Data Sources and Methodology
EWG’s National Tap Water Quality Database includes information about drinking water quality for nearly 50,000 water companies or utilities nationwide. The database displays information about the chemical and radioactive contaminants detected in drinking water, and how these concentrations compare to the federal legal limits and to health guidelines. It also includes violations of drinking water regulations by water utilities across the country.
EWG's database includes water quality tests conducted by utilities nationwide from 2010 to 2015. To build this database, EWG requested the water quality data for community water systems from authorities responsible for drinking water quality oversight in all 50 states and Washington, D.C. State drinking water authorities, typically health or environmental departments, maintain the records on water testing from drinking water utilities or associated water testing laboratories.
Altogether, EWG's drinking water quality database includes nearly 30 million test results for 502 contaminants, making it the largest resource of free and openly accessible data on U.S. drinking water.
Where we got the data
EWG obtained water testing data for 2010 to 2015 from all 50 states and the district. EWG’s database does not include water quality information for private wells because there are no federal or state requirements for private wells to be tested.
Additionally, EWG obtained the results of drinking water testing for unregulated contaminants – substances known to contaminate drinking water but that have no national legal limits – from the Environmental Protection Agency. The EPA’s third Unregulated Contaminant Monitoring Rule (UCMR3) required that utilities serving more than 10,000 people test water for 30 unregulated contaminants from 2013 to 2015. The EWG database also includes water quality violations information from the EPA Enforcement and Compliance History Online, or ECHO, website.
The EPA classifies the water systems that purchase, or otherwise receive, some or all of their finished water from one or more public water utilities as “consecutive systems.” These systems are only required to test for disinfection byproducts, lead and copper. For consecutive systems, EWG indicated, based on publicly accessible records, where the system likely purchases its water, and provided links to the water quality information for supplier systems. For example, DC Water, the tap water supplier for the nation’s capital, purchases its water from the Washington Aqueduct, a federally owned and operated public water supply agency under the authority of the U.S. Army Corps of Engineers.
After assembling our database, EWG reviewed the data for inconsistencies, systematically searching for test results that could indicate potential errors, such as measurement units different from what is typically used for a specific contaminant. EWG contacted state water agencies to discuss potential errors, and corrected or removed many thousands of test results flagged as potentially erroneous.
EWG also briefed the largest drinking water utility trade association, the American Water Works Association, or AWWA, and provided its members a two-month window to review the state agency data and make any necessary corrections. More than 200 water utilities reviewed the testing and violation data for their systems and either verified the accuracy of the data or provided corrections.
EWG assessed both the yearly and quarterly average levels of contaminants found in U.S. tap water against the federal legal limits and non-enforceable health guidelines published by health and environmental agencies and those from our own research:
- EPA Maximum Contaminant Level (MCL) — The enforceable federal standard that defines the highest level of a contaminant allowed in drinking water. Available at www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations
- EPA Maximum Contaminant Level Goal (MCLG) — A non-enforceable federal health guideline. For cancer-causing chemicals, these health goals are typically set to zero. Available at www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations
- California public health goal (PHG) — Defined by the California Office of Environmental Health Hazard Assessment as the level of contaminant that poses no significant health risk if consumed for a lifetime. Public health goals are based solely on health effects without regard to cost. For carcinogens, the level represents one additional case of cancer per million people drinking the water over a lifetime. Available at oehha.ca.gov/water/phg/allphgs.html
- EPA one-in-a-million cancer risk level — The concentration of a chemical in drinking water corresponding to an excess estimated lifetime cancer risk of one in a million, as calculated by the EPA Integrated Risk Information System. Available at www.epa.gov/iris
- Reference concentration under EPA Unregulated Contaminants Monitoring Program — A non-enforceable health guideline for drinking water contaminants, it represents a benchmark for testing unregulated contaminants included in the EPA monitoring program. Available at www.epa.gov/sites/production/files/2017-02/documents/ucmr3-data-summary-january-2017.pdf
- EPA lifetime health advisory, non-cancer risk — A non-enforceable federal health advisory that is the concentration of a chemical in drinking water that is not expected to cause any adverse health effects for a lifetime of exposure. The lifetime advisory is based on exposure for a 154-pound adult consuming two liters of water per day and only accounts for potential health risks other than cancer. Available at www.epa.gov/sites/production/files/2015-09/documents/dwstandards2012.pdf
- EPA children's health advisory for 10-day exposure — A non-enforceable federal health advisory for a concentration of a chemical in drinking water that is not expected to cause any adverse effects for up to 10 days of exposure. The 10-day advisory is typically set to protect a 22-pound child – typically, a one-year-old – who consumes one liter of water per day. Available at www.epa.gov/sites/production/files/2015-09/documents/dwstandards2012.pdf
- Minnesota Risk Assessment Advice – This health guideline is a risk assessment advisory level, which is a non-enforceable recommendation. Available at www.health.state.mn.us/divs/eh/risk/guidance/waterguidance.html
- EWG Standards – When official health guidelines are not available or are insufficient to protect public health, we developed our own health benchmarks using publicly available scientific research.
