National Water-Quality Assessment (NAWQA) Program

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Frequently Asked Questions

USGS Role Regarding Drinking Water

Is USGS responsible for monitoring 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, such as for compliance. 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. This is the first NAWQA study that also looks at the quality of water at public water-supply intakes and of treated (or “finished”) water, represented in this study by sampling of water after treatment but before distribution. The goal of such efforts is to understand and compare occurrence patterns in source water to patterns that may also occur in treated water. The assessments are intended to complement drinking-water monitoring required by Federal, State, and local programs, which focus primarily on post-treatment compliance monitoring. In addition, findings are intended to evaluate what is in the source water to help guide those involved in decisions on treatment processes in the future.

Does USGS regulate drinking water?
No. The U.S. Geological Survey (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.

What does “regulated” refer to?
Under the authority of the Safe Drinking Water Act, the USEPA establishes drinking-water standards, such as Maximum Contaminant Levels (MCLs), to limit the level of contaminants in the Nation’s drinking water. After reviewing studies of health effects, the USEPA sets a Maximum Contaminant Level Goal (MCLG), the maximum level of a contaminant in drinking water at which no known or anticipated adverse effect on the health of persons would occur, and which allows an adequate margin of safety. MCLGs are non-enforceable public health goals. Because MCLGs consider only public health and not the limits of detection and treatment technology, sometimes they are set at a level that water systems cannot meet. When determining an MCLG, the USEPA considers the risk to sensitive subpopulations (infants, children, the elderly, and those with compromised immune systems) of experiencing a variety of adverse health effects.

Once the MCLG is determined, the USEPA sets an enforceable standard. In most cases, the standard is a Maximum Contaminant Level (MCL), the maximum permissible level of a contaminant in water that is delivered to any user of a public water system.

The MCL is set as close to the MCLG as feasible, which the Safe Drinking Water Act defines as the level that may be achieved with the use of the best available technology, treatment techniques, and other means that the USEPA finds are available (after examination for efficiency under field conditions and not solely under laboratory conditions), taking cost into consideration.
Additionally, the Federal government generally delegates to States the responsibility for enforcing primary standards. Therefore, States also may have enforceable drinking-water regulations that are more stringent than Federal standards. These standards may vary among States; specific information can be obtained from State drinking-water programs. More information on this topic is included at

About the Study

Where can I learn more about the detailed study design and results?
A summary of the study design and objectives is provided in a USGS Fact Sheet, available at

Technical findings are presented in a USGS Scientific Investigations Report (, and an overview of findings is provided in a USGS Fact Sheet (

Detailed technical information on the study design and analytical methods are available in Carter and others (2007) (

Please note that USGS findings and information (as described above) represent drinking-water results at only nine community water systems in the United States. Specific local information on drinking water may be found in a Consumer Confidence Report for which utilities are required to provide general information regarding the source(s) of drinking water, the treatment process, and the levels of detected contaminants that are regulated by a primary drinking-water regulation. In addition, supporting information on drinking water and regulations can be found at  Additional information on selected chemicals in the environment can be found at the USEPA’s Integrated Risk Information System ( or at the Agency for Toxic Substances and Disease Registry Web site (

Please also note that this study includes some preliminary comparisons to existing human-health benchmarks for drinking water, such as benchmarks for regulated contaminants by the USEPA. The study did not look at implications to aquatic ecosystems or health and, specifically, no comparisons were done between concentrations in source water and aquatic-life criteria. This study, however, did include many of the same compounds measured in ambient water samples collected from 186 streams during 1992-2001 by Gilliom and others (2006). Information on the potential significance of pesticides to aquatic life can be accessed at

What sites are included in the study?
Sampling and assessments were completed at community water systems with intakes on nine streams, including on White River in Indiana; Elm Fork Trinity River in Texas; Potomac River in Maryland; Neuse River in North Carolina; Chattahoochee River in Georgia; Running Gutter Brook in Massachusetts; Clackamas River in Oregon; Truckee River in Nevada; and Cache La Poudre in Colorado. Characteristics of the community water systems and their watersheds are provided on Table 1 in the USGS Scientific Investigations Report

Specific sampling locations of river intakes are not disclosed in reports to meet policy and source-water security purposes under USGS Homeland Security activities. Participating community water systems, however, do receive USGS results and may use and distribute the data as they choose.

