National Water-Quality Assessment (NAWQA) Program
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Domestic wells are an important source of water supply in the United States. They supply 15 percent of the U.S. population, or more than 43 million people with drinking water. These wells are the primary source of drinking water in many rural areas. Although a key component of our National water supply, domestic wells are not regulated by the Federal Safe Drinking Water Act nor, in most cases, by State or local laws after the wells are installed. The lack of regular monitoring of domestic wells makes assessments such as the present study important tools for understanding the potential health effects of contaminants in this drinking-water supply source.
This study of the quality of water from domestic wells had three main objectives. First, the study was intended to assess the occurrence and distribution of water-quality conditions for domestic wells in the United States and to evaluate contaminant concentrations in relation to human-health benchmarks for drinking water in a screening level assessment. Second, the study described the variation in water quality among principal aquifers and rock types. Third, the study investigated the co-occurrence of contaminants as mixtures in the sampled domestic wells.
A screening-level assessment provides an initial perspective on the potential significance of contaminant occurrence to human health and can help prioritize future studies. A screening-level assessment is not designed to evaluate specific effects of contaminants in domestic well water on human health, nor is it suitable for a comprehensive risk assessment, which generally includes additional factors such as multiple avenues of exposure.
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. Senate, 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 its toxicity and concentration in drinking water. In fact, many contaminants are beneficial at certain concentrations. For example, many naturally occurring inorganic contaminants are required in small amounts for normal physiologic function, even though higher amounts may cause adverse health effects. On the other hand, man-made organic contaminants, such as pesticides, are not required by humans and may or may not have adverse effects on people, depending on concentrations, exposure, and toxicity. As a first step toward evaluating whether a particular contaminant may adversely affect human health, its concentrations measured in water can be compared to a USEPA Maximum Contaminant Level (MCL) or a USGS Health-Based Screening Level (HBSL). MCLs are Federal drinking-water standards for public water supplies under the SDWA (they do not legally apply to domestic wells), and HBSLs are nonenforceable health guidelines (see http://water.usgs.gov/nawqa/HBSL). Although these standards and guidelines are not enforceable standards for domestic wells, concentrations greater than MCLs or HBSLs may indicate the potential for human-health effects.
Contaminants originate from a wide range of natural and man-made sources. Most inorganic chemicals, nutrients, and microbial contaminants measured in this study occur naturally, although their concentrations in ground water may be altered by human activities. For example, nitrate is present from natural sources in many wells, but concentrations are often increased by contributions from man-made sources in agricultural and urban areas. In contrast, the organic contaminants measured in this study are all man-made, though some also may form in ground water through various chemical and biological transformation processes.
U.S. Senate, 2002, Title XIV of the public health service act; safety of public water systems (Safe Drinking Water Act); Part A - definitions (as amended through P.L. 107-377): Updated December 31, 2002, Accessed July 29, 2008, at URL http://epw.senate.gov/sdwa.pdf
As many as 214 contaminants and 5 properties were measured in the sampled domestic wells, including water properties such as pH and temperature, major ions, trace elements, nitrate and other nutrients, radon and other radionuclides, pesticides, volatile organic compounds, and microbial contaminants (fecal indicator bacteria).
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 state and Federal standards and guidelines for the protection of water quality. Detections of contaminants, therefore, do not necessarily indicate a concern to human health, but rather identify the environmental presence of a wide variety of chemicals, some of which are not commonly monitored in water resources. These findings complement ongoing drinking-water monitoring required by Federal, state, and local programs, which focus primarily on post-treatment compliance monitoring of contaminants regulated by USEPA in drinking water. Many of the chemicals analyzed by USGS are not included in other source water and finished-water monitoring programs.
The domestic wells in this study were sampled for NAWQA ground-water studies that were parts of integrated assessments of the water resources within selected hydrologic systems across the Nation. The NAWQA study areas, account for more than half of the Nation’s population and water use. The ground-water studies represent typical hydrogeological settings within more regionally extensive aquifers used for water supply (principal aquifers). The wells were randomly located within the targeted sampling areas of the ground-water studies. The sampled wells represent typical aquifer conditions and were not focused in areas of known contamination.
The study included samples from 2,167 domestic wells.
The wells were located in 48 States. They also were distributed among 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 1991 and 2004, such that the combined results represent a composite snapshot for the time period that focuses on geographic patterns. Generally, ground water moves very slowly and substantial trends within the study period were not expected. The wells were sampled as close to the wellhead as possible, before any in-home treatment systems that may have been in place or in-home plumbing.