EWG’s assessment of the toxicity of drinking water contaminants
For individual water utilities and specific contaminants, EWG compared the 2015 annual averages to state and national averages for the same time period. For contaminants that were tested under the EPA's UCMR3 program – including perfluorinated compounds and 1,4-dioxane, and radioactive compounds not tested annually – EWG analyzed the available data collected between 2010 and 2015. Test results were averaged by quarter using this data and compared with health guidelines and legal limits. For every water sample, the date, laboratory identification number where available and numerical result are provided.
EWG calculated state and national averages using the averages of the contaminant measurements for each utility after assigning a value of zero for non-detects. Some water systems blend water from different water sources in differing amounts and this mixing may reduce the average contaminant amount in the finished tap water served to consumers. The water blending may be adjusted seasonally or more frequently based on overall water demand, supply changes, and contaminant levels. Utilities may employ different methods for averaging and the values published within a water quality report may not necessarily match what is shown in the database.
EWG also included health guidelines for many contaminants based on one of two sets of health benchmarks:
- 1. Health guidelines published by federal or state health and environmental agencies.
- 2. Health guidelines EWG developed based on recent research.
For the first category, EWG identified a health guideline for over 90 of the most concerning and most common chemical contaminants in drinking water. These health guidelines represent the maximum concentration of a contaminant in water that scientists consider safe. This value is based only on protecting health and does not consider technical feasibility or the cost of water treatment.
For chemicals that can cause cancer, health guidelines typically represent one excess case of cancer per million people drinking the water over a lifetime. For chemicals that are not considered carcinogens, health guidelines are set using toxicity information from animal or human studies, and applying additional safety factors. These factors are used to provide a margin of safety based on differences between people and animals, differences among people in their susceptibility to chemical exposure, and the fact that for many chemicals, scientists don’t fully know the potential health effects. Additionally, the safety factors typically incorporate the increased likelihood of harm from exposure to toxic chemicals during pregnancy or childhood.
For each contaminant that has a health guideline, the EWG database specifies the critical health effect that was used to calculate the safe concentration. Most of the critical health effects listed in the database, such as cancer or internal organ damage, do not specify the specific body part where harm occurs because of differences between how chemicals impact humans and the laboratory animals used to study toxicity.
The majority of health guidelines provided in the drinking water database are reflective of the public health goal values published by the California Office of Environmental Health Hazard Assessment. Where available, EWG utilized more recent health assessments completed by federal or state health agencies.
Secondly, for a subset of drinking water contaminants for which no official health guidelines are available, or where we found that the existing health guidelines are insufficient to protect public health, we developed our own health benchmarks that we consider EWG standards for drinking water. This category currently includes:
- Perfluorinated chemicals such as PFOA, PFOS and related compounds detected in drinking water. EWG adopted a health guideline derived from the best available peer-reviewed literature. Because of sparse testing data, the health guideline of 0.001 parts per billion, or 1 part per trillion, for the entire family of PFCs was based on the most-studied member of the perfluorinated chemical group, PFOA.
- The fertilizer chemical nitrate. The health guideline of 5 parts per million for nitrate was defined by EWG based on studies scientists at the National Cancer Institute and other independent researchers. This health-based guideline protects against the risk of cancer, and harm to fetal growth and development.
Exposure to contaminants at concentrations above the health guidelines increases the possibility of additional adverse health effects. For chemicals with health guidelines, we have identified both a critical health effect as well as a list of the other health effects that were observed in research studies. The primary sources of information for the other health effects were drinking water toxicity assessments completed by the state of California or the EPA.
