U.S. Department of the Interior

U.S. Geological Survey

Circular 1332

National Water-Quality Assessment Program

The Quality of Our Nation’s Waters

Quality of Water from Domestic Wells 

in Principal Aquifers of the United States,

1991–2004

Overview of Major Findings
Front cover. Photographs from iStock Photos (middle photograph) and U.S. Geological Survey.
The Quality of Our Nation’s Waters

Quality of Water from Domestic Wells 

in Principal Aquifers of the United States, 

1991–2004

Overview of Major Findings

By Leslie A. DeSimone, Pixie A. Hamilton, and Robert J. Gilliom

National Water-Quality Assessment Program

Circular 1332

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

Revised and reprinted, May, 2009

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Suggested reference:

DeSimone, L.A., Hamilton, P.A., and Gilliom, R.J., 2009, The quality of our nation’s waters—Quality of water from 

domestic wells in principal aquifers of the United States, 1991–2004—Overview of major findings: U.S. Geological Survey Circular 1332, 48 p.

Library of Congress Cataloging-in-Publication Data

The quality of our nation’s waters—Quality of water from domestic wells in principal aquifers of the United 

States, 1991–2004—Overview of major findings/ by Leslie A. DeSimone,... [et al.].

Includes bibliographical references.

2009922297

ISBN: 9781411323506
Foreword iii

Foreword

The U.S. Geological Survey (USGS) is committed to providing the Nation with reliable scientific 

information that helps to enhance and protect the overall quality of life and that facilitates effective 

management of water, biological, energy, and mineral resources (http://www.usgs.gov/). Information on the Nation’s water resources is critical to ensuring long-term availability of water that is safe 

for drinking and recreation and is suitable for industry, irrigation, and fish and wildlife. Population 

growth and increasing demands for water make the availability of that water, measured in terms 

of quantity and quality, even more essential to the long-term sustainability of our communities 

and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 to 

support national, regional, state, and local information needs and decisions related to water-quality 

management and policy (http://water.usgs.gov/nawqa). The NAWQA Program is designed to answer: 

What is the quality of our Nation’s streams and ground water? How are conditions changing over 

time? How do natural features and human activities affect the quality of streams and ground water, 

and where are those effects most pronounced? By combining information on water chemistry, 

physical characteristics, stream habitat, and aquatic life, the NAWQA Program aims to provide 

science-based insights for current and emerging water issues and priorities. From 1991 to 2001, the 

NAWQA Program completed interdisciplinary assessments and established a baseline understanding 

of water-quality conditions in 51 of the Nation’s river basins and aquifers, referred to as Study Units 

(http://water.usgs.gov/nawqa/studyu.html).

National and regional assessments are ongoing in the second decade (2001–2012) of the NAWQA 

Program as 42 of the 51 Study Units are selectively reassessed. These assessments extend the 

findings in the Study Units by determining water-quality status and trends at sites that have been 

consistently monitored for more than a decade, and filling critical gaps in characterizing the quality 

of surface water and ground water. For example, increased emphasis has been placed on assessing 

the quality of source water and finished water associated with many of the Nation’s largest community water systems. During the second decade, NAWQA is addressing five national priority topics 

that build an understanding of how natural features and human activities affect water quality, and 

establish links between sources of contaminants, the transport of those contaminants through the 

hydrologic system, and the potential effects of contaminants on humans and aquatic ecosystems. 

Included are studies of the fate of agricultural chemicals, effects of urbanization on stream ecosystems, bioaccumulation of mercury in stream ecosystems, effects of nutrient enrichment on aquatic 

ecosystems, and transport of contaminants to public-supply wells. In addition, national syntheses of 

information on pesticides, volatile organic compounds (VOCs), nutrients, trace elements, and aquatic 

ecology are continuing.

The USGS aims to disseminate credible, timely, and relevant science information to address practical and effective water-resource management and strategies that protect and restore water quality. 

We hope this NAWQA publication will provide you with insights and information to meet your 

needs, and will foster increased citizen awareness and involvement in the protection and restoration 

of our Nation’s waters.

The USGS recognizes that a national assessment by a single program cannot address all waterresource issues of interest. External coordination at all levels is critical for cost-effective management, regulation, and conservation of our Nation’s water resources. The NAWQA Program, therefore, 

depends on advice and information from other agencies—Federal, state, regional, interstate, tribal, 

and local—as well as nongovernmental organizations, industry, academia, and other stakeholder 

groups. Your assistance and suggestions are greatly appreciated.

Matthew C. Larsen

Associate Director for Water
Donna N. Myers 

NAWQA Program Chief 

U.S. Geological Survey

Introduction to this report and the NAWQA series iv

The Quality of Our Nation’s Waters

This report is one of a series of publications, The Quality of Our Nation’s Waters, which 

describes major findings of the NAWQA Program on water-quality issues of regional and 

national concern. This report presents a national assessment of the quality of water in 

private domestic wells based on samples from about 2,100 wells located in 48 states and 

tapping 30 regional aquifers used for water supply. Compared to previous investigations of 

domestic well water quality, “Quality of Water from Domestic Wells in Principal Aquifers 

of the United States, 1991–2004—Overview of Major Findings” greatly expands the 

number of contaminants assessed in domestic wells and improves geographic coverage 

of wells sampled. Other reports in this series focus on specific water-quality constituents 

of concern, such as nutrients, pesticides, and volatile organic compounds in ground water 

and surface water, as well as on the effects of contaminants and habitat conditions on 

aquatic ecosystem health. Each report builds toward a more comprehensive understanding 

of regional and national water resources.

The information in this series is intended primarily for those interested or involved in 

resource management and protection, conservation, regulation, and policymaking at 

regional and national levels. In addition, the information is relevant to those at a local 

level who wish to know more about the general quality of streams and ground water 

in areas near where they live and how that quality compares with other areas across 

the Nation.
Contents v

Contents

Summary of Major Findings and Implications .................................................................................... 1

Introduction ............................................................................................................................................... 7

NAWQA’s Approach to Assessing the Quality of Water from Domestic Wells .......................... 11

Major Findings ........................................................................................................................................ 17

More than one in five (23 percent) of the sampled wells contained one or more 

contaminants at a concentration greater than a human-health benchmark ......... 17

Contaminants most often found at concentrations greater than human-health 

benchmarks were inorganic chemicals, with all but nitrate derived primarily 

from natural sources ....................................................................................................... 18

Nitrate is the only contaminant derived primarily from man-made sources that was 

found at concentrations greater than a human-health benchmark in more 

than 1 percent of wells .................................................................................................... 25

Man-made organic compounds were detected in more than half (60 percent) of the 

sampled wells, but concentrations were seldom greater than humanhealth benchmarks (less than 1 percent of wells) ...................................................... 28

Microbial contaminants were detected in as many as one-third of the 

approximately 400 wells sampled .................................................................................. 31

About half (48 percent) of the sampled wells contained at least one contaminant at 

a level or concentration outside the range of values recommended by 

USEPA for the aesthetic quality of water ..................................................................... 32

Contaminants usually co-occurred with other contaminants as mixtures .......................... 35

References Cited .................................................................................................................................... 41

Abbreviations, Acronyms, and Units of Measure ............................................................................. 48
This page intentionally left blank
Summary of Major Findings 

and Implications

More than 43 million people—about 15 percent of the U.S. population—

rely on domestic wells as their source of drinking water (Hutson and 

others, 2004). The quality and safety of water from domestic wells, 

also known as private wells, are not regulated by the Federal Safe 

Drinking Water Act or, in most cases, by state laws. Rather, individual 

homeowners are responsible for maintaining their domestic well systems 

and for monitoring water quality. The lack of regular monitoring of 

domestic wells makes periodic assessments at national, regional, and 

local scales important sources of information about this key source of 

drinking water.

This study from the National Water-Quality Assessment (NAWQA) 

Program of the U.S. Geological Survey (USGS) assesses water-quality 

conditions for about 2,100 domestic wells. The sampled wells are located 

in 48 states and in parts of 30 regionally extensive aquifers used for water 

supply in the United States. As many as 219 properties and contaminants, 

including pH, major ions, nutrients, trace elements, radon, pesticides, 

and volatile organic compounds (VOCs), were measured. Fecal indicator 

bacteria and additional radionuclides were analyzed for a smaller 

number of wells. The large number of contaminants assessed and the 

broad geographic coverage of the present study provides a foundation 

for an improved understanding of the quality of water from the major 

aquifers tapped by domestic supply wells in the United States.

Major findings and implications of this study are summarized on 

pages 2–5 and results are described in greater detail in the remainder 

of the report. 

Additional Information

For more information on domestic well 

water quality, refer to DeSimone (2009).

The report is available at 

http://pubs.usgs.gov/sir/2008/5227. 
2 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Major Findings

•	 More than one in five (23 percent) of the sampled wells contained one or more

contaminants at a concentration greater than a human-health benchmark (p 17).

Concentrations of at least one chemical contaminant were greater than humanhealth benchmarks—U.S. Environmental Protection Agency (USEPA) Maximum 

Contaminant Levels (MCLs) or USGS Health-Based Screening Levels (HBSLs)—in 

23 percent of 1,389 domestic wells sampled in this study for which all contaminants 

were measured. Concentrations of at least one chemical contaminant were greater 

than an established MCL in 13 percent of the 1,389 wells. These results do not include 

microbial contaminants, which were measured in only about 400 wells. 

•	 Contaminants most often found at concentrations greater than human-health

benchmarks were inorganic chemicals, with all but nitrate derived primarily from

natural sources (p. 18–24).

The contaminants most frequently found at concentrations greater than benchmarks 

were radon, several trace elements (including arsenic and uranium), nitrate, and 

fluoride. Each of these was individually greater than its benchmark in about 1 to 

7 percent of wells nationally (using the higher of two USEPA proposed MCLs for 

radon). Except for nitrate, these contaminants in ground water all originate primarily 

from natural geologic sources. Regional patterns in concentrations were apparent for 

many contaminants, such as radon, as the result of the geographic distributions of 

natural sources.

•	 Nitrate is the only contaminant derived primarily from man-made sources that was

found at concentrations greater than a human-health benchmark in more than

1 percent of wells (p. 25–27).

Nitrate was measured at concentrations greater than the USEPA MCL of 

10 milligrams per liter as N in 4.4 percent of all wells and was most frequently 

found at these concentrations in wells in agricultural areas. Nitrate occurs naturally, 

but elevated concentrations usually originate from man-made sources, including 

fertilizers, livestock, and septic systems. Nitrate concentrations followed regional 

patterns consistent with those found in other studies and were highest in the upper 

Midwest, Great Plains, and the Central Valley in California. A separate analysis 

of domestic wells in areas of relatively intense agricultural land use showed that 

concentrations of nitrate were greater than the MCL in nearly 25 percent of about 

400 wells sampled in those areas.

•	 Man-made organic compounds were detected in more than half (60 percent) of

the sampled wells, but concentrations were seldom greater than human-health

benchmarks (less than 1 percent of wells) (p. 28–30).

Low-level occurrences of man-made organic compounds in 60 percent of the wells 

sampled—including herbicides, insecticides, solvents, disinfection by-products, 

gasoline hydrocarbons and oxygenates, refrigerants, and fumigants—indicate the 

diverse industrial, agricultural, and urban sources that can affect the quality of the 
Summary of Major Findings and Implications Summary of Major Findings and Implications 3

source water to domestic wells. Only 7 of the 168 organic compounds that were 

analyzed—two insecticides, one herbicide, two solvents, and two fumigants—were 

found in one or more wells at concentrations greater than human-health benchmarks, 

and organic compounds were found in less than 1 percent of sampled wells at 

these concentrations.

•	 Microbial contaminants were detected in as many as one-third of the approximately

400 wells sampled (p. 31).

Escherichia coli (E. coli) was detected in 7.9 percent of 397 sampled wells. E. coli

typically is not harmful but is an indicator of fecal contamination and, therefore, the 

possible presence of pathogens. Total coliform bacteria, a broader group that includes 

bacteria from soil, water, and animal feces, were detected in 34 percent of 378 

sampled wells.

•	 About half (48 percent) of the sampled wells contained at least one contaminant at a

level or concentration outside the range of values recommended by USEPA for the

aesthetic quality of water (p. 32–34).

