U.S. Department of the Interior

U.S. Geological Survey

Circular 1346

 The Quality of Our Nation’s Waters

National Water-Quality Assessment Program

Quality of Water from Public-Supply 

Wells in the United States, 1993–2007 

Overview of Major Findings
Cover illustration: Map showing the 932 public-supply wells included in this study. Worldimagery base map from ESRI (© 2009), based on source data from ESRI, i-cubed, USDA 

FSA, USGS, AEX, GeoEye, and Getmapping (http://server.arcgisonline.com/ArcGIS/rest/

services/World_Imagery/MapServer).
The Quality of Our Nation’s Waters

Quality of Water from Public-Supply 

Wells in the United States, 1993–2007

Overview of Major Findings

By Patricia L. Toccalino and Jessica A. Hopple

National Water-Quality Assessment Program

Circular 1346

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested reference:

Toccalino, P.L., and Hopple, J.A., 2010, The quality of our Nation’s waters—Quality of water from public-supply wells 

in the United States, 1993–2007—Overview of major findings: U.S. Geological Survey Circular 1346, 58 p.

Additional information about this study is available at http://water.usgs.gov/nawqa/studies/public_wells/

Library of Congress Cataloging-in-Publication Data

Toccalino, Patricia, 1965–

 The quality of our Nation’s waters : quality of water from public-supply wells in the United States, 1993–2007 : 

overview of major findings / by Patricia L. Toccalino and Jessica A. Hopple. 

-- 1st ed.

 58 p. cm. -- (Circular ; 1346)

 Includes bibliographical references.

 ISBN 978-1-4113-2780-1

 1. Water quality--United States--Measurement. 2. Well water--United States. 3. Wells--United States. 

I. Hopple, Jessica A. II. Geological Survey (U.S.) III. Title. IV. Series: U.S. Geological Survey circular ; 1346.

 TD223.T63 2010

 553.7 ' 9097309049--dc22 

 2010013913 
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, now 

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 groundwater? How are conditions 

changing over time? How do natural features and human activities affect the quality of streams and 

groundwater, 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–

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 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 groundwater. For example, increased emphasis has been placed on assessing the quality 

of source water and finished water associated with the Nation’s 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 

water-resource 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
iv

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

describe 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 source 

(untreated) water from 932 public-supply wells and source and finished (treated) water 

from a subset of 94 wells. The 932 public-supply wells are widely distributed nationally; 

they are located in selected parts of 41 states, and withdraw water from parts of 

30 regionally extensive aquifers used for public water supply. Compared to previous 

investigations of public-well water quality, "Quality of Water from Public-Supply Wells 

in the United States, 1993–2007—Overview of Major Findings" greatly expands the 

number of individual contaminants and contaminant mixtures assessed in groundwater 

from public-supply wells. Other reports in this series focus on specific water-quality 

constituents of concern, such as pesticides and volatile organic compounds in 

groundwater and surface water, the effects of contaminants and habitat conditions on 

aquatic ecosystem health, and on the quality of water from private domestic wells. 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 groundwater 

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

the Nation.

Donna N. Myers

Chief, Office of Water Quality

U.S. Geological Survey

Introduction to this report and the NAWQA series

The Quality of Our Nation’s Waters
Contents v

Contents

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

Introduction ............................................................................................................................................. 11

NAWQA’s Approach to Assessing the Quality of Water from Public Wells................................ 14

Major Findings........................................................................................................................................ 20

More than one in five source-water samples from public wells contained

one or more contaminants at concentrations greater than

human-health benchmarks............................................................................................. 20

Contaminants from natural sources accounted for about three-quarters of

contaminant concentrations greater than human-health benchmarks

in source-water samples ................................................................................................ 24

Contaminants that originate entirely or primarily from man-made sources 

accounted for about one-quarter of contaminant concentrations greater

than human-health benchmarks in source-water samples and were

detected in nearly two-thirds of the samples, predominantly in

samples from unconfined aquifers................................................................................ 29

Many organic contaminants detected in source water also were detected in

finished water at similar concentrations ..................................................................... 32

Human-health benchmarks are not yet available for many organic contaminants,

including some that were frequently detected in source water.............................. 34

