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National Drinking Water Database


National Drinking Water Database - Study Findings

Study findings

December 2009

Testing Finds Hundreds of Contaminants in America's Drinking Water

The Environmental Working Group's analysis of nearly 20 million drinking water tests conducted by water suppliers nationwide between 2004 and 2009 revealed hundreds of pollutants in U.S. tap water. For most, the government has set no safety-based legal limits. Many other contaminants were found in drinking water at concentrations above government-issued advisory health guidelines.

EWG obtained this test data from state water authorities over the past three years, compiling it into the largest database of tap water quality in existence. EWG's analysis shows wide variations in water quality. Generally, large water utilities test more often and supply water with lower levels of common pollutants than smaller utilities. But even some large utilities' water is contaminated with multiple pollutants at levels that exceed government health guidelines.

EWG Rated the Water Utilities

EWG rated big city (population over 250,000) water utilities based on three factors: the total number of chemicals detected since 2004; the percentage of chemicals found of those tested; and the highest average level for an individual pollutant, relative to legal limits or national average amounts for the most common pollutants (disinfection byproducts, nitrate and arsenic). [read more on rating methodology]

  • Top-rated large water utilities included Arlington, TX (Arlington Water Utilities), Providence, RI (Providence Water), and Fort Worth, TX (Fort Worth Water Department). These utilities all reported testing for a large number of chemicals beyond those required by the federal government and found relatively low levels or none of some of the most common toxic pollutants, including arsenic; nitrate, a fertilizer ingredient that is harmful to infants; and cancer-causing disinfection byproducts called trihalomethanes.

  • Low-rated large utilities included Pensacola, FL (Emerald Coast Water Utility), Riverside, CA (City of Riverside Public Utilities), and Las Vegas, NV (Las Vegas Valley Water District). All reported many pollutants at levels exceeding government health guidelines.

  • Most small water utilities tested for fewer contaminants. Utilities serving less than 10,000 people tested for an average of 67 pollutants, while larger utilities tested for an average of 89.

  • Among the large utilities, those finding the highest average levels of cancer-causing water disinfection byproducts were Plano, TX (City of Plano Utility Operations Department), Pittsburgh, PA (Pittsburgh Water & Sewer Authority and Pennsylvania American Water Company), and Monroe County, NY (Monroe County Water Authority- Shoremont Water Treatment Plant). For arsenic, the three with the highest levels were Reno, Nev. (Truckee Meadows Water Authority, 7 ppb), Omaha, Neb. (5 ppb), and Chino Hills, Calif. (4 ppb). While these levels comply with the federal government's legal limit, they are far above non-enforceable health guidelines.

  • Most water utilities comply with all legal limits for chemical contaminants, which are usually based on yearly averages, but many also commonly detect short-term spikes above the legal limits. Ten percent of all utilities reported temporary spikes in levels of disinfection byproducts (trihalomethanes and/or haloacetic acids) above legal limits.

  • Among utilities serving at least 250,000 people, the highest reported levels for common pollutants in a single test were:
    • Trihalomethanes — Massachusetts Water Resources Authority, service in the Boston area, in 2005: 132 parts per billion (ppb; the legal limit is 80 ppb). This utility subsequently changed its water treatment process by adding a filtration step, which resulted in drastically lower levels of disinfection byproducts in tap water.
    • Haloacetic acids - Cleveland, Ohio in 2006: 90 ppb (legal limit 60 ppb).
    • Arsenic - Chino Hills, Calif. in 2004: 9 ppb (legal limit 10 ppb).
    • Nitrate - Riverside, Calif.: 5 parts per million (ppm; the legal limit is 10 ppm). Peak levels for small utilities are frequently even higher. The highest level of nitrate was detected in Manassas Park, Virg in 2008: 30 ppm.

Americans Are Justifiably Concerned about Drinking Water Safety

A Gallup poll published in March 2009 found that Americans rank water pollution as the number one environmental concern facing the country: 84 percent of respondents said they worried a "great deal" or a "fair amount" about pollution of drinking water; 83 percent worried about pollution of rivers, lakes, and reservoirs; and 80 percent worried the maintenance of the nation's supply of fresh water for household needs (The Gallup Poll 2009).

There is a deep disconnect between what people care about and what the government is willing to act on. From agricultural pollution to industrial waste to pollution stemming from sprawl and urban runoff, a lack of political will means poor planning and scarce funding and ultimately leads to pollution that begins upstream and ends up at the tap.