The study did not evaluate trends over time, which requires more intensive sampling of wells over long periods of time and detailed evaluation of ground-water flow systems, often using age-dating and models.
The 2,100 wells sampled were selected as part of regional-scale studies of major aquifers throughout the Nation and represent a sampling of typical water-quality conditions in many parts of the country that depend on ground water. The broad geographic distribution of sampled wells, including 30 major aquifers and 48 states, make it likely that many, if not most, of the basic types of water quality conditions expected throughout the nation have been characterized. The study was not, however, based on a national statistical sample of all 15 million private wells and did not include wells in all important areas. It is not intended to support regional or national risk assessment, although the results can serve to guide the design of more detailed studies where they may be needed.
In addition to the omission of some major aquifers and certain areas of the country, the complex and often localized nature of ground-water flow and quality that can result in contaminant occurrence that can vary over short distances and with depth--making it common for individual wells to vary from the apparent regional pattern. On the other hand, study findings show that contaminants in domestic wells follow distinct geographic patterns, related to geology, geochemical conditions, land use, and other human influences. The study thus provides an assessment of the kinds of contaminants that can be expected in certain principal aquifers, geographic areas, and land-use settings across the Nation. Findings from the study underscore the continuing need for public education and testing of domestic wells, particularly in those aquifers and areas where concentrations are highest relative to human-health benchmarks and where high proportions of the population depend on domestic wells.
Concentrations of contaminants are compared to U.S. Environmental Protection Agency (USEPA) MCLs or USGS Health-Based Screening Levels (HBSLs); HBSLs were developed by the USGS and others as a first step towards understanding whether these concentrations may be of potential human-health concern. Concentrations of contaminants with MCLs are compared to their MCLs, and concentrations of contaminants without MCLs are compared to their HBSLs, when available. MCLs and HBSLs are referred to as human-health benchmarks in this study. Human-health benchmarks used in this study also included USEPA proposed MCLs for radon, and Safe Drinking Water Act action levels defined for copper, lead, and several other contaminants.
A USEPA MCL is a legally enforceable, Federal standard for public water supplies that sets 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 in drinking water at which no known or anticipated adverse effects on human health would occur, taking into account the best available technology, treatment techniques, cost considerations, expert judgment, and public comments. MCLs are not enforceable for water from domestic wells.
HBSLs are non-enforceable guidelines based on health effects alone and do not consider cost and technical limitations. HBSLs were developed for those contaminants for which Federal standards (MCLs) have not been established. HBSLs were developed by the USGS in collaboration with USEPA and others using standard USEPA methods for establishing drinking-water guidelines for the protection of human health. The most current, USEPA peer-reviewed, publicly available human-health risk assessments are used to develop HBSLs. As a result, HBSL values are equivalent to existing USEPA Lifetime Health Advisory values (for noncarcinogens) and Cancer Risk Concentration values (for carcinogens), when they exist, except for compounds for which more recent toxicity information has become available. Thus, 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.
The most recent USEPA toxicity information is used to calculate HBSLs, therefore, HBSLs provide a mechanism for the timely incorporation of updated toxicity information in the interpretation of water-quality data. Also, because HBSLs supplement existing USEPA drinking-water standards and guidelines, they provide a basis for a more comprehensive evaluation of contaminant-occurrence data in a human-health context than by using USEPA benchmarks alone. Prior to the calculation of HBSLs for unregulated contaminants without existing drinking-water guideline values, the ability to evaluate the human-health context of their occurrence on a basis consistent with USEPA benchmarks was limited
Neither MCLs nor HBSLs are enforceable standards for the quality of water from domestic wells.
MCLs were available for 58, or 27 percent, of the 214 contaminants measured in this study. HBSLs were available for an additional 96 (45 percent) contaminants. MCLs or HBSLs were not available for 60, or 28 percent, of the contaminants measured in this study, and so concentrations of these contaminants could not be evaluated in a human-health context.
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.
For individual contaminants, concentrations within one-tenth of a human-health benchmark are often used to identify contaminants that may warrant additional monitoring, to analyze trends in their occurrence, and to provide an early indication of contaminant concentrations that approach their benchmarks. Early attention to potential ground-water contamination, in particular, is warranted because the movement of ground water is usually slow and contamination is difficult to reverse. As a result, the presence of contaminants at concentrations less than, but approaching benchmarks can inform water-resource managers about needs for preventive actions for sources of these contaminants.