Are sites representative of other community water systems?
A sample set of nine sites cannot possibly serve to represent the population of more than 50,000 community water systems across the United States. However, the findings do provide a preliminary assessment of the kinds of compounds that can be expected in relatively large community water systems with conventional treatment. Specifically, the nine community water systems generally represent conventional treatment, consisting of coagulation, flocculation, sedimentation, filtration, chlorine or sodium hypochlorite disinfection, and clear well storage. Additional treatment steps, such as chemical oxidation, pH adjustment, and use of powdered activated carbon (PAC) for seasonal taste and odor issues or removal of organic compounds, are used at several of the systems. Eight of the nine community water systems are categorized as “large” or “very large” water systems, defined by the USEPA as providing water to more than 10,000 and 100,000 people, respectively ( The size of the contributing watershed to these eight stream sites ranges from about 480 to 11,500 square miles (mi²), with most between about 1,000 and 1,500 mi².  Land use varies among the sites, ranging from relatively undeveloped and forested to intensely agricultural.

Will additional sites be sampled?
The NAWQA Program is planning an additional 21 surface-water assessments through 2013 ( The 30 systems, which is a small number relative to the number of community water systems across the country, are not intended to comprehensively portray the quality of our Nation’s source waters. They are, however, intended to improve understanding of ambient resource conditions in a drinking-water-supply context. Specific information on the study design is available in Carter and others (2007) (

How are the sites selected for study?
In general, the community water systems are selected to meet several criteria. Specifically, sites are located in study areas previously sampled by the NAWQA Program during 1992-2001 where data on source water and treated water can be compared to other NAWQA data collected from streams across the country. The community water systems also are generally located on free-flowing reaches of streams. In addition, they are generally single-source systems, with little or no blending of other source waters and are relatively large (serving more than 10,000 people). The type of water treatment or prior monitoring results, including those for compliance monitoring, are not considered in the selection process.

What compounds were monitored in this study?
This study characterizes the occurrence of about 260 anthropogenic (or man-made) organic compounds, including pesticides, solvents, gasoline hydrocarbons, personal-care and domestic-use products, disinfection by-products, and manufacturing additives. The compounds included in this study do not include pharmaceuticals, hormones, steroids, and various other chemicals often associated with wastewaters (including municipal wastewater discharges and livestock agricultural facilities). A national reconnaissance study for pharmaceuticals and other organic wastewater contaminants in untreated drinking-water sources was completed in a separate USGS study by Focazio and others (FAQsdoi:10.1016/j.scitotenv.2008.02.021).  

Do facilities generally monitor for these compounds?
Most of the compounds are unregulated in drinking water and therefore are not required compounds for monitoring by drinking-water suppliers.

What water was sampled at the community water systems?
Source-water samples were collected as close as practical to the drinking-water intake or at a raw water tap. Water samples also were collected following all water treatment, prior to the water entering the distribution system, and after the source-water samples were collected, to account for the residence time in the water-treatment plants.

Do facilities treat the water for removal of the compounds included in this study?
In general, the types of treatment steps used by the systems (and most across the Nation) are not designed specifically to remove most of the organic compounds monitored in this study. Findings on the occurrence in treated water are therefore not intended to characterize treatment efficacy. Rather, they provide a preliminary indication of the potential significance of the presence of organic compounds most commonly detected in source water to the quality of treated water. Findings are intended to evaluate what is in the source water to help guide those involved in decisions on treatment processes in the future. Treatment used by the systems generally is considered conventional, including steps of coagulation, flocculation, sedimentation, filtration, and disinfection. Three of the systems differ in that one water system uses slow sand filtration and disinfection; one adds ozone to the source water as a preliminary step to conventional treatment; and a third system uses a direct filtration treatment plant that follows steps used in conventional treatment. Four of the systems treat their water intermittently with powdered activated carbon (PAC), primarily to remove taste and odor compounds.

How did samples of source water relate to samples of treated water regarding timing?
The study was designed to allow time between collection of the source-water and treated-water samples and account for the retention time in the treatment plant (which ranged from 1 hour to 5 days). However, the timing is not perfect (for example, retention time can vary as a result of changes in water demand), which can introduce some uncertainty in comparisons between compounds in source and treated water. It also is possible that some compounds detected in source water degrade or transform during the treatment process into other compounds, some of which may not have been monitored as part of this study.

Does the study link findings to specific sources?
No. The study was not designed to examine specific sources and (or) factors causing and affecting the occurrence and concentrations of compounds in source water. However, additional perspective is added by highlighting general patterns and associations as appropriate, including those related to land use, wastewater discharge, streamflow, and seasonality.