Values or concentrations of pH, dissolved solids, iron, and manganese in samples 

collected prior to any in-home treatment were individually outside the ranges 

defined by USEPA Secondary Maximum Contaminant Levels (SMCLs) in 15 to 

21 percent of wells. Fluoride was greater than its SMCL in 4 percent of wells. Low 

pH was prevalent in aquifers in the eastern United States. About 60 percent of the 

sampled wells, mostly in the central United States, contained water that generally is 

considered to be “hard.” Undesirable effects of elevated values or concentrations of 

these properties and contaminants include corrosion or scale deposition in pipes and 

plumbing, laundry staining, unpleasant water color or taste, and teeth staining.

•	 Contaminants usually co-occurred with other contaminants as mixtures (p. 35–39).

Contaminants found in domestic wells usually co-occurred with other contaminants 

as mixtures, rather than alone, which is a potential concern because the total 

toxicity of a mixture can be greater than that of any single contaminant. Although 

only 4 percent of domestic wells contained mixtures of two or more contaminants 

at concentrations individually greater than human-health benchmarks (excluding 

microbial contaminants), most of the sampled wells (73 percent) contained mixtures 

of multiple contaminants with concentrations greater than one-tenth of their 

individual benchmarks. The most common mixtures were composed of inorganic 

contaminants, including nitrate, arsenic, radon, and uranium. Unique mixtures 

composed of specific combinations of two or more of these four contaminants were 

found in 12 to 32 percent of wells at concentrations greater than one-tenth of their 

individual benchmarks. Nitrate or radon, each at concentrations greater than one-tenth 

of their respective benchmarks, co-occurred with the organic contaminants atrazine, 

deethylatrazine, or chloroform in 10 to 15 percent of wells.
4 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Implications

•	 The presence of contaminants at levels greater than human-health benchmarks in 

23 percent of domestic wells, which are not routinely monitored by public agencies, 

underscores the importance of public education, testing, and additional assessment 

regarding the water quality of domestic wells.

•	 Naturally occurring contaminants, such as arsenic and radon, may be present in 

ground water at concentrations of potential concern for human health in relatively 

undeveloped settings that otherwise may not be perceived as vulnerable to 

contamination. Moreover, traditional well-head protection approaches to preventing 

contamination generally are not effective for contaminants that occur naturally in 

ground water.

•	 Contaminants from man-made sources, such as nitrate and selected organic 

compounds that persist in ground water, may be present from previous land uses, 

pointing to the importance of public education and water-quality testing in areas 

where land use has changed.

•	 The widespread occurrence and variety of man-made organic compounds detected in 

domestic wells, although usually at levels below available human-health benchmarks, 

indicates the presence of contaminant sources and transport pathways for all principal 

aquifers. The ubiquitous presence of sources and transport pathways needs to be 

considered for long-term monitoring and protection of water quality. Continued 

water-quality assessment, monitoring, and research are needed to more fully 

understand natural and man-made factors and transport mechanisms associated with 

the movement of contaminants to domestic wells.

•	 The common occurrence of mixtures of multiple contaminants in domestic wells 

requires further investigation to evaluate the potential significance to human health. 

The combined toxicity of contaminant mixtures can be greater than that of any 

single contaminant and the potential for additive or synergistic effects of mixtures 

of contaminants at low levels is not well understood. Available human-health 

benchmarks do not allow full assessment of the potential health effects of contaminant 

mixtures because benchmarks have been established for only a few specific mixtures 

and are not yet available for all individual contaminants.

•	 Regional patterns in concentrations of naturally occurring trace elements and radionuclides indicate that customized approaches for monitoring, management, and treatment for some specific geographic areas could be effective. Contaminant occurrence, 

however, also can vary over short distances and with depth within aquifers because 

of variability in contaminant sources, aquifer characteristics, and geochemical conditions, again pointing to the importance of water-quality testing of individual wells.

•	 The greatest potential for successfully addressing water-quality concerns for domestic 

wells is with targeted approaches in specific areas where (1) concentrations of specific 

contaminants are highest in relation to human-health benchmarks, and (2) high proportions of the population depend on domestic wells. Improved information is needed 

on the number of people consuming water from domestic wells in specific regions 

and aquifers, and on water-quality conditions in the particular aquifer zones that are 

tapped by wells. Such information is essential for evaluating the potential humanhealth implications and possible mitigation approaches.
Understanding Study Results

Findings from this study are based on a synthesis of data from National WaterQuality Assessment (NAWQA) Program ground-water resource assessments of key 

hydrologic systems across the Nation. Several aspects of the original NAWQA study 

design and how results are interpreted in relation to potential human-health concerns are 

important to understand:

•	The NAWQA assessments focused on ground-water aquifers used for water supply, 

and the sampled wells were selected to represent typical conditions in these aquifers. 

Because the sampled domestic wells are located only in NAWQA ground-water 

study areas, they may not be representative of all domestic wells throughout the 

United States.

•	Water samples were collected prior to in-home plumbing or treatment, and thus 

represent the quality of source water from the domestic wells, rather than water 

consumed in homes after passing through distribution pipes and any treatment systems 

that may be used.

•	In this report, following the terminology of the Safe Drinking Water Act, all constituents 

measured in water are referred to as “contaminants,” regardless of concentration or 

potential health effects. Thus, the presence of a contaminant does not necessarily mean 

that there is a human-health concern (see sidebar on top of page 13).

•	U.S. Geological Survey (USGS) analytical methods were designed to measure 

contaminants at as low a concentration as possible, frequently resulting in detections at 

concentrations far below human-health benchmarks for drinking water (see sidebar on 

bottom of page 13).

•	To place findings in a human health context, concentrations are compared to benchmark 

values for human health that include current (2009) U.S. Environmental Protection 

Agency (USEPA) Maximum Contaminant Levels (MCLs) and USGS Health-Based 

Screening Levels (HBSLs) (see sidebar on page 16).

Additional details on NAWQA’s approach to assessing the quality of water from 

domestic wells are described on pages 11–16.

Summary of Major Findings and Implications Summary of Major Findings and Implications 5
This page intentionally left blank
Introduction 7

Introduction

More than 43 million people—about 

15 percent of the population of the United 

States—rely on more than 15 million privately 

owned domestic wells for their drinking water 

(Solley and others, 1998; Hutson and others, 2004; 

U.S. Census Bureau, 2008). These wells are the 

sole source of drinking water and water for other 

household needs for most people in many rural 

areas (fig. 1).

Although most ground water in the 

United States is generally considered of good 

quality and safe to drink (U.S. Environmental 

Protection Agency, 2000, 2002), water-quality 

problems associated with domestic wells have 

been reported. For example, although few in 

number (averaging fewer than 20 outbreaks 

per year), 20 to 40 percent of the waterbornedisease outbreaks from drinking water that were 

reported to the U.S. Centers for Disease Control 

and Prevention (CDC) from 1999 to 2002 were 

attributed to contaminants in domestic well 

water, and gastrointestinal illness associated 

with domestic well water is likely underreported 

(Blackburn and others, 2004; Liang and 

others, 2006). Health risks associated with 

contaminants in domestic well water have also 

been noted to include exposure to elevated 

concentrations of nitrate, arsenic, radon, lead, and 

organic compounds (U.S. Centers for Disease 

Control and Prevention, 2003).

The quality and safety of water from 

privately owned domestic wells is not regulated 

under Federal or, in most cases, state laws. 

State and local regulations, where they exist, 

provide for a minimum amount of water testing 

(see sidebar on top of page 9). Individual 

homeowners are primarily responsible for 

maintaining their domestic well systems and for 

any water-quality monitoring that is conducted. 

Federal regulations under the Safe Drinking 

Water Act (SDWA) for public water supplies, 

although not directly applicable to the regulation 

of domestic wells, often are used to evaluate the 

implications of water-quality monitoring results 

EXPLANATION

Population (1995) supplied by

 domestic wells, in percent of

 total county population

≥ 80

≥ 60 and < 80

≥ 40 and < 60

≥ 20 and < 40

< 20

Figure 1. In many parts of the United States, domestic wells supply drinking water for large percentages of the population. 

Nationwide, more than 43 million people rely on domestic wells. Data shown are from Solley and others (1998).
8 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

for domestic wells. The SDWA provides useful 

concentration benchmarks for contaminants and 

concepts for evaluating the quality of water from 

domestic wells in a human-health context (see 

sidebar on top of page 13).

When the SDWA was initiated in 1974, it 

mandated a national study of rural water systems, 

including domestic wells. The study, which 

focused on indicator bacteria and inorganic 

contaminants, found that concentrations were 

greater than the health benchmarks available at 

that time in more than 15 percent of domestic 

wells in the United States (National Statistical 

Assessment of Rural Water Conditions, or NSA; 

U.S. Environmental Protection Agency, 1984). 

A variety of contaminants in domestic wells 

have been documented in more recent studies, 

although usually at levels unlikely to have 

adverse effects. Pesticides in domestic wells 

were investigated in national and regional studies 

(U.S. Environmental Protection Agency, 1990; 

Holden and others, 1992; U.S. Centers for 

Disease Control and Prevention 1998). A recent 

retrospective analysis of all water-quality 

data for domestic wells from USGS studies 

during 1986 to 2001 (including those in known 

problem areas) described concentrations of 

26 contaminants that are regulated under the 

SDWA (Focazio and others, 2006). Numerous 

studies, many targeted at agricultural areas, 

have investigated nitrate and bacteria 

concentrations in domestic wells in states or 

regions across the United States (including, for 

example, Hallberg and others, 1992; Kolpin 

and others, 1994; Mehnert and others, 1995; 

Richards and others, 1996; Gosselin and 

others, 1997; Townsend and others, 1998; see 

additional references in DeSimone, 2009). 

A smaller number of regional or statewide 

studies have investigated the occurrence of other 

contaminants in domestic wells, such as arsenic 

and radon (for example, Maine Department 

of Human Services, 1998; Peters and others, 

1999; Hagan, 2004; Shiber, 2005). However, as 

reported to Congress in 1997, available data on 

contaminants other than nitrate and bacteria in 

domestic wells in the United States generally 

have been sparse (U.S. General Accounting 

Office, 1997). 

Past studies have provided important 

information on the distribution of specific 

contaminants and contaminant groups, but are 

limited in that they do not describe the overall 

quality of water from domestic wells, have 

been restricted to contaminants with regulatory 

standards, or are targeted at potential problem 

areas. The present study expands the scope of the 

previous investigations by considering a much 

larger number of contaminants and water-quality 

properties, and by assessing the co-occurrence of 

contaminants as mixtures (see sidebar on bottom 

of page 9). 

Domestic wells are usually within a few hundred feet of the houses that 

they supply. Examples shown are from houses in Maine, Massachusetts, 

and Florida.

Photographs by Joe Ayotte, Leslie DeSimone, and Terry Petrosky, USGS.
Introduction 9

Water-Quality Testing in Domestic Wells

State and local regulations, where they exist, typically require a minimum amount of water testing for domestic wells. Regulations apply 

primarily at the time of well installation and are limited in scope. Less than half of the states require testing of water from new domestic 

wells, typically for bacteria and nitrate. County or other local testing requirements for new wells also may exist. Water-quality testing at the 

time of home sales is a condition of some home loans and is required by some states (Oregon Department of Human Services, 2003; Veterans 

Benefits Administration, 2005; New Jersey Department of Environmental Protection, 2008). A few states also conduct free voluntary testing 

programs or test high-risk wells (Riding and Quilter, 2004; State Water Resources Control Board, 2005; Florida Department of Health, 2006). 

Many health and environmental agencies and non-profit organizations advise homeowners on testing procedures and recommend annual testing (U.S. Environmental Protection Agency, 2002; National Ground Water Association, 2006).

These programs, regulations, and recommendations all contribute to the safety of drinking water supplied by domestic wells. However, 

the limited number of contaminants assessed, the small numbers of wells tested, and the infrequent and voluntary nature of the testing do 

not provide domestic well users with the same level of protection afforded to users of public water systems. On the other hand, enormous 

resources would be needed to monitor the millions of domestic wells in the United States in a manner similar to the monitoring conducted for 

public water systems, of which there were 170,000 in the United States in 2000—about 100 times fewer than the number of domestic wells 

(U.S. Environmental Protection Agency, 2001).

