resource. In 1983, the Florida legislature passed the Water Quality Assurance Act, and in 1984 adopted the State and Regional Planning Act. These and subsequent legislative actions provide the legal basis and guidance for the Ground Water Strategy developed by the Florida Department of Environmental Regulation (DER).

Ground water protection programs in Florida are implemented at federal, state, regional, and local levels and involve both regulatory and nonregulatory approaches. The most significant nonregulatory effort involves more than 30 ground water studies being conducted in collaboration with the Water Resources Division of the U.S. Geological Survey. At the state level, Florida statutes and administrative codes form the basis for regulatory actions. Although DER is the primary agency responsible for rules and statutes designed to protect ground water, the following state agencies participate to varying degrees in their implementation: five water management districts, the Florida Geological Survey, the Department of Health and Rehabilitative Services (HRS), the Department of Natural Resources, and the Florida Department of Agriculture and Consumer Services (DACS). In addition, certain interagency committees help coordinate the development and implementation of environmental codes in the state. A prominent example is the Pesticide Review Council which offers guidance to the DACS in developing pesticide use regulation. A method for screening pesticides in terms of their chronic toxicity and environmental behavior has been developed through collaborative efforts of the DACS, the DER, and the HRS (Britt et al. 1992). This method will be used to grant registration for pesticide use in Florida or to seek additional site-specific field data.

Selecting an Approach

The emphasis of the DER ground water program has shifted in recent years from primarily enforcement activity to a technically based, quantifiable, planned approach for resource protection.

The administrative philosophy for ground water protection programs in Florida is guided by the following principles:

Ground water is a renewable resource, necessitating a balance between withdrawals and natural or artificial recharge.

Ground water contamination should be prevented to the maximum degree possible because cleanup of contaminated aquifers is technically or economically infeasible.

It is impractical, perhaps unnecessary, to require nondegradation standards for all ground water in all locations and at all times.

The principle of ''most beneficial use" is to be used in classifying ground water into four classes on the basis of present quality, with the goal of attaining the highest level protection of potable water supplies (Class I aquifers).

Part of the 1983 Water Quality Assurance Act requires Florida DER to "establish a ground water quality monitoring network designed to detect and predict contamination of the State's ground water resources" via collaborative efforts with other state and federal agencies. The three basic goals of the ground water quality monitoring program are to:

Establish the baseline water quality of major aquifer systems in the state,

Detect and predict changes in ground water quality resulting from the effects of various land use activities and potential sources of contamination, and

Disseminate to local governments and the public, water quality data generated by the network.

The ground water monitoring network established by DER to meet the goals stated above consists of two major subnetworks and one survey (Maddox and Spicola 1991). Approximately 1,700 wells that tap all major potable aquifers in the state form the Background Network, which was designed to help define the background water quality. The Very Intensively Studied Area (VISA) network was established to monitor specific areas of the state considered highly vulnerable to contamination; predominant land use and hydrogeology were the primary attributes used to evaluate vulnerability. The DRASTIC index, developed by EPA, served as the basis for statewide maps depicting ground water vulnerability. Data from the VISA wells will be compared to like parameters sampled from Background Network wells in the same aquifer segment. The final element of the monitoring network is the Private Well Survey, in which up to 70 private wells per county will be sampled. The sampling frequency and chemical parameters to be monitored at each site are based on several factors, including network well classification, land use activities, hydrogeologic sensitivity, and funding. In Figure 5.3 , the principal aquifers in Florida are shown along with the distribution of the locations of the monitoring wells in the Florida DER network.

The Preservation 2000 Act, enacted in 1990, mandated that the Land Acquisition Advisory Council (LAAC) "provide for assessing the importance

FIGURE 5.3 Principal aquifers in Florida and the network of sample wells as of March 1990 (1642 wells sampled). (Adapted from Maddox and Spicola 1991, and Maddox et al. 1993.)

of acquiring lands which can serve to protect or recharge ground water, and the degree to which state land acquisition programs should focus on purchasing such land." The Ground Water Resources Committee, a subcommittee of the LAAC, produced a map depicting areas of ground water significance at regional scale (1:500,000) (see Figure 5.4 ) to give decision makers the basis for considering ground water as a factor in land acquisition under the Preservation 2000 Act (LAAC 1991). In developing maps for their districts, each of the five water management districts (WMDs) used the following criteria: ground water recharge, ground water quality, aquifer vulnerability, ground water availability, influence of existing uses on the resource, and ground water supply. The specific approaches used by

FIGURE 5.4 General areas of ground water significance in Florida. (Map provided by Florida Department of Environmental Regulation, Bureau of Drinking Water and Ground Water Resources.)

the WMDs varied, however. For example, the St. Johns River WMD used a GIS-based map overlay and DRASTIC-like numerical index approach that rated the following attributes: recharge, transmissivity, water quality, thickness of potable water, potential water expansion areas, and spring flow capture zones. The Southwest Florida WMD also used a map overlay and index approach which considered four criteria, and GIS tools for mapping. Existing databases were considered inadequate to generate a DRASTIC map for the Suwannee River WMD, but the map produced using an overlay approach was considered to be similar to DRASTIC maps in providing a general depiction of aquifer vulnerability.

In the November 1988, Florida voters approved an amendment to the Florida Constitution allowing land producing high recharge to Florida's aquifers to be classified and assessed for ad valorem tax purposes based on character or use. Such recharge areas are expected to be located primarily in the upland, sandy ridge areas. The Bluebelt Commission appointed by the 1989 Florida Legislature, studied the complex issues involved and recommended that the tax incentive be offered to owners of such high recharge areas if their land is left undeveloped (SFWMD 1991). The land eligible

for classification as "high water recharge land" must meet the following criteria established by the commission:

The parcel must be located in the high recharge areas designated on maps supplied by each of the five WMDs.

The high recharge area of the parcel must be at least 10 acres.

The land use must be vacant or single-family residential.

The parcel must not be receiving any other special assessment, such as Greenbelt classification for agricultural lands.

Two bills related to the implementation of the Bluebelt program are being considered by the 1993 Florida legislation.

THE SAN JOAQUIN VALLEY

Pesticide contamination of ground water resources is a serious concern in California's San Joaquin Valley (SJV). Contamination of the area's aquifer system has resulted from a combination of natural geologic conditions and human intervention in exploiting the SJV's natural resources. The SJV is now the principal target of extensive ground water monitoring activities in the state.

Agriculture has imposed major environmental stresses on the SJV. Natural wetlands have been drained and the land reclaimed for agricultural purposes. Canal systems convey water from the northern, wetter parts of the state to the south, where it is used for irrigation and reclamation projects. Tens of thousands of wells tap the sole source aquifer system to supply water for domestic consumption and crop irrigation. Cities and towns have sprouted throughout the region and supply the human resources necessary to support the agriculture and petroleum industries.

Agriculture is the principal industry in California. With 1989 cash receipts of more than $17.6 billion, the state's agricultural industry produced more than 50 percent of the nation's fruits, nuts, and vegetables on 3 percent of the nation's farmland. California agriculture is a diversified industry that produces more than 250 crop and livestock commodities, most of which can be found in the SJV.

Fresno County, the largest agricultural county in the state, is situated in the heart of the SJV, between the San Joaquin River to the north and the Kings River on the south. Grapes, stone fruits, and citrus are important commodities in the region. These and many other commodities important to the region are susceptible to nematodes which thrive in the county's coarse-textured soils.

While agricultural diversity is a sound economic practice, it stimulates the growth of a broad range of pest complexes, which in turn dictates greater reliance on agricultural chemicals to minimize crop losses to pests, and maintain productivity and profit. Domestic and foreign markets demand high-quality and cosmetically appealing produce, which require pesticide use strategies that rely on pest exclusion and eradication rather than pest management.

Hydrogeologic Setting

The San Joaquin Valley (SJV) is at the southern end of California's Central Valley. With its northern boundary just south of Sacramento, the Valley extends in a southeasterly direction about 400 kilometers (250 miles) into Kern County. The SJV averages 100 kilometers (60 miles) in width and drains the area between the Sierra Nevada on the east and the California Coastal Range on the west. The rain shadow caused by the Coastal Range results in the predominantly xeric habitat covering the greater part of the valley floor where the annual rainfall is about 25 centimeters (10 inches). The San Joaquin River is the principal waterway that drains the SJV northward into the Sacramento Delta region.

The soils of the SJV vary significantly. On the west side of the valley, soils are composed largely of sedimentary materials derived from the Coastal Range; they are generally fine-textured and slow to drain. The arable soils of the east side developed on relatively unweathered, granitic sediments. Many of these soils are wind-deposited sands underlain by deep coarse-textured alluvial materials.

From the mid-1950s until 1977, dibromochloropropane (DBCP) was the primary chemical used to control nematodes. DBCP has desirable characteristics for a nematocide. It is less volatile than many other soil fumigants, such as methylbromide; remains active in the soil for a long time, and is effective in killing nematodes. However, it also causes sterility in human males, is relatively mobile in soil, and is persistent. Because of the health risks associated with consumption of DBCP treated foods, the nematocide was banned from use in the United States in 1979. After the ban, several well water studies were conducted in the SJV by state, county and local authorities. Thirteen years after DBCP was banned, contamination of well waters by the chemical persists as a problem in Fresno County.

Public concern over pesticides in ground water resulted in passage of the California Pesticide Contamination Prevention Act (PCPA) of 1985. It is a broad law that establishes the California Department of Pesticide Regulation

as the lead agency in dealing with issues of ground water contamination by pesticides. The PCPA specifically requires:

pesticide registrants to collect and submit specific chemical and environmental fate data (e.g., water solubility, vapor pressure, octanol-water partition coefficient, soil sorption coefficient, degradation half-lives for aerobic and anaerobic metabolism, Henry's Law constant, hydrolysis rate constant) as part of the terms for registration and continued use of their products in California.

establishment of numerical criteria or standards for physical-chemical characteristics and environmental fate data to determine whether a pesticide can be registered in the state that are at least as stringent as those standards set by the EPA,

soil and water monitoring investigations be conducted on:

pesticides with properties that are in violation of the physical-chemical standards set in 2 above, and

pesticides, toxic degradation products or other ingredients that are:

contaminants of the state's ground waters, or

found at the deepest of the following soil depths:

2.7 meters (8 feet) below the soil surface,

below the crop root zone, or

below the microbial zone, and

creation of a database of wells sampled for pesticides with a provision requiring all agencies to submit data to the California Department of Pesticide Regulation (CDPR).

Difficulties associated with identifying the maximum depths of root zone and microbial zone have led to the establishment of 8 feet as a somewhat arbitrary but enforceable criterion for pesticide leaching in soils.

Selection and Implementation of an Approach

Assessment of ground water vulnerability to pesticides in California is a mechanical rather than a scientific process. Its primary goal is compliance with the mandates established in the PCPA. One of these mandates requires that monitoring studies be conducted in areas of the state where the contaminant pesticide is used, in other areas exhibiting high risk portraits (e.g., low organic carbon, slow soil hydrolysis, metabolism, or dissipation), and in areas where pesticide use practices present a risk to the state's ground water resources.

The numerical value for assessments was predetermined by the Pesticide Use Report (PUR) system employed in the state. Since the early

1970s, California has required pesticide applicators to give local authorities information on the use of restricted pesticides. This requirement was extended to all pesticides beginning in 1990. Application information reported includes names of the pesticide(s) and commodities, the amount applied, the formulation used, and the location of the commodity to the nearest section (approximately 1 square mile) as defined by the U.S. Rectangular Coordinate System. In contrast to most other states that rely on county pesticide sales in estimating pesticide use, California can track pesticide use based on quantities applied to each section. Thus, the section, already established as a political management unit, became the basic assessment unit.

The primary criteria that subject a pesticide to investigation as a ground water pollutant are:

detection of the pesticide or its metabolites in well samples, or

its failure to conform to the physical-chemical standards set in accordance with the PCPA, hence securing its position on the PCPA's Ground Water Protection List of pesticides having a potential to pollute ground water.