Did the study look at public supply wells?
Yes. A companion study is scheduled for release in 2009 that summarizes the occurrence of the same organic compounds in ground water supplying high-production wells for 15 community water systems and the associated finished water.

Selected Findings

Are man-made organic compounds in source and treated water?
Yes. About one-half (134) of the organic compounds analyzed in this study were detected in at least one source-water sample collected at the nine source-water intakes. A total of 119 compounds were not detected at all. The most commonly detected compounds in source water were the disinfection by-product, chloroform; the herbicides simazine, atrazine, metolachlor, prometon, 2,4-D, and deethylatrazine (DEA); and the fragrance hexahydrohexamtheylcyclopentabenzopyran (HHCB).

About two-thirds of the compounds detected in 10 percent or more of source-water samples also were detected in treated-water samples. However, concentrations in source and treated water generally were low—generally less than 1 part per billion (microgram per liter) (which is equivalent to one thimble of water in an Olympic pool), and annual mean concentrations of all compounds were less than human-health benchmarks. On the basis of the NAWQA screening-level assessment of concentrations detected in this study, adverse effects to human health are not expected, subject to limitations of available human-health benchmarks.

Are the findings surprising?
No. Many of the compounds detected most commonly in source- and treated-water samples at the community water systems also are among those most commonly detected in ambient stream water sampled across the Nation by NAWQA during 1992-2001 (Gilliom and others, 2006, Many of the compounds detected generally are associated with streams that drain considerable agricultural and urban land use (as reported in Gilliom and others, 2006) and with upstream wastewater facilities.

What factors complicate the patterns of occurrence?
Occurrence is complex because of mixtures or the “co-occurrence” of many compounds in individual samples. More than 75 percent of source- and treated-water samples contained five or more organic compounds, often including parent compounds and degradates. The common occurrence of compound mixtures means that the total combined toxicity in source water may be greater than that of any single compound that is present. Continued research is needed because human-health benchmarks are based on toxicity data for individual compounds, and the additive or synergistic effects of mixtures of compounds at low levels are not well understood.

Do compounds occur all year?
Multiple samples were collected to characterize variability in source- and treated-water samples. Although the sampling strategy can be characterized as fixed frequency, a range of flow conditions were sampled at most sites.  Specifically, samples generally were collected monthly with as many as four additional samples collected during high flow or base-flow conditions when water-quality changes might be expected or when higher concentrations of some compounds are most likely to occur.

Findings indicate that variability depends on the compound. Some compounds were detected year-round in source- and treated-water samples, such as, chloroform, and the musk fragrance hexahydrohexamtheylcyclopentabenzopyran (HHCB). Wastewater discharge from upstream may be a relatively constant source for these types of compounds. Selected herbicides, like atrazine and simazine, also were detected year-round, but the magnitude and timing varied considerably. For example, concentrations of atrazine at sites with significant agriculture upstream (such as the White River) varied by tenfold or more during the year. The highest concentrations of these herbicides generally occurred in the spring following chemical applications in row-crop areas.

Is the water getting better or worse?
This study is not designed to look at trends. Repeated sampling and re-assessments for trends at the individual sites are not planned.

Implications to Human Health and Ecosystems

Do detections automatically mean that harmful effects will occur?
No. The presence of "detectable" organic compounds does not mean that harmful human-health effects will occur because detections are often at low concentrations that were well below benchmarks that are protective of human health. USGS analytical methods are designed to measure low levels of compounds—typically as low as 0.02 part per billion and sometimes as low as parts per trillion—which is commonly 100 to 1,000 times lower than drinking-water standards. For perspective, reporting limits for public drinking-water commonly are set through Federal regulations at 0.5 microgram per liter, and water utilities generally are not required to measure below this limit. Detections reported in this study, therefore, do not necessarily indicate a concern to human health, but rather provide a characterization of the low-level environmental occurrence of a wide variety of chemicals not commonly monitored in sources of drinking water. The USGS low-level approach helps to identify emerging issues; to track changes in concentrations over time; and to provide a comprehensive screening for the types of compounds we can expect to see in source waters in different environmental settings across the country.

