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The Quality of Water from Public-Supply Wells in the United States, 19932007


Frequently Asked Questions

BACK TO Quality of Water from Public Wells in the United States Materials Page

USGS Role Regarding Drinking Water

About the Study

Human-Health Context for Study Findings

Selected Findings About Contaminants in the Sampled Public Wells

More Information for People Served by Public Water Utilities

More Information on this Study

Abbreviations and Units of Measure Used in this FAQ

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USGS Role Regarding Drinking Water

No. The U.S. Geological Survey (USGS), and specifically, the National Water-Quality Assessment (NAWQA) Program, does not assess the quality of the Nation’s drinking water or conduct regulatory compliance monitoring. Rather, NAWQA assessments focus mainly on the quality of the available, untreated resource (source water), such as water upstream from treatment plants and water from public-supply and domestic wells. For a subset of samples, this NAWQA study also compares water-quality conditions in untreated source water and finished treated water (after treatment but before distribution).

No. The USGS is a non-regulatory agency under the U.S. Department of Interior and is the primary federal agency responsible for providing scientific information on the quality of the Nation’s water resources. USGS information is intended to facilitate effective management of water resources and ensure long-term availability of water that is safe for drinking and recreation and is suitable for industry, irrigation, and fish and wildlife.

The quality of finished (treated) drinking water from the Nation’s public water systems is regulated by the U.S. Environmental Protection Agency (USEPA) under the Safe Drinking Water Act (SDWA) (U.S. Environmental Protection Agency, 2004).  The SDWA defines a public water system as one that serves piped drinking water to at least 25 people or 15 service connections for at least 60 days a year (U.S. Environmental Protection Agency, 2003).  The SDWA, originally passed by Congress in 1974 and amended in 1986 and 1996, requires many actions to protect drinking water and its sources—rivers, lakes, reservoirs, springs, and groundwater.  For example, SDWA authorizes the USEPA to set national health-based standards for drinking water to protect against both naturally occurring and man-made contaminants that may be detected in drinking water (U.S. Environmental Protection Agency, 2004).  USEPA oversees the states, localities, and water suppliers who implement the drinking-water standards.  Information about how USEPA decides which contaminants to regulate, how drinking water standards are set, and when to revise existing regulations is available online at: http://www.epa.gov/safewater/standard/setting.html.

References:
U.S. Environmental Protection Agency, 2003, Water on tap: what you need to know: U.S. Environmental Protection Agency, Office of Water EPA 816-K-03-007, October 2003, 36 p.  Available at http://www.epa.gov/safewater/wot/pdfs/book_waterontap_full.pdf

U.S. Environmental Protection Agency, 2004, Understanding the safe drinking water act: U.S. Environmental Protection Agency, Office of Water EPA 816-F-04-030, June 2004, 4 p.  Available at http://www.epa.gov/safewater/sdwa/pdfs/fs_30ann_sdwa_web.pdf

The Safe Drinking Water Act (SDWA) directs USEPA to regulate contaminants in drinking water that present the greatest public-health concern.  As part of determining whether drinking-water regulations are needed, the SDWA requires USEPA to publish a list of unregulated contaminants called the Contaminant Candidate List (CCL) every 5 years.  The CCL includes contaminants that currently are not subject to any proposed or promulgated national primary drinking water regulations, which are known or anticipated to occur in public water systems, and which may require regulation under the SDWA in the future.  Following the publication of each CCL, USEPA decides whether to regulate at least five contaminants from the CCL in drinking water (called regulatory determinations) on the basis of the contaminant’s potential for adverse human-health effects and occurrence in public water systems, and a meaningful opportunity to protect public health (U.S. Environmental Protection Agency, 2010).  The USEPA uses USGS data on the occurrence of unregulated contaminants in water resources as part of both the CCL and regulatory determination processes.

Reference:
U.S. Environmental Protection Agency, 2010, Drinking water contaminant candidate list and regulatory determinations: U.S. Environmental Protection Agency, Office of Water, Updated February 22, 2010, Accessed February 24, 2010, at http://www.epa.gov/safewater/ccl/

No.   USGS did not assess the safety of drinking water. As mentioned above, the quality of finished drinking water is regulated by the USEPA under the SDWA. Most of the samples included in this study were collected prior to any treatment or blending that potentially could alter contaminant concentrations.  As a result, the sampled groundwater represents the quality of the source water and not necessarily the quality of finished water ingested by the people served by these public wells. 