In the absence of routine monitoring of individual wells, water-quality information from a wide variety of sources is commonly used to 

assess the potential occurrence and distribution of contaminants in domestic wells. Sources of information for assessing specific aspects of 

domestic-well water quality include one-time statewide or regional surveys, voluntary testing programs, and ambient ground-water monitoring 

programs (see references in DeSimone, 2009). Results from these studies are useful for prioritizing contaminants for further study, identifying areas of concern, guiding local testing recommendations, and identifying factors that potentially affect the occurrence of contaminants 

in wells. However, differences in study designs and methods make available survey studies difficult to compare and extrapolate (Ray and 

Schock, 1996). Many statewide programs have targeted vulnerable aquifers, and voluntary testing programs may be biased toward wells with 

higher contaminant concentrations (Peters and others, 1999).

Comparison With Previous Studies

Results of this study are generally consistent with previous studies of water quality in domestic wells, but they also more fully 

characterize a comprehensive suite of contaminants and contaminant mixtures than previously reported. Specifically, this study expands 

findings on domestic wells in several ways, including national-scale information about:

•	 low-level occurrences of pesticides and volatile organic compounds;

•	 a broad suite of naturally occurring trace elements and their potential significance to human health;

•	water properties, such as pH and hardness, that affect the aesthetic quality of water;

•	mixtures of contaminants, which may be more toxic than individual occurrences; and

•	contaminant occurrence within regional aquifers, which provides a framework for understanding water-quality conditions within units of 

similar hydrogeology.

Detailed comparisons with previous regional, statewide, and national studies of the quality of water from domestic wells are provided 

in DeSimone (2009). Some examples include widespread detections of microbial contaminants in this study (34 percent of sampled wells) 

and the U.S. Environmental Protection Agency’s National Statistical Analysis (NSA) of Rural Water Conditions (42 percent), conducted in 

the late 1970s (U.S. Environmental Protection Agency, 1984). Both of these studies also reported dissolved solids, iron, and manganese 

as relatively common nuisance contaminants, and nitrate as one of the contaminants most frequently present at concentrations greater 

than human-health benchmarks (4 percent of wells with concentrations greater than the Maximum Contaminant Level (MCL) in the NSA, 

compared to 4.4 percent in the present study). Results of the present study indicate slightly lower concentrations of some contaminants than 

a previous national retrospective study by Focazio and others (2006), which most likely reflects differences in well selection and study design. 

Specifically, the present study includes wells targeted in hydrogeologic settings without regard to land use or known water-quality problems. 

Focazio and others (2006), by comparison, included all domestic wells available in the U.S. Geological Survey’s water-data information system, 

regardless of study design or study objective, which could result in the inclusion of wells near contaminated sites or from studies that targeted 

agricultural areas.
10 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Types of Domestic Wells

There are several different types of 

domestic wells used by homeowners across the 

United States. The oldest wells were dug by 

hand. These wells typically are shallow and of 

large diameter (several feet), and may be lined 

with stones, brick, or tile. They typically extend 

only a short distance below the water table 

(Waller, 1991). Driven wells are constructed 

by driving small-diameter pipe into saturated 

unconsolidated sand and gravel. Driven wells 

typically are deeper than dug wells but are 

still relatively shallow. Most modern wells 

are drilled using truck-mounted machines 

and rotary or percussion methods. Drilled 

wells may be several hundred to more than 

a thousand feet in depth, and can penetrate 

consolidated sediments and bedrock. Drilled 

wells in unconsolidated sediments have well 

casing and a well screen to prevent collapse of 

the borehole; drilled wells in bedrock typically 

are cased through the overlying soil and thin 

sediment, where present, and are open holes 

below. The domestic wells included in this 

study were mostly (more than 90 percent) drilled 

wells, with steel or PVC casings 4 to 8 inches 

in diameter. A few dug and driven wells also 

were sampled. The wells averaged 174 ft in 

depth, with most (80 percent) between 50 and 

335 ft in depth. The average age of the wells 

was about 20 years, with most between 10 and 

45 years old.

Soil and

unconsolidated

sediment

Well

casing

Water table

Soil and

unconsolidated

sediment

Well

casing

Water table

Pressure

tank

Basement

of home

Pump

Electrical

control box

Bedrock

Soil and

unconsolidated

sediment

Well

casing

Open hole

Water table

Drilled well

Not to scale

Modified from University of Rhode Island Cooperative Extension Program (2003).

Most of the domestic wells sampled in this study were drilled 

wells, which can penetrate hundreds of feet into consolidated 

sediments or bedrock.
NAWQA’s Approach to Assessing the Quality of Water from Domestic Wells 11

NAWQA’s Approach to Assessing the Quality of Water from Domestic Wells

This analysis provides an assessment 

of water-quality conditions for about 2,100 

domestic wells across the United States. The 

wells are located in 48 states and represent 

selected parts of 30 regionally extensive aquifers 

used for water supply (fig. 2). As many as 

219 properties and contaminants, including pH, 

major ions, nutrients, trace elements, radon, 

pesticides, and VOCs, were measured. Fecal 

indicator bacteria, some trace elements, and 

additional radionuclides were analyzed for 

smaller numbers of wells (table 1). Data were 

collected using consistent sampling, analytical, 

and quality assurance methods. All constituents 

are considered contaminants by the SDWA 

(see sidebar on top of page 13), regardless 

of concentration or potential health effects. 

A subset of 1,389 of the sampled wells, located in 

45 states and 25 principal aquifers, had uniform 

and complete chemical analyses (no data gaps for 

any contaminant groups). These 1,389 wells are 

used for analysis of contaminant co-occurrence 

and mixtures.

Findings from this study are based on a 

synthesis of data for domestic wells that were 

collected as part of multiple NAWQA groundwater assessments conducted across the Nation. 

Several aspects of the study design and data 

characteristics, as well as the approach to data 

analysis and interpretation, are important to 

consider when interpreting the findings in 

this report.

•	 Principal aquifers, which are regionally 

extensive aquifers used for potable 

water supply (U.S. Geological Survey, 

2003; fig. 2), are used as a framework 

for regional synthesis of water-quality 

data within units of broadly similar 

permeability, rock type, geologic setting, 

and climate (Lapham and others, 2005). 

However, water-quality conditions vary 

substantially within principal aquifers, 

reflecting the heterogeneity of natural 

systems. In some areas, principal 

aquifers also may overlie or underlie 

less extensive local aquifers that have 

different water-quality characteristics, 

such that water-quality conditions 

Table 1. Water properties and concentrations of inorganic, organic, and 

microbial contaminants were measured in as many as 2,160 domestic wells in 

this study.

Contaminant or contaminant group Approximate numbers of 

wells sampled1

Water properties2 2,050–2,140

Major ions 2,160

Trace elements

 Arsenic and uranium 1,730–1,780

 Boron, lithium, strontium, thallium, and vanadium 490–660

 Iron and manganese 2,160

 Other trace elements3 1,550–1,640

Nutrients 1,780–2,140

Radionuclides

 Radon 1,960

 Radionuclides other than radon 380–440

Organic compounds

 Pesticides 1,370–2,120

 Volatile organic compounds 1,540–1,960

Fecal indicator bacteria 380–400

Contaminant co-occurrence and mixtures 1,390

1

 Ranges indicate differences in number of samples for individual contaminants.

2

 Specific conductance, temperature, pH, dissolved oxygen, and alkalinity.

3

 Aluminum, antimony, barium, beryllium, cadmium, chromium, cobalt, copper, lead, 

molybdenum, nickel, selenium, silver, and zinc.

described for a principal aquifer do not describe the quality of water 

from all domestic wells in the geographic area. 

•	 NAWQA ground-water assessments, conducted from 1991 through 

2004 in 51 study areas, focused on important water-supply aquifers 

that account for more than half of the Nation’s population and water 

use. The study areas represent typical hydrogeologic settings within 

the regionally extensive principal aquifers. All wells sampled in 

NAWQA ground-water assessments, including the domestic wells 

selected for this analysis, were randomly selected to represent 

typical aquifer conditions and were not focused in areas of known 

contamination. Because the sampled domestic wells are located only 

in the 51 study areas, they may not be representative of all domestic 

wells throughout the United States.
12 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

•	 Water samples were collected prior to any plumbing or treatment, 

and thus represent the quality of source water from the domestic 

wells rather than water consumed in homes after passing through 

distribution pipes and any treatment systems that may be used.

•	 Each well was sampled once during 1991–2004. Trends within the 

study period were not assessed, but broad changes are not expected 

because of the relatively slow movement of ground water.

•	 Detections of contaminants do not necessarily indicate a concern 

to human health, which depends on concentration, exposure, and 

toxicity. Samples were analyzed by methods with low detection 

levels—typically at concentrations 100 to 1,000 times lower than 

state and Federal standards and guidelines for drinking water. These 

low-level concentration data help to identify and better define the 

occurrence of a wide variety of contaminants in domestic wells 

(see sidebar on bottom of page 13).

•	 Comparisons between measured concentrations and human-health 

benchmarks, which include USEPA MCLs and USGS HBSLs, 

are used to provide a screening-level assessment of the potential 

significance to human health of a broad suite of contaminants 

(see sidebar on page 16).

Data used in the present study also have been selectively included 

in national-scale analyses of nutrients, pesticides, VOCs, trace elements, 

radionuclides, and microbial contaminants in the water resources of the 

United States and have been used to describe water-quality conditions 

in individual NAWQA assessment areas and in principal aquifers. 

These previous NAWQA studies are detailed investigations of the water 

quality of specific areas, the occurrence of specific contaminants in the 

environment, and the natural and human factors affecting water quality. 

Publications from these previous NAWQA studies can be accessed at 

http://water.usgs.gov/nawqa.

Water samples were collected prior to any 

treatment system that may have been in use 

and prior to most plumbing in the home.

, and Lane Simmons, USGS. yotte, Cathy Munday Photographs by Joe A
13

What is a “Contaminant”?

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, such as selenium, are required in small amounts for normal physiologic function, 

even though higher amounts may cause adverse health effects (Eaton and Klaassen, 2001). 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 were compared to a U.S. Environmental Protection 

Agency (USEPA) Maximum Contaminant Level (MCL) or a U.S. Geological Survey (USGS) Health-Based Screening Level 

(HBSL). Concentrations greater than these water-quality benchmarks indicate the potential for human-health effects (see 

sidebar on page 16).

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. 

Importance of Low-Level Detections 

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 not commonly 

monitored in water resources, and serve to track changes in their 

occurrence and concentrations over time. 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 sourcewater and finished-water monitoring programs such as the Unregulated 

Contaminant Monitoring Program (U.S. Environmental Protection Agency, 

2007) and the U.S. Department of Agriculture Pesticide Data Program 

(U.S. Department of Agriculture, 2008). 

U.S. Geological Survey analyst at the 

National Water-Quality Laboratory in 

Lakewood, Colorado.

NAWQA’s Approach to Assessing the Quality of Water from Domestic Wells Photograph by USGS.
14 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Figure 2. Domestic wells sampled in this study are located in parts of 30 principal aquifers of the United States. (A) The 

30 principal aquifers represented by wells in this study are shown. These are 30 of the more than 60 principal aquifers 

that have been mapped across the Nation (U.S. Geological Survey, 2003). Each principal aquifer is regionally extensive 

and has the potential to be used as a source of potable water. (B) Locations of domestic wells sampled in this study are 

shown by dots that are colored to correspond to the color-coding of principal aquifers (see explanation).

A

B

BR

BR

RG

OP

CV

BR

BR

SPf

M

WL

VRs

ET

NR

HP

F

NA

Gw CL

MV

CL

TC SC

SD

LT

P

EM

P

SPb

PBx

M

M

SD

O

O

CO

ET

F

VRs

VRc

VRc

VRs

NEx

NA

EM

SC
Principal aquifers and well locations—Principal aquifers sampled in this study are shown with

 colored areas. Domestic well locations are shown with colored dots. The full spatial extent of some aquifers 

 is not shown in areas where the aquifers partially underly other aquifers.

 

Coastal plain aquifers in

 semi-consolidated sand

Sandstone aquifers

Sandstone and carbonate-rock 

 aquifers

Carbonate-rock aquifers

Crystalline-rock aquifers—Aquifers

 in New England and New York are present

 at depth; aquifers in Colorado are not shown

Basin-fill and other non-glacial 

 sand and gravel aquifers

Glacial sand and gravel aquifers—

 Aquifers are discontinuous within area shown.