Is this a risk assessment? If not, why is USGS reporting on human-health effects?
No. The USGS report is NOT a risk assessment and findings should not be confused with risk assessments for specific compounds by the U.S. Environmental Protection Agency (USEPA). The USEPA and States have primary responsibility for risk assessment and risk communication at the Federal and State levels, respectively. Since the implementation of NAWQA in 1991, the USGS has been asked with increasing frequency about the public-health implications of its findings. Therefore, to help place findings in a context for understanding potential effects on human health, measured concentrations by NAWQA were compared to water-quality benchmarks derived from standards and guidelines established by the USEPA, toxicity values from USEPA risk assessments, and selected guidelines from other sources. The water-quality benchmarks are estimates of the concentrations below which adverse effects on humans are not expected to occur. The screening-level assessment is primarily intended to identify and prioritize needs for further investigation. It is not designed to evaluate specific effects of contaminants on human health and is not a substitute for comprehensive risk assessment, which includes consideration of many more factors, such as additional avenues of exposure.

What benchmarks are used to assess potential human-health effects?
Potential human-health effects are not directly assessed in this study, but a screening level assessment was done to identify and prioritize compounds that may warrant further investigation. To help place detections of compounds in a human-health context, annual mean concentrations were compared to the U.S. Environmental Protection Agency's (USEPA) Maximum Contaminant Levels (MCLs) for regulated compounds, and to U.S. Geological Survey's (USGS) Health-Based Screening Levels (HBSLs) for unregulated compounds. HBSLs are non-enforceable guidelines that were developed by the USGS in collaboration with USEPA and others using USEPA methodologies and the most current USEPA peer-reviewed, publicly available human-health risk assessments and toxicity information (see USGS Fact Sheet 2005-3059, and the Web site, USGS Health-Based Screening Levels.

Were human-health benchmarks available for all compounds included in this study?
No, only about one-half have human-health benchmarks. Specifically, 38 of the compounds monitored in this study have an established MCL (U.S. Environmental Protection Agency) and 110 have an HBSL (USGS Health-Based Screening Levels).  HBSLs have not been developed for the remaining 129 unregulated compounds because of a lack of toxicity information. The potential human-health significance of these compounds, therefore, cannot be evaluated at this time.

How do detections in this study compare to existing human-health benchmarks?
Concentrations of detected compounds generally were less than 1 part per billion, and annual mean concentrations of all compounds were less than human-health benchmarks, which are available for about one-half of the compounds. As such, adverse effects to human health are unlikely to be caused by exposure to individual contaminants, subject to limitations of available human-health benchmarks.

What are the uncertainties and limitations to using a screening-level assessment to evaluate what detections may mean to human health?
Uncertainties are associated with screening-level assessments such as used in this study. Specifically, concentrations less than human-health benchmarks, such as USEPA Maximum Contaminant Levels (MCLs) and USGS Health-Based Screening Levels (HBSLs), indicate that adverse effects are unlikely to occur, even if water with such concentrations were to be ingested over a lifetime. Water containing concentrations greater than these benchmarks might be of potential human-health concern if the water were to be ingested as the primary drinking-water source without treatment for many years. The likelihood for adverse effects generally increases as concentrations increase above their benchmark values. If water containing concentrations greater than their benchmarks is ingested, however, it does not mean that adverse human-health effects will occur because: (1) human-health benchmarks are intentionally conservative (protective) and incorporate safety factors and conservative assumptions to account for uncertainty in the underlying toxicity information; and (2) models used to develop human-health benchmarks assume lifetime exposure of 2 liters of tap water per day, whereas actual exposure may be lower and for less than a lifetime.

Several limitations also are associated with screening-level assessments. First, some compounds analyzed in this assessment (total of 129) do not have human-health benchmarks (MCLs and HBSLs) because of a lack of peer-reviewed health risk assessments and toxicity information. Second, most of the available human-health benchmarks (MCLs and HBSLs) are based on toxicity information for individual chemicals, whereas NAWQA results indicate that mixtures of compounds occur in many samples. The long-term cumulative effects of low concentrations of multiple contaminants on human health currently are unknown.