Contaminants usually co-occurred with other contaminants in source and finished

water, and mixtures of two or more contaminants at concentrations greater

than one-tenth of individual human-health benchmarks were dominated

by inorganic contaminants ............................................................................................. 37

The most complex mixtures in source water—those with the greatest number of

contaminants—were most often detected in samples from unconfined

aquifers .............................................................................................................................. 42

References Cited .................................................................................................................................... 44

Appendixes—Lists of contaminants analyzed ................................................................................. 51

Abbreviations, Acronyms, and Units of Measure............................................................................. 58
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1

Annual consumer confidence reports provided by water utilities are a source of information about 

finished‐water quality in local communities. See "More Information for People Served by Public Water Utilities" 

on page 43.

About 105 million people in the United States—more than one-third of the 

Nation’s population—receive their drinking water from about 140,000 public 

water systems that use groundwater as their source. Although the quality of 

finished drinking water (after treatment and before distribution) from these 

public water systems is regulated by the U.S. Environmental Protection Agency 

(USEPA) under the Safe Drinking Water Act (SDWA), long-term protection 

and management of groundwater, a vital source of drinking water, requires 

an understanding of the occurrence of contaminants in untreated source 

water. Sources of drinking water potentially are vulnerable to a wide range of 

man-made and naturally occurring contaminants, including many that are not 

regulated in drinking water under the SDWA.

Examples of wellhead plumbing typical of public wells sampled in this study.

(Photographs by Michael Rosen and Nicholas Smith, U.S. Geological Survey.)

In this study by the National Water-Quality Assessment (NAWQA) Program of the 

U.S. Geological Survey (USGS), chemical water-quality conditions were assessed 

in source (untreated) groundwater from 932 public-supply wells, hereafter referred 

to as public wells, and in source and finished water from a subset of 94 wells. The 

public wells are located in selected parts of 41 states and withdraw water from 

parts of 30 regionally extensive water-supply aquifers, which constitute about 

one-half of the principal aquifers in the United States. Although the wells sampled 

in this study represent less than 1 percent of all groundwater-supplied public 

water systems in the United States, they are widely distributed nationally and 

were randomly selected within the sampled hydrogeologic settings to represent 

typical aquifer conditions. All source-water samples were collected prior to any 

treatment or blending that potentially could alter contaminant concentrations. 

As a result, the sampled groundwater represents the quality of the source water 

and not necessarily the quality of finished water ingested by the people served by 

these public wells1.

Summary of Major Findings 

and Implications
A greater number of chemical contaminants—as many as 337—both naturally occurring 

and man-made, were assessed in this study than in any previous national study of 

public wells (Appendixes 1 and 2). Consistent with the terminology used in the SDWA, all 

constituents analyzed in water samples in this study are referred to as "contaminants," 

regardless of their source, concentration, or potential for health effects (see sidebar 

on page 3). Eighty-three percent (279) of the contaminants analyzed in this study are 

not regulated in drinking water under the SDWA. The USEPA uses USGS data on the 

occurrence of unregulated contaminants to fulfill part of the SDWA requirements for 

determining whether specific contaminants should be regulated in drinking water 

in the future. By focusing primarily on source-water quality, and by analyzing many 

contaminants that are not regulated in drinking water by USEPA, this study complements 

the extensive sampling of public water systems that is routinely conducted for the 

purposes of regulatory compliance monitoring by federal, state, and local drinking-water 

programs.

The objectives of this study were to evaluate (1) the occurrence of contaminants 

in source water from public wells and their potential significance to human health, 

(2) whether contaminants that occur in source water also occur in finished water 

after treatment, and (3) the occurrence and characteristics of contaminant mixtures. 

To evaluate the potential significance of contaminant occurrence to human health, 

contaminant concentrations were compared to regulatory Maximum Contaminant Levels 

(MCLs) or non-regulatory Health-Based Screening Levels (HBSLs)—collectively referred 

to as human-health benchmarks in this study (see sidebars on pages 4 and 19).

The major findings and implications of this study are summarized below and the results 

are described in greater detail in the remainder of the report. These findings build upon 

water-quality data from previous public-well studies and provide new information to 

agencies and organizations that manage the protection of drinking water and human 

health.