 

About the Study

Public wells are a critical source of water supply in the United States. They supply 34 percent of the U.S. population, or more than 105 million people with drinking water. These wells are the primary source of drinking water in many areas of the United States. Finished drinking water is regulated by the U.S. Environmental Protection Agency (USEPA) under the Safe Drinking Water Act (SDWA).  Public water systems typically are required to monitor finished water for about 90 contaminants that are regulated in drinking water under the SDWA, but generally do not monitor finished water for most (83 percent) of the contaminants assessed in this study.  The USEPA uses USGS data on the occurrence of unregulated contaminants to identify contaminants that may require drinking-water regulation in the future.  By focusing primarily on source-water quality, and by analyzing many contaminants that are not regulated in drinking water by USEPA, this study complements the extensive sampling of public water systems that is routinely conducted for the purposes of regulatory compliance monitoring by federal, state, and local drinking-water programs.

The objectives of this public-well study were to evaluate (1) the occurrence of contaminants in source water from public wells and their potential significance to human health, (2) whether contaminants that occur in source water also occur in finished water after treatment, and (3) the occurrence and characteristics of contaminant mixtures. 

A contaminant is defined by the Safe Drinking Water Act (SDWA) as “any physical, chemical, biological, or radiological substance or matter in water” (U.S. Code, 2002; 40 CFR 141.2). This broad definition of contaminant includes every substance that may be found dissolved or suspended in water—everything but the water molecule itself. The presence of a contaminant in water does not necessarily mean that there is a human-health concern.  Whether a particular contaminant in water is potentially harmful to human health depends on the contaminant’s toxicity and concentration in drinking water.  Other factors include the susceptibility of individuals, amount of water consumed, and duration of exposure.

Contaminants originate from a wide range of natural and man-made sources. Most inorganic contaminants and nutrients analyzed in this study occur naturally, although concentrations of some inorganic contaminants in groundwater may be altered by human activities.  For example, nitrate from natural sources is present in most wells, but concentrations often are increased by contributions from man-made sources in agricultural and urban areas.  By contrast, most of the organic contaminants analyzed in this study are man-made, although some also may form in groundwater through various chemical and biological transformation processes.

Reference:
U.S. Code, 1996, Safe Drinking Water Act amendments of 1996: Title 42 The Public Health and Welfare, Chapter 6A Public Health Service, SubChapter XII Safety of Public Water Systems, Public Law 104-182, Enacted August 6, 1996, Accessed January 6, 2010, at http://uscode.house.gov/download/pls/42C6A.txt

A total of six water-quality properties (such as pH) and as many as 337 contaminants were analyzed in source-water samples from public wells in this study. The 44 inorganic contaminants included major ions, trace elements, radionuclides, and nutrients.  The 293 organic contaminants included pesticide compounds, volatile organic compounds, personal-care and domestic-use products, and other organic contaminants such as manufacturing additives.

Recent advances in laboratory analytical methods have given scientists increasingly refined tools to detect a wide range of chemicals in the environment at low concentrations. In particular, the analytical methods used in this study have low detection levels—often 100 to 1,000 times lower than human-health benchmarks for the protection of water quality. Detections of contaminants, therefore, do not necessarily indicate a concern for human health, but rather identify the environmental presence of a wide variety of chemicals, many of which are not commonly monitored in water resources. 

The reporting levels for the analytical methods used by the USEPA, water utilities, or commercial laboratories for the analysis of finished-water samples for compliance monitoring typically are higher than reporting levels for analytical methods used by USGS.  As a result, contaminant detection frequencies in USGS reports may be greater than detection frequencies in annual water-quality reports (consumer confidence reports) provided by water utilities.