Basaltic-rock aquifers

EXPLANATION

Aquifer not known

CL

NA

TC

SC

New England crystalline-rock aquifers— 

 Extent at depth of crystalline-rock aquifers

 in New York also is shown

NEx

Piedmont and Blue Ridge crystalline-rock

 aquifers

PBx

RFx Rocky Mountain Front Range crystalline-rock

 aquifers—Aquifer extent, in Colorado, not shown

SPb Snake River Plain basaltic-rock aquifers

ET Edwards-Trinity aquifer

M Mississippian aquifers

F Floridan aquifer system

O Ordovician aquifers

OP Ozark Plateaus aquifer system

SD Silurian-Devonian aquifers

VRc Valley and Ridge carbonate-rock aquifers

BR Basin and Range basin-fill aquifers

CV Central Valley aquifer system

HP High Plains aquifer

MV Mississippi River Valley alluvial aquifer

NR Northern Rocky Mountains Intermontane

 Basins aquifer system

OS Other stream valley alluvial aquifers—

 Aquifer extent not shown

RG Rio Grande aquifer system

SPf Snake River Plain basin-fill aquifers

WL Willamette Lowland basin-fill aquifers

Coastal lowlands aquifer system

North Atlantic Coastal Plain aquifer system

Southeastern Coastal Plain aquifer system

Texas coastal uplands aquifer system

CO Cambrian-Ordovician aquifer system

EM Early Mesozoic basin aquifers

LT Lower Tertiary aquifers and Upper

 Cretaceous aquifers

P Pennsylvanian aquifers

VRs Valley and Ridge sandstone and shale aquifers

NAWQA’s Approach to Assessing the Quality of Water from Domestic Wells 15
16

Human-Health Benchmarks 

Used in This Assessment

Concentrations of contaminants measured in this study were 

compared to human-health benchmarks to place concentration 

data in a human-health context. Specifically, concentrations were 

compared to U.S. Environmental Protection Agency (USEPA) Maximum 

Contaminant Levels (MCLs) or U.S. Geological Survey (USGS) HealthBased Screening Levels (HBSLs). USEPA MCLs are legally enforceable 

standards for drinking water that specify the maximum permissible 

level of a contaminant that is delivered by public water systems 

(U.S. Environmental Protection Agency, 2006a). HBSLs are nonenforceable benchmark concentrations that were developed by USGS in 

collaboration with USEPA and others, using standard USEPA methods 

and current toxicity information, for many unregulated contaminants 

for which MCLs have not been established (Toccalino and others, 2004, 

2006; Toccalino, 2007; values used in this report were current as of 

March 2009, see http://water.usgs.gov/nawqa/HBSL). Unregulated 

contaminants include several (for example, boron and manganese) that 

USEPA has evaluated for regulation but determined that no regulatory 

action was appropriate (U.S. Environmental Protection Agency, 2003a, 

2008a). HBSLs are equivalent to existing USEPA Lifetime Health 

Advisory and Cancer Risk concentration values (when they exist), 

except for unregulated compounds for which more recent toxicity 

information has become available. It is important to note that the 

presence of contaminants at concentrations greater than benchmarks 

does not necessarily indicate that adverse effects are certain to occur. 

Conversely, concentrations that are less than benchmarks do not 

guarantee that adverse effects will not occur, but indicate that adverse 

effects are unlikely. Human-health benchmarks were available for 154 

of the 214 contaminants measured in this study.

Comparison of the water-quality data from domestic wells in this 

study to MCLs or HBSLs provides an initial perspective on the potential 

significance of contaminant occurrence to human health and can help 

prioritize further studies. A screening-level assessment of this kind also 

provides a perspective on areas where adverse effects of contaminants 

in domestic well water are more likely to occur and the contaminants 

that may be responsible for such effects (Gilliom and others, 2006). 

A screening-level assessment is not designed to evaluate specific 

effects of contaminants in domestic well water on human health, nor 

is it a substitute for a comprehensive risk assessment, which generally 

includes many additional factors, such as multiple avenues of exposure 

(Toccalino and others, 2006).

A concentration greater than one-tenth of a 

human-health benchmark was used in this study 

to provide an indication of contaminants that may 

approach concentrations of potential humanhealth concern, either individually or as mixtures, 

and to identify those that may warrant additional 

monitoring and study. Although a variety of 

criteria may be selected to identify contaminants 

that may be approaching concentrations of 

concern, a criterion of one-tenth of a benchmark 

is consistent with a number of state and 

Federal risk-assessment practices (for example, 

see U.S. Environmental Protection Agency, 1998).

Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

28 %

45 %

27 %

EXPLANATION

Human-health benchmarks 

used in this study

(number in parentheses)

MCLs (58)

HBSLs (96)

None available (60)
Major Findings 17

Major Findings

Concentrations of at least one chemical proposed MCL for radon of 4,000 picocuries 

contaminant were greater than human-health per liter (pCi/L). Microbial contaminants are not 

benchmarks (USEPA MCLs or USGS HBSLs) included because they were measured in only 

in 23 percent of 1,389 domestic wells sampled about 400 wells. Wells with contaminant concenin this study for which all contaminants were trations greater than human-health benchmarks 

measured (fig. 3). Concentrations of at least were located throughout the United States and in one chemical contaminant were greater than nearly all of the principal aquifers included in the an established MCL in 13 percent of the study (fig. 3). 1,389 wells. These results are based on the 

More than one in five (23 percent) of the sampled wells contained one or more 

contaminants at a concentration greater than a human-health benchmark

EXPLANATION

No contaminant concentration greater than a human-health benchmark

At least one contaminant concentration greater than a human-health benchmark

Figure 3. Concentrations of at least one chemical contaminant were greater than a human-health 

benchmark in 23 percent of 1,389 domestic wells sampled in 25 principal aquifers. Human-health 

benchmarks include U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels 

(MCLs) and U.S. Geological Survey Health-Based Screening Levels. Results are based on the USEPA 

proposed MCL for radon of 4,000 picocuries per liter. Microbial contaminants are not included.
18

Contaminants most often found at concentrations greater than human-health 

benchmarks were inorganic chemicals, with all but nitrate derived primarily 

from natural sources

Radon (using the higher of two proposed MCLs for radon), several trace elements (such 

as arsenic and uranium), nitrate, and fluoride occurred individually at concentrations greater 

than human-health benchmarks in about 1 to 7 percent of all domestic wells sampled in this 

study. The naturally occurring contaminants, in many cases, followed regional patterns that 

were related to the geologic and geochemical characteristics of the principal aquifers.

Major ions, trace elements, and radionuclides in ground water originate primarily 

from the geochemical weathering of rocks 

and minerals. The concentrations of these inorganic contaminants in ground water reflect, in 

large part, their abundance in aquifer materials, 

length of time that the water is in contact with 

those materials, and physical and geochemical conditions, such as permeability, pH, and 

dissolved oxygen concentrations, that govern 

solubility and mobility. Natural water quality, 

therefore, varies from place to place and with 

the types of soils, rocks, and aquifer materials 

through which the ground water moves.

Inorganic contaminants measured in this 

study include 9 major ions, 23 trace elements, 

6 nutrients (including nitrate), 4 radionuclides 

(including radon), and total dissolved solids. 

Human-health benchmarks were available for 

about two-thirds of these. The individual inorganic contaminants found in 1 percent or more of 

the domestic wells in this study at concentrations 

greater than available human-health benchmarks 

are listed in table 2. Several other inorganic 

contaminants were found at concentrations greater 

than benchmarks in less than 1 percent of wells, 

including antimony (0.06 percent, MCL), beryllium (0.06 percent, MCL), cadmium (0.1 percent, 

MCL), copper (0.06 percent, USEPA Action 

Level), molybdenum (0.6 percent, HBSL), nickel 

(0.06 percent, HBSL), selenium (0.2 percent, 

MCL), and radium-226 plus radium-228 

(0.8 percent, MCL). The seven contaminants in 

table 2 that are derived primarily from natural 

sources—radon, strontium, arsenic, manganese, 

uranium, boron, and fluoride—are discussed in 

detail below. Nitrate, which has important manmade sources, is discussed on pages 25–27.

Radon

Radon is a naturally occurring, radioactive gas, and is part of the decay series of uranium, which occurs in trace amounts in aquifer 

sediment and rocks. Radon dissolved in water 

was detected in samples from nearly all wells. 

USEPA classifies radon as a human carcinogen 

(U.S. Environmental Protection Agency, 2006a), 

and people who are exposed to radon, mostly 

in the air but also through drinking water, may 

have an increased risk of getting cancer over 

their lifetime, especially lung cancer (National 

Research Council, 1999). Health effects from 

radon in drinking water are primarily through 

inhalation, after the gas is released from solution—for example, in the shower. 

Federal regulation of radon was proposed 

by USEPA in 1999 with an MCL of 4,000 pCi/L 

that would apply to public water systems or states 

that have programs to reduce radon risks from 

Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Radon in water from domestic wells may present a 

health risk after it is released from solution during use, 

for example, in the shower. The red dots represent radon, 

and the red arrows represent ground-water and radon 

flow directions.

Modified from Otten and others (1993).
Major Findings 19

Figure 4. Concentrations of radon greater than the lower U.S. Environmental Protection Agency proposed 

Maximum Contaminant Level (MCL) of 300 picocuries per liter (pCi/L) were found in 65 percent of wells, which 

are located throughout the United States and in all 30 principal aquifers sampled. Concentrations greater 

than the proposed MCL of 4,000 pCi/L were found in 4.4 percent of wells, and generally were associated 

with crystalline-rock aquifers located in the Northeast, the central and southern Appalachians, and 

central Colorado.

all sources, and with a lower MCL of 300 pCi/L 

that would apply to systems or states that do not 

have such programs in place (U.S. Environmental Protection Agency, 1999). Both proposed 

MCLs are used as human-health benchmarks in 

the present study. Several states also have their 

own standards or guidelines for radon in drinking water (Ayotte and others, 2007), including 

Connecticut (5,000 pCi/L), Maine (4,000 pCi/L), 

Massachusetts (10,000 pCi/L), and New Hampshire (2,000 pCi/L).

Concentrations of radon were greater than 

the lower of the proposed MCLs (300 pCi/L) in 

65 percent of the domestic wells sampled in this 

study, and concentrations were greater than the 

higher of the proposed MCLs (4,000 pCi/L) in 

4.4 percent of wells (table 2). The occurrence 

of radon in ground water is controlled, in large 

part, by the presence of uranium-bearing rocks, 

and, although it was widely distributed, radon 

was found at higher concentrations in some 

areas of the United States than others (fig. 4). 

Concentrations were highest in crystalline-rock 

aquifers located in the Northeast, in the central 

and southern Appalachians, and in central 

Colorado; in these aquifers, about 30 percent 

of the sampled wells had concentrations greater 

than 4,000 pCi/L. High radon concentrations 

in ground water and their relation to regional 

aquifer lithology in these areas are well 

documented (Brutsaert and others, 1981; Hess 

and others, 1985; Loomis, 1987; Longtin, 1988; 

Lawrence and others, 1991; Senior, 1998; Ayotte 

and others, 2007).

Radon, in picocuries per liter

> 4,000

EXPLANATION

> 300 and ≤ 4,000 ≤ 300
20 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Table 2. Eight inorganic contaminants were found at concentrations greater than human-health benchmarks in 1 percent or more of domestic

[Human-health benchmarks: MCLs are U.S. Environmental Protection Agency Maximum Contaminant Levels for public water supplies and HBSLs are U.S. Geological 

Sources of contaminant in drinking water: modified from U.S. Environmental Protection Agency (2008b) and from references cited in the text. pCi/L, picocurie per 

Contaminant Contaminant 

type

Number of 

wells sampled

Frequency of 

concentrations 

greater than 

benchmark 

(percent of wells)

Human-health benchmark

Potential health effects from exposure above 

the human-health benchmark 

Value Type

Radon Radionuclide 1,958 4.4 

65

4,000 pCi/L

300 pCi/L

Proposed 

MCLs

Increased risk of lung cancer 1,2 

Strontium Trace element 488 7.3 4,000 μg/L HBSL Abnormal bone development 3 

Arsenic Trace element 1,774 6.8 10 μg/L MCL Skin damage or problems with circulatory 

systems, and may have increased risk of 

getting cancer 4,5

Manganese Trace element 2,159 5.2 300 μg/L HBSL Neurological effects 6 

Nitrate Nutrient 2,132 4.4 10 mg/L 

as N

MCL Infants below the age of six months who 

drink water containing nitrate in excess 

of the MCL could become seriously ill 

and, if untreated, may die. Symptoms 

include shortness of breath and bluebaby syndrome 4,5

Uranium Trace element 1,725 1.7 30 μg/L MCL Increased risk of cancer, kidney toxicity 4,5 

Boron Trace element 535 1.3 1,000 μg/L HBSL Gastrointestinal, reproductive, and 

developmental effects 7, 8

Fluoride Major ion 2,157 1.2 4 mg/L MCL Bone disease (pain and tenderness of the 

bones); children may get mottled teeth 4,5

1

 National Research Council, 1999, Risk Assessment of Radon in Drinking Water: Washington, D.C., National Academy Press, 296 p.