2 Quality of Water from Public-Supply Wells in the United States, 1993–2007
What is a Contaminant?

A contaminant is defined in the Safe Drinking Water Act (SDWA) as "any physical, 

chemical, biological, or radiological substance or matter in water" (U.S. Code, 1996). 

This broad definition of contaminant includes every substance that may be dissolved 

or suspended in water—everything but the water molecule itself. The presence of a 

contaminant in water does not necessarily mean that there is a human-health concern.

Whether a particular contaminant in water is potentially harmful to human 

health depends on the contaminant’s toxicity and concentration in drinking water. 

Other factors include the susceptibility of individuals, amount of water consumed, 

and duration of exposure (U.S. Environmental Protection Agency, 2008a). For example, 

some contaminants that typically occur naturally, such as selenium and chromium, are 

essential trace elements and are required in low doses for normal physiologic function, 

but high doses can cause adverse health effects (Eaton and Klaassen, 2001). By contrast, 

man-made organic contaminants, such as pesticides, are not required by humans at any 

dose, and may or may not cause adverse effects in humans depending on factors such as 

exposure and toxicity.

A total of six water-quality properties (such as pH) and as many as 337 chemical 

contaminants were analyzed in source-water samples from public wells in this study 

(Appendixes 1 and 2); the contaminants originate from a wide range of natural and 

man-made sources. Most of the 44 inorganic contaminants analyzed in this study—

including trace elements, radionuclides, and nutrients—occur naturally, although 

concentrations of some inorganic contaminants in groundwater may be altered by 

human activities. For example, nitrate from natural sources is present in most wells, 

but concentrations often are increased by contributions from man-made sources in 

agricultural and urban areas. By contrast, most of the 293 organic contaminants analyzed 

in this study—including pesticide compounds, volatile organic compounds, personal-care 

and domestic-use products, and other organic contaminants such as manufacturing 

additives—are man-made, although some also may form in groundwater through various 

chemical and biological transformation processes. 

Summary of Major Findings and Implications Summary of Major Findings and Implications 3
Human-Health Benchmarks Used in This Study: 

Regulatory Maximum Contaminant Levels and 

Non-Regulatory Health-Based Screening Levels

As used in this study, regulated contaminants are those contaminants for which the U.S. 

Environmental Protection Agency (USEPA) has established drinking-water standards (Maximum 

Contaminant Levels or MCLs) under the Safe Drinking Water Act (SDWA), and unregulated 

contaminants are those that are not regulated in drinking water under the SDWA and therefore 

do not have MCLs. Contaminants that are not federally regulated in drinking water under the 

SDWA may be regulated in drinking water by some states, and also may be regulated in other 

contexts and under other statutes, such as the Federal Insecticide, Fungicide, and Rodenticide Act.

For the purposes of placing study findings in the context of human health, concentrations 

of contaminants that are regulated by USEPA in drinking water under the SDWA were 

compared to MCLs, and concentrations of unregulated contaminants were compared to 

U.S. Geological Survey (USGS) Health-Based Screening Levels (HBSLs), when available. Of 

the 337 contaminants analyzed in this study, MCLs are available for 58 contaminants and 

HBSLs are available for 135 contaminants (see sidebar on page 19). Because regulatory 

MCLs and non-regulatory HBSLs both are used in this study to provide an initial perspective 

on the potential significance of detected contaminants to human health, they are henceforth 

collectively referred to as "human-health benchmarks".

MCLs are legally enforceable USEPA drinking-water standards that set the maximum 

permissible level of a contaminant in water that is delivered to any user of a public water 

system. MCLs are set as close as feasible to the maximum level of a contaminant at which no 

known or anticipated adverse effects on human health would occur over a lifetime, taking into 

account the best available analytical and treatment technologies, cost considerations, expert 

judgment, and public comments (U.S. Environmental Protection Agency, 2009a). As a result, not 

all MCLs are based on health-effects data alone. 