Each public well was part of a NAWQA groundwater assessment study (sampling network).  Assessment studies typically were designed to describe the water quality of major aquifers used for drinking water within study areas throughout the United States.  NAWQA studies were designed as integrated water-resource assessments of critical and regionally extensive hydrologic systems of the Nation, but were not designed as a statistically representative sampling of all public wells nationwide.  Within the sampled hydrogeologic settings, however, the public wells were randomly selected to represent typical aquifer conditions, and areas with known contamination were not targeted.  Public wells were excluded from this study if the well was only associated with a limited special study, or wells were less than 1,000 meters apart from each other in order to eliminate wells that may hydraulically influence each other.

The study included groundwater samples from 932 public wells.

No.  The occurrence and composition of contaminant mixtures were assessed in various subsets of wells. Detailed analysis of mixtures focused on source-water samples in which most contaminants—major ions, trace elements, nutrients, radon, pesticide compounds, and volatile organic compounds (VOCs)—were analyzed.  Mixtures of organic contaminants also were evaluated in a subset of 814 source-water samples, and in a subset of 94 paired samples of source and finished water.

A groundwater-supply public-water system may be comprised of one or more (sometimes hundreds) public wells.  The 932 public wells sampled in this study represented 629 unique public water systems.  For most of these 629 public water systems, one well was sampled per system, but as many as 16 wells per system were sampled.  The 629 public water systems with one or more wells sampled in this study represent less than 1 percent of the approximately 140,000 groundwater-supplied public water systems in the United States. 

Samples from the 629 systems represent source water used by about 26 million people—about 25 percent of the U.S. population served by groundwater-supplied public water systems—or about 9 percent of the entire U.S. population in 2008.  This is because (1) most people are customers of large and very large publicly owned systems (U.S. Environmental Protection Agency, 2002); (2) about one-half of the sampled wells were associated with large and very large systems; and (3) most large and very large systems were sampled in urban areas that tend to be densely populated.

Reference:
U.S. Environmental Protection Agency, 2002, Community water system survey 2000, volume 1: overview: U.S. Environmental Protection Agency, Office of Water EPA 815-R-02-005A, December 2002, 58 p.  Available at http://www.epa.gov/safewater/consumer/pdf/cwss_2000_volume_i.pdf

The wells were located in parts of 41 states. They also were distributed among parts of 30 regionally extensive aquifers used for water supply (principal aquifers), or about half of the more than 60 principal aquifers in the United States that have been mapped by USGS.

Each well was sampled once, between 1993 and 2007, such that the combined results represent a composite snapshot for the time period that focuses on geographic patterns.  All source-water samples were collected prior to any treatment or blending that potentially could alter contaminant concentrations.  All samples were collected and analyzed following a nationally consistent study design.

The study did not evaluate trends over time, which requires more intensive sampling of wells over long periods of time and detailed evaluation of groundwater flow systems, often using age-dating and models.  Changes in contaminant occurrence over time were not expected to be large compared to geographic variability because of the relatively slow movement of groundwater.  The occurrence of some contaminants, however, may have changed over time in some wells because of variability in contaminant sources and aquifer characteristics and because pumping and other human activities can enhance the mobility of contaminants.

An aquifer is a saturated, permeable geologic formation that can transmit substantial quantities of water, such as to wells.  The amount of groundwater that can flow through a geologic formation primarily depends on the permeability (the size and arrangement of the connected spaces in the materials that comprise the formation) and the hydraulic gradient. 

A confined aquifer is one that is located beneath a relatively impermeable (confining) layer, and an unconfined aquifer is one in which the water table forms the upper boundary.  Confined aquifers tend to be deeper than unconfined aquifers, which commonly are near the land surface.  Accordingly, of the public wells sampled in this study, the 397 wells that withdraw water from confined aquifers generally were deeper than the 502 wells that withdraw water from unconfined aquifers.  In general, an aquifer that is less susceptible to contamination tends to have a deep water table (for an unconfined aquifer), a thick, low-permeability layer such as clay-rich material between the aquifer and the land surface (for a confined aquifer), and no highly permeable fractures that can act as conduits for fluid flow.  By contrast, a highly susceptible aquifer tends to have a shallow water table, no low-permeability protecting layer, and may contain features that facilitate the movement of water, such as fractures and nearby wellbores.