The public has reason to be concerned. Between 2004 and 2009, water suppliers across the U.S. detected 316 contaminants in water supplied to the public. The list of detected contaminants includes:

  • 97 agricultural pollutants, including pesticides and chemicals from fertilizer- and manure-laden runoff;

  • 204 industrial chemicals from factory discharges and consumer products;

  • 86 contaminants linked to sprawl and urban areas or from polluted runoff and wastewater treatment plants;

  • 42 pollutants that are byproducts of the water treatment processes or that leach from pipes and storage tanks.

See all detected contaminants, by frequency of occurrence.

For regulated pollutants that carry government-set, health-based mandatory limits, EWG's analysis found 92 percent compliance by the nation's water utilities, showing their clear commitment to adhere to safety standards when they exist. These data support EPA's recently published review of annual performance goals, which reported that in 2007, 91.5 percent of the population served by community water systems received drinking water that met all enforceable standards (EPA 2009t).

Still, that leaves millions of people drinking water that does not meet drinking water standards. Infants and other vulnerable populations may be most at risk from these elevated levels of pollutants. And hundreds of chemicals found in drinking water remain unregulated.

Table 1. 150 drinking water pollutants exceed health standards and guidelines.

 ChemicalYearsUtilities above EPA's Maximum Contaminant Level (MCL)Utilities Above Health Guidelines
1Total trihalomethanes (TTHMs)2004-20094,135 systems
53,126,324 people
15,425 systems
206,475,520 people
2Manganese2004-20093,246 systems
42,169,574 people
3,246 systems
42,169,574 people
3Total haloacetic acids (HAAs)2004-20092,411 systems
33,251,759 people
17,547 systems
205,677,872 people
4Arsenic (total)2004-20091,724 systems
11,149,291 people
11,173 systems
113,356,829 people
5Chloroform2004-20091,455 systems
9,045,096 people
9,312 systems
157,153,669 people

Note: MCL - Maximum Contaminant Level, EPA-enforced maximum allowable concentrations of chemical contaminants in drinking water. Health Guidelines - recommended levels in non-mandatory government advisories.

See all contaminants above health standards and guidelines.


The Most Common Sources: Disinfection byproducts and agriculture

EWG's analysis of tap water test results revealed 42 pollutants that are residues of water treatment, storage and distribution, including chemical byproducts of water disinfection, in water supplied to 237 million people in 45 states. The water provided to 97 percent of them contained one or more of these contaminants at concentrations above non-mandatory, advisory health guidelines issued by government agencies. Moreover, 24 of these disinfection byproducts are unregulated, with no enforceable health-based limits for drinking water.

Tap water disinfection is crucial for controlling waterborne disease, but the chemicals used can form harmful, DNA-damaging or cancer-causing byproducts in the treated water. These byproducts form when disinfectants react with organic pollution from agriculture or urban and sprawl runoff. EPA requires testing and restricts levels for 11 disinfection byproducts: four trihalomethanes, five haloacetic acids, bromate and chlorite. This is a tiny fraction of the more than 600 chemicals that can form as disinfection byproducts in tap water, according to scientists from EPA, academia and water utilities (Boorman 1999; Chowdhury 2009; Krasner 2006, 2009; Richardson 1998, 1999a,b, 2003, 2007). Neither the states nor the federal government require testing for the vast majority of these compounds, and most water systems do not test for them.

Among EWG's findings:

  • Nationally, two groups of disinfection byproducts — trihalomethanes (chloroform, bromodichloromethane, dibromochloromethane) and haloacetic acids — top the list of the most commonly detected tap water contaminants. Disinfection byproducts are associated with damage to DNA and elevated risk of cancer (Richardson 2007; EPA 2005d; EPA 2007f).

  • The fifth frequently detected water pollutant is nitrate. Excess nitrate (and/or the related chemical nitrite) in water used to make infant formula can cause "blue baby syndrome," a potentially fatal condition in which an infant's blood is less able to carry oxygen (Knobeloch 2000; Manassaram 2006; National Research Council 1995). The greatest use of nitrates is as a fertilizer in agriculture (USGS 1999; EPA 2009b).

  • Many commonly detected contaminants have been found at concentrations exceeding mandatory federal drinking water standards.

  • Water quality varies widely from one community to the next (Figure 4). For two groups of regulated disinfection byproducts, trihalomethanes and haloacetic acids, mid-sized utilities serving between 10,000 and 250,000 people had the highest average contaminant levels, while the smallest and largest utilities fared better.