HBSLs are non-enforceable benchmark concentrations of contaminants in water that were 

developed by the USGS in collaboration with the USEPA and others using: (1) USEPA Office 

of Water methodologies for establishing drinking-water guidelines, and (2) the most recent, 

USEPA peer-reviewed, publicly available human-health toxicity information (Toccalino and 

others, 2006; Toccalino, 2007). As a result, HBSL values are consistent with existing USEPA 

drinking-water guideline values, such as Lifetime Health Advisory values and Cancer Risk 

Concentration values (when they exist), except for unregulated contaminants for which more 

recent toxicity information has become available (Toccalino, 2007). 

Neither an MCL nor an HBSL is available for radon, but USEPA has proposed both an MCL 

of 300 picocuries per liter (pCi/L) and an Alternative MCL of 4,000 pCi/L for radon in public 

water systems (U.S. Environmental Protection Agency, 2006a). The lower proposed MCL for 

radon applies to states and public water systems that do not develop programs to address 

health risks from radon in indoor air; the higher proposed Alternative MCL applies to states 

and public water systems that have established such programs (U.S. Environmental Protection 

Agency, 1999a, 2009e). Except where indicated otherwise in this report, radon activities were 

compared to the higher proposed Alternative MCL of 4,000 pCi/L.

4 Quality of Water from Public-Supply Wells in the United States, 1993–2007
More than one in five source-water samples from public wells contained 

one or more contaminants at concentrations greater than human-health 

benchmarks (p. 20–23).

One or more chemical contaminants were detected at concentrations greater than 

MCLs or HBSLs in more than one in five (22 percent) of the 932 source-water 

samples from public wells. Ten contaminants—seven from natural sources and three 

primarily from man-made sources—individually were detected at concentrations 

greater than human-health benchmarks in at least 1 percent of samples and 

collectively accounted for most concentrations greater than benchmarks. Five of 

these 10 contaminants currently are regulated in drinking water under the SDWA

(MCLs are available)—overall, concentrations of at least one contaminant were 

greater than an MCL in 15 percent of the samples. Eighty percent of the source-water 

samples contained one or more contaminants at concentrations greater than one-tenth 

of human-health benchmarks, but most individual contaminant detections were at 

much lower concentrations, and many were several orders of magnitude less than 

benchmarks. Public wells yielding water in which contaminant concentrations were 

greater than benchmarks, as well as those in which concentrations were greater than 

one-tenth of benchmarks, were distributed throughout the United States and included 

wells that withdraw water from all principal aquifer rock types included in this study. 

Implications:

• Source water from a substantial proportion of public wells—about one of every five 

sampled—would require treatment or blending with higher-quality water sources to 

decrease contaminant concentrations to less than human-health benchmarks. Water 

utilities, however, are not required to treat water for unregulated contaminants, 

which accounted for about 40 percent of the concentrations greater than 

benchmarks. Contaminant concentrations greater than benchmarks in source water 

are potential human-health concerns, but do not necessarily indicate that adverse 

effects will occur because the benchmarks are conservative (protective) and samples 

were collected prior to any treatment or blending of water.

• Most public wells sampled—four of every five—contained one or more 

contaminants at concentrations greater than one-tenth of a benchmark. Contaminant 

concentrations greater than one-tenth of a benchmark provide an early and 

conservative indication of contaminant concentrations that may at some time 

approach or exceed benchmarks and identify contaminants that may warrant 

additional monitoring. Early attention to potential groundwater contamination is 

vital because groundwater contamination is difficult and costly to reverse once it occurs.

• The widespread occurrence of contaminants at concentrations greater than one‐tenth 

of benchmarks, including the relatively common occurrence of concentrations 

greater than benchmarks, indicates the ubiquitous nature of natural and man-made 

contaminant sources and that all principal aquifer rock types included in this 

study are vulnerable to contamination. The occurrence of contaminants of primary 

concern, however, differed among regions and aquifers.

Summary of Major Findings and Implications Summary of Major Findings and Implications 5
Contaminants from natural sources accounted for about three-quarters of 

contaminant concentrations greater than human-health benchmarks in sourcewater samples (p. 24–28).