 

Human-Health Context for Study Findings

As used in this study, regulated contaminants are those contaminants for which the U.S. Environmental Protection Agency (USEPA) has established drinking-water standards (Maximum Contaminant Levels or MCLs) under the Safe Drinking Water Act (SDWA), and unregulated contaminants are those that are not regulated in drinking water under the SDWA and therefore do not have MCLs.  Contaminants that are not federally regulated in drinking water under the SDWA may be regulated in drinking water by some states, and their use also may be regulated in other contexts and under other statutes, such as the Federal Insecticide, Fungicide, and Rodenticide Act.

To evaluate the potential significance of contaminant occurrence to human health, concentrations of contaminants that are regulated by USEPA in drinking water under the SDWA were compared to regulatory Maximum Contaminant Levels (MCLs), and concentrations of unregulated contaminants were compared to non-regulatory Health-Based Screening Levels (HBSLs), when available.  MCLs and HBSLs are collectively referred to as human-health benchmarks in this study.  These comparisons provide an initial perspective on the potential significance of detected contaminants to human health and to help prioritize further investigations, but are not designed to evaluate specific effects of contaminants on human health, and are not a substitute for comprehensive risk assessments, which generally include many additional factors such as multiple avenues of exposure.

Concentrations greater than benchmarks are potential human-health concerns, but do not mean that adverse effects are certain to occur because the benchmarks are conservatively protective, and most samples were collected prior to any treatment or blending of water that potentially could alter contaminant concentrations.  Exposure to individual contaminants detected at concentrations less than benchmarks is unlikely to result in adverse human-health effects because the benchmarks typically are concentrations in drinking water that are not anticipated to cause adverse effects from a lifetime of exposure. 

MCLs are legally enforceable USEPA drinking-water standards that set the maximum permissible level of a contaminant in water that is delivered to any user of a public water system.  MCLs are set as close as feasible to the maximum level of a contaminant at which no known or anticipated adverse effects on human health would occur over a lifetime, taking into account the best available analytical and treatment technologies, cost considerations, expert judgment, and public comments.  As a result, not all MCLs are based on health-effects data alone.

HBSLs are non-enforceable benchmark concentrations of unregulated contaminants in water that were developed by the USGS in collaboration with the USEPA and others using: (1) USEPA Office of Water methodologies for establishing drinking-water guidelines, and (2) the most recent, USEPA peer-reviewed, publicly available human-health toxicity information.  As a result, HBSL values are consistent with existing USEPA drinking-water guideline values, such as Lifetime Health Advisory values and Cancer Risk Concentration values (when they exist), except for unregulated contaminants for which more recent toxicity information has become available.  HBSLs are based on health effects alone and do not consider cost and technical limitations.  HBSLs supplement established federal drinking-water standards and guidelines, thereby providing a basis for a more comprehensive evaluation of contaminant-occurrence data in the context of human health.

MCLs are legally enforceable drinking-water standards that apply to finished drinking water from public water systems.  However, in this study, contaminant concentrations greater than MCLs do not represent MCL violations because MCLs apply only to finished water and the majority of the samples were collected from source waters.  None of the source-water or finished-water samples were collected for regulatory compliance purposes; further, compliance with most MCLs is based on running average concentrations, not on concentrations detected in single samples as collected in this study.  The detection of a contaminant in source water at a concentration greater than an MCL does not imply that the corresponding public water system delivered finished water to its customers at a concentration greater than the regulatory MCL. HBSLs are not enforceable drinking water standards.  

If contaminant concentrations are greater than MCLs, public water systems typically treat or blend source water with higher-quality water sources in order to decrease concentrations to less than MCLs.  In this study, 58 contaminants are regulated in drinking water under the SDWA and have MCLs.  Water utilities, however, are not required to treat water for unregulated contaminants (that is, those without MCLs).  In a subset of 94 public wells in this study, disinfection was the primary water treatment used at about three-quarters of the sampled systems, which is about the same proportion as for all groundwater-supplied public water systems nationwide.  Disinfection destroys harmful organisms, but generally is not designed to remove the chemical contaminants analyzed in this study.

MCLs and HBSLs used in this study were current as of September 2009.  MCL values were obtained from the USEPA (U.S. Environmental Protection Agency, 2006), and HBSL values were obtained from the HBSL website (Toccalino and others, 2008).  USEPA updated their drinking-water standards and guidelines document in late 2009 (U.S. Environmental Protection Agency, 2009), but data from the updated USEPA report were not available in time for inclusion in this study. 