Many drinking water contaminants are unregulated

Nearly two-thirds of the 315 chemicals found in the nation's drinking water over the last five years — a total of 201 — are unregulated. Public health officials have not set safety standards for them, even though millions ingest them in their tap water every day. Among the most common are some that have been linked to serious health concerns:

  • Bromochloroacetic acid, a tap water disinfection byproduct, was found in the water supplied to 40 million consumers. It induces gene mutations and is associated with damage to DNA.

  • Perchlorate, a rocket fuel ingredient, was found in water provided to 26 million people. It is toxic to the thyroid gland.

  • MTBE (methyl tert-butyl ether), a gasoline additive and groundwater pollutant scheduled to be phased out nationwide, was found in water supplied to 12 million people. It is associated with liver and kidney damage, and nervous system effects.

  • Di-n-butylphthalate, a chemical from a group of industrial plasticizers called phthalates, was found in water used by 5 million people. Phthalates have been linked to birth defects and reproductive toxicity.

See all unregulated contaminants in drinking water

Despite the potential health risks, there is no legal limit on these chemicals — no matter how high the concentrations — in drinking water. Among the unregulated contaminants detected in water supplies between 2004 and 2008, 168 have been linked to cancer, 54 to reproductive toxicity, 67 to developmental toxicity, and 35 to immune system damage, according to EWG analysis of chemical contaminant toxicity data provided by more than 50 standard government and industry toxicity references and the open scientific literature. These databases are described in the Data Source and Methodology section of this report.

For 96 of these unregulated contaminants, the government has not even issued non-binding, health-based advisories on limits in tap water, let alone set a mandatory safety standard.

Some unregulated contaminants were found in the water of hundreds of communities, while others were found in very few. Some were detected at levels of health concern, while others were not. Overall, the statistics reported here represent an underestimate of consumers' exposures to unregulated contaminants in the tap water. The full spectrum of unregulated chemicals in drinking water is likely to be much more extensive than we currently know from the limited testing data. EPA scientists, for example, recently identified approximately 6,000 potential drinking water contaminants (EPA 2008h).


Unregulated contaminants — the more we test, the more we find

As concern about chemical contaminants has grown and scientific methods have improved, the number of contaminants that drinking water systems test for has risen. The datasets that state water officials provided to EWG contained test results for a total of 446 contaminants in 2007, up from 363 in 2000. The increased testing is good news, as it provides a more complete understanding of the water pollution challenges facing the country.

With water utilities testing for more chemicals than ever before, the obvious next question is: Are they finding more contaminants? EWG's analysis of data from 1995 to 2008 found that the number of contaminants detected has been rising, but the percentage of tests producing detections has remained fairly stable at around 60 percent.

These findings suggest that there are likely many more chemical contaminants in drinking water than we know. But they are also good news in that the rate at which testing is finding new contaminants does not appear to be rising.

Water utilities in some states have been advancing the knowledge on drinking water contaminants much more than others. EWG's analysis found that between 2004 and 2009:

  • Municipal water utilities in three states each tested for more than 250 contaminants: California, New York, and Missouri.
  • Municipal water utilities in three states each tested for 90 or fewer chemical contaminants: Kentucky, Connecticut, and South Dakota.
  • 2,116 water utilities each tested for at least 150 different pollutants.

See number of contaminants tested by all states

With many municipal water utilities facing budgets strained by a weak economy, states might choose not to require testing beyond that mandated by the federal government. Water utilities that did additional testing should be commended for their efforts to better understand the public health threats posed by contaminants in drinking water.

Table 2. Number of chemical contaminants tested for by the utilities, as reported in the datasets provided to EWG by state health and environmental departments.


Number of chemical contaminants testedNumber of utilities testing at this levelPercent of utilities in the EWG database testing at this level
More than 2002220.5 %
150-2001,8944.0 %
100-15011,63724.5 %
50-10016,53534.7 %
25-505,49611.6 %
Fewer than 2511,79924.8 %

Most of the testing data in the EWG database reflects the results of tests for individual chemicals conducted by water utilities to satisfy state and federal requirements. A relatively small percentage of utilities test for large numbers of contaminants. In their filings to state authorities, a majority (59.2 percent) of the utilities represented in the EWG database report testing for 50-to-150 chemicals, as required by the federal regulations. However, nine states and the District of Columbia did not provide EWG with affordable, high quality data for the 2004-2009 period; some other states provided incomplete records.

Due to gaps in the state drinking water quality data provided to EWG, it is impossible to determine from the current dataset whether utilities that report testing for the fewest contaminants actually test for such small numbers of chemicals or whether state offices did not correctly record the utility testing data. To address this data gap and ensure that it has the information needed to guarantee that drinking water is safe, EPA should require broader testing and create and maintain a comprehensive national water quality database.