Naturally occurring trace elements and radionuclides accounted for about 

three‐quarters (74 percent) of all contaminant concentrations greater than 

human‐health benchmarks in source-water samples from public wells. Four trace 

elements (arsenic, manganese, strontium, and boron) and three radionuclides (radon, 

radium, and gross alpha-particle radioactivity) each were detected at concentrations 

greater than MCLs or HBSLs in at least 1 percent of source-water samples. Radon 

activities were greater than the higher proposed Alternative MCL of 4,000 picocuries 

per liter (pCi/L) in less than 1 percent of samples, but were greater than the lower 

proposed MCL of 300 pCi/L in 55 percent of samples. Each of the remaining six trace 

elements and radionuclides was detected at concentrations greater than human-health 

benchmarks in 3 to 19 percent of samples. Manganese and boron have undergone 

USEPA’s regulatory determination process under the SDWA, but were not selected 

for regulation in drinking water. Collectively, concentrations of one or more trace 

elements or radionuclides were greater than MCLs or HBSLs in 19 percent of 

samples (when radon activities were compared to 4,000 pCi/L). Trace elements and 

radionuclides were detected at concentrations greater than benchmarks in samples 

from both confined and unconfined aquifers, consistent with the fact that these 

contaminants originate primarily from aquifer materials, rather than from man-made 

sources at the land surface. Regional patterns of occurrence were apparent for some 

contaminants, such as arsenic, as a result of the geographic distributions of natural 

sources. 

Implications:

• Naturally occurring trace elements and radionuclides may be present in groundwater 

at concentrations of potential human-health concern even in undeveloped areas or 

confined aquifers where contamination usually is not expected. 

• Traditional wellhead protection approaches designed to reduce man-made sources 

of contaminants to groundwater generally are not designed to protect against natural 

sources of contaminants and, therefore, most occurrences of naturally occurring 

contaminants at concentrations greater than benchmarks are unlikely to be affected 

by these approaches. 

Contaminants that originate entirely or primarily from man-made sources 

accounted for about one-quarter of contaminant concentrations greater than 

human-health benchmarks in source-water samples and were detected in 

nearly two-thirds of the samples, predominantly in samples from unconfined 

aquifers (p. 29–31).

Nitrate, pesticide compounds, and volatile organic compounds (VOCs) accounted 

for about one-quarter (25 percent) of all contaminant concentrations greater than 

MCLs or HBSLs in source-water samples from public wells. Nitrate, dieldrin, and 

6 Quality of Water from Public-Supply Wells in the United States, 1993–2007
perchloroethene (PCE) each were detected at concentrations greater than MCLs or 

HBSLs in 1 to 3 percent of source-water samples. Nitrate is naturally occurring, but 

most nitrate concentrations greater than 1 milligram per liter as nitrogen (which is 

one-tenth of the nitrate MCL) originate from man-made sources such as fertilizers, 

livestock, and wastewater. Dieldrin has undergone USEPA’s regulatory determination 

process under the SDWA, but was not selected for regulation in drinking water. 

Collectively, pesticide compounds or VOCs were detected in nearly two-thirds 

(64 percent) of the samples, and concentrations of one or more of these contaminants 

were greater than human-health benchmarks in 4.5 percent of samples, primarily 

in the highly populated areas of East-Coast states. Considering all 293 man-made 

organic contaminants analyzed in this study (including personal-care products 

and other organic contaminants), 169 (58 percent) were detected in at least one 

sample. Nitrate, pesticide compounds, and VOCs were detected in a significantly 

greater proportion of samples from unconfined aquifers than in samples from 

confined aquifers, and almost all concentrations greater than benchmarks were 

detected in samples from unconfined aquifers, consistent with the fact that these 

contaminants originate from man-made sources at the land surface. Contaminant 

detections represent the influence of both past and current land-use practices and 

contaminant‐use patterns.

Implications:

• The frequent detections of man-made contaminants in samples from aquifers used 

for public water supply, particularly in samples from unconfined aquifers, indicate 

the vulnerability of many water-supply aquifers to contamination from human 

activities at the land surface. This finding underscores the importance of wellhead 

protection programs designed to reduce groundwater contamination from man-made 

sources. Reduction of contaminant sources potentially reduces needs for water 

treatment and blending, and may avoid the loss of some public wells as sources of 

drinking water. 