References:
Toccalino, P.L., Norman, J.E., Booth, N.L., and Zogorski, J.S., 2008, Health-based screening levels: a tool for evaluating what water-quality data may mean to human health: U.S. Geological Survey, National Water-Quality Assessment Program, Updated April 10, 2008, Accessed February 24, 2010, at http://water.usgs.gov/nawqa/HBSL

U.S. Environmental Protection Agency, 2006, 2006 Edition of the drinking water standards and health advisories: U.S. Environmental Protection Agency, Office of Water EPA 822-R-06-013, August 2006, 18 p.  Available at http://www.epa.gov/waterscience/criteria/drinking/dwstandards.pdf

U.S. Environmental Protection Agency, 2009, 2009 Edition of the drinking water standards and health advisories: U.S. Environmental Protection Agency, Office of Water EPA 822-R-09-011, October 2009, 18 p.  Available at http://www.epa.gov/waterscience/criteria/drinking/dwstandards2009.pdf

MCLs were available for 58, or 17 percent, of the 337 contaminants analyzed in this study. HBSLs were available for an additional 135 (40 percent) contaminants. Neither MCLs nor HBSLs were available for 144, or 43 percent, of the contaminants analyzed in this study, and so concentrations of these contaminants could not be evaluated in a human-health context.

Neither an MCL nor an HBSL is available for radon, but USEPA has proposed both a lower MCL and a higher Alternative MCL for radon in public water systems.  The lower proposed MCL for radon applies to states and public water systems that do not develop programs to address health risks from radon in indoor air; the higher proposed Alternative MCL applies to states and public water systems that have established such programs. 

Yes.  The MCL for arsenic changed from 50 micrograms per liter (µg/L) to 10 µg/L in 2001, and the new MCL became effective in January 2006 (U.S. Environmental Protection Agency, 2001).  Arsenic concentrations in all source-water samples in this study were compared to the current MCL of 10 µg/L, regardless of when the samples were collected.  As a result, source-water samples collected prior to January 2006 with arsenic concentrations greater than 10 µg/L (but less than 50 µg/L) did not represent concentrations greater than the MCL at that time.

Reference:
U.S. Environmental Protection Agency, 2001, National primary drinking water regulations; arsenic and clarifications to compliance and new source contaminants monitoring: U.S. Environmental Protection Agency, Federal Register, 40 CFR Parts 9, 141, and 142, v. 66, no. 14, January 22, 2001, 6975–7066 p. Available at http://www.epa.gov/fedrgstr/EPA-WATER/2001/January/Day-22/w1668.htm.

Information about MCLs is available from the USEPA’s Safe Drinking Water Hotline at 1-800-426-4791 or http://www.epa.gov/safewater/hotline. Information about MCLs is also available at http://www.epa.gov/safewater/contaminants/index.html. Information about HBSLs is available from the USGS at http://water.usgs.gov/nawqa/HBSL.

Contaminant concentrations greater than one-tenth of human-health benchmarks were used in this study to provide an early and conservative indication of contaminant concentrations that approach benchmarks, to identify contaminants that may warrant additional monitoring, and in analyses of contaminant mixtures.  Various federal and state agencies use the criteria of one-tenth (or one-half) of a human-health benchmark for a variety of purposes; examples include: (1) reporting contaminant occurrence in groundwater, (2) reporting pesticide detections in water to USEPA under the Federal Insecticide, Fungicide, and Rodenticide Act, (3) ranking the susceptibility of wells to contamination, and (4) identifying contaminants of potential human-health concern for risk assessment evaluations.

 

Selected Findings About Contaminants in the Sampled Public Wells

One or more chemical contaminants were detected at concentrations greater than MCLs or HBSLs in more than one in five (22 percent) of the 932 source-water samples from public wells.  Contaminants from natural sources accounted for about three-quarters of contaminant concentrations greater than human-health benchmarks in source-water samples, and contaminants that originate entirely or primarily from man-made sources accounted for about one-quarter of concentrations greater than benchmarks.