In either case, relatively large areas surrounding the original detection site or, in the latter case, high use regions are monitored via well surveys. Positive findings automatically increase the scope of the surveys, and since no tolerance levels are specified in the PCPA, any detectable and confirmed result establishes a pesticide as a contaminant.

When a pesticide or its degradation products is detected in a well water sample and the pesticide is judged to have contaminated the water source as a result of a legal agricultural use, the section the well is in is declared a Pesticide Management Zone (PMZ). Further application of the detected pesticide within PMZ boundaries may be prohibited or restricted, depending on the degree of contamination and subject to the availability of tried and tested modifications in management practices addressing environmental safety in use of the pesticide. PMZs are pesticide-specific—each contaminant pesticide has its own set of PMZs which may or may not overlap PMZs assigned another pesticide. Currently, consideration is being given to the extension of PMZs established for one chemical to other potential pesticide pollutants. In addition to monitoring activities in PMZs, protocols have been written to monitor ground water in sections adjacent to a PMZ. Monitoring of adjacent sections has resulted in many new PMZs. Currently, California has 182 PMZs involving five registered pesticides.

California has pursued this mechanical approach to assessing ground water vulnerability to pesticides for reasons that cover a spectrum of political, economic, and practical concerns. As noted earlier, the scale of the assessment unit was set at the section level because it is a well-defined

geopolitical unit used in the PUR system. Section boundaries frequently are marked by roads and highways, which allows the section to be located readily and makes enforcement of laws and regulations more practical. California law also requires that well logs be recorded by drillers for all wells in the state. Well-site information conforms to the U.S. Rectangular Coordinate System's township, range, and section system.

The suitability and reliability of databases available for producing vulnerability assessments was a great concern before passage of the PCPA in 1985. Soil survey information holds distinct advantages for producing assessments and developing best management practices strategies, but it was not available in a format that could work in harmony with PUR sections. To date, several areas of the SJV are not covered by a modern soil survey; they include the western part of Tulare County, which contains 34 PMZs. Other vadose zone data were sparse, it available at all.

The use of models was not considered appropriate, given the available data and because no single model could cope with the circumstances in which contaminated ground water sources were being discovered in the state. While most cases of well contamination were associated with the coarse-textured soils of the SJV and the Los Angeles Basin, several cases were noted in areas of the Central Valley north of the SJV, where very dense fine-textured soils (vertisols and other cracking clays) were dominant.

The potential vagaries and uncertainties associated with more scientific approaches to vulnerability assessment, given the tools available when the PCPA was enacted, presented too large a risk for managers to consider endorsing their use. In contrast, the basic definition of the PMZ is difficult to challenge (pesticide contamination has been detected or not detected) in the legal sense. And the logic of investing economic resources in areas immediately surrounding areas of acknowledged contamination are relatively undisputable. The eastern part of the SJV contains more than 50 percent of the PMZs in the state. Coarse-textured soils of low carbon content are ubiquitous in this area and are represented in more than 3,000 sections. The obvious contamination scenario is the normal scenario in the eastern SJV, and because of its size it creates a huge management problem. While more sophisticated methods for assessing ground water vulnerability have been developed, a question that begs to be asked is "How would conversion to the use of enhanced techniques for evaluating ground water vulnerability improve ground water protection policy and management in the SJV?"

More than 90 percent of the population of Hawaii depends on ground water (nearly 200 billion gallons per day) for their domestic supply (Au 1991). Ground water contamination is of special concern in Hawaii, as in other insular systems, where alternative fresh water resources are not readily available or economically practical. Salt water encroachment, caused by pumping, is by far the biggest source of ground water contamination in Hawaii; however, nonpoint source contamination from agricultural chemicals is increasingly a major concern. On Oahu, where approximately 80 percent of Hawaii's million-plus population resides, renewable ground water resources are almost totally exploited; therefore, management action to prevent contamination is essential.

Each of the major islands in the Hawaiian chain is formed from one or more shield volcanoes composed primarily of extremely permeable thin basaltic lava flows. On most of the Hawaiian islands the margins of the volcanic mountains are overlapped by coastal plain sediments of alluvial and marine origin that were deposited during periods of volcanic quiescence. In general, the occurrence of ground water in Hawaii, shown in Figure 5.5 , falls into three categories: (1) basal water bodies floating on and displacing salt water, (2) high-level water bodies impounded within compartments formed by impermeable dikes that intrude the lava flows, and (3) high-level water bodies perched on ash beds or soils interbedded with

FIGURE 5.5 Cross section of a typical volcanic dome showing the occurrence of ground water in Hawaii (After Peterson 1972. Reprinted, by permission, from Water Well Journal Publishing Company, 1972.)

thin lava flows on unconformities or on other relatively impervious lava flows (Peterson 1972).

A foundation of the tourist industry in Hawaii is the pristine environment. The excellent quality of Hawaii's water is well known. The public has demanded, and regulatory agencies have adopted, a very conservative, zero-tolerance policy on ground water contamination. The reality, however, is that past, present, and future agricultural, industrial, and military activities present potentially significant ground water contamination problems in Hawaii.

Since 1977 when 1,874 liters of ethylene dibromide (EDB) where spilled within 18 meters of a well near Kunia on the island of Oahu, the occurrence and distribution of contaminants in Hawaii's ground water has been carefully documented by Oki and Giambelluca (1985, 1987) and Lau and Mink (1987). Before 1981, when the nematocide dibromochloropropane (DBCP) was found in wells in central Oahu, the detection limit for most chemicals was too high to reveal the low level of contamination that probably had existed for many years.

Concern about the fate of agriculture chemicals led the Hawaii State Department of Agriculture to initiate a large sampling program to characterize the sources of nonpoint ground water contamination. In July 1983, 10 wells in central Oahu were closed because of DBCP and EDB contamination. The public has been kept well informed of possible problems through the publication of maps of chemicals detected in ground water in the local newspaper. Updated versions of these maps are shown in Figures 5.6a , b , c , and d .

In Hawaii, interagency committees, with representation from the Departments of Health and Agriculture, have been formed to address the complex technical and social questions associated with ground water contamination from agricultural chemicals. The Hawaii legislature has provided substantial funding to groups at the University of Hawaii to develop the first GIS-based regional scale chemical leaching assessment approach to aid in pesticide regulation. This effort, described below, has worked to identify geographic areas of concern, but the role the vulnerability maps generated by this system will play in the overall regulatory process is still unclear.

Agrichemicals are essential to agriculture in Hawaii. It is not possible to maintain a large pineapple monoculture in Hawaii without nematode control using pesticides. Pineapple and sugar growers in Hawaii have generally employed well controlled management practices in their use of fertilizers, herbicides, and insecticides. In the early 1950s, it was thought that organic chemicals such as DBCP and EDB would not leach to ground water

FIGURE 5.6a The occurrence and distribution of ground water contamination on the Island of Oahu. (Map provided by Hawaii State Department of Health.)

FIGURE 5.6b The occurrence and distribution of ground water contamination on the Island of Hawaii. (Map provided by Hawaii State Department of Health.)

FIGURE 5.6c The occurrence and distribution of ground water contamination on the Island of Maui. (Map provided by Hawaii State Department of Health.)

FIGURE 5.6d The occurrence and distribution of ground water contamination on the Island of Kauai. (Map provided by Hawaii State Department of Health.)

because (1) the chemicals are highly sorbed in soils with high organic carbon contents, (2) the chemicals are highly volatile, and (3) the water table is several hundred meters below the surface. Measured concentrations of DBCP and EDB down to 30 meters at several locations have shown the original assessment to be wrong. They have resulted in an urgent need to understand processes such as preferential flow better and to predict if the replacement chemicals used today, such as Telon II, will also leach to significant depths.

Leaching of pesticides to ground water in Hawaii could take decades. This time lag could lead to a temporary false sense of security, as happened in the past and potentially result in staggering costs for remedial action. For this reason, mathematical models that permit the user to ask ''what if" questions have been developed to help understand what the future may hold under certain management options. One needs to know what the fate of chemicals applied in the past will be and how to regulate the chemicals considered for use in the future; models are now being developed and used to help make these vulnerability assessments.

Researchers have embarked on several parallel approaches to quantitatively assess the vulnerability of Hawaii's ground water resources, including: (1) sampling, (2) physically-based numerical modeling, and (3) vulnerability mapping based on a simple chemical leaching index. Taken together these approaches have provided insight and guidance for work on a complex, spatially and temporally variable problem.

The sampling programs (Wong 1983 and 1987, Peterson et al. 1985) have shown that the chemicals applied in the past do, in fact, leach below the root zone, contrary to the original predictions, and can eventually reach the ground water. Experiments designed to characterize the nuances of various processes, such as volatilization, sorption, and degradation, have been conducted recently and will improve the conceptualization of mathematical models in the future.

The EPA's Pesticide Root Zone Model (PRZM), a deterministic-empirical/conceptual fluid flow/solute transport model, has been tested by Loague and co-workers (Loague et al. 1989a, b; Loague 1992) against measured concentration profiles for DBCP and EDB in central Oahu. These simulations illustrate that the chemicals used in the past can indeed move to considerable depths. Models of this kind, once properly validated, can be used to simulate the predicted fate of future pesticide applications. One must always remember, however, that numerical simulations must be interpreted in terms of the limiting assumptions associated with model and data errors.

Ground water vulnerability maps and assessments of their uncertainty were pioneered at the University of Hawaii in the Department of Agriculture Engineering (Khan and Liang 1989, Loague and Green 1990a). These pesticide leaching assessments were made by coupling a simple mobility index to a geographic information system. Loague and coworkers have investigated the uncertainty in these maps owing to data and model errors (Loague and Green 1988; Loague et al. 1989c, 1990; Loague and Green 1990b, 1990c; Loague 1991; Kleveno et al. 1992; Yost et al. 1993). The Hawaiian database on soils, climate, and chemicals is neither perfect nor poor for modeling applications; it is typical of what exists in most states—major extrapolations are required to estimate the input parameters required for almost any chemical fate model.

Sampling from wells in Hawaii has shown the concentrations of various chemicals, both from agriculture and industrial sources, which have leached to ground water in Hawaii. These concentrations, in general, are low compared to the levels detected in other states and for the most part are below health advisory levels established by EPA. In some instances contamination has not resulted from agriculture, but rather from point sources such as chemical loading and mixing areas and possibly from ruptured fuel lines. The widespread presence of trichloropropane (TCP) in Hawaii's ground water and deep soil cores at concentrations higher than DBCP was totally unexpected. TCP was never applied as a pesticide, but results from the manufacture of the fumigant DD, which was used until 1977 in pineapple culture. The occurrence of TCP illustrates that one must be aware of the chemicals applied as well as their components and transformation products.

Wells have been closed in Hawaii even though the measured contaminant concentrations have been below those considered to pose a significant health risk. At municipal well locations in central Oahu, where DBCP, EDB, and/or TCP have been detected, the water is now passed through carbon filters before it is put into the distribution system. The cost of this treatment is passed on to the water users, rather than to those who applied the chemicals.

The pesticide leaching assessment maps developed by Khan and Liang (1989) are intended for incorporation into the regulatory process. Decisions are not made on the basis of the red and green shaded areas for different chemicals (see Plate 3 ), but this information is considered. The uncertainty analysis by Loague and coworkers has shown some of the limitations of deterministic assessments in the form of vulnerability maps and provided initial guidance on data shortfalls.

APPLICATION OF A VULNERABILITY INDEX FOR DECISION-MAKING AT THE NATIONAL LEVEL

Need for a vulnerability index.

A vulnerability index for ground water contamination by pesticides has been developed and used by USDA as a decision aid to help attain the objectives of the President's Water Quality Initiative (see Box 1.1 ). A vulnerability index was needed for use in program management and to provide insight for policy development. Motivation for the development of the vulnerability index was provided by two specific questions:

Given limited resources and the geographic diversity of the water quality problems associated with agricultural production, what areas of the country have the highest priority for study and program implementation?

What policy implications emerge from the spatial patterns of the potential for conamination from a national perspective, given information currently available about farming practices and chemical use in agriculture?