2

 U.S. Environmental Protection Agency, 1999, Proposed radon in drinking water rule: Washington, D.C., U.S. Environmental Protection Agency, Office of Water, 

EPA 815- F-99-006, 6 p., accessed October 26, 2005, at http://www.epa.gov/safewater/radon/proposal.html.

3

 Agency for Toxic Substances and Disease Registry, 2004a. Toxicological profile for strontium: U.S. Department of Health and Human Services, 

Public Health Service, accessed November 12, 2008, at http://www.atsdr.cdc.gov/toxprofiles/tp159.html.

4

 U.S. Environmental Protection Agency, 2008b, Drinking water contaminants, accessed November 25, 2008, at http://www.epa.gov/safewater/contaminants/index.html.

5

 U.S. Environmental Protection Agency, 2003b, Water on tap—What you need to know: Office of Water, EPA 816-K-03-007, accessed November 25, 2008, at 

http://www.epa.gov/safewater/wot/pdfs/book_waterontap_full.pdf.
Major Findings 21

 wells in this study. Except for nitrate, these contaminants are derived primarily from natural sources.

 Survey Health-Based Screening Levels. Potential health effects: see references cited in footnotes.

 liter; μg/L, microgram per liter; mg/L, milligram per liter; 1 mg/L is equal to 1,000 μg/L]

Principal aquifers with concentrations most frequently Sources of contaminant in drinking water greater than human-health benchmarks

Radioactive decay of uranium in aquifer materials Crystalline rock aquifers in the Northeast, central and southern 

Appalachians, and central Colorado (about 30 percent of wells 

> 4,000 pCi/L).

Aquifer materials Some basin-fill and sandstone/carbonate-rock aquifers in the 

southwest and south-central United States.

Aquifer materials; pesticide application in orchards; some industrial wastes Crystalline-rock aquifers in New England; basin-fill aquifers in the 

western and south-central United States, and basaltic-rock aquifer 

in Idaho (more than 10 percent of wells > 10 μg/L).

Aquifer materials; some industrial wastes Glacial aquifers; some sandstone aquifers in central Appalachian 

region; some coastal plain aquifers in Southeast (about 8 to 

60 percent of wells > 300 μg/L). In all aquifers, in wells with low 

dissolved oxygen concentrations.

Fertilizer use; manure; sewage and septic-system effluent; aquifer materials Some basin-fill aquifers in the Southwest and Cailfornia; 

glacial aquifers in the Upper Midwest; some coastalplain and crystalline-rock aquifers in the central 

Appalachian region (more than 10 percent of wells 

> 10 mg/L as N). Generally, in areas of agricultural land use.

Aquifer materials Basin-fill aquifers in the West; crystalline-rock aquifers in the Rocky 

Mountains and in the Northeast (about 3 to 26 percent of wells 

> 30 μg/L).

Aquifer materials; sewage or septic-system effluent; fertilizer application; Some basin-fill and sandstone aquifers in the west and 

some industrial wastes south-central United States.

Water additive which promotes strong teeth; aquifer materials; discharge Some basin-fill and sandstone aquifers in the western and southfrom fertilizer and aluminum factories central United States (about 4 percent of wells > 4 mg/L).

6

 Agency for Toxic Substances and Disease Registry, 2008, Public health statement, manganese: U.S. Department of Health and Human Services, Public Health 

Service, September 2008, accessed November 12, 2008, at http://www.atsdr.cdc.gov/toxprofiles/tp151-c1-b.pdf.

7

 World Health Organization, 2004, Guidelines for Drinking-Water Quality (3d ed.), Volume 1, Recommendations: World Health Organization, Geneva, 

Switzerland, 515 p.

8

 U.S. Environmental Protection Agency, 2008a, Regulatory determinations support document for selected contaminants from the second drinking water 

contaminant candidate list (CCL 2), chap. 3: boron: Washington, D.C., U.S. Environmental Protection Agency, Office of Ground Water and Drinking Water, EPA 

Report 815-R-08-012, 4 p., accessed September 17, 2008, at http://www.epa.gov/safewater/ccl/pdfs/reg_determine2/report_ccl2-reg2_supportdocument_full.pdf.
22 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Arsenic

Arsenic is a nonmetallic trace element 

that has multiple geologic sources, including 

sulfide minerals and geothermal deposits; it 

also was used in some pesticides. Arsenic is 

toxic to humans and, in drinking water, can 

contribute to skin, bladder, and lung cancers 

(National Research Council, 2001). Arsenic was 

found at concentrations greater than the USEPA 

MCL of 10 micrograms per liter (μg/L) in 

6.8 percent of wells nationwide (table 2). Arsenic 

concentrations were greater than the MCL in 

more than 10 percent of wells in several aquifer 

types, including crystalline-rock aquifers in 

New England; basin-fill aquifers in California, 

Nevada, Arizona, New Mexico, Texas, and 

Nebraska; and the Snake River basaltic-rock 

aquifers in Idaho (fig. 5). These findings 

are consistent with previous national-scale 

studies of arsenic in ground water that showed 

elevated concentrations in the West, Midwest, 

and Northeast and the lowest concentrations 

in the Southeast (Welch and others, 2000). 

The percentages of domestic wells and public 

supply wells with arsenic concentrations greater 

than the MCL appear to be similar, based 

on findings of this study and the estimate of 

7.6 percent for public supply systems in the 

United States (Focazio and others, 2000). 

Figure 5. Concentrations of arsenic were greater than the U.S. Environmental Protection Agency Maximum 

Contaminant Level of 10 micrograms per liter in 6.8 percent of wells nationally and were found at these 

concentrations in wells in several aquifer types.

Arsenic, in micrograms per liter

EXPLANATION

> 10 ≥ 1 and ≤10 < 1
23

Geochemical conditions and the distribution 

of arsenic in source rocks have important roles 

in controlling arsenic in ground water, through 

processes that differ among aquifers. For 

example, processes that result in elevated arsenic 

in ground water include arsenic release from 

iron oxides, which is favored by low dissolved 

oxygen concentrations and high pH, and is 

common in glacial aquifers and some crystallinerock aquifers (Welch and others, 2000; Ayotte 

and others, 2003; Erickson and Barnes, 2005; 

Thomas, 2007). In addition, oxidation of sulfide 

minerals in some sandstone aquifers and mined 

areas, evaporative concentration in some basinfill aquifers in the Southwest, and the effects 

of geothermal water locally in the West also 

may result in elevated arsenic concentrations 

in ground water (Welch and others, 2000; 

Ryker, 2001; Schreiber and others, 2003).

Uranium

Uranium is a metallic trace element that is 

radioactive. Health effects of uranium in drinking 

water include possible harm to kidneys (World 

Health Organization, 2004). Concentrations of 

uranium were greater than the USEPA MCL 

of 30 μg/L in 1.7 percent of domestic wells 

(table 2). Concentrations varied regionally, 

with the highest concentrations occurring most 

frequently in basin-fill aquifers in the West and in 

crystalline-rock aquifers in the Rocky Mountains 

and the Northeast. Concentrations were generally 

less than 2 μg/L in the coastal plain and 

carbonate-rock aquifers in the Southeast and in 

the glacial aquifers in the east-central parts of the 

United States.

Uranium is a common trace element in 

many rock types, but is particularly enriched 

in certain types of rocks, such as granites 

(Hess and others, 1985). Like arsenic, its presence in ground water is affected by geochemical 

conditions, including dissolved oxygen concentrations. The combination of elevated sourcerock concentrations and the conditions that 

favor uranium solubility—dissolved oxygen 

and carbonate alkalinity—have led to relatively 

high concentrations of uranium in ground water 

in various hydrogeologic settings, including 

the Central Valley of California, the Newark 

Basin sediments of northern New Jersey, and 

the crystalline-rock aquifers of New England 

(Szabo and Zapecza, 1991; Ayotte and others, 

2007; Jurgens and others, 2008). 

Manganese

Manganese is one of the more abundant metallic elements and is a 

minor constituent of many common minerals (Hem, 1985). Manganese 

is an essential trace element for human health but may have adverse 

neurological effects at high doses, although food is the primary source 

of exposure (Agency for Toxic Substances and Disease Registry, 2008). 

Manganese concentrations were greater than the HBSL of 300 μg/L 

in about 5 percent of wells. Manganese was present in samples from 

nearly every principal aquifer but occurred at elevated concentrations 

most often in the glacial aquifers, in sandstone aquifers in Pennsylvania 

and West Virginia, and in some coastal-plain aquifers in Louisiana and 

Texas. Relatively high concentrations of manganese coincided with 

low dissolved oxygen concentrations and low pH, because manganesecontaining minerals are more soluble in water under these geochemical 

conditions. The distribution of relatively high manganese concentrations 

among principal aquifers also was documented by McMahon and 

Chapelle (2007), who showed through an analysis of geochemical 

conditions in domestic wells sampled by NAWQA that anoxic conditions 

occurred most frequently in ground water in the glacial and sandstone 

aquifers as compared to other aquifer types.

Pyrolusite

Arsenopyrite Granite

, 

m 

Fluorite

Borax

Photographs by Andrew Young, Brigham Young University and USGS, from USGS Photographic Library.

Major Findings 

Major ions, trace elements, and radionuclides in ground water originate 

primarily from the dissolution and 

weathering of the rocks and minerals that make up aquifers and soils. 

Example sources include arsenopyrite

a sulfide mineral containing arsenic; 

granite, an igneous rock sometimes 

enriched in uranium; fluorite, a common fluoride mineral; pyrolusite, a 

manganese oxide; and borax, a sodiu

borate compound.
24 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Fluoride

Fluoride is a naturally occurring ion that 

is beneficial to people in small amounts, but 

potentially harmful if concentrations are too high. 

According to the CDC, small amounts of fluoride 

are beneficial to prevent tooth decay, and a concentration range of 0.7 to 1.2 milligrams per liter 

(mg/L) in drinking water is recommended (U.S. 

Centers for Disease Control and Prevention, 

Fluoride Recommendations Work Group, 2001). 

Fluoride is often added to public drinking-water 

systems to raise the concentration to within that 

range. Most (85 percent or more) of the wells 

sampled in this study had fluoride concentrations 

lower than CDC’s recommended range. 

USEPA has established two drinking-water 

benchmarks for fluoride: an MCL of 4 mg/L to 

prevent acute and chronic illness, such as bone 

damage, and a Secondary Maximum Contaminant Level (SMCL) of 2 mg/L to prevent tooth 

staining (U.S. Environmental Protection Agency, 

2006a). About 1 and 4 percent of the fluoride 

concentrations in the samples analyzed were 

greater than the MCL and SMCL, respectively.

Strontium

Strontium is a trace element that is geochemically similar to calcium, a common 

constituent of ground water. Health effects of 

elevated doses of strontium include abnormal 

bone development (Agency for Toxic Substances 

and Disease Registry, 2004a). Concentrations 

of strontium were greater than the HBSL of 

4,000 μg/L in 7.3 percent of the 488 wells in 

which it was measured. Relatively high concentrations of strontium were most common in the 

southwestern and south-central United States—in 

the Basin and Range basin-fill aquifer in Arizona, 

the southern High Plains basin-fill aquifer, and 

the Edwards-Trinity sandstone/carbonate aquifer 

in Texas. Ground water in these aquifers were 

characterized by relatively high concentrations of 

dissolved solids. 

Boron

Boron is an accessory element in 

several common minerals and a component 

of evaporate deposits; it is used in industry 

and in some cleaning agents. Health effects 

of elevated doses include gastrointestinal, 

reproductive, and developmental effects 

(World Health Organization, 2004; 

U.S. Environmental Protection Agency, 2008a). 