What are the potential implications to aquatic health and ecosystems?
This study did not look at implications to aquatic ecosystems or health and, specifically, no comparisons were done between concentrations in source water and aquatic-life criteria. However, this study did include many of the same compounds measured in ambient water samples collected from 186 streams during 1992-2001 by Gilliom and others, 2006. Information on the potential significance of pesticides to aquatic life can be accessed at

Related Information and Contacts

How can I retrieve more technical information on NAWQA’s methodology, design, and findings?
Access for USGS publications. This includes (1) a USGS technical report (66 pages), Anthropogenic Organic Compounds in Source Water of Nine Community Water Systems that Withdraw from Streams, 2002—05, by Kingsbury and others, 2008; (2) two USGS Fact Sheets (each 6 pages), Man-made Organic Compounds in Source Water of Nine Community Water Systems that Withdraw from Streams, 2002—05 by Kingsbury and others, and Organic Compounds in Potomac River Water Used for Public Supply near Washington, D.C., 2003—05 by Brayton and others; and (3) a USGS data report containing information on the study design and analytical methods and tables of concentration data by Carter and others.

How do other agencies and community water systems use NAWQA information on source-water quality assessments?
USGS works closely with the U.S. Environmental Protection Agency (USEPA), providing information to inform decisions related to various pieces of environmental legislation governing water quality and human health. For example, information is used by the USEPA to evaluate the drinking water Contaminant Candidate List ( and to develop or revise drinking-water standards under the Safe Drinking Water Act. Information is also used by the USEPA for decisions on registration or re-registration of new or existing pesticides under the Food Quality Protection Act.  Managers associated with community water systems also use the information to continue to manage and track the quality of public drinking-water supplies, and to help guide decisions on treatment processes in the future.

How can I find out what is in my drinking water?
If drinking water is supplied by a community water system, the water supplier has the results of all the tests that are performed on water samples from the water system. Furthermore, results of testing are summarized in a Consumer Confidence Report that water suppliers must supply to their customers annually. Monitoring schedules differ according to the type of contaminant and the population that the community water system serves. In the Consumer Confidence Reports, community water systems are required to provide general information regarding the source(s) of drinking water, the treatment process, and the levels of detected contaminants that are regulated by a primary drinking-water regulation. More information about Consumer Confidence Reports, including links to reports for some water systems, is available at

Where can I get information on individual chemicals?
Additional information on selected chemicals in the environment can be found at the Agency for Toxic Substances and Disease Registry Web site (

How do I learn more about how drinking water is regulated?
The USEPA, together with States, Tribes, and its many partners, protects public health by ensuring safe drinking water and protecting ground water. The agency oversees implementation of the Safe Drinking Water Act, which is the national law safeguarding drinking water provided by public water systems in America. Access the USEPA Web site on safe water and learn about drinking water programs authorized under the Safe Drinking Water Act, such as how to find information about water quality; how to test your water quality; more information on drinking-water standards; how to find information about specific contaminants and public water systems; and more (

What other USGS programs study source water and drinking water?
Contaminants like pesticides and volatile organic compounds have been researched and monitored in streams and ground water by the USGS since the 1980s. Initial research on these substances is often done by the Toxic Substances Hydrology Program (or “TOXICs”) in USGS ( The Toxics Program started in the 1980s with research on pesticides and volatile organic compounds, topics about which little was known at the time. Later in the 1990s, research by the TOXICs Program expanded to what we now call “contaminants of emerging concern,” including many compounds used in our homes, businesses and industries, such as human and veterinary pharmaceuticals, detergents, fragrances, fire retardants, disinfectants, plastics, and insect repellants. The research answers many questions, including the role of different sources such as wastewater-treatment plants, livestock production and animal feedlot wastes, aquaculture, onsite septic systems, combined sewer overflows; transport in different environmental settings; environmental health effects and assimilation in organisms; and persistence of contaminants in drinking-water sources.

Who can I contact for more information on the NAWQA assessment?

Greg Delzer, Coordinator, Source Water-Quality Assessments
Phone: (605) 394-3230
Jim Kingsbury, Lead scientist of the national assessment

NAWQA staff in individual facilities:

Cache la Poudre, Colorado –
Lori Sprague,, (303)-236-4882, ext. 262

Chattahoochee River, Georgia –
William Hughes,, (770)-903-9162

Clackamas River, Oregon –
Hank Johnson,, (503) 251-3472

Elm Fork Trinity River, Texas –
Patricia B Ging,, (512) 927-3581

Neuse River, North Carolina –
Jerad D Bales,, (919) 571-4048

Potomac River, Maryland –
Michael Brayton,, (443)-498-5598

Running Gutter Brook, Massachusetts –
Craig Brown,, (860) 291-6766

Truckee River, Nevada –
Michael Rosen,, (775) 887-7683

White River, Indiana –
Jeffrey W Frey,, (317) 290-3333, ext. 151

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