Description of the Vulnerability Index

A vulnerability index was derived to evaluate the likelihood of shallow ground water contamination by pesticides used in agriculture in one area compared to another area. Because of the orientation of Initiative policies to farm management practices, it was necessary that the vulnerability measure incorporate field level information on climate, soils, and chemical use. It also needed to be general enough to include all areas of the country and all types of crops grown.

A Ground Water Vulnerability Index for Pesticides (GWVIP) was developed by applying the Soil-Pesticide Interaction Screening Procedure (SPISP) developed by the Soil Conservation Service to the National Resource Inventory (NRI) land use database for 1982 and the state level pesticide use database created by Resources for the Future (Gianessi and Puffer 1991). Details of the computational scheme and databases used are described by Kellogg et al. (1992). The 1982 NRI and the associated SOIL-5 database provide information on soil properties and land use at about 800,000 sample points throughout the continental United States. This information is sufficient to apply the SPISP to each point and thus obtain a relative measure of the soil leaching potential throughout the country. The RFF pesticide use database was used to infer chemical use at each point on the basis of the crop type recorded in the NRI database. By taking advantage of the statistical properties of the NRI database, which is based on a statistical survey

sampling design, the GWVIP score at each of the sample points can be statistically aggregated for making comparisons among regions.

Since the GWVIP is an extension of a screening procedure, it is designed to minimize the likelihood of incorrectly identifying an area as having a low potential for contamination—that is, false negatives are minimized and false positives are tolerated. The GWVIP is designed to classify an area as having a potential problem even if the likelihood is small.

GWVIP scores were graphically displayed after embedding them in a national cartographic database consisting of 13,172 polygons created by overlaying the boundaries of 3,041 counties, 189 Major Land Resource Areas (MLRAs), 2,111 hydrologic units, and federal lands.

Three caveats are especially important in using the GWVIP and its aggregates as a decision aid:

Land use data are for 1982 and do not represent current cropping patterns in some parts of the country. Although total cropland acreage has remained fairly stable over the past 10 years, there has been a pronounced shift from harvested cropland to cropland idled in government programs.

The approach uses a simulation model that predicts the amount of chemical that leaches past the root zone. In areas where the water table is near the surface, these predictions relate directly to shallow ground water contamination. In other areas a time lag is involved. No adjustment was made for areas with deep water tables.

No adjustment in chemical use is made to account for farm management factors, such as chemical application rates and crop rotations. The approach assumes that chemical use is the same for a crop grown as part of a rotation cropping system as for continuous cropping. Since the chemical use variable in the GWVIP calculation is based on acres of land treated with pesticides, application rates are also not factored into the analysis.

Application to Program Management

By identifying areas of the country that have the highest potential for leaching of agrichemicals, the GWVIP can serve as a basis for selecting sites for implementation of government programs and for more in-depth research on the environmental impact of agrichemical use. These sites cannot be selected exclusively on the basis of the GWVIP score, however, because other factors, such as surface water impacts and economic and demographic factors, are also important.

For example, the GWVIP has been used as a decision aid in selecting sites for USDA's Area Study Program, which is designed to provide chemical use and farming practice information to aid in understanding the relationships among farming activities, soil properties, and ground water quality.

The National Agricultural Statistics Service interviews farm operators in 12 major watersheds where the U.S. Geological Survey is working to measure the quality of surface and ground water resources under its National Water Quality Assessment Program. At the conclusion of the project, survey information will be combined with what is learned in other elements of the President's Water Quality Initiative to assess the magnitude of the agriculture-related water quality problem for the nation as a whole and used to evaluate the potential economic and environmental effects of Initiative policies of education, technical assistance, and financial assistance if implemented nationwide.

To meet these objectives, each Area Study site must have a high potential for ground water contamination relative to other areas of the country. A map showing the average GWVIP for each of the 13,172 polygons comprising the continental United States, shown in Plate 3 , was used to help select the sites. As this map shows, areas more likely to have leaching problems with agrichemicals than other areas of the country occur principally along the coastal plains stretching from Alabama and Georgia north to the Chesapeake Bay area, the corn belt states, the Mississippi River Valley, and the irrigated areas in the West. Sites selected for study in 1991 and 1992 include four from the eastern coastal plain (Delmarva Peninsula, southeastern Pennsylvania, Virginia and North Carolina, and southern Georgia), four from the corn belt states (Nebraska, Iowa, Illinois, and Indiana), and two from the irrigated areas in the West (eastern Washington and southeastern Idaho). Four additional sites will be selected for study in 1993.

Application to Policy Analysis and Development

The GWVIP has also been used by USDA to provide a national perspective on agricultural use of pesticides and the potential for ground water contamination to aid in policy analysis and development.

The geographic distribution of GWVIP scores has shown that the potential for ground water contamination is diverse both nationally and regionally. Factors that determine intrinsic vulnerability differ in virtually every major agricultural region of the country. Whether an impact is realized in these intrinsically vulnerable areas depends on the activities of producers—such as the type of crop planted, chemical use, and irrigation practices—which also vary both nationally and regionally. High vulnerability areas are those where a confluence of these factors is present. But not all cropland is vulnerable to leaching. About one-fourth of all cropland has GWVIP scores that indicate very low potential for ground water contamination from the use of agrichemicals. Nearly all agricultural states have significant acreage that meets this low vulnerability criterion. Areas of the country identified as being in a high vulnerability group relative to potential

for agrichemical leaching also have significant acreages that appear to have low vulnerability.

This mix of relative vulnerabilities both nationally and regionally has important policy implications. With the potential problem so diverse, it is not likely that simple, across-the-board solutions will work. Simple policies—such as selective banning of chemicals—may reduce the potential for ground water contamination in problem areas while imposing unnecessary costs on farming in nonproblem areas. The geographic diversity of the GWVIP suggests that the best solutions will come from involvement of both local governments and scientists with their state and national counter-parts to derive policies that are tailored to the unique features of each problem area.

In the future, USDA plans to use vulnerability indexes, like the GWVIP, in conjunction with economic models to evaluate the potential for solving agriculture-related water quality problems with a nationwide program to provide farmers with the knowledge and technical means to respond voluntarily to water quality concerns.

These six case studies illustrate how different approaches to vulnerability assessment have evolved under diverse sets of management requirements, data constraints, and other technical considerations. In addition, each of these examples shows that vulnerability assessment is an ongoing process through which information about a region's ground water resources and its quality can be organized and examined methodically.

In Iowa, the Iowa DNR staff elected to keep their vulnerability characterization efforts as simple as possible, and to use only properties for which data already existed or could be easily checked. They assumed that surficial features such as the soil are too thin and too disrupted by human activities (e.g., tillage, abandoned wells) to provide effective ground water protection at any particular location and sought to identify a surrogate measure for average travel time from the land surface to the aquifer. Thus, a ground water vulnerability map was produced which represents vulnerability primarily on the basis of depth to ground water and extent of overlying materials. Wells and sinkholes are also shown. The results are to be used for informing resource managers and the public of the vulnerability of the resource and to determine the type of information most needed to develop an even better understanding of the vulnerability of Iowa's ground water.

The Cape Cod approach to ground water vulnerability assessment is perhaps one of the oldest and most sophisticated in the United States. Driven by the need to protect the sole source drinking water aquifer underlying this sandy peninsula, the vulnerability assessment effort has focused on the identification

and delineation of the primary recharge areas for the major aquifers. This effort began with a simple mass balance approach which assumed even recharge within a circular area around each drinking water well. It has since evolved to the development of a complex, particle-tracking three-dimensional model that uses site-specific data to delineate zones of contribution. Bolstered by strong public concern, Cape Cod has been able to pursue an ambitious and sophisticated agenda for resource protection, and now boasts a sophisticated differential management ground water protection program.

In Florida, ground water resource managers rely on a combination of monitoring and vulnerability assessment techniques to identify high recharge areas the develop the state ground water protection program. Overlay and index methods, including several modified DRASTIC maps were produced to identify areas of ground water significance in support of decision making in state land acquisition programs aimed at ground water protection. In addition, several monitoring networks have been established to assess background water quality and monitor actual effects in areas identified as highly vulnerable. The coupling of ground water vulnerability assessments with monitoring and research efforts, provides the basis of an incremental and evolving ground water protection program in Florida.

The programs to protect ground water in California's intensely agricultural San Joaquin Valley are driven largely by compliance with the state Pesticide Contamination Prevention Act. The California Department of Pesticide Regulation determined that no model would be sufficient to cover their specific regulatory needs and that the available data bases were neither suitable nor reliable for regulatory purposes. Thus, a ground water protection program was built on the extensive existing pesticide use reporting system and the significant ground water monitoring requirements of the act. Using farm sections as management units, the state declares any section in which a pesticide or its degradation product is detected as a pesticide management zone and establishes further restrictions and monitoring requirements. Thus, the need to devise a defensible regulatory approach led California to pursue a mechanistic monitoring based approach rather than a modeling approach that would have inherent and difficult to quantify uncertainties.

In contrast, the approach taken in Hawaii involves an extensive effort to understand the uncertainty associated with the assessment models used. The purpose of this is to provide guidance to, but not the sole basis for, the pesticide regulation program. The combined use of sampling, physically-based numerical modeling, and a chemical leaching index has led to extensive improvements in the understanding of the fate of pesticides in the subsurface environment. Uncertainty analyses are used to determine where additional information would be most useful.

Finally, USDA's Ground Water Vulnerability Index for Pesticides illustrates a national scale vulnerability assessment developed for use as a decision aid and analytical tool for national policies regarding farm management and water quality. This approach combines nationally available statistical information on pesticide usage and soil properties with a simulation model to predict the relative likelihood of contamination in cropland areas. USDA has used this approach to target sites for its Area Study Program which is designed to provide information to farmers about the relationships between farm management practices and water quality. The results of the GWVIP have also indicated that, even at the regional level, there is often an mix of high and low vulnerability areas. This result suggests that effective ground water policies should be tailored to local conditions.

Au, L.K.L. 1991. The Relative Safety of Hawaii's Drinking Water. Hawaii Medical Journal 50(3): 71-80.

Barlow, P.M. 1993. Particle-Tracking Analysis of Contributing Areas of Public-Supply Wells in Simple and Complex Flow Systems, Cape Cod, Massachusetts. USGS Open File Report 93-159. Marlborough, Massachusetts: U.S. Geological Survey.

Britt, J.K., S.E. Dwinell, and T.C. McDowell. 1992. Matrix decision procedure to assess new pesticides based on relative ground water leaching potential and chronic toxicity. Environ. Toxicol. Chem. 11: 721-728.

Cape Cod Commission (CCC). 1991. Regional Policy Plan. Barnstable, Massachusetts: Cape Cod Commission.

Cape Cod Planning and Economic Development Commission (CCPEDC). March 1978a. Draft Area Wide Water Quality Management Plan for Cape Cod. Barnstable, Massachusetts: Cape Cod Commission.

Cape Cod Planning and Economic Development Commission (CCPEDC). September 1978b. Final Area Wide Water Quality Management Plan for Cape Cod. Barnstable, Massachusetts: Cape Cod Commission.

Department of Environmental Protection, Division of Water Supply (DEP-WS). 1991. Guidelines and Policies for Public Water Supply Systems. Massachusetts Department of Environmental Protection.

Gianessi, L.P., and C.A. Puffer. 1991. Herbicide Use in the United States: National Summary Report. Washington, D.C.: Resources for the Future.

Guswa, J.H., and D.R. LeBlanc. 1981. Digital Models of Ground water Flow in the Cape Cod Aquifer System, MA. USGS Water Supply Paper 2209. U.S. Geological Survey.

Heath, D.L. 1988. DRASTIC mapping of aquifer vulnerability in eastern Barnstable and western Yarmouth, Cape Cod, Massachusetts. In Appendix D, Cape Cod Aquifer Management Project, Final Report, G.A. Zoto and T. Gallagher, eds. Boston: Massachusetts Department of Environmental Quality Engineering.

Horsely, S.W. 1983. Delineating zones of contribution of public supply wells to protect ground water . In Proceedings of the National Water Well Association Eastern Regional Conference, Ground-Water Management, Orlando, Florida.