Federal and state agencies use the criteria of one-tenth of a human-health benchmark for a variety of purposes—examples include: (1) reporting contaminant occurrence in ground water (New Jersey Department of Environmental Protection, 2003), (2) reporting pesticide detections in water to USEPA under the Federal Insecticide, Fungicide, and Rodenticide Act (U.S. Environmental Protection Agency, 1998; Nebraska Department of Agriculture, 1997), (3) ranking the susceptibility of wells to contamination (New Jersey Department of Environmental Protection, 2003, 2004), and (4) identifying contaminants of potential human-health concern for risk assessment evaluations (U.S. Environmental Protection Agency, 1993, 1994). Thus, use of concentrations within one-tenth of a human-health benchmark to identify contaminants that may warrant additional monitoring is consistent with a number of state and federal practices.
For contaminant mixtures, concentrations that approach human-health benchmarks have added importance because toxicologic interactions can occur between contaminants at these concentrations, and some interactions can result in greater adverse effects than from exposure to the individual contaminants in the mixture (Carpenter and others, 2002; Yang, 1994). Further, most human-health benchmarks do not allow for the evaluation of the occurrence of contaminant mixtures in the context of human health because most benchmarks have been established for individual contaminants. Little is currently known about the potential health effects associated with exposure to multiple contaminants at concentrations observed in drinking water.
For risk assessments of contaminant mixtures, the USEPA and the Agency for Toxic Substances and Disease Registry identify those contaminants in a mixture for which estimated exposures are more than one-tenth of a noncancer toxicity value (Agency for Toxic Substances and Disease Registry, 2004; U.S. Environmental Protection Agency, 1994). If two or more contaminants in a mixture have exposures within one-tenth of a toxicity value, then further assessment of the joint toxic action is taken to determine whether additivity or other interactions between contaminants may results in a significant health hazard (Agency for Toxic Substances and Disease Registry, 2004).
Agency for Toxic Substances and Disease Registry, 2004, Guidance manual for the assessment of joint toxic action of chemical mixtures: U.S. Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry, 107 p., accessed March 25, 2009, at http://www.atsdr.cdc.gov/interactionprofiles/IP-ga/ipga.pdf.
Carpenter, D.O., Arcaro, K., and Spink, D.C., 2002, Understanding the human health effects of chemical mixtures: Environmental Health Perspectives, v. 110, no. Supplement 1, p. 25-42.
Nebraska Department of Agriculture, 1997, Nebraska pesticides and groundwater generic state management plan: Pesticide Program, March 18, 1997, variously paginated, http://www.agr.state.ne.us/division/bpi/pes/gsmp.pdf.
New Jersey Department of Environmental Protection, 2003, Susceptibility of source water to community water-supply wells in New Jersey to contamination by volatile organic compounds: Trenton, NJ, 21 p., http://www.state.nj.us/dep/swap/reports/gw_voc.pdf.
New Jersey Department of Environmental Protection, 2004, New Jersey source water assessment program statewide summary: Trenton, NJ, December 2004, 20 p., http://www.state.nj.us/dep/swap/reports/swap_sum200412.pdf.
U.S. Environmental Protection Agency, 1993, Risk Assessment—Technical Guidance Manual: Philadelphia, Penn., U.S. Environmental Protection Agency, Region 3, EPA/093/R-98-001, accessed March 25, 2009, at http://www.epa.gov/reg3hwmd/risk/human/info/guide2.htm.
U.S. Environmental Protection Agency, 1994, Region 8 Superfund Technical Guidance—Evaluating and identifying contaminants of concern for human health: Denver, Colo., U.S. Environmental Protection Agency, Region 8, accessed March 25, 2009, at http://www.epa.gov/region8/r8risk/pdf/r8_ra03-cocs.pdf.
U.S. Environmental Protection Agency, 1998, Code of Federal Regulations, title 40—Protection of environment, chapter 1—Environmental Protection Agency, subchapter E—Pesticide Programs, part 159—Statements of policies and interpretations, subpart D—Reporting requirements for risk/benefit information, 40 CFR 159: National Archives and Records Administration, September 19, 1997; amended June 19, 1998.
Yang, R.S.H., 1994, Introduction to the toxicology of chemical mixtures, in Yang, R.S.H., ed., Toxicology of chemical mixtures: San Diego, CA, Academic Press, p. 1-10.
Concentrations of contaminants like chloride, iron, and manganese and of properties like the pH of the water were compared to USEPA National Secondary Drinking Water Regulations (NSDWRs or secondary standards; http://www.epa.gov/safewater/contaminants/index.html). NSDWRs are non-enforceable guidelines regarding contaminants that may cause cosmetic effects (such as skin or tooth discoloration) or aesthetic effects (such as taste, odor, or color) in drinking water.