Ten contaminants—seven from natural sources and three primarily from man-made sources—individually were detected at concentrations greater than human-health benchmarks in at least 1 percent of source-water samples from public wells and collectively accounted for most concentrations greater than benchmarks.  The seven contaminants from natural sources included four trace elements (arsenic, manganese, strontium, and boron) and three radionuclides (radon, radium, and gross alpha-particle radioactivity).  Radon activities were greater than the higher proposed Alternative MCL of 4,000 picocuries per liter (pCi/L) in less than 1 percent of samples, but were greater than the lower proposed MCL of 300 pCi/L in 55 percent of samples.  Each of the remaining six trace elements and radionuclides was detected at concentrations greater than human-health benchmarks in 3 to 19 percent of samples. 

The three contaminants from primarily man-made sources were nitrate (a nutrient), dieldrin (an insecticide that is no longer used), and perchloroethene (or PCE, a solvent), which each were detected at concentrations greater than MCLs or HBSLs in 1 to 3 percent of source-water samples.  Nitrate occurs naturally, but most nitrate concentrations greater than 1 milligram per liter (which is one-tenth of the nitrate MCL) originate from man-made sources.

None of the finished-water samples analyzed in this study had contaminant concentrations greater than MCLs or HBSLs.  

The contaminants that were detected most frequently at concentrations greater than human-health benchmarks in source-water samples from public wells—the seven contaminants from natural sources described above—were from natural geologic sources. These contaminants originate from the rocks and sediments that make up the aquifers from which the public wells withdraw water. Nitrate can originate from man-made sources such as fertilizers, livestock, and wastewater.  Pesticides are released into the environment primarily through their application to agricultural lands, such as croplands, and to non-agricultural lands, such as lawns, golf courses, and commercial areas.  Volatile organic compounds (VOCs) are a broad class of organic contaminants that have numerous uses in industry, commerce, households, and military sites, and consequently have several transport pathways into groundwater.

Trace elements and radionuclides were detected at concentrations greater than human-health benchmarks in source-water samples from both unconfined and confined aquifers, consistent with the fact that these contaminants originate primarily from aquifer materials.  By contrast, most man-made contaminants were detected at concentrations greater than human-health benchmarks only in samples from unconfined aquifers, consistent with the fact that these contaminants originate at the land surface.
Some of the contaminants detected in source-water samples from public wells also showed geographic patterns of occurrence that were related to aquifer geology or land use.  For example, although individual samples with arsenic concentrations greater than its MCL were distributed across the United States, about three-quarters of these samples were from public wells in the western half of the United States, which is consistent with findings from previous national-scale studies. About two-thirds of the samples with pesticide or VOC concentrations greater than benchmarks were from public wells in the highly populated areas of states bordering the East Coast.

Collectively, pesticide compounds or VOCs were detected in nearly two-thirds (64 percent) of the source-water samples from public wells.  Of those detections, concentrations of one or more of these contaminants were greater than human-health benchmarks in 4.5 percent of samples.  Considering all 293 man-made organic contaminants analyzed in this study, 169 (58 percent) were detected in at least one sample.  Pesticides and VOCs were detected in a significantly greater proportion of samples from unconfined aquifers than in samples from confined aquifers.

Organic contaminants detected in source water generally occurred in finished water after treatment at similar concentrations.  As many as 272 organic contaminants were analyzed in a subset of 94 paired source- and finished-water samples from public wells (inorganic contaminants were not analyzed in these paired samples).   Considering all organic contaminants that were detected in at least 10 percent of source-water samples, concentrations generally were similar in source and finished water, except for concentrations of disinfection by-products, which were greater in finished water because these contaminants are by-products that form during the disinfection process.  Two of the source-water samples, and none of the finished-water samples, contained at least one organic contaminant at a concentration greater than a human-health benchmark. 

Contaminants detected in source- and finished-water samples from public wells usually co-occurred with other contaminants as mixtures.  Although few human-health benchmarks have been established for mixtures of contaminants, concentrations of the contaminants in mixtures were compared to individual benchmarks.  About 4 percent of source-water samples contained mixtures of two or more contaminants at concentrations greater than individual human-health benchmarks, whereas most samples (84 percent) contained mixtures of two or more contaminants at concentrations greater than one-tenth of individual benchmarks. 