Concentrations of boron were greater than 

the HBSL of 1,000 μg/L in 1.3 percent of the 

535 wells in which it was measured. Relatively 

high concentrations of boron occurred most 

often in basin-fill aquifers in California, the 

southern High Plains basin-fill aquifer, and the 

Lower Tertiary/Upper Cretaceous sandstone 

aquifers underlying the Yellowstone River Basin 

in Wyoming, where the high concentrations 

may be associated with evaporate deposits or 

saline ground water (Bartos and others, 2005; 

Bruce and others, 2007).
Major Findings 25

Nitrate is the only contaminant derived primarily from man-made sources that 

was found at concentrations greater than a human-health benchmark in more 

than 1 percent of wells

Nitrate was found at concentrations greater than the USEPA MCL in 4.4 percent of all wells, and 

most frequently in wells in agricultural areas. Concentrations greater than the MCL are primarily the 

result of man-made sources.

Nitrate is the most common nutrient in septic systems, in many parts of the United States 

ground water and was the only nutrient that was (Nolan and Hitt, 2003). 

found at concentrations with potential human- Concentrations of nitrate were greater than 

health effects. Excessive nitrate can result in the USEPA MCL of 10 mg/L as N in 4.4 percent 

restriction of oxygen transport in the blood- of wells (fig. 6). Nitrate concentrations followed 

stream, which is of particular concern in infants regional patterns consistent with those found in 

who lack the enzyme needed to correct this other NAWQA studies on nitrate occurrence in 

condition (resulting in methemoglobinemia, or ambient ground water (Nolan and Hitt, 2006; 

“blue baby syndrome”). Nitrate occurs naturally, McMahon and Chapelle, 2007). Concentrations 

but concentrations greater than 1 mg/L as N were greater than the MCL most frequently 

usually are indicative of human activities, such in several aquifers, including the Basin and 

as fertilizer application, animal production, and Range and Central Valley basin-fill aquifers 

Figure 6. Concentrations of nitrate were greater than the U.S. Environmental Protection Agency Maximum 

Contaminant Level of 10 milligrams per liter (mg/L) as N in 4.4 percent of the wells. Elevated concentrations of 

nitrate occurred throughout the United States, but least commonly in the Southeast. Concentrations greater than 

1 mg/L as N, which is usually indicative of human activities in many areas (Nolan and Hitt, 2003), were found in 

about 40 percent of wells located throughout the sampled areas.

Nitrate, in milligrams per liter as N

EXPLANATION

> 10 > 1 and ≤10 ≤ 1
26 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Median

25th percentile

75th percentile

90th percentile

10th percentile

Number of samples

EXPLANATION

52

Undeveloped Mixed Agricultural Urban

445

651 193

843

10

0

8

6

4

2

NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER AS N

DOMINANT LAND USE AROUND WELL

Figure 7. Concentrations of nitrate varied with land use surrounding the sampled wells. 

Concentrations were highest in wells surrounded by agricultural land use and lowest in wells 

in predominantly undeveloped areas. Land-use categories are described in Gilliom and others 

(2006), Price and others (2007), and DeSimone (2009).

in the Southwest and in California, the westcentral glacial aquifers in the Upper Midwest, 

the North Atlantic coastal plain aquifers in the 

central Appalachian region, and the Piedmont 

crystalline-rock aquifers, also in the central 

Appalachian region. Concentrations were lowest 

in coastal plain aquifers in the Southeast; the low 

concentrations may result from aquifer and soil 

conditions that promote denitrification of nitrate 

to harmless nitrogen gas.

Nitrate concentrations varied by land use 

and were higher in ground water near agricultural 

land than in other areas (fig. 7; wells are 

classified according to the dominant land use 

within 500 m of the well; Gilliom and others, 

2006; Price and others, 2007; DeSimone, 2009). 

Concentrations were greater than 10 mg/L as N in 

7.1 percent of domestic wells in areas dominated 

by agriculture, compared to 3.1 percent in areas 

of urban land use, 3.7 percent in areas of mixed 

land use, and less than 1 percent in undeveloped 

areas. A separate analysis of an additional 

436 domestic wells, targeted in shallow ground 

water beneath relatively intense agricultural land 

use, showed that concentrations of nitrate were 

greater than 10 mg/L as N in nearly 25 percent 

of the sampled wells (these 436 wells were not 

included in other data analyses). These findings 

are consistent with the many previous studies that 

have found relatively high nitrate concentrations 

in some agricultural areas of the Nation 

(for example, Kolpin and others, 1994; Mehnert 

and others, 1995; Townsend and others, 1998; see 

additional references in DeSimone, 2009). Other 

factors found to be associated with high nitrate 

concentrations are the presence of well-drained 

soils, fractured or porous-rock aquifers, and 

oxygenated ground water (Nolan and Hitt, 2006; 

McMahon and Chapelle, 2007).
Major Findings 27

Photograph by Tim McCabe, Natural Resources Conservation Service, 

U.S. Department of Agriculture.

Photograph by Jeff Vanuga, Natural Resources Conservation Service, 

U.S. Department of Agriculture.

Drain

field Septic tank

Pumping

level

Water table

Percolation

Well Evaporation

Septic tank

Pumping

level

Water table

Percolation

Drain

field

Modified from Waller (1991).

Although nitrate can occur naturally in ground 

water, high concentrations result primarily 

from human activities. Sources of nitrate to 

ground water include fertilizers, livestock, 

and septic systems.
28 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Man-made organic compounds were detected in more than half (60 percent) 

of the sampled wells, but concentrations were seldom greater than humanhealth benchmarks (less than 1 percent of wells)

Only seven man-made organic compounds were found at concentrations greater than a humanhealth benchmark in one or more wells, and none in more than 1 percent of wells. However, one or 

more compounds—including herbicides, insecticides, solvents, disinfection by-products, gasoline 

hydrocarbons and oxygenates, refrigerants, and fumigants—were detected at low levels in 60 percent 

of wells, indicating the diverse industrial, agricultural, and urban sources that can affect the quality of 

source water to domestic wells. 

About 1 billion pounds of pesticides are 

used each year in the United States to control weeds, insects, and other pests, primarily 

in agricultural and urban areas (Gilliom and 

others, 2006). VOCs are produced in large volumes and are associated with a myriad of products, such as plastics, adhesives, paints, gasoline, 

fumigants, refrigerants, and dry-cleaning fluids 

(Zogorski and others, 2006). The concentrations 

of 168 organic compounds, including 83 pesticides and 85 VOCs, were measured in samples 

collected from the domestic wells in this study. 

Organic compounds seldom were found 

at concentrations greater than human-health 

benchmarks, which were available for 80 percent 

of the compounds measured. Thus, the potential 

for adverse effects on human health from these 

compounds is expected to be low (see sidebars 

on bottom of page 13 and on page 16). Collectively, only 0.8 percent of wells contained 

any organic compound at concentrations greater 

than benchmarks (0.3 percent for compounds 

with MCLs and 0.5 percent for compounds with 

HBSLs). Only 3 percent of wells contained any 

organic compound at concentrations greater 

than one-tenth of a benchmark (2.3 percent for 

MCLs and 0.7 percent for HBSLs). Seven of the 

168 organic compounds analyzed occurred at 

concentrations greater than an MCL or HBSL, 

each in less than 1 percent of wells, and these 

included two insecticides, one herbicide, two solvents, and two fumigants (table 3). Ten additional 

compounds were found at concentrations greater 

than one-tenth of benchmarks; each of these also 

was found in less than 1 percent of the wells at 

these concentrations.

A wide variety of organic compounds 

was detected at low concentrations. Twentythree pesticides and VOCs were each detected 

at concentrations greater than 0.02 μg/L in 

1 to 18 percent of wells (fig. 8). The most 

frequently detected compounds are derived 

from diverse sources and include six solvents, 

Photograph by Jeff Vanuga, Natural Resources Conservation Service, 

U.S. Department of Agriculture.

Sources of man-made organic compounds, such as pesticides and volatile 

organic compounds (VOCs), are various and widespread. Over a billion pounds 

of pesticides are applied each year in the United States to control weeds, 

insects, and other pests, particularly in agricultural and urban areas. VOCs are 

used in numerous industrial, commercial, and domestic activities, and sources 

to ground water include landfills, leaking storage tanks and spills, septic 

systems, leaking water and sewer lines, stormwater runoff, vehicular exhaust, 

and the atmosphere. 

Photograph from EGDigital, iStock Photos
Major Findings 29

five herbicides plus an herbicide degradate, 

three trihalomethanes, two refrigerants, two 

VOCs used in organic synthesis, two gasoline 

hydrocarbons, one gasoline oxygenate, and one 

fumigant (fig. 8). If all detections are considered, 

including those as low as 0.001 μg/L, at least one 

pesticide or VOC was detected in 60 percent of 

the wells. 

The frequency of detection of organic 

compounds in the domestic wells included in 

the present study is consistent with previous 

studies of low-level concentrations of organic 

compounds in ground water (Shapiro and 

others, 2004; Plummer and others, 2008) and of 

detection frequencies of pesticides and VOCs 

from all well types sampled by the NAWQA 

Program (Gilliom and others, 2006; Zogorski 

and others, 2006). Detection frequencies for 

organic compounds may differ among supply 

well types, however, with less frequent detections 

in domestic wells than in public supply wells 

(Zogorski and others, 2006; P.E. Stackelberg, 

U.S. Geological Survey, written commun., 

2008). The widespread occurrence of organic 

compounds suggests the pervasiveness of manmade sources and the potential for exposure to 

low-level mixtures of contaminants.

Table 3. Seven organic compounds were found at least once at concentrations greater than available human-health benchmarks but 

each in less than 0.5 percent of wells. Ten additional compounds were found at concentrations greater than one-tenth of benchmarks. 

No compound was present at concentrations greater than one-tenth of benchmarks in more than 1 percent of wells.

[Human-health benchmarks: MCLs are U.S. Environmental Protection Agency Maximum Contaminant Levels for public water supplies and HBSLs are 

U.S. Geological Survey Health-Based Screening Levels. μg/L, microgram per liter]

Compound name Use Number of 

wells sampled

Frequency of 

concentrations 

greater than 

benchmark 

(percent of wells)

Frequency of 

concentrations less 

than, but greater 

than one-tenth 

of, benchmark 

(percent of wells)

Human-health benchmark

Value 

(μg/L) Type

Dieldrin Insecticide 2,115 0.43 0.00 0.002 HBSL

Ethylene dibromide Fumigant 1,955 0.10 0.00 0.05 MCL

Trichloroethene Solvent 1,956 0.10 0.20 5 MCL

Dinoseb Herbicide 1,371 0.07 0.00 7 MCL

Diazinon Insecticide 2,115 0.05 0.09 1 HBSL

Dibromochloropropane 

(DBCP)

Fumigant 1,956 0.05 0.00 0.2 MCL

Perchloroethene Solvent 1,927 0.05 0.47 5 MCL

Atrazine Herbicide 2,116 0.00 0.85 3 MCL

Methylene chloride Solvent 1,956 0.00 0.26 5 MCL

Diuron Herbicide 1,374 0.00 0.15 2 HBSL

Chloroform Trihalomethane 1,956 0.00 0.15 801 MCL

1,2-Dichloroethane Solvent 1,945 0.00 0.10 5 MCL

1,2-Dichloropropane Fumigant 1,956 0.00 0.10 5 MCL

Alachlor Herbicide 2,115 0.00 0.09 2 MCL

Benzene Gasoline 

hydrocarbon

1,957 0.00 0.05 5 MCL

1,1-Dichloroethene Solvent 1,956 0.00 0.05 7 MCL

Vinyl chloride Organic synthesis 1,957 0.00 0.05 2 MCL

1

MCL is for sum of four trihalomethanes.
30 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Two of the most frequently detected organic 

compounds were chloroform and atrazine. 

Chloroform, a by-product from the disinfection 

of water, may originate in ground water from 

infiltration of treated wastewater or drinking 

water, from shock chlorination of domestic 

wells, from chemicals disposed of in septic 

systems, or from chemical spills (Ivahnenko 

and Zogorski, 2006). Chloroform was detected 

throughout the United States and in most 

principal aquifers sampled. Concentrations of 

chloroform were low and less than one-hundredth 

of the USEPA MCL, or undetected, in 90 percent 

of wells. Atrazine and its degradation product 

deethylatrazine were the most frequently 

detected pesticide compounds (fig. 8). Atrazine 

is a relatively persistent and mobile herbicide 

used primarily on corn, and hence, was most 

commonly detected in agricultural areas. Like 

chloroform, atrazine concentrations were less 

than one-hundredth of the USEPA MCL in 

most wells.