One or more contaminants were present at concentrations greater than human-health benchmarks (MCLs or HBSLs) in 23 percent of the sampled domestic wells. The contaminants included in this finding included trace elements, nitrate, fluoride, radon (at concentrations greater than the higher of two proposed MCLs), pesticides, and volatile organic compounds. These results do not include microbial contaminants, which were measured for only about 400 wells.
Radon, several trace elements (arsenic, boron, manganese, strontium, and uranium), nitrate, and fluoride were found individually at concentrations greater than human health benchmarks in about 1 to 7 percent of sampled wells nationally (using the higher of two proposed MCLs for radon). Pesticides, volatile organic compounds, several other trace elements were found at concentrations greater than human-health benchmarks in less than 1 percent of the sampled wells.
Except for nitrate, which is primarily from man-made sources, the contaminants that were present most frequently at concentrations greater than human-health benchmarks—for example, radon, arsenic, and uranium—were from natural geologic sources. These contaminants originate from the rocks and sediments that make up the aquifers from which the domestic wells withdraw water. Nitrate can originate from man-made sources such as fertilizer or septic systems.
Many of the contaminants found in the sampled domestic wells showed geographic patterns of occurrence that were related to aquifer geology or land use. For example, radon was most frequently at higher concentrations in wells that tapped crystalline rock aquifers, in the Northeast, central and southern Appalachian region, and in Colorado. These rocks are enriched in uranium-bearing minerals, which are the precursor to radon in ground water. Nitrate, which is primarily from man-made sources, was most frequently at higher concentrations in wells in areas of agricultural land use.
Contaminants were commonly found in mixtures with other contaminants. Although only 4 percent of wells had more than one contaminant at concentrations greater than health benchmarks, nearly three-fourths (73 percent) of the sampled wells had mixtures of multiple contaminants at concentrations greater than one-tenth of their individual health benchmarks. The criterion of one-tenth of a benchmark was used to indicate concentrations that might be approaching levels of potential concern in mixtures. The mixtures were most frequently made up of nitrate, arsenic, radon, and uranium, and, to a lesser extent, molybdenum and manganese.
Little is known about the potential health effects of most mixtures of contaminants. Health effects, if there are any, of the most frequently occurring mixtures of contaminants found in the sampled domestic wells, such as mixtures of nitrate, arsenic, radon, and uranium, are not known. The possible health effects of mixtures is an area of current research by health scientists. There are a few mixtures for which the health effects have been investigated—for example, mixtures of triazine herbicides, mixtures of triazine herbicides and nitrate, and mixtures of several solvents. These types of mixtures did occur in some of the sampled wells, but at concentrations that were below levels thought to be of concern.
The possible health effects of contaminant mixtures is an area of active research. Information on research topics regarding contaminant mixtures is available from several Federal agencies, and information on mixtures also may be found in the scientific literature. The Agency for Toxic Substances & Disease Registry of the U.S. Department of Agriculture provides information on human health effects of chemical mixtures at http://www.atsdr.cdc.gov/mixtures.html. The Children’s Environmental Health Centers of USEPA provides information on research topics on chemical mixtures at http://es.epa.gov/ncer/childrenscenters/chemical.html.
Results of the present study are consistent with the findings of previous regional, statewide, and national studies of various aspects of the quality of water from domestic wells. For contaminants in common, results are similar to those of the USEPA National Statistical Analysis of Rural Water Conditions (NSA), conducted in the late 1970s (U.S. Environmental Protection Agency, 1984). Both studies found widespread detections of microbial contaminants (42 percent of wells for total coliform in the NSA and 34 percent in the present study) and found that nitrate was one of the contaminants most frequently present at concentrations greater than human-health benchmarks (4 percent of wells with concentrations greater than the MCL in the NSA and 4.4 in the present study). Both studies also identified dissolved solids, iron, and manganese as relatively common nuisance contaminants (15 to 30 percent of wells in the NSA and 15 to 21 percent in the present study).