Hoyer, B.E. 1991. Ground water vulnerability map of Iowa. Pp. 13-15 in Iowa Geology, no. 16. Iowa City, Iowa: Iowa Department of Natural Resources.

Hoyer, B.E., J.E. Combs, R.D. Kelley, C. Cousins-Leatherman, and J.H. Seyb. 1987. Iowa Ground water Protection Strategy. Des Moines: Iowa Department of Natural Resources.

Kellogg, R.L., M.S. Maizel, and D.W. Goss. 1992. Agricultural Chemical Use and Ground Water Quality: Where Are the Potential Problems? Washington, D.C.: U.S. Department of Agriculture, Soil Conservation Service.

Khan, M.A., and T. Liang. 1989. Mapping pesticide contamination potential. Environmental Management 13(2):233-242.

Kleveno, J.J., K. Loague, and R.E. Green. 1992. An evaluation of a pesticide mobility index: Impact of recharge variation and soil profile heterogeneity. Journal of Contaminant Hydrology 11(1-2):83-99.

Land Acquisition Advisory Council (LAAC). 1991. Ground Water Resources Committee Final Report: Florida Preservation 2000 Needs Assessment. Tallahassee, Florida: Department of Environmental Regulation. 39 pp.

Lau, L.S., and J.F. Mink. 1987. Organic contamination of ground water: A learning experience. J. American Water Well Association 79(8):37-42.

LeBlanc, D.R., and J.H. Guswa. 1977. Water-Table Map of Cape Cod, MA. May 23-27, 1976, USGS Open File Report 77-419, scale 1:48,000.

Loague, K. 1991. The impact of land use on estimates of pesticide leaching potential: Assessments and uncertainties. Journal of Contaminant Hydrology 8: 157-175.

Loague, K. 1992. Simulation of organic chemical movement in Hawaii soils with PRZM: 3. Calibration. Pacific Science 46(3):353-373.

Loague, K.M., and R.E. Green. 1988. Impact of data-related uncertainties in a pesticide leaching assessment. Pp. 98-119 in Methods for Ground Water Quality Studies, D.W. Nelson and R.H. Dowdy, eds. Lincoln, Nebraska: Agricultural Research Division, University of Nebraska.

Loague, K., and R.E. Green. 1990a. Comments on "Mapping pesticide contamination potential," by M.A. Khan and T. Liang. Environmental Management 4:149-150.

Loague, K., and R.E. Green. 1990b. Uncertainty in Areal Estimates of Pesticide Leaching Potential. Pp. 62-67 in Transactions of 14th International Congress of Soil Science. Kyoto, Japan: International Soil Science Society.

Loague, K., and R.E. Green. 1990c. Criteria for evaluating pesticide leaching models. Pp. 175-207 in Field-Scale Water and Solute Flux in Soils, K. Roth, H. Flühler, W.A. Jury, and J.C. Parker, eds. Basel, Switzerland: Birkhauser Verlag.

Loague, K.M., R.E. Green, C.C.K. Liu, and T.C. Liang. 1989a. Simulation of organic chemical movement in Hawaii soils with PRZM: 1. Preliminary results for ethylene dibromide. Pacific Science 43(1):67-95.

Loague, K., T.W. Giambelluca, R.E. Green, C.C.K. Liu, T.C. Liang, and D.S. Oki. 1989b. Simulation of organic chemical movement in Hawaii soils with PRZM: 2. Predicting deep penetration of DBCP, EDB, and TCP. Pacific Science 43(4):362-383.

Loague, K.M., R.S. Yost, R.E. Green, and T.C. Liang. 1989c. Uncertainty in a pesticide leaching assessment for Hawaii. Journal of Contaminant Hydrology 4:139-161.

Loague, K., R.E. Green, T.W. Giambelluca, T.C. Liang, and R.S. Yost. 1990. Impact of uncertainty in soil, climatic, and chemical information in a pesticide leaching assessment. Journal of Contaminant Hydrology 5:171-194.

Maddox, G., and J. Spicola. 1991. Ground Water Quality Monitoring Network. Tallahassee, Florida: Florida Department of Environmental Regulation. 20 pp.

Maddox, G., J. Lloyd, T. Scott, S. Upchurch, and R. Copeland, eds. 1993. Florida's Ground Water Quality monitoring Program: Background Hydrogeochemistry. Florida Geological Survey Special Publication #34. Tallahassee, Florida: Florida Department of Environmental Regulation in cooperation with Florida Geological Survey.

National Research Council (NRC). 1986. Ground Water Quality Protection: State and Local Strategies. Washington, D.C.: National Academy Press.

Oki, D.S., and T.W. Giambelluca. 1985. Subsurface Water and Soil Quality Data Base for State of Hawaii: Part 1. Spec. Rept. 7. Manoa, Hawaii: Water Resources Research Center, University of Hawaii at Manoa.

Oki, D.S., and T.W. Giambelluca. 1987. DBCP, EDB, and TCP contamination of ground water in Hawaii. Ground Water 25:693-702.

Olimpio, J.C., E.C. Flynn, S. Tso, and P.A. Steeves. 1991. Use of a Geographic Information System to Assess Risk to Ground-Water Quality at Public-Supply Wells, Cape Cod, Massachusetts. Boston, Massachusetts: U.S. Geological Survey.

Peterson, F.L. 1972. Water development on tropic volcanic islands—Type example: Hawaii. Ground Water 5:18-23.

Peterson, F.L., K.R. Green, R.E. Green, and J.N. Ogata. 1985. Drilling program and pesticide analysis of core samples from pineapple fields in central Oahu. Water Resources Research Center, University of Hawaii at Manoa, Special Report 7.5. Photocopy.

Southwest Florida Water Management Districts (SFWMD). 1991. The Bluebelt Commission. Brooksville, Florida: Southwest Florida Water Management Districts.

U.S. Bureau of the Census. 1991. Statistical Abstracts of the United States: 1991, 111th edition. Washington, D.C.: U.S. Government Printing Office.

Wong, L. 1983. Preliminary report on soil sampling EDB on Oahu. Pesticide Branch, Div. of Plant Industry, Department of Agriculture, State of Hawaii. Photocopy.

Wong, L. 1987. Analysis of ethylene dibromide distribution in the soil profile following shank injection for nematode control in pineapple culture. Pp. 28-40 in Toxic Organic Chemicals in Hawaii's Water Resources, P.S.C. Rao and R.E. Green, eds. Ser. 086. Honolulu: Hawaii Inst. Trop Agric. Hum. Resources Res. Exten. University of Hawaii.

Yost, R.S., K. Loague, and R.E. Green. 1993. Reducing variance in soil organic carbon estimates—soil classification and geostatistical approaches. Geoderma 57(3):247-262

Since the need to protect ground water from pollution was recognized, researchers have made progress in understanding the vulnerability of ground water to contamination. Yet, there are substantial uncertainties in the vulnerability assessment methods now available.

With a wealth of detailed information and practical advice, this volume will help decision-makers derive the most benefit from available assessment techniques. It offers:

• Three laws of ground water vulnerability.
• Six case studies of vulnerability assessment.
• Guidance for selecting vulnerability assessments and using the results.
• Reviews of the strengths and limitations of assessment methods.
• Information on available data bases, primarily at the federal level.

This book will be indispensable to policymakers and resource managers, environmental professionals, researchers, faculty, and students involved in ground water issues, as well as investigators developing new assessment methods.

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Impact of drought on drinking water contamination: Disparities found affecting Latino/a communities

by American Public Health Association

Long-term exposure to contaminants such as arsenic and nitrate in water is linked to an increased risk of various diseases, including cancers, cardiovascular diseases, developmental disorders and birth defects in infants.

In the United States, there is a striking disparity in exposure to contaminants in tap water provided by community water systems (CWSs), with historically marginalized communities at greater risks compared to other populations. Often, CWSs that distribute water with higher contamination levels exist in areas that lack adequate public infrastructure or sociopolitical and financial resources.

In a study published in the American Journal of Public Health , Ms. Sandy Sum, a Ph.D. candidate at the Bren School of Environmental Science & Management, UC Santa Barbara, investigated the drinking water quality in California's CWSs serving majority Latino/a communities.

Ms. Sum analyzed trends in nitrate and arsenic concentrations in drinking water sourced from both surface and groundwater, using a varied set of data, including water sampling data, historical drought records, sociodemographic characteristics of the populations, measures of agricultural intensity and CWS characteristics from the period 2007–2020.

Her study found that these systems consistently exhibit higher and more variable levels of nitrate and arsenic compared to those serving non-majority Latino/a populations. She also found that instances of drought increased the contamination in CWSs serving these communities.

"Drought increased nitrate concentrations in majority Latino/a communities, with the effect doubling for CWSs with more than 75% Latino/a populations served. Arsenic concentrations in surface sources also increased during drought for all groups," explains Ms. Sum.

Nitrate concentrations in groundwater-sourced drinking water increased from a baseline of 2.5 mg/L in 1998 to a peak of 3.1 mg/L in 2018 for majority Latino/a CWSs.

In contrast, nitrate levels in non-majority Latino/a CWSs decreased from 2.1 mg/L to 1.8 mg/L over the same period. This widening disparity in nitrate exposure is particularly pronounced in surface-sourced water, where majority Latino/a CWSs show a mean nitrate concentration of 2.2 mg/L, significantly higher than the 1.2 mg/L observed in non-majority Latino/a CWSs as of 2020.

Drought conditions exacerbated these disparities, with a notable impact on surface-sourced drinking water. For majority Latino/a CWSs, drought conditions led to an increase in nitrate levels, with a 2-unit increase in the normalized drought index resulting in a 0.04 mg/L rise in nitrate concentrations for CWSs serving more than 25% Latino/a populations.

The increase is more pronounced in systems serving over 75% Latino/a populations, with a 0.16 mg/L rise. This effect is particularly evident in very small (<500 connections) and privately operated CWSs, where nitrate concentrations are more susceptible to drought conditions.

Surface-sourced water shows a drought-related increase in nitrate levels of 0.17 mg/L, more than double the increase observed in groundwater sources (0.07 mg/L).

"[The findings] are concerning when we consider that although more CWSs, about 77%, are supplied by groundwater, more people, almost 80%, are served by CWSs that use surface water as their primary source," Ms. Sum notes.

"Impending droughts driven by climate change may further increase drinking water disparities and arsenic threats. This underscores the critical need to address existing inequities in climate resilience planning and grant making," she explains further.

Additionally, arsenic concentrations in drinking water also exhibited variability under drought conditions. Drought increases overall arsenic concentrations in surface-sourced drinking water for both majority and non-majority Latino/a CWSs.

However, for majority Latino/a communities, drought leads to a statistically insignificant decrease in arsenic levels in groundwater-sourced drinking water. This trend contrasts with recent findings in the San Joaquin Valley, where drought-related intensified agricultural groundwater pumping has significantly increased nitrate prevalence by three to five times in public supply wells, highlighting a broader regional issue.

The study underscores the need for enhanced drought resilience measures, particularly for very small and privately operated CWSs serving Latino/a communities.

"[The differential] effects I found suggest that CWSs serving Latino/a communities are not mitigating elevated nitrate concentrations during drought conditions, which exacerbates existing disparities. This may reflect a lack of treatment infrastructure, resource constraints or other operational or technical differences," says Ms. Sum.

"Although I focused on only arsenic and nitrate concentrations, these vulnerable CWSs may also be at increased risk for contamination from other sources like pesticides, waste disposal sites and manufacturing plants, under stressors such as drought, floods and other natural events," Ms. Sum concludes on a cautionary note.

Future research should explore how drought impacts arsenic and nitrate levels to help understand the public health implications and guide policy initiatives for ensuring safe and equitable drinking water access.