All of the most common mixtures in source water in which contaminant concentrations were greater than one-tenth of individual benchmarks were composed of one or more trace elements (arsenic, strontium, or uranium were most common), nitrate, and (or) radon (concentrations greater than 300 pCi/L).  Three-quarters of the organic-contaminant mixtures contained an herbicide (atrazine or simazine) or an herbicide degradate (deethylatrazine), two-thirds contained a disinfection by-product (chloroform), and about 40 percent contained the solvents perchlorethene or trichloroethene.  Organic-contaminant mixtures were detected somewhat more frequently in finished water than in source water because of the formation of disinfection by-products in finished water.

The most complex mixtures—those with the greatest number of contaminants—were detected more frequently in source-water samples from public wells that withdraw water from shallower unconfined aquifers than in samples from deeper confined aquifers.  For example, about two-thirds of the mixtures containing three contaminants, and nearly all mixtures containing 10 contaminants, were detected in samples from unconfined aquifers. 

Few human-health benchmarks have been established for mixtures of contaminants.  The widespread and frequent detections of contaminant mixtures in source water is a matter of increasing concern and attention because the total combined toxicity of contaminants in water may be greater than that of any individual contaminant.  Little is known about the potential health effects, if any, associated with exposure to multiple contaminants at concentrations detected in the environment, and more investigation is needed to evaluate the potential toxicity of contaminant mixtures to humans. This study identifies which contaminant mixtures may be of most concern in groundwater used for public-water supply on the basis of frequency of occurrence and comparisons of contaminant concentrations to individual human-health benchmarks.  This information can help human-health researchers to target and prioritize toxicity assessments of contaminant mixtures.

Most drinking-water regulations are for single contaminants.  In recent years, however, several federal agencies have focused more attention on health risks from exposure to contaminant mixtures, including the USEPA (http://www.epa.gov/risk/guidance.htm), the Agency for Toxic Substances and Disease Registry of the U.S. Centers for Disease Control (http://www.atsdr.cdc.gov/mixtures.html), and others. 

This study complements and expands upon findings reported in previous national-scale studies of public wells in several ways, and findings from this study generally confirm and reinforce previous conclusions about many contaminants.  For example, detection frequencies and the percentages of samples with contaminant concentrations greater than benchmarks determined in this study generally were similar to those observed in previous national-scale U.S. Geological Survey (USGS) studies of specific contaminant groups.  These similarities occur because one-quarter to one-third of the public wells sampled in this study also were included in some of the previous USGS studies and because of similarities in study designs.

Those contaminants that were most and least frequently detected in this study also generally were most and least frequently detected in several previous national-scale U.S. Environmental Protection Agency (USEPA) monitoring studies (Westrick and others, 1984; Longtin, 1988; U.S. Environmental Protection Agency, 1999, 2001, 2003, 2008).  Many individual contaminants were detected more frequently in this study than were reported in the USEPA monitoring studies because USGS uses lower analytical reporting levels than those used in most USEPA monitoring reports.  USGS also typically collects untreated source-water samples whereas USEPA typically collects treated finished-water samples.  In this study and in USEPA monitoring studies, many regulated contaminants were detected at concentrations greater than one-tenth of Maximum Contaminant Levels (MCLs), but concentrations greater than MCLs were not common—the same few contaminants generally were most frequently detected at concentrations greater than MCLs.

References:
Longtin, J.P., 1988, Occurrence of radon, radium, and uranium in groundwater: Journal American Water Works Association, v. 80, no. 7, p. 84–93.