The more frequent occurrence of elevated nitrate concentrations in agricultural areas, which was found in the present study, is consistent with the findings of many regional or statewide studies; for example, nitrate concentrations were greater than the MCL in 2 to 32 percent of wells in the Midwest (see references in the USGS reports on domestic water quality that are the subject of this FAQ). Results of the present study for pesticides also are similar, though more extensive, to those of the USEPA National Pesticide Study (NPS) of domestic wells (U.S. Environmental Protection Agency, 1990). The NPS, conducted in the late 1980s, determined that about 4 percent of wells contained one or more detectable pesticides (with detection limits of 0.1 to 0.7 μg/L) and 0.6 percent of wells contained pesticides at concentrations greater than human-health benchmarks. These results are comparable to the finding of the present study that 3 percent of wells contained one or more pesticides at concentrations greater than 0.2 μg/L and 0.5 percent of wells contained pesticides at concentrations greater than benchmarks.
Results of the present study indicate slightly lower percentages of wells with contaminants greater than benchmarks than the percentages found by a recent retrospective of all USGS data on domestic wells (Focazio and others, 2006). The frequencies for the present study and for Focazio and others (2006) are, 6.8 and 10.6 percent for arsenic, 65 and 75 percent for radon greater than 300 pCi/L, 4.4 and 9.0 percent for radon greater than 4,000 pCi/L, 1.7 and 3.7 percent for uranium, 4.4 and 8.4 percent for nitrate, and 1.2 and 0.81 for fluoride. These differences may result, in part, from the inclusion of wells near contamination sites or from studies that targeted agricultural areas in the retrospective of all USGS data on domestic wells in Focazio and others (2006). The present study includes only NAWQA data from assessment studies that targeted major hydrogeologic settings without regard to land use or known water-quality problems.
Focazio, M.J., Tipton, Deborah, Shapiro, S.D., and Geiger, L.H., 2006, The chemical quality of self-supplied domestic well water in the United States: Ground Water Monitoring & Remediation, v. 26, no. 3, p. 1-13
U.S. Environmental Protection Agency, 1984, National statistical assessment of rural water conditions, Technical summary: Washington, D.C., U.S. Environmental Protection Agency, Office of Drinking Water, EPA570/9-84-004, 111 p.
U.S. Environmental Protection Agency, 1990, National Pesticide Survey, Summary results of EPA's national survey of pesticides in drinking water wells: Washington, D.C., U.S. Environmental Protection Agency, Office of Water and Office of Pesticides and Toxic Substances, EPA 570/9-90-NPS5, 17 p.
Domestic well owners may contact local and state health agencies for information about the characteristics of well water in their area and may have their water tested for compounds that may be of local potential concern. Additionally, individuals may use in-home water treatment devices designed to remove particular compounds, following manufacturer’s directions. Domestic well owners can find out what contaminants are in their well water and at what concentrations by having their water tested. Many local and state health agencies or state environmental protection agencies maintain lists of state-certified laboratories that can conduct water-quality testing of domestic well water.
Domestic well owners may contact local and state health agencies for information about the characteristics of well water in their area and may have their water tested for compounds that may be of local potential concern. Additionally, individuals may use in-home water treatment devices designed to remove particular compounds, following manufacturer’s directions.
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. The U.S. Centers for Disase Control and Prevention provides information about contaminants specifically in domestic wells at http://www.cdc.gov/ncidod/dpd/healthywater/privatewell.htm.
There are many sources of information about water-quality testing of domestic wells. Many state environmental or public-health agencies provide information and recommendations for homeowners about testing and water-quality of domestic wells. The USEPA also provides such information, and provides links to many state agency web sites on private wells. The U.S. Centers for Disease Control and Prevention provides information on water-quality testing and the health effects of selected contaminants in private wells. The U.S.Department of Agriculture, in cooperation with USEPA, provides information and resources for domestic-well owners through its Farm*A*Syst/Home*A*Syst and Cooperative State Research, Education, and Extension (CREES) Program. Local health departments, in many cases, are a source of information about private wells. Recommendations for water-quality testing and other information about domestic wells also are provided by several non-governmental organizations. Sources of information available on the World Wide Web from some of these agencies and organizations are listed below.
Leslie DeSimone, USGS Hydrologist and lead scientist of the USGS study
Phone: (508) 490-5023
Data used in this report are available at: http://water.usgs.gov/nawqa/studies/domestic_wells/DomQW_DataArchive_031609.zip
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.
HBSL Health-Based Screening Level
MCL Maximum Contaminant Level
mg/L milligram per liter
µg/L microgram per liter
NAWQA National Water Quality Assessment Program
NPS National Pesticide Study
NSA National Statistical Analysis of Rural Water Conditions
SDWA Safe Drinking Water Act
SMCL Secondary Maximum Contaminant Level
USGS U.S. Geological Survey
USEPA U.S. Environmental Survey