Journal information: American Journal of Public Health

Provided by American Public Health Association

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Water Pollution and its Sources, Effects & Management: A Case Study of Delhi

Shahid Ahmed and Saba Ismail (2018) 'Water Pollution and its Sources, Effects & Management: A Case Study of Delhi', International Journal of Current Advanced Research, 07(2), pp. 10436-10442

7 Pages Posted: 31 Mar 2018

Shahid Ahmed

Jamia Millia Islamia - Economics

Saba Ismail

Jamia Millia Islamia

Date Written: 2018

Water pollution is a national and global issue. Humans and all living species in the world are facing worst results of polluted water. The present study investigates the level of awareness about water pollution in Delhi, its causes, its health effects and solutions among the youth in Delhi. The paper has used primary data collected through a schedule from university/college students in Delhi. The study concludes that the majority of educated youth (94%) perceives water pollution as environmental challenge and 52% respondents ranked it (1-3) as most important threat. The study identified dumping of waste as one of the most important causes of water pollution; untreated sewage as the second most important cause of water pollution and industrial discharge as the third most important cause of water pollution. The study identified Typhoid, Diarrhoea, Dengue, Cholera, Jaundice, Malaria, Chikungunya, etc are associated with water pollution on the basis of survey. The study suggests awareness campaign involving citizens and strict enforcement of environmental laws by concerned agencies as the appropriate solution to control environment degradation. It is recommended that there should be proper waste disposal system and waste should be treated before entering in to river and water bodies.

Keywords: Environment Sustainability, Water Pollution, Health Effects

JEL Classification: Q50, Q53, I12

Suggested Citation: Suggested Citation

Shahid Ahmed (Contact Author)

Jamia millia islamia - economics ( email ).

Jamia Nagar New Delhi, 110025 India

HOME PAGE: http://jmi.ac.in/economics/faculty-members/Prof_Shahid_Ahmed-1783

Jamia Millia Islamia ( email )

Jamia Nagar, New Delhi Delhi, 110025 India

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For the Instructor

• First Publication: July 15, 2016
• Fix Links: August 7, 2024 -- Links were updated and downloadable files of linked materials were added when those materials were determined to be open access permission obtained for some of the materials.

Case Study 3: My Water Smells (and Tastes) Like Gasoline!

Compiled and modified for instructional use by: Mike Phillips, Illinois Valley Community College, [email protected]

Wedron, Illinois — LUST, dust, and other sensory impacts

This case study is an examination of the chemical and sensory impacts of a variety of environmental issues including sand mining, leaking underground gasoline storage tanks (LUSTs), and a railroad.

Wedron is a small, unincorporated town of approximately 100 residents in north central Illinois. A large sand mine has been in operation on the south and west sides of the town for many decades, and the mine property includes several abandoned and active pits, a processing plant, and a train car loading facility. Railroad tracks are located along the east side of town as is a grain elevator (storage and loading facility).

The following information was obtained from the US EPA web site about Wedron , personal visits to the town, and published news articles.

In April 1982, Illinois EPA began a groundwater investigation in Wedron after the Illinois Department of Public Health received complaints from several residents of gasoline-type odors in their private well water. Illinois EPA collected groundwater samples from several private wells in April 1982, June 1983, and August 1983, and confirmed the presence of chemicals found in gasoline. At that time, a new deeper well was drilled to provide clean drinking water to the affected homes. In addition, an investigation of several potential sources of contamination was completed; one possible source, an old gasoline station, was identified; and the gas station property was cleaned up.

In 2011, residents of Wedron reported gasoline odors from their water. As a result, the Illinois Department of Public Health collected groundwater samples in October 2011 and found two homes with benzene levels above the health standard. In November 2011, the LaSalle County Health Department told these residents to no longer drink or use their well water. Illinois EPA then contacted the US EPA, which began the current investigation. Beginning in December 2011 and continuing through 2015, the US EPA collected groundwater samples from residential and commercial wells in the town. These samples were tested for the presence of a family of chemicals called volatile organic compounds (VOCs), semi-volatile organic compounds and metals. A cluster of wells were found to be contaminated with VOCs commonly associated with gasoline. Residences with wells showing contaminant levels above drinking water standards were first provided with a filtration system and then connected to wells drilled into a deeper, uncontaminated aquifer. The US EPA identified several possible sources, including abandoned underground gasoline storage tanks and former gasoline stations (including the previously identified one), and began working with the responsible parties to develop a cleanup plan.

The sand mine and attendant facilities had a role in altering the direction of groundwater flow, and leaking gasoline storage tanks were found on the property; however, the contamination found in the residential wells was not linked to the sources at the sand mine's facility. The investigation did reveal community concerns about silica dust associated with the mine, and an investigation of those concerns was begun.

In the early months of 2013, environmental activist Erin Brockovich and associates from the law firm where she works were contacted and began working with some local residents to ensure that the groundwater contamination was removed.

Discussion Questions:

• What is the problem?
• What might have caused the problem?
• How did the problem come to the attention of local residents? Local government officials? The state and US EPAs?
• What data would you want to assess and address the problem?

Data Sources for Group Analysis and Presentation Activity:

The following data sources each provide information about gasoline (and other) contamination in Wedron, Illinois.

• US EPA website about Wedron site (preferred resources are from this site)
• to access the file, visit: https://semspub.epa.gov/src/collection/05/SC30194
• enter "photo" in the search box under "Document Title"
• Key Communication Documents dated February 2014 (Acrobat (PDF) 356kB Jul17 24) , June 2013 (Acrobat (PDF) 303kB Jul17 24) , February 2013 (Acrobat (PDF) 146kB Jul17 24) , and October 2012 (Acrobat (PDF) 262kB Jul17 24) (documents may be downloaded by clicking on date or by visiting this site and searching by date).
• p. 1–26 (main body of report)
• p. 36–44 (color maps)
• p. 263 and 269 (color photos of contaminated and clean sand; contaminated sand is green while clean sand is white)
• Potential Source and Water Table Elevation Contour Map ( document (Acrobat (PDF) 406kB Aug1 24) / USEPA archive ) (PDF, 1 pp, 401K)
• Water Table Elevation Contour Map ( document (Acrobat (PDF) 579kB Aug1 24) / USEPA archive )(PDF, 1 pp, 574K)
• Illustration of a geologic cross section using well information at Thompson Park in Wedron, Illinois
• Potentiometric Surface Map, Wedron Groundwater Site ( document (Acrobat (PDF) 211kB Aug1 24) / USEPA archive ) (PDF, 4pp, 177K) November 2012
• Small Town Consumed By Toxic Water Mess (Acrobat (PDF) 221kB Aug1 24) ( direct link ); by Mike Moen, April 29, 2013
• Small Town Attracts Big Name To Help With Toxic Water Mess (Acrobat (PDF) 237kB Aug1 24) ( direct link ); by Mike Moen, March 14, 2013
• Environmental scientist calls Wedron water pollution an 'emergency' (Acrobat (PDF) 92kB Aug1 24) ( direct link) , an article from Ottawa Times/MyWebTimes by Steve Stout, January 28, 2013
• Wedron residents file suit over groundwater contamination (Acrobat (PDF) 293kB Aug1 24) , an article from MyWebTimes by Steve Stout, January 22, 2014
• Wedron, Illinois website from Erin Brockovich
• US EPA Envirofacts map and data (Enter "Wedron, IL" in search box.)

Post-Group Analysis/Presentation Reflection:

• What data/information is most likely to be used to make decisions about cleanup?
• What data/information is most likely to get the attention of the impacted residents?
• What data/information is the most difficult to quantify?
• What data were excluded? Why do you think the EPA chose to use the data it did?

Additional resources:

Documentary: The Price of Sand

• The movie touches on a variety of sensory inputs related to mining (although, not gasoline odors & contamination).
• The movie is available to stream on a variety of platforms.

« Previous Page      

• Module Overview
• Unit 1: Data Set Analysis
• Unit 2: Sensory Log & Holistic Reflection
• Unit 4: Case Study Analysis
• Unit 5: Sensory Map Development
• Student Materials
• Case Study 1: El Paso Smelter
• Case Study 2: The Salton Sea
• Case Study 3: Water Contamination

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JSmol Viewer

Groundwater lnapl contamination source identification based on stacking ensemble surrogate model.

1. Introduction

2.1. simulation model for multiphase flow, 2.2. quasi-monte carlo sampling, 2.3. surrogate model, 2.3.1. stacking ensemble method, 2.4. inversion method, 2.4.1. overview of bayesian theory, 2.4.2. differential evolutionary markov chain, 3. case study, 3.1. case description and generalization, 3.2. variable to be identified.

Click here to enlarge figure

3.3. Establish Surrogate Model

3.4. application of the de-mc method, 4.1. performance of the stacking ensemble surrogate model, 4.2. de-mc inversion results, 5. conclusions and discussion, author contributions, data availability statement, conflicts of interest.

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Share and Cite

Bai, Y.; Lu, W.; Wang, Z.; Xu, Y. Groundwater LNAPL Contamination Source Identification Based on Stacking Ensemble Surrogate Model. Water 2024 , 16 , 2274. https://doi.org/10.3390/w16162274

Bai Y, Lu W, Wang Z, Xu Y. Groundwater LNAPL Contamination Source Identification Based on Stacking Ensemble Surrogate Model. Water . 2024; 16(16):2274. https://doi.org/10.3390/w16162274

Bai, Yukun, Wenxi Lu, Zibo Wang, and Yaning Xu. 2024. "Groundwater LNAPL Contamination Source Identification Based on Stacking Ensemble Surrogate Model" Water 16, no. 16: 2274. https://doi.org/10.3390/w16162274

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• Published: 01 March 2024

Assessing exposure and health consequences of chemicals in drinking water in the 21st Century

• Nicole C. Deziel   ORCID: orcid.org/0000-0002-5751-9191 1 , 2 &
• Cristina M. Villanueva 2  

Journal of Exposure Science & Environmental Epidemiology volume  34 ,  pages 1–2 ( 2024 ) Cite this article

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Populations worldwide are exposed to a myriad of chemicals via drinking water, yet only a handful of chemicals have been extensively evaluated with regard to human exposures and health impacts [ 1 , 2 ]. Many chemicals are generally “invisible” in that they do not alter the color or odor of drinking water, and many of the associated effects are not observable for decades, making linkages between exposure and disease difficult. The articles included in the Journal of Exposure Science and Environmental Epidemiology Special Topic “Assessing Exposure and Health Consequences of Chemicals in Drinking Water in the 21st Century” cover a range of topics, including: (i) new exposure and health research for regulated and emerging chemicals, (ii) new methods and tools for assessing exposure to drinking water contaminants, (iii) issues of equity and environmental justice, (iv) drinking water issues within the context of a changing climate. This Special Topic includes articles authored by experts across multiple disciplines including environmental engineering, hydrology, exposure science, epidemiology, toxicology, climate science, and others. Many of these papers emerged from an international symposium organized by ISGlobal and Yale scientists held in Barcelona in September 2022 [ 3 ].

Regulated chemicals

Chemicals that have been the focus of environmental health research include disinfection by-products (DBPs), nitrate, and metals. Although many of these chemicals are regulated, there is concern about low-dose exposures at concentrations below standards and guidelines, and risks of health endpoints not yet studied. Kaufman et al. explore new ways to assess DBP exposure, considering concentrations and specific toxicity potential in relation to birth defects risk [ 4 ]. Long-term exposure to DBPs and nitrate is addressed by Donat-Vargas et al. in relation to chronic lymphocytic leukaemia in Spain [ 5 ]. Friedman et al. examine temporal and spatial variability of manganese concentrations in a case study in the United States (US) [ 6 ]. Hefferon et al. evaluated sociodemographic inequalities in fluoride concentrations across the US [ 7 ]. Spaur et al. evaluate the contribution of water arsenic to biomarker levels in a prospective study in the US [ 8 ].

Chemicals of emerging concern

Many emerging chemicals, such as per- and polyfluoroalkyl substances (PFAS), microplastics, and 1,4-dioxane, have drinking water as the dominant exposure pathway for many populations. Yet, these remain largely unregulated or have standards and guidelines that vary widely across states and countries. Because only small percentages of the universe of contaminants are regulated in drinking water, routine monitoring data for many chemicals of emerging concern is frequently absent or very limited. To advance understanding of drinking water exposures to PFAS, Cserbik et al. [ 9 ]. and Kotlarz et al. [ 10 ]. evaluate and compare PFAS in drinking water and blood serum samples in two different settings: an urban setting not impacted by PFAS pollution in Spain [ 9 ] and among well water users living near a fluorochemical facility in the US [ 10 ], respectively.