U.S. Environmental Protection Agency, 1999, A review of contaminant occurrence in public water systems: U.S. Environmental Protection Agency, Office of Water EPA 816-R-99-006, November 1999, 78 p.  Available at http://www.epa.gov/safewater/occur/occur.html

U.S. Environmental Protection Agency, 2001, Occurrence of unregulated contaminants in public water systems—an initial assessment: U.S. Environmental Protection Agency, Office of Water EPA 815-P-00-001, May 2001, 508 p.  Available at http://www.epa.gov/ogwdw000/ucmr/data/report_ucm1-2_no_uc.pdf

U.S. Environmental Protection Agency, 2003, Occurrence estimation methodology and occurrence findings report for the six-year review of existing national primary drinking water regulations: U.S. Environmental Protection Agency, Office of Water EPA-815-R-03-006, June 2003, 874 p.  Available at http://www.epa.gov/safewater/standard/review/pdfs/support_6yr_occurancemethods_final.pdf

U.S. Environmental Protection Agency, 2008, Factoids: drinking water and ground water statistics for 2008: U.S. Environmental Protection Agency, Office of Water EPA 816-K-08-004, November 2008, 16 p.  Available at http://www.epa.gov/safewater/databases/pdfs/data_factoids_2008.pdf

Westrick, J.J., Mello, J.W., and Thomas, R.F., 1984, The groundwater supply survey: Journal American Water Works Association, v. 76, no. 5, p. 52–59.

There are three particularly important information gaps:  First, more information is needed on the potential health effects of individual contaminants that do not have benchmarks because of insufficient toxicity data. Research also is needed on the potential health effects of contaminant mixtures. Third, there are thousands of contaminants beyond the 337 analyzed in this study that may be present in source water, including industrial and pharmaceutical chemicals. 

The USGS shares its water-quality findings with the USEPA and others including which unregulated contaminants are most frequently detected but do not have human-health benchmarks, and which contaminant mixtures are most frequently detected in source water.  The USGS continues to develop new analytical methods to be able to measure man-made contaminants that are expected to occur in source water, but have not previously been assessed.

 

More Information for People Served by Public Water Utilities

The best source of specific information about drinking-water quality for people served by public water systems is their water supplier. Water suppliers that serve the same people year-round are required by the U.S. Environmental Protection Agency (USEPA) to provide their customers an annual water-quality report (also called a consumer confidence report, or CCR).  CCRs may be obtained directly from water utilities and some are available online at http://www.epa.gov/safewater/ccr/index.html.  Each CCR provides consumers with fundamental information about their drinking water including (1) the source of the water, (2) a brief summary of the susceptibility of the local drinking-water source to contamination, (3) the concentrations of any contaminants detected in local drinking water, as well as their Maximum Contaminant Levels, and (4) resources for additional information.  Many other agencies and organizations also provide consumers and homeowners with information about their drinking-water quality, including:  USEPA’s Safe Drinking Water Hotline (http://www.epa.gov/safewater/hotline), local and state environmental and public-health agencies, and non-governmental organizations such as the American Water Works Association (http://www.awwa.org or http://www.drinktap.org) and the National Ground Water Association (http://www.ngwa.org).

The USEPA publishes fact sheets for many contaminants with MCLs at http://www.epa.gov/safewater/hfacts.html, and the Agency for Toxic Substances and Disease Registry publishes ToxFAQs (summaries about chemical exposure and the effects of exposure on human health) at http://www.atsdr.cdc.gov/toxfaq.html.Information on the health effects of contaminants also is available from the USEPA Integrated Risk Information System (IRIS) at http://www.epa.gov/iris.

 

More Information on this Study

The results of this study are described in three USGS publications, including an overview of the major findings and implications (Circular 1346) and two detailed technical reports on data sources, analyses, and results (Scientific Investigations Reports 2009-5200 and 2010-5024).

Patricia Toccalino, USGS Hydrologist and lead scientist of the USGS study
Email: ptocca@usgs.gov
Phone: (916) 278-3090

Data used in this report are available online at http://water.usgs.gov/nawqa/studies/public_wells/

Access http://water.usgs.gov/nawqa for information about the USGS NAWQA Program and other assessments in the Program about the water-resources of the Nation.

 

Abbreviations and Units of Measure Used in this FAQ

CCL            contaminant candidate list
CCR           consumer confidence report
HBSL          Health-Based Screening Level
MCL           Maximum Contaminant Level
NAWQA      National Water Quality Assessment Program
PCE           perchloroethene
pCi/L          picocuries per liter
SDWA        Safe Drinking Water Act
µg/L            micrograms per liter
USGS         U.S. Geological Survey
USEPA       U.S. Environmental Protection Agency
VOC           volatile organic compound

 

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