Displaying lead testing results
Unlike for other contaminants, water utilities test for lead and copper levels inside their customers’ homes to account for the fact that lead corrosion and leaching occurs within water lines and household plumbing. Lead testing results indicate the potential magnitude of lead hazards within a water system, but are not a reliable predictor of the specific levels of lead for tap water within individual homes served by a water utility. For this reason, EWG displays both the individual lead testing results and the 90th percentile lead testing values where available.
The EPA’s Lead and Copper Rule allows 10 percent of homes sampled to exceed the federal lead action level of 15 ppb lead; 90 percent of the sampled homes must have lead levels below 15 ppb for the utility to be in compliance with the federal regulations. EWG obtained information about the 90th percentile lead testing from the EPA’s Envirofacts database. We display the data for the most recent years available, generally between 2013 and 2016.
Water utilities also report individual test results to their state drinking water program. Where available, EWG obtained individual household lead test results from states directly and displayed this information in our database. We also received lead data for the state of Wyoming from the EPA Region 9 office and were unable to obtain lead results for the District of Columbia, which we requested but did not receive from the EPA Region 3 office.
Whenever a utility collected at least 10 household lead water samples in 2015, the most recent year analyzed by EWG, we displayed the test results in a pie chart to help viewers distinguish the proportion of tests above public health guidelines.
EWG analysis of drinking water compliance data
To examine whether tap water provided by different water utilities complied with federal drinking water standards, EWG analyzed information from the EPA's Enforcement and Compliance History Online, or ECHO, database. The EWG database presents ECHO data for October 2015 to September 2018, the latest three-month period with the EPA assessment information online, as well as information on the compliance status over the last three years.
Database pages for individual utilities indicate with a “Yes” or “No” whether the utility was listed by the EPA as being “in violation” or “a serious violator” of drinking water standards for the October 2015 to September 2018 quarter. Where data were available, EWG also listed the number of quarters over the last three years a specific water utility was designated as having been “in violation” or “a serious violator” of drinking water standards.
Drinking water quality is regulated on a state level, with state agencies in all but one state responsible for compiling and reviewing drinking water utility data, confirming that water utilities are in compliance with federal laws, and addressing noncompliance issues through enforcement actions. Wyoming is an exception because the EPA is directly responsible for the oversight of drinking water utilities in this state.
The EPA described the reporting process as follows:
States report information about [Safe Drinking Water Act] violations quarterly to EPA. After a violation has been reported, there is typically a three-to-six-month review period before the official quarterly status of the [public water system] is updated. The most recent "official" compliance status is displayed in the 12th quarter on the Detailed Facility Report ... Until the next quarter becomes official, these data are considered draft, have not been fully quality assured, and are subject to change.
When the water utility is classified as being “in violation” or as “a serious violator” of water quality standards, state authorities work with the utility to help it return to compliance. Where provided by the law, formal enforcement action might be applied.
Violations can be of several types, of which the most serious type is a health-based violation, which indicates a lack of compliance with the federal Maximum Contaminant Levels or other legal requirements limiting the levels of contaminants in the water, such as application of appropriate treatment techniques for lead. The EPA assigns a greater number of violations points, 5 or 10 points, for such significant compliance issues.
Other types of violations include monitoring and reporting violations, public notice violations, or failure to keep required records of water treatment. Such compliance issues typically receive 1 violation point.
Based on the compliance history for a specific utility, violation points are accrued and remain associated with a specific utility until specific violations are corrected. Click here to see types of health-based violations, other types of violations, and the weighted point system the EPA uses to reflect a water system’s degree of noncompliance.
EWG’s database displays the utilities with violations on our map of water results by state. On each state page, utilities with violations are ranked by the number of current uncorrected violation points, according to information accessible on the ECHO website.
The EPA’s ECHO site also contains detailed facility information that can be accessed by following the link at the bottom of each utility page in the EWG tap water database. This detailed facility report includes a summary graphic for the most recent 12-quarter period, where each quarter is identified as “no violation,” “noncompliance,” “significant violation” or “unknown.” These reports also provide detailed violation histories for each quarter and each drinking water quality regulation. For the purposes of the EPA’s summary graphic in the detailed facility reports, quarters in violation are classified as “noncompliance,” while quarters as a serious violator are classified as “significant violation.”