New methods and tools for exposure assessment

There is a need for improved tools, methods, and data to evaluate drinking water related exposures. These tools and techniques remain somewhat limited and lag behind those of other stressors (e.g., air pollution). Also, despite water contaminants occurring in mixtures, most of the evaluations (and policies and regulations) are conducted chemical by chemical, ignoring potential interactions. Schullehner et al. present case studies of three approaches of exposure assessment of drinking water quality: use of country-wide routine monitoring databases, wide-scope chemical analysis, and effect-based bioassay methods [ 11 ]. Luben et al. elaborate and compare different exposure assessment metrics to trihalomethanes in epidemiological analyses of reproductive and developmental outcomes [ 12 ]. Escher et al. present in vitro assays to evaluate biological responses of including neurotoxicity, oxidative stress, and cytotoxicity in different types of drinking water samples (tap, bottled, filtered) [ 13 ] Isaacs et al. present newly developed automated workflows to screen contaminants of concern based on toxicity and exposure potential [ 14 ]. Dorevitch et al. develop a novel method to improve detection of particulate lead spikes [ 15 ].

Issues of equity, environmental justice, and vulnerable populations

A substantial portion of the population (e.g., 20% in the United States) have private water supplies (e.g., a household domestic drinking water well), which are not subject to any federal regulatory oversight or monitoring [ 16 ]. This presents an equity issue in access to data on drinking water quality, as discussed in Levin et al. [ 2 ]. and heterogeneity in state-based policies for drinking water prevention, as discussed by Schmitt et al. [ 17 ]. Spaur et al. [ 8 ], observed that water from unregulated private wells and regulated municipal water supplies contributes substantially to overall exposures (as measured by urinary arsenic and uranium concentrations) in both rural, American Indian populations and urban, racially/ethnically diverse populations nationwide. Hefferon et al. evaluated environmental justice issues with respect to fluoride and found that 2.9 million US residents are served by public water systems with average fluoride concentrations exceeding the World Health Organization’s guidance limit [ 7 ]. Friedman et al. show that manganese in drinking water frequently exceeds current guidelines in the US, and occur at concentrations shown to be associated with adverse health outcomes, especially for vulnerable and susceptible populations like children [ 6 ].

Chemical contamination may also pose a serious threat in the developing world. Today, around 2.2 billion people – or 1 in 4 – still lack safely managed drinking water at home [ 18 ]. In most of the world, microbial contamination is the biggest challenge. Because it has been understudied, the chemical risks remain obscure [ 19 ], and regulators often require local data to take action. Praveena et al. reviews the quality of different drinking water types in Malaysia (tap water, ground water, gravity feed system) and its implications on policy, human health, management, and future research [ 20 ].

Water quality in a changing climate

There is an urgent need to anticipate and prepare for current and future challenges in a rapidly changing world. We also need to foresee new challenges to address issues of water scarcity (e.g., increasing desalination, use of treated wastewater in densely populated urban areas to meet water use demands), and aging infrastructure for many middle- and high-income countries constructed in the nineteenth and twentieth centuries. The impacts of climate change on the water cycle are direct and observable, such as more frequent droughts and floods, sea level rise, and ice/snow melt. These events will challenge drinking water quality and availability through direct and indirect mechanisms [ 21 ]. There is still very limited knowledge on how climate events will affect the quality of finished drinking water. In our special issue, Oliveras et al. conducts a new analysis on the impacts of drought and heavy rain surrogates on the quality of drinking water in Barcelona, Spain [ 22 ].

Chemical contamination of drinking water is widespread. Although our knowledge on chemical risks in drinking water is increasing, there are knowledge gaps that make a slow translation to public health protection. We hope this issue highlights, elevates, and motivates research on chemical exposures via drinking water.

Villanueva CM, Kogevinas M, Cordier S, Templeton MR, Vermeulen R, Nuckols JR, et al. Assessing exposure and health consequences of chemicals in drinking water: current state of knowledge and research needs. Environ health Perspect. 2014;122:213–21.

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Levin R, Villanueva CM, Beene D, Cradock AL, Donat-Vargas C, Lewis J, et al. US drinking water quality: exposure risk profiles for seven legacy and emerging contaminants. J Expo Sci Environ Epidemiol. 2024;34:3–22.

ISGlobal. https://www.isglobal.org/en/-/advancing-the-science-for-drinking-water-chemical-exposure-assessment-and-health-research ; 2022.

Kaufman JA, Wright JM, Evans A, Rivera-Núñez Z, Meyer A, Reckhow DA, et al. Risks of obstructive genitourinary birth defects in relation to trihalomethane and haloacetic acid exposures: expanding disinfection byproduct mixtures analyses using relative potency factors. J Expo Sci Environ Epidemiol. 2024;34:34–46.

Donat-Vargas C, Kogevinas M, Benavente Y, Costas L, Campo E, Castaño-Vinyals G, et al. Lifetime exposure to brominated trihalomethanes in drinking water and swimming pool attendance are associated with chronic lymphocytic leukemia: a Multicase-Control Study in Spain (MCC-Spain). J Expo Sci Environ Epidemiol. 2024;34:47–57.

Friedman A, Boselli E, Ogneva-Himmelberger Y, Heiger-Bernays W, Brochu P, Burgess M, et al. Manganese in residential drinking water from a community-initiated case study in Massachusetts. J Expo Sci Environ Epidemiol. 2024;34:58–67.

Hefferon R, Goin DE, Sarnat JA, Nigra AE. Regional and racial/ethnic inequalities in public drinking water fluoride concentrations across the US. J Expo Sci Environ Epidemiol. 2024;34:68–76.

Spaur M, Glabonjat RA, Schilling K, Lombard MA, Galvez-Fernandez M, Lieberman-Cribbin W, et al. Contribution of arsenic and uranium in private wells and community water systems to urinary biomarkers in US adults: the strong heart study and the multi-ethnic study of atherosclerosis. J Expo Sci Environ Epidemiol. 2024;34:77–89.

Cserbik D, Casas M, Flores C, Paraian A, Haug LS, Rivas I, et al. Concentrations of per- and polyfluoroalkyl substances (PFAS) in paired tap water and blood samples during pregnancy. J Expo Sci Environ Epidemiol. 2024;34:90–6.

Kotlarz N, Guillette T, Critchley C, Collier D, Lea CS, McCord J, et al. Per- and polyfluoroalkyl ether acids in well water and blood serum from private well users residing by a fluorochemical facility near Fayetteville, North Carolina. J Expo Sci Environ Epidemiol. 2024;34:97–107.

Schullehner J, Cserbik D, Gago-Ferrero P, Lundqvist J, Nuckols JR. Integrating different tools and technologies to advance drinking water quality exposure assessments. J Expo Sci Environ Epidemiol. 2024;34:108–14.

Luben TJ, Shaffer RM, Kenyon E, Nembhard N, Weber KA, Nuckols J, et al. Comparison of Trihalomethane exposure assessment metrics in epidemiologic analyses of reproductive and developmental outcomes. J Expo Sci Environ Epidemiol. 2024;34:115–25.

Escher BI, Blanco J, Caixach J, Cserbik D, Farré MJ, Flores C, et al. In vitro bioassays for monitoring drinking water quality of tap water, domestic filtration and bottled water. J Expo Sci Environ Epidemiol. 2024;34:126–35.

Isaacs KK, Wall JT, Paul Friedman K, Franzosa JA, Goeden H, Williams AJ, et al. Screening for drinking water contaminants of concern using an automated exposure-focused workflow. J Expo Sci Environ Epidemiol. 2024;34:136–47.

Dorevitch S, Geiger SD, Kelly W, Jacobs DE, Demirtas H. Repeated home drinking water sampling to improve detection of particulate lead spikes: a simulation study. J Expo Sci Environ Epidemiol. 2024;34:148–54.

Lee D, Murphy HM. Private wells and rural health: groundwater contaminants of emerging concern. Curr Environ Health Rep. 2020;7:129–39.

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Mary Ward DM, Schmitt K, Loeb S. A state-by-state comparison of policies that protect private well users. J Expo Sci Environ Epidemiol. 2024;34:155–60.

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Damania R, Desbureaux S, Rodella A-S, Russ J, Zaveri E. Quality unknown: the invisible water crisis: World Bank Publications; 2019.

Praveena SM, Aris AZ, Hashim Z, Hashim JH. Drinking water quality status in Malaysia: a scoping review of occurrence, human health exposure, and potential needs. J Expo Sci Environ Epidemiol. 2024;34:161–74.

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Benítez-Cano D, González-Marín P, Gómez-Gutiérrez A, Marí-Dell’Olmo M, Oliveras L. Association of drought conditions and heavy rainfalls with the quality of drinking water in Barcelona (2010–2022). J Expo Sci Environ Epidemiol. 2024;34:175–83.

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Deziel, N.C., Villanueva, C.M. Assessing exposure and health consequences of chemicals in drinking water in the 21st Century. J Expo Sci Environ Epidemiol 34 , 1–2 (2024). https://doi.org/10.1038/s41370-024-00639-0

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Published : 01 March 2024

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How a US health agency became a shield for polluters

Companies and others responsible for some of America's most toxic waste sites are using a federal health agency’s faulty reports to save money on cleanups, defend against lawsuits and deny victims compensation, a Reuters investigation found. A Missouri neighborhood's tale.

By JAIMI DOWDELL , M.B. PELL , BENJAMIN LESSER , MICHELLE CONLIN , PHOEBE QUINTON and WAYLON CUNNINGHAM

Filed Aug. 7, 2024, 11 a.m. GMT

BRIDGETON, Missouri

The 43,000 tons of radioactive waste and soil came from a top-secret initiative: The Manhattan Project, which built the atomic bombs America dropped on Japan in 1945.

In 1973, that waste ended up in an unlined landfill in Bridgeton, Missouri, a St. Louis suburb. Workers spread it to cover trash and construction debris. In 1990, the U.S. Environmental Protection Agency declared the West Lake Landfill one of the nation’s most contaminated sites requiring cleanup. Still, many who lived near the dump didn’t know about West Lake’s toxic history.

It wasn’t until 2012, when garbage was burning underground, that the landfill burst fully into public view. The stench smothered nearby neighborhoods. Parents shut their kids inside. Emergency responders drew up evacuation plans, worried the smoldering waste would cause a nuclear catastrophe. Residents mobilized, spotlighting stories of children dying from cancer. And they pressed waste-management giant Republic Services, the dump’s owner, to remove the radioactive waste. In 2017, HBO aired a documentary about their cause.

Testing for radioactive material

The EPA has taken more than 1,000 soil samples from the West Lake Landfill and the surrounding area and found some radioactive material outside the site. The agency continues to investigate and says it will use the findings in deciding on a cleanup plan.

Sources: EPA, Google Earth

For all the radioactive publicity, though, Republic beat back neighbors’ claims. The nuclear waste is still there, and the government hasn’t said when a cleanup will begin.

In refuting neighbors’ complaints, Republic tapped an unlikely ally that U.S. corporations have leaned on for decades: a federal health agency set up to protect people from environmental hazards just like the West Lake dump.

A 2015 report by that small bureaucracy, the Agency for Toxic Substances and Disease Registry (ATSDR), did not identify any radioactive material outside the landfill. It declared that the landfill posed no health risk to the community and that radioactive gas would not leave the site. Its assessment contradicted findings from two sets of scientists: some hired by Missouri’s Attorney General and others from an environmental consulting firm working with residents. Republic still uses the ATSDR report to argue for a less expensive cleanup of the contamination, despite mounting evidence that the agency’s assessment was wrong.

Some residents told Reuters they resent the agency’s part in the drawn-out saga. Deborah Mitchell grew up in Spanish Village, less than a mile from the dump. She lost both parents to cancer and battled the disease herself. Dozens of neighbors have similar stories. Three cancer researchers told Reuters the number of cases in the neighborhood is worrisome and requires comprehensive study. That’s never been done.