Figure 8. Six pesticides (brown text) and 17 volatile organic compounds (blue text) were 

detected in more than 1 percent of wells at concentrations greater than a common threshold of 

0.02 micrograms per liter (μg/L). The wide variety of compounds detected, including herbicides, 

solvents, disinfection by-products, gasoline hydrocarbons and oxygenates, refrigerants, and 

fumigants, reflect the diverse industrial, agricultural, and domestic sources that can affect the 

quality of source water to domestic wells. Concentrations greater than 0.02 μg/L are shown 

here to eliminate the effects of different analytical detection limits among compounds.

DETECTION FREQUENCY AT CONCENTRATIONS

GREATER THAN 0.02 MICROGRAMS PER LITER, IN PERCENT

0 2 4 181614121086 20

Chloroform

Carbon disulfide

Deethylatrazine

Toluene

Chloromethane

Atrazine

Perchloroethene

Methyl tert-butyl ether

Methylene chloride

1,1,1-Trichloroethane

1,2,4-Trimethylbenzene

Dichlorodifluoromethane

Trichloroethene

Bromodichloromethane

1,1-Dichloroethane

1,2-Dichloropropane

Trichlorofluoromethane

Bentazon

Simazine

Iodomethane

Metolachlor

Prometon

Dibromochloromethane
Major Findings 31

Microbial contaminants were detected in as many as one-third of the 

approximately 400 wells sampled

Escherichia coli (E. coli), a coliform bacteria that typically is not harmful but is an indicator of 

fecal contamination, was detected in 7.9 percent of about 400 sampled wells. Total coliform bacteria, a 

broad group that includes bacteria from soil, water, and animal feces, were detected in 34 percent of 

the sampled wells.

Bacteria of various kinds are ubiquitous in 

ground water, but some types indicate possible 

contamination by human or animal fecal matter. 

Two fecal indicator bacteria were measured 

in samples from about 400 wells, located in 

about half of the principal aquifers included in 

this study. Total coliform bacteria and E. coli

are bacteria that live in the intestines of warmblooded animals and are present in feces. 

Coliform bacteria are a broad group that also can 

live in water and soil and may be present in the 

environment from sources other than fecal or 

sewage contamination. These bacteria typically 

are not harmful but are used to signal the possible 

presence of pathogenic bacteria and (or) viruses 

that also can occur in feces and can cause illness. 

Federal drinking-water standards and those 

of many states use total coliform bacteria and 

E. coli bacteria as indicators of microbiological 

water quality.

Total coliform bacteria were detected in 

34 percent of 397 domestic wells, and E. coli

were detected in 7.9 percent of 378 domestic 

wells. Detections occurred in samples from at 

least one well from each of 15 principal aquifers 

for which bacteria were measured. Detection 

frequencies varied among principal aquifers but 

were greatest in several carbonate-rock, crystalline-rock, and sandstone aquifers in the central 

Appalachian region. These aquifers generally 

are porous and fractured and can be associated 

with relatively rapid infiltration (Embrey and 

Runkle, 2006). Wells in which total coliform and 

E. coli were detected also tended to have open 

intervals that were closer to land surface than 

wells in which these bacteria were not detected. 

Wells in which total coliform bacteria were 

detected were typically older and had higher 

percentages of agricultural land surrounding 

them than wells without total coliform detections. In the present study, available information 

about other well characteristics was limited, 

but several previous studies have shown that 

well type and construction characteristics are 

important factors for detections of total coliform 

bacteria (U.S. Environmental Protection Agency, 

1984; Hallberg and others, 1992; U.S. Centers for 

Disease Control and Prevention, 1998). 

Fecal indicator bacteria, such as Escherichia coli, which were detected in 7.9 percent of 

the domestic wells sampled in this study, are commonly measured by membrane-filtration 

methods. A water sample is passed through a membrane filter, and the bacteria on the 

filter are cultured and counted, such as on the culture plates shown above.
32 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

About half (48 percent) of the sampled wells contained at least one 

contaminant at a level or concentration outside the range of values 

recommended by USEPA for the aesthetic quality of water 

Dissolved solids, pH, iron, and manganese were found at levels outside of Secondary Maximum 

Contaminant Levels (SMCLs) recommended by USEPA for drinking water in 15 to 21 percent of wells. 

Collectively, 48 percent of the sampled wells contained at least one contaminant or property at levels 

outside of SMCLs. Regional patterns were apparent in dissolved solids, pH, and hardness. 

A number of contaminants or properties can adversely affect the aesthetic quality 

of drinking water or can result in effects that 

are undesirable but are not health problems. 

The USEPA has established SMCLs as nonenforceable guidelines for these contaminants 

and properties in drinking water (U.S. Environmental Protection Agency, 1992, 2006a). 

Undesirable effects of elevated concentrations of 

these properties and contaminants include corrosion or scale deposition in pipes and plumping, 

laundry staining, unpleasant water color or taste, 

and teeth staining.

Dissolved Solids

Dissolved solids in water include major 

ions, such as calcium, sodium, bicarbonate, 

sulfate, and chloride; nutrients, such as nitrogen 

and phosphorus; and trace elements, such as 

selenium, chromium, and arsenic. Dissolved 

solids originate primarily from the dissolution 

of soil and aquifer materials but can be augmented by the effects of human activities, such 

as irrigation, waste disposal, and road salting. 

The USEPA SMCL recommends dissolved solids concentrations less than 500 mg/L to avoid 

unpleasant taste or color or greater costs because 

of mineral or sediment deposits in plumbing 

(U.S. Environmental Protection Agency, 1992).

Concentrations of dissolved solids were 

greater than the SMCL in 15 percent of the 

domestic wells. Relatively high concentrations (greater than the SMCL) occurred most 

frequently in the West, Southwest, and southcentral United States, such as in some basin-fill, 

sandstone/carbonate, and sandstone aquifers 

underlying Texas, New Mexico, Arizona, Utah, 

and Wyoming. Such high concentrations are 

expected in these parts of the country because of 

easily eroded and soluble rock types and sediments, a dry climate with low precipitation, and 

heavy irrigation seepage. Concentrations generally were less than the SMCL in the East, such as 

in the coastal plain and crystalline-rock aquifers, 

and in some of the glacial aquifers in the northern 

parts of the United States.

pH

pH is a property of water and is an important 

control on the solubility of many metals. Highly 

acidic (low pH) or alkaline (high pH) water can 

corrode pipes and may have an unpleasant taste 

(World Health Organization, 2004). The USEPA 

recommends a pH between 6.5 and 8.5 for 

drinking water (U.S. Environmental Protection 

Agency, 2006a). The pH of water from most of 

the domestic wells sampled in this study was 

within the recommended range; pH in 80 percent 

of the samples ranged from 6.1 to 7.6, with a 

median of 7.3. Water from about 14 percent of 

the wells was more acidic than recommended, 

and water from about 2 percent of wells was 

more alkaline than recommended. Low pH 

values occurred in the East (fig. 9); this pattern is 

similar to the geographic pattern in the acidity of 

precipitation across the United States (National 

Atmospheric Deposition Program, 2005). The 

wells with low pH generally were completed in 

crystalline-rock and coastal plain sand aquifers, 

which do not weather easily and have relatively 

little capacity to neutralize acid precipitation. 

These aquifers also had low concentrations of 

dissolved solids.

Iron and Manganese

Iron and manganese were each detected 

in about half of the wells. USEPA SMCLs 

for iron and manganese were established 

to prevent poor water taste and the 

staining of laundry and plumbing fixtures 

(U.S. Environmental Protection Agency, 1992). 

Concentrations of iron were greater than the 

SMCL of 300 μg/L in 19 percent of wells, and 
Major Findings 33

Figure 9. The pH of water was outside of the range of 6.5 to 8.5 recommended by the U.S. Environmental 

Protection Agency Secondary Maximum Contaminant Level in 16 percent of wells. Low pH values occurred mostly 

in the East, consistent with the regional pattern of acid precipitation and the distribution of aquifers that are 

relatively resistant to weathering, with little buffering capacity.

pH, in standard units 

EXPLANATION

< 6.5 ≥ 6.5 and ≤ 8.5 > 8.5
34 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

concentrations of manganese were greater than 

the SMCL of 50 μg/L in 21 percent of wells. The 

elevated concentrations occurred throughout the 

United States; such concentrations were found in 

samples from nearly all principal aquifers. Iron 

and manganese are common elements in most 

aquifer materials but are mobilized under certain 

geochemical conditions, such as low dissolved 

oxygen concentrations and low pH.

Hardness

Hardness is a physical property that is used 

to describe the mineral content of water, and 

primarily reflects concentrations of dissolved 

calcium and magnesium. Water is considered 

“soft” if hardness is less than 60 mg/L as calcium 

carbonate (CaCO3

), and “hard” if hardness is 

greater than 120 mg/L as CaCO3

. Hard and very 

hard water (more than 180 mg/L as CaCO3

) 

often requires treatment (Hem, 1985). Hard 

water is not a health risk, but it is a nuisance 

because it can cause mineral buildup in plumbing 

fixtures, pipes, and hot water heaters, and poor 

performance of soaps and detergents. Water 

that is too soft also can be a nuisance, because 

ions react with the soap to produce a residue 

that is difficult to wash off. Hardness varied 

regionally and among most aquifers (fig. 10). 

Hard water (>120 mg/L as CaCO3

) was found in 

about 60 percent of the sampled wells and was 

prevalent in the east-central and western United 

States, reflecting the distribution of carbonate 

aquifers and aquifers with relatively high 

dissolved solids. Aquifers with hard water also 

contained relatively high alkalinity.

Hardness, in milligrams per liter as calcium carbonate

EXPLANATION

> 180 > 120 and ≤ 180 > 60 and ≤ 120 ≤ 60

Figure 10. Hard water was prevalent in the east-central and western United States, reflecting the distribution of 

carbonate aquifers and aquifers with relatively high concentrations of dissolved solids.
Major Findings 35

Contaminants usually co-occurred with other contaminants as mixtures

Mixtures of two or more contaminants with concentrations individually greater than their humanhealth benchmarks (excluding microbial contaminants) occurred in only 4 percent of wells, but 

mixtures of two or more contaminants at concentrations individually greater than one-tenth of their 

human-health benchmarks were found in 73 percent of the wells. Possible health effects of most of 

these mixtures have not been extensively studied and are largely unknown. Identification of the most 

common mixtures found in this study can be used to prioritize future investigations.

Contaminants generally do not occur in 

isolation in domestic or other supply wells 

but usually co-occur as mixtures with other 

contaminants. Contaminants co-occur as 

mixtures in part because well water itself is a 

mixture of ground water from multiple sources, 

and each source may contribute different and 

multiple contaminants. Sources of water may 

include recharge from precipitation or irrigation 

in different types of land-use settings, infiltration 

of surface water, or inflow from adjoining 

aquifers. Along the flow pathways by which 

ground water reaches a well, naturally occurring 

contaminants, such as trace elements, may be 

contributed by various minerals from the aquifer 

sediments and rocks. Man-made contaminants 

may be introduced from many sources, such as 

wastewater disposal, recharge in agricultural 

or urban areas, or chemical spills. Thus, the 

compositions of contaminant mixtures that occur 

in domestic wells depend on the unique sources 

and flow paths that affect each well.

Analysis of individual contaminants, as 

summarized in the previous sections, is essential 

for identifying those of potential concern for 

human health and for assessing where and 

why they occur. However, contaminants in 

mixtures—even at individual concentrations less 

than benchmarks for individual compounds—

may present additional potential concerns for 

human health. The health effects of a mixture 

might be greater than, equal to, or less than 

the effects of its components individually 

(Carpenter and others, 2002). With a few 

exceptions for pesticides with common modes 

of action, however, health benchmarks generally 

are not available for specific mixtures. The 

presence of mixtures in domestic wells also is 

important to homeowners investigating water 

treatment options because a single treatment 

device may not be available to remove all 

contaminants of potential concern in a mixture 

(U.S. Environmental Protection Agency, 2002). 