“You just feel like you’re being gaslighted by your own government,” Mitchell said of the ATSDR’s role.

Republic Services, in an emailed response to Reuters questions, said it agrees with the ATSDR’s finding that the landfill poses no risk to the community. Its own experts reached a similar conclusion in 2015, it said.

The company has “always advocated for the responsible remediation of West Lake,” says the statement from Republic subsidiary Bridgeton Landfill LLC. The company has spent “tens of millions of dollars” studying the landfill and has “fully complied with every EPA directive.”

Also responsible for the site are mining firm Cotter Corp and the U.S. Department of Energy, manager of the U.S. nuclear weapons arsenal. The DOE declined to comment, and Cotter did not respond to questions about the cleanup plans.

Republic’s use of the ATSDR to argue for a less extensive cleanup of the West Lake Landfill is a strategy some companies wield at toxic sites across the U.S.

The ATSDR, part of the U.S. Department of Health and Human Services, is a public health agency that advises the EPA. It was created in 1980 by a landmark federal law that required polluters to clean up their toxic messes. The agency was meant to safeguard the public by identifying risks from those sites. Instead, it regularly downplays and disregards neighbors’ health concerns, a Reuters investigation found. Companies and other polluters wield the agency’s work against the people it is meant to protect.

The agency issued 428 reports containing 1,582 health-related findings from 2012 to 2023. In 68% of its findings, it declared communities safe from hazards or did not make any determination at all, a Reuters review found.

ATSDR’s record of finding little harm at the nation’s most contaminated waste sites strains credulity, said Judith Enck, a former regional administrator for the EPA. She is now president and founder of Beyond Plastics, a nonprofit that seeks to end plastics pollution.

“This is not at all surprising and why I advise community groups not to request ATSDR involvement,” she said. “ATSDR has a long history of minimizing environmental health problems, and that needs an independent investigation by Congress.”

The agency’s frequent declarations of no harm often are rooted in faulty research, Reuters found. At least 38% of the time, agency reports show, its researchers relied on old or flawed data.

Reuters consulted with 15 sources with experience in environmental and public health for its analysis.

ATSDR officials did not respond to questions about its overall performance, errors in its work, or how polluters use its reports. In an emailed statement to Reuters, it noted that its report on the West Lake Landfill did identify one potential harm: that radioactive dust particles could be released if the surface of the landfill were disturbed. Those particles could be inhaled by workers and harm their health, the statement said.

“At the West Lake Landfill Site, ATSDR did not have evidence that residents were drinking landfill contaminated groundwater, eating or incidentally ingesting landfill contaminated soils, breathing landfill-related radon, or absorbing radiation emitted by landfill contaminants,” the statement said.

It is impossible to know how often the agency has been correct in declaring communities safe, because the search for harm often ends once the ATSDR reports its findings. Still, Reuters found at least 20 instances from 1996 to 2017 where the agency declared that a potential hazard posed no health risk – only to be refuted later by other government agencies or the ATSDR itself. Those reports relied on outdated or limited data, contained math errors or provided overly optimistic conclusions.

Patrick Breysse, who led the agency from 2014 until 2022, said that out of the hundreds of reports the ATSDR has published, 20 is a small number. And he noted that not all polluted sites are dangerous.

But he acknowledged that the agency often bases decisions on whatever information happens to be at hand rather than its own well-constructed studies.

“ATSDR often lacked the resources to collect data needed to fill in gaps in our understanding of the potential health risks,” Breysse said. “We didn’t have the staff.”

The agency told Reuters it uses “the best available science to protect communities from harmful health effects related to exposure to hazardous substances.”

“Because ATSDR is a non-regulatory federal public health agency, typically ATSDR relies on other agencies for environmental sampling data, including regulatory agencies such as the U.S. Environmental Protection Agency (EPA), whose purview is to perform environmental sampling,” the agency said.

Breysse highlighted instances in which the ATSDR has intervened to help the public. Among its successes, the agency’s recommendations helped prevent increased pollution from a proposed metal-recycling plant expansion in Chicago. It also helped identify a 2016 public health crisis in Flint, Michigan, where drinking water was contaminated with high levels of lead, triggering a federal emergency.

The Reuters analysis is the most comprehensive ever conducted of the agency, which has been the target of previous probes by Congress and the Government Accountability Office. Despite decades of criticism, the agency continues to publish research that relies upon practices its own review board, in a 2010 evaluation , called “virtually useless” and “not very good.”

Some companies have seized upon the agency’s weakness. They’ve used its research to fend off lawsuits, deny victims compensation, criticize their opponents, and argue to delay, reduce or cancel cleanup of their toxic messes. ATSDR’s work has helped polluters save at least tens of millions of dollars on cleanups, delayed billions of dollars in medical claims and exposed millions of people to potential harm.

"I thought, my God, this is going to hurt one of my kids."

Dawn Chapman

In addition to Republic Services – the nation’s second-largest waste management operator – companies that have touted faulty ATSDR research include aerospace giant Boeing and Drummond Co Inc, an international coal company based in Alabama. The U.S. Navy and other government entities have benefited, too. ( See related article ).

Their tactics follow a familiar corporate practice of trumpeting flawed science. The tobacco industry touted shabby research to claim cigarettes were safe. The fossil fuel industry used the strategy to call climate change a myth.

Republic began leveraging the 2015 ATSDR report about West Lake the day it was released. The trash giant had formed a group called the Coalition to Keep Us Safe to assert grassroots support for capping the radioactive waste rather than removing it. Republic says this cheaper option poses fewer potential health risks. The coalition blasted social media with the health agency’s conclusions. The coalition’s leader, Molly Teichman, taunted local activists through her personal account on Twitter, the social media platform since rebranded as X.

“Dear mombots of #westlakelandfill, your reality tv show is over," she tweeted. "Go home and hangout with your kids – they miss you."

The posts were deleted after Reuters attempted to contact Teichman, who did not respond to multiple requests for comment. Republic Services sent Reuters a written statement disassociating the company from her remarks.

“These comments were not in alignment with our views, objectives or approach,” the statement said.

Using the agency to defend polluters isn’t what lawmakers intended when they wrote the 1980 Comprehensive Environmental Response, Compensation, and Liability Act, commonly known as the Superfund law, said Rena Steinzor, a former congressional staff counsel who advised the law’s authors after it was enacted.

“It is a perversion,” said Steinzor, now a law professor at the University of Maryland. “I’m saddened but not shocked they’ve been subverted.”

‘Highly suggestive’ of a problem

In June 2006, Deborah Mitchell returned to her childhood home in Spanish Village to help her father after her mother, Janice Majka, died of pancreatic cancer at age 64.

Mitchell, who wasn’t feeling well herself, says she went for a run in the neighborhood and stopped to use the restroom at a hotel. What she saw there scared her. First, blood in her stool. Then a notice about an EPA meeting to discuss radioactive waste buried in the landfill.

She was shocked. Mitchell and other kids grew up playing in the dirt around Spanish Village, a hilly enclave of cul-de-sacs, carports and above-ground swimming pools. She and others talk of riding their bikes to climb a giant pile known as Sand Mountain. They swung from a vine into a creek that flowed downhill from the direction of the landfill toward their homes. At the edges of the dump, they dug for Native American beads and arrowheads. They’d be so dirty when they got home, they’d have to hose off before going inside.

Mitchell attended the EPA meeting in 2006 with her father and asked the small gathering how many had cancer. Five people raised their hands, she says. Within a year, she and her father would be among them.

In July 2006, at age 38, Mitchell was diagnosed with colorectal cancer. The following year, her father, Robert Majka, a nonsmoker, was diagnosed with lung cancer at age 68. Around the same time, Valerie Shelton, a 59-year-old who lived across the street, was diagnosed with kidney cancer.

Mitchell’s father died in 2008, Shelton in 2013. In the houses surrounding them, five other neighbors died of cancer between 2019 and 2022.

Though Mitchell’s cancer has been in remission for more than a decade, she suffers from multiple sclerosis. There is no evidence linking the autoimmune disease to environmental radiation exposure, but she and other residents worry that could be the cause. Her health problems and frequent doctor visits make it impossible for her to work full time. Once an extremely active woman and avid cyclist, she now struggles to walk.

Reuters reporters tracked down current or former residents of half of the 92 homes in Spanish Village. At least 33 of those people have been diagnosed since the 1980s with cancers covered by the Radiation Exposure Compensation Act, a federal law that has compensated people exposed to nuclear waste from early atomic weapons programs. To verify the cases, reporters interviewed family members, friends and neighbors and reviewed medical records, death certificates, obituaries or social media posts.

Proving the existence of a cancer cluster is difficult, if not impossible, cancer researchers say, even when there's a glaring suspected cause such as radiation. That's because many factors are at play, including genetics, people moving in and out of areas, and duration of exposure to various carcinogens. Scientists also don’t fully understand what causes cancer.

Still, the Reuters finding of 33 cancer cases in Spanish Village is “highly suggestive” of a problem, said Sarah Chavez, a public health scientist at Washington University in St. Louis who reviewed the news organization’s research. Chavez heads the Missouri Cancer Consortium, a group that studies and tracks cancer disparities.

The health of people living near the landfill has never been comprehensively studied. In its statement to Reuters, Republic’s Bridgeton subsidiary pointed to a 2014 state review of cancer counts among current residents. It found some zip codes near the site had higher-than-expected rates of adult leukemia or childhood brain cancer. The study did not find elevated cancer rates in the zip code that includes Spanish Village. Chavez said the 2014 report covered too few illnesses and failed to track residents such as Mitchell who had moved away.

Testing of dust samples in Spanish Village homes resulted in dueling results, each the subject of criticism from opposing parties. Republic points to a 2017 EPA-funded study of dust samples from two Spanish Village homes, which reported no radiation from the landfill. But another scientist found high levels of radioactivity in at least eight Spanish Village homes. He conducted some of his testing for plaintiff’s attorneys.

Many in the community interviewed by Reuters recalled their hopes that the ATSDR would bring relief. But the agency’s 2015 report about West Lake said the community was not exposed to radiation from the dump. It did not investigate concerns about cancer and other illnesses.

In its statement to Reuters, the ATSDR said it relied on data the EPA provided in 2015 to form its conclusions about the landfill.

Earlier this year, led by Sen. Josh Hawley of Missouri, the U.S. Senate approved federal legislation that would compensate residents near the landfill as part of a $50 billion program for victims of Manhattan Project radiation. President Biden vowed to sign the bill into law if it passes the House, but Speaker Mike Johnson has not scheduled it for a vote.

“Locals have been telling the government that its studies were wrong for years,” Hawley said in an emailed response to Reuters questions. “The ATSDR’s faulty findings just underscore the blatant negligence at play here.”

‘Doubt is what they want’

The ATSDR is responsible for evaluating health hazards at Superfund sites. It is supposed to help prevent or reduce exposure to those hazards and the illnesses they can cause. The agency issues findings and makes recommendations for reducing risk, but it has no rule-making or regulatory authority.

The EPA, which does have regulatory authority, determines how much waste polluters must clean up at Superfund sites. It considers the ATSDR’s findings and recommendations when making those decisions.

The reviews have the potential to protect the health of millions of Americans. Approximately 78 million live within three miles of a Superfund site.

Toxic sites across the US

The EPA estimates approximately 78 million Americans live within three miles of a Superfund site. Taken together, the areas cover about 65,000 square miles, roughly the size of Wisconsin.

Source: EPA

From the outset, two influential industry groups became unlikely champions of the agency. The Chemical Manufacturers Association and the American Petroleum Institute joined an environmental group in a 1982 lawsuit to force the federal government to create the ATSDR. Its work, they said in court filings, was important to determine whether contaminants were making people sick.

In reality, however, studies of pollution often yield inconclusive results about risks to human health. Using that uncertainty to deflect regulation and liability is a long-standing corporate strategy, six professors who study corporate influence on public health told Reuters.

In the case of toxic waste sites, weak and inconclusive ATSDR reports give polluters authoritative, government-backed uncertainty to argue against extensive cleanups, Reuters found.

Spokespersons for the two industry groups that joined the 1982 lawsuit declined to comment on the case because it was filed so long ago.

“The last thing industry wants is an effective ATSDR that can say with more precision how dangerous these chemicals are,” said Thomas Burke, who was deputy assistant administrator of the EPA’s Office of Research and Development until 2017. Burke helped write the 1980 Superfund law and now teaches at Johns Hopkins University’s School of Public Health.

“Doubt is what they want,” Burke said.

Republic Services took exception to Burke’s assessment. “Bridgeton Landfill, LLC wants an effective ATSDR that helps the public understand who is truly exposed and who is not, and what genuine risks exist and do not exist,” a company spokesman said in an email to Reuters. “We believe ATSDR has done that at West Lake."

By 1993, the Washington Legal Foundation was encouraging industry to get involved in the ATSDR’s process to try to influence the agency’s health assessments. Funded by companies like Koch Industries and Philip Morris, the legal organization has filed briefs in lawsuits supporting companies like Exxon and Monsanto.

In a 1993 publication , the WLF said that tough ATSDR assessments could be costly for industry but that “early participation” in ATSDR reviews “can be highly advantageous.” The WLF noted that some reports had led to less extensive cleanups.

That same year, the foundation held an off-the-record luncheon for industry titled, "How to Minimize Superfund Liability by Successfully Handling the ATSDR Health Assessment Process.” Among the promoted speakers: Barry Johnson, then ATSDR’s assistant administrator. In recent interviews, Johnson said he does not remember the event or efforts by industry to influence the agency.

The WLF declined to comment on its earlier activities and said the ATSDR is no longer a focus for the organization.

Agency employees meet often with industry representatives while collecting information for assessments, said Burt Cooper, who retired in 2016 after 20 years as an ATSDR environmental health scientist. It was not unusual for companies or the military to pressure the agency, he said, though he could not recall specific incidents.

“You get beat up,” Cooper said. “These are high-paid attorneys, and you’re there trying to do the best you can with more meager resources.”

Despite the pressure, Cooper said agency officials never directed him to make a change in a report that wasn’t based on public health. “I was very proud of the ethics and devotion to public health that the agency had,” he said.

Outside influence

A 2008 congressional inquiry into the ATSDR’s performance found that industry and political influence was contributing to deficient research. Among the examples the committee found: The ATSDR bowed to pressure from a company executive and elected officials while studying contamination from a Brush Wellman beryllium plant in Elmore, Ohio.

The multinational company, now called Materion, is the sole U.S. producer of beryllium, a soft metal used in the aerospace, automotive and other industries. In 2006, the company was considering expanding its Elmore plant.

Beryllium is a carcinogen and can also cause lung disease. At the request of then-U.S. Sen. Mike DeWine, the ATSDR assessed air emissions from the plant in 2001 and found no health concerns. But the report's authors said more work was necessary to be sure. In 2006, the ATSDR announced plans to test blood from up to 200 people in the community to look for signs of exposure to the metal.

The company’s president and chief operating officer, Richard J. Hipple, didn’t like that plan. He wrote to DeWine and then-Governor Bob Taft.

The ATSDR’s plans “are likely to seriously damage Brush Wellman’s reputation in the Elmore community, unfairly elevate concerns about potential beryllium health effects … and increase the likelihood of litigation by plaintiff’s lawyers using the ATSDR as justification,” Hipple told DeWine in a March 2006 letter reviewed by Reuters.

After hearing from Hipple, Taft sent a handwritten note to the secretary of the U.S. Department of Health and Human Services. Taft complained that the ATSDR’s actions could dissuade Brush Wellman from expanding the plant. “Please have someone look into this and get back to me,” Taft wrote.

Brush Wellman announced its decision to expand in Elmore in September of that year. In November, the ATSDR announced that it had tested the blood of 18 people – 91% fewer than it had planned before politicians intervened. The tests showed no major health issues linked to beryllium from the site, the agency reported.

The agency’s approach was “scaled back significantly” based partly on concerns raised by elected officials and the company, according to a 2006 ATSDR document titled "ATSDR Brush Wellman Background Paper."

Emissions at the company's Elmore plant have remained "well-below" state and federal safety regulations for decades, said Materion spokesman Jason Saragian. He said the company could not speculate about past events.

DeWine, Hipple and Taft declined to comment on the 2006 events.

Finding ‘no effect’

The Reuters review of 428 agency reports found the agency commonly cites outdated data and often lacks adequate air, water and soil samples. In 83 reports, the agency based assurances of no harm on studies, samples or equipment it admitted were flawed. This includes its West Lake Landfill report. Despite acknowledging the use of faulty equipment to collect air samples, the report said those samples showed no radiation leaving the site.

The lack of accurate and current information makes it difficult to reach definitive conclusions about health risks, more than a dozen independent scientists told Reuters.

“The methods they use and assumptions they apply often result in studies that find no effect,” said David Michaels, an epidemiologist at the George Washington University School of Public Health.

Breysse, the former ATSDR director, said the agency’s lack of adequate funding makes it a challenge to conduct more thorough studies. The agency will operate with $86 million this year and a staff of roughly 200 workers – half the number it had in 2004, federal payroll data shows. It responds to about 700 requests a year to address health risks. The companies it examines often dwarf ATSDR: Republic Services has about 42,000 employees and posted revenue of $15 billion last year.

When the agency receives resources, it can do groundbreaking work, Breysse said. He cited its work to link contaminated drinking water to cancers and other illnesses suffered by Marines and civilians who worked at the Camp Lejeune military base in North Carolina. The Camp Lejeune studies, which began in 2014, cost about $40 million, according to congressional testimony and federal contracting data. That is equal to about half the agency’s annual budget. It took an act of Congress, followed by threats from two North Carolina senators, to force the U.S. Navy to fund the work.

The agency rarely receives such support at other sites. Even with this political backing, Breysse said the Defense Department tried to dictate how to conduct studies at Camp Lejeune and insisted on seeing all findings in advance. “We had to fight to maintain our independence,” Breysse said.

The Defense Department declined to comment for this story, referring questions to the Navy. The Navy did not respond to questions about whether it tried to influence the ATSDR’s work on Camp Lejeune.

The agency’s finding meant up to 1 million former base residents and workers now have an opportunity for compensation. Still, the Camp Lejeune report came long after a flawed ATSDR report in 1997 in which the agency declared the water did not harm adults.

‘Living in a graveyard’

The Missouri Attorney General sued Republic Services in 2013, accusing the company of allowing toxic, black water to run off the landfill and emitting highly offensive odors and hazardous substances into the air. In October 2015, the ATSDR published its report declaring that radioactive material in the West Lake Landfill had not left the site and posed no significant risk to the surrounding community. Two weeks later, Republic Services filed 15 expert reports in defense of the suit. Five mentioned the ATSDR health assessment.

Two press releases on a Republic website use the ATSDR report to buttress the company’s own scientific reports that declare the landfill safe. The federal public health agency “concluded that ‘groundwater, air, and soil data do not indicate a health risk to communities surrounding West Lake Landfill,’ ” one release says.

The EPA also embraced the report. Nine days after it came out, the St. Louis Post-Dispatch published an opinion piece by the EPA’s then-regional administrator, Mark Hague.

“I want to assure you that we at the Environmental Protection Agency are committed to protecting public health from the radiological contamination buried at the West Lake Landfill,” Hague wrote. “Recently, the federal Agency for Toxic Substances and Disease Registry completed a health consultation for EPA that confirmed there is no current offsite health risk.”

In a recent interview with Reuters, Hague said he understands the community concerns but is confident the site is being managed appropriately. “I think the EPA are working really hard to get this site remedied,” he said.

"No one told us what we were moving into."

Tonya Mason

Less than two weeks after publication of Hague’s opinion piece, the Missouri Department of Natural Resources, working with the EPA, found soil contaminated with radioactive material at a private business near the landfill. It had been carried there by rainwater, said Christine Jump, EPA remedial project manager for West Lake. The area was covered with rock to prevent exposure. The EPA has found other contaminated soil and groundwater near the site and says it is still assessing potential risks.

The EPA’s cleanup plan has evolved as it received results of new scientific testing, agency officials told Reuters. They said they are aware of community concerns, but based on air, soil and water testing they are confident that radioactive material in the site “as it sits today” does not pose a harm to nearby residents. Conditions at the site could change, they said. And they have not finished a health analysis of groundwater outside the landfill where they found elevated levels of radium and dioxane. The EPA classifies the solvent as a likely carcinogen. It can damage the central nervous system and kidneys.

Republic, in its responses to Reuters, said the groundwater samples appeared to contain “naturally-occurring” radium “consistent with other groundwater in the region.” EPA officials told Reuters that Republic lacks enough information to make such a claim. Ongoing testing is meant to determine if radium detected in surrounding groundwater is coming from the landfill.

"I joke sometimes that that’s what I have for breakfast is a handful of pills."

Republic says none of those tests show unsafe levels of radiation that would cause people harm. The company continues to cite scientific studies, including the 2015 ATSDR report, as evidence that there is no need for a full cleanup.

Some current and past residents nearby don’t buy it. Melissa Mitchell – unrelated to cancer patient and neighbor Deborah Mitchell – moved with her two children into a three-bedroom ranch in Bridgeton in 1994. Eight years later, she was diagnosed with Grave’s disease, a thyroid condition. Her son developed a benign tumor the size of a tennis ball on his femur in 2012. Her dog died of stomach cancer. Mitchell wonders if all of their maladies were caused by radiation exposure.

Over the years, she says, she watched as neighbors fell ill all around her – next door, across the street, catty corner to her house. In five houses on the block, 10 people had cancer.

“I felt like I was living in a graveyard,” she told Reuters.

The EPA says it is now considering a plan that would require Republic and the other responsible parties to remove 94,200 cubic yards of radioactive-contaminated waste. That’s enough to fill 82 Boeing 747s.

Thirty-four years after the landfill’s designation as a Superfund site and more than eight years after the ATSDR published its report, the waste remains and trash continues to smolder underground. Because more off-site contamination has been uncovered, EPA administrators told Reuters they don’t know when the cleanup will begin. They say they hope it will be in the next decade.

How Reuters identified weaknesses in a US health agency’s work

It is a little-known federal health agency with an unfamiliar name: the Agency for Toxic Substances and Disease Registry. Congress conceived it as part of the 1980 Comprehensive Environmental Response, Compensation, and Liability Act, commonly known as the Superfund law, meant to hold polluters responsible for the nation’s most toxic messes.

The law requires the agency to identify potential health risks at such sites to protect the people living and working around them. Reuters reporters had encountered several examples of the ATSDR failing its mission and decided to take a closer look.

To assess the agency’s work, they reviewed 428 health reports published on its website between 2012 and 2023. Those reports contained 1,582 conclusions regarding potential harms at Superfund sites or other concerns brought to the agency by communities or government agencies.

The agency often produces the reports in partnership with state agencies.

Reporters examined the ATSDR risk classification listed for each finding and categorized the conclusion: harm or potential harm found, no harm or no potential harm found, inconclusive findings. A handful of findings could not be categorized.

Fifteen people with expertise in environmental and public health vetted the Reuters methodology. They included former ATSDR employees, and state environmental health officials, academic researchers and community health advocates.

The data collected from the reports, available for download here , enabled the reporters to document:

How frequently the agency declared potential health hazards, found no hazard or failed to issue a conclusive finding.

How often the authors used outdated data to support their findings.

Reports in which the authors detailed serious limitations in the data or analysis that undermined their conclusions.

Reporters flagged reports for using outdated data when conclusions relied on data that was at least four years old. No federal standard exists for how fresh data must be to support health assessments like those the ATSDR produces. But generally, relying on data four years old or older would result in questionable findings, five public health and environmental regulatory specialists told Reuters.

Toxic Twist

By Jaimi Dowdell, M.B. Pell, Benjamin Lesser, Michelle Conlin, Phoebe Quinton and Waylon Cunningham

Contributors: Peter Eisler and Charlie Szymanski

Graphics: Sam Hart

Photo editing: Corinne Perkins

Video: Eric Cox and Angela Johnston

Visual Editing: Feilding Cage

Edited by Janet Roberts

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