The analysis in this study identified 

mixtures of naturally occurring contaminants 

that individually exceeded or approached levels 

of potential concern for human health, with 

additional consideration of all man-made organic 

compounds detected at any level. A concentration 

greater than one-tenth of a human-health 

benchmark was used to provide an indication of 

contaminants that may approach concentrations 

of potential human-health concern when 

occurring with other contaminants in mixtures, 

and to identify those mixtures that may warrant 

additional monitoring and study (see sidebar on 

page 16). The higher proposed USEPA MCL 

was used for radon in mixtures of contaminants 

at concentrations greater than benchmarks, and 

the lower proposed MCL was used for radon 

in mixtures of contaminants at concentrations 

greater than one-tenth of benchmarks. Fluoride 

was not included in analyses of mixtures 

at concentrations greater than one-tenth of 

benchmarks (but below benchmarks) because, 

as discussed on page 24, such concentrations for 

fluoride are often in a beneficial range. These 

analyses were used to identify mixtures of 

potential importance and to provide a perspective 

on patterns in co-occurrence that may be 

important as more is learned about sources and 

potential health effects of mixtures.
36 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Occurrence and Distribution 

of Mixtures

Although samples from 23 percent of wells 

contained at least one contaminant individually 

greater than a human-health benchmark, mixtures 

of two or more contaminants with concentrations 

greater than their individual benchmarks were 

relatively uncommon, occurring in only 4 percent 

of wells (fig. 11). However, about 73 percent of 

wells had two or more contaminants greater than 

one-tenth of their individual benchmarks, and 

mixtures that additionally included detections 

of any organic contaminants (regardless of 

concentration) were found in 89 percent of wells. 

Most mixtures were primarily composed of 

naturally occurring inorganic contaminants. 

Mixtures of contaminants at concentrations 

greater than one-tenth of their individual 

benchmarks occurred in most sampled areas, but 

mixtures with the largest number of contaminants 

were most common in several aquifers in western 

and south-central United States, including the 

Basin and Range, Central Valley, and High Plains 

basin-fill aquifers and the Lower Tertiary/Upper 

Cretaceous sandstone aquifers (fig. 12). Mixtures 

of two or more contaminants at concentrations 

greater than individual benchmarks, although 

less frequent, were similarly distributed. This 

occurred, in large part, because the mixtures 

were composed mainly of naturally occurring 

trace elements and other inorganic contaminants 

frequently detected in those aquifers. These 

contaminants are likely associated with 

relatively old ground water, easily weathered 

aquifer materials, or evaporative concentration. 

Typically, these mixtures were found in water 

that also contained relatively high concentrations 

of dissolved solids. 

Figure 11. Mixtures of two or more contaminants with concentrations individually greater than a 

benchmark were not common, occurring in 4 percent of wells. Mixtures of two or more contaminants 

with concentrations greater than one-tenth of individual benchmarks were found in 73 percent of 

wells. Contaminants included were major ions (except fluoride), trace elements, nitrate, radon, and 

organic compounds.

Mixtures of contaminants

At concentrations greater than

 human-health benchmarks

EXPLANATION

At concentrations greater than one-tenth

 of human-health benchmarks

At concentrations greater than one-tenth

 of human-health benchmarks plus any

 detectable concentration of 

 an organic compound

≥1 ≥2 ≥3 ≥4 ≥5

100

0

80

60

40

20

≥6

NUMBER OF CONTAMINANTS

FREQUENCY OF OCCURRENCE,

IN PERCENT OF WELLS
Major Findings 37

EXPLANATION

Number of contaminants in a sampled well with concentrations

greater than one-tenth of human-health benchmarks

Greater than 6 4 to 6 2 to 3 1 None

Figure 12. Mixtures with the largest number of contaminants at concentrations greater than one-tenth of 

human-health benchmarks were most common in several aquifers in the western and south-central United 

States. Typically, these mixtures were found in water that also contained relatively high concentrations of 

dissolved solids. Contaminants included were major ions (except fluoride), trace elements, nitrate, radon, and 

organic compounds.
38 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Composition of Mixtures

The composition of mixtures is described 

in terms of the occurrence of unique mixtures, 

which are specific combinations of two, three, 

or more contaminants, regardless of the presence of other contaminants. There were many 

unique mixtures (nearly 5,000) of contaminants that occurred at concentrations greater 

than one-tenth of their individual benchmarks, 

but only 15 unique mixtures occurred in more 

than 10 percent of wells. The 15 mixtures were 

all composed of two or more of only six contaminants––all inorganic and, except for nitrate, 

primarily derived from natural sources (table 4). 

The six contaminants were nitrate, arsenic, radon, 

and uranium, and, to a lesser extent, molybdenum 

and manganese. The most common unique mixture consisted of nitrate and radon (where radon 

was greater than 300 pCi/L), which occurred 

in 32 percent of sampled wells. These contaminants also occurred in mixtures at concentrations 

greater than individual benchmarks, but none in 

more than 1 percent of wells.

Concentrations of most organic 

contaminants in mixtures were low compared to 

available benchmarks, but organic contaminants 

at low concentrations in certain mixtures may, 

nevertheless, be a concern. The appropriate level 

of concern is difficult to evaluate because of 

the lack of human-health benchmarks for some 

individual compounds and the potential for 

additive or synergistic effects for some mixtures. 

Eight unique mixtures of multiple contaminants 

with concentrations greater than one-tenth of 

human-health benchmarks, plus a detectable 

organic compound, occurred in 10 percent or 

more of wells (table 5). These mixtures were 

composed of combinations of two or three of the 

following: atrazine, deethylatrazine, chloroform, 

nitrate, or radon. Combinations of nitrate with 

atrazine and deethylatrazine, its degradation 

product, likely originate from similar sources of 

these contaminants (for example, agricultural 

activities); combinations of organic compounds 

with radon reflect the ubiquitous occurrence 

of radon at concentrations greater than the 

proposed MCL of 300 piC/L. Unique mixtures 

Table 4. Mixtures of contaminants at concentrations individually greater than one-tenth of human-health 

benchmarks that were found in 10 percent or more of wells and were composed of various combinations 

of six contaminants, most of which were derived primarily from natural sources.

[Contaminants included were major ions (except fluoride), trace elements, nitrate, radon, and organic compounds. 

--, not applicable]

Components of mixture

Frequency of mixtures 

with component 

concentrations 

greater than one-tenth 

of benchmarks 

 (percent of wells)

Frequency of 

mixtures with 

component 

concentrations 

greater than 

benchmarks 

(percent of wells)

Nitrate

Arsenic

Nitrate

Uranium

Arsenic

Nitrate

Nitrate

Nitrate

Arsenic

Nitrate

Arsenic

Nitrate

Radon

Manganese

Uranium

Radon

Radon

Arsenic

Radon

Uranium

Uranium

Arsenic

Arsenic

Uranium

Uranium

Molybdenum

Arsenic

Molybdenum

Radon

Molybdenum

--

--

--

--

--

--

Radon

Uranium

Radon

Radon

--

Uranium

--

--

--

--

--

--

--

--

--

--

--

--

--

--

Radon

--

--

--

32

30

24

21

21

19

18

16

16

15

13

12

12

11

11

0.1

0.1

0.2

0.5

0.3

0.6

0.0

0.1

0.0

0.0

0.2

0.0

0.0

0.1

0.1
Major Findings 39

Table 5. Eight mixtures of contaminants at concentrations individually greater than one-tenth of humanhealth benchmarks, plus organic compounds detected at any concentration, were found in 10 percent or 

more of wells.

[Contaminants included were major ions (except fluoride), trace elements, nitrate, radon, and organic compounds. 

--, not applicable]

Components of mixture

Frequency of mixtures with 

component concentrations greater 

than one-tenth of benchmarks, plus 

organic compounds detected 

at any concentration 

(percent of wells)

Radon Chloroform -- 15

Nitrate Deethylatrazine -- 14

Radon Deethylatrazine -- 13

Atrazine Deethylatrazine -- 12

Nitrate Atrazine -- 11

Nitrate Chloroform -- 11

Nitrate Radon Deethylatrazine 11

Nitrate Atrazine Deethylatrazine 10

containing only organic compounds were 

not common, except for the combination of 

atrazine and deethylatrazine. The common 

presence of the degradate deethylatrazine with 

its parent compound atrazine (in 84 percent 

of wells that contained atrazine) is consistent 

with previous findings on the prevalence of 

some pesticide degradates in ground water 

(Gilliom and others, 2006).

Assessing the Potential Significance 

of Mixtures to Human Health

Little is known about the potential health 

effects of most mixtures of contaminants. 

Depending on the specific compounds, the 

toxicity of a mixture may result from additive 

effects among the compounds, antagonistic 

effects (less than additive), or synergistic effects 

(greater than additive), or there may be no special 

effects that result from the mixture (independent 

effects). Additive or synergistic toxicity results in 

a mixture toxicity that is greater than that of any 

of its individual components. 

Several combinations of organic compounds 

in mixtures for which possible health effects have 

been evaluated were identified in the sampled 

wells—specifically: atrazine and deethylatrazine; atrazine or simazine with nitrate; and 

perchloroethene and three other solvents (Agency 

for Toxic Substances and Disease Registry, 

2004b, 2004c). Combined concentrations of the 

contaminants in these mixtures, however, were 

less than available benchmarks, or no benchmarks were available for the mixtures. Atrazine commonly co-occurred with its degradate 

deethylatrazine (84 percent of atrazine detections) and with simazine (17 percent of atrazine 

detections), but combined concentrations were all 

less than a USEPA screening value of 12.5 μg/L 

for combined triazine concentrations in community public water systems (U.S. Environmental 

Protection Agency, 2006b). Nitrate (at concentrations greater than 1 mg/L as N) co-occurred 

with atrazine, deethylatrazine, and (or) simazine 

in up to 14 percent of all sampled wells, and 

even more frequently in wells included in USGS 

targeted agricultural studies, but currently there 

are no numerical benchmarks for these mixtures (Agency for Toxic Substances and Disease 

Registry, 2006). Similarly, although there are 

currently no benchmarks for mixtures of perchlorethene with trichloroethene, 1,1-dichloroethane, 

or 1,1,1-trichloroethane, which are considered 

to have additive toxic effects (Agency for Toxic 

Substances and Disease Registry, 2004c), one 

or more of these three solvents was present with 

36 percent of perchlorethene detections. 
 40 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 40 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

 More Information for Homeowners

Many government agencies and other organizations provide information 

for homeowners on domestic wells and water-quality testing. Local health 

departments also can provide information on domestic wells in many 

cases. These agencies and organizations include: the U.S. Environmental 

Protection Agency; the U.S. Centers for Disease Control and Prevention; 

the U.S. Department of Agriculture’s National Farm*A*Syst/Home*A*Syst 

Program and Cooperative State Research, Education, and Extension 

Service (CREES) National Water Program; state environmental and public 

health agencies; and several non-governmental organizations. Links to 

these sources of information on the World Wide Web are provided at 

http://water.usgs.gov/nawqa.

NAWQA Reports and Data

are Readily Available

The U.S. Geological Survey (USGS) promotes public access to scientific 

information and strives to communicate and disseminate credible, timely, 

and relevant information about water resources. USGS Circulars from the 

National Water-Quality Assessment (NAWQA) Program, along with hundreds 

of other publications that describe individual study-unit and national-scale 

assessments, are available on the NAWQA Web site at http://water.usgs.

gov/nawqa. The Web site also includes maps and water-quality data. The 

large NAWQA database on national water-quality conditions at http://water.

usgs.gov/nawqa/data can be used for a wide range of analyses at national, 

regional, state, and local scales.
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Abbreviations, Acronyms, and Units of Measure

CaCO3 calcium carbonate

CDC U.S. Centers for Disease Control and Prevention

CFU/100mL colony-forming units per 100 milliliters

ft foot

mi mile

mi2 square mile

HBSL Health-Based Screening Level

MCL Maximum Contaminant Level

mg/L milligrams per liter

μg/L micrograms per liter

NAWQA National Water-Quality Assessment

NPS National Pesticide Study

NSA National Statistical Analysis of Rural Water Conditions

pCi/L picocuries per liter

SDWA Safe Drinking Water Act

SMCL Secondary Maximum Contaminant Level

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VOCs volatile organic compounds

48 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004
Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

Director

U.S. Geological Survey

Massachusetts-Rhode Island Water Science Center

10 Bearfoot Road

Northborough, MA 01532

dc_ma@usgs.gov

or visit our Web site at:

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DeSimone and others—Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004—Overview of Major Findings Circular 1332 —