• The occurrence of man-made contaminants in some samples from confined 

aquifers indicates that the large pumping rates typical for public wells, combined 

with the proximity of the wells to developed areas and the possible presence of 

short‐circuiting flow paths to such wells, make even some deep wells that withdraw 

water from confined aquifers vulnerable to contamination from man-made sources.

• Based only on comparisons of contaminant concentrations to individual 

human‐health benchmarks, contaminants from man-made sources may have 

less potential human-health significance in public wells than contaminants from 

natural sources. Human-health benchmarks are not available for many organic 

contaminants analyzed in this study, however, and the full significance of their 

occurrence to human health cannot yet be assessed (see below).

• More than 20 years after being banned, dieldrin is still detected at concentrations 

greater than its benchmark in some source waters because of historical use. 

Source‐water protection strategies that rely on changes in human activities and 

practices at the land surface to achieve water-quality objectives can take many 

decades to affect the quality of water in some public wells.

 Summary of Major Findings and Implications 7
Many organic contaminants detected in source water also were detected in 

finished water at similar concentrations (p. 32–33). 

As many as 272 organic contaminants were analyzed in a subset of 94 paired source- and 

finished-water samples from public wells (inorganic contaminants were not analyzed 

in these paired samples). Considering all organic contaminants that were detected in 

at least 10 percent of source-water samples, concentrations generally were similar in 

source and finished water, except for concentrations of disinfection by-products (DBPs), 

which were greater in finished water because these contaminants are by-products that 

form during the disinfection process. Two of the source-water samples, and none of the 

finished-water samples, contained at least one organic contaminant at a concentration 

greater than a human-health benchmark. Comparisons between source and finished 

water were not intended to characterize treatment efficacy. Disinfection was the primary 

water treatment used at about three-quarters of the sampled systems, which is about the 

same proportion as for all groundwater-supplied public water systems nationwide. 

Implications:

• The occurrence of organic contaminants in both source and finished water at similar 

concentrations indicates that commonly used disinfection practices do not reduce 

concentrations of many organic contaminants. Because disinfection is not designed to 

treat organic contaminants, and because pesticide compounds or VOCs were detected 

in 64 percent of the source-water samples (see above), the frequent occurrence of these 

organic contaminants in finished water is likely, although usually at concentrations less 

than benchmarks. 

• Because concentrations of many man-made organic contaminants were not affected by 

commonly used treatment processes, protection of source-water quality is an essential 

component of ensuring high-quality drinking water. Source-water protection can help 

to reduce potential concerns about the presence of man-made contaminants in source 

water, particularly unregulated contaminants that do not require treatment in finished 

water under the SDWA.

Human-health benchmarks are not yet available for many organic contaminants, 

including some that were frequently detected in source water (p. 34–36).

Human-health benchmarks are not available for 144 (43 percent) of the contaminants 

analyzed in this study (mostly organic contaminants) because of insufficient toxicity 

information, including nine contaminants that were detected in 6 to 35 percent of 

source‐water samples from public wells. The nine contaminants are degradates of the 

herbicides atrazine, alachlor, and metolachlor, a gasoline oxygenate (methyl tert-butyl 

ether), and a solvent (1,1-dichloroethane). Several of these contaminants are on USEPA’s 

most recent (third) Contaminant Candidate List (CCL), which is a list of unregulated 

contaminants that are known or anticipated to occur in public water systems and may 

require regulation under the SDWA in the future. Most herbicide degradates analyzed 

8 Quality of Water from Public-Supply Wells in the United States, 1993–2007
in this study are not currently regulated by the USEPA in drinking water under the 

SDWA, but some herbicide degradates are regulated by USEPA under the Federal 

Insecticide, Fungicide, and Rodenticide Act. Concentrations of the parent herbicides 

atrazine, alachlor, and metolachlor were less than MCLs or HBSLs in all source-water 

samples. 

Implications:

• The occurrence of contaminants for which human-health benchmarks are not 

yet available could not be evaluated in the context of potential human-health 

significance. Continued development of human-health benchmarks is needed 

to fill this information gap and to help interpret monitoring data, especially for 

contaminants that frequently occur in water resources.

• Continued monitoring of contaminants for which human-health benchmarks are 

not yet available is essential for identifying those contaminants for which toxicity 

assessments are most needed, as well as to track emerging issues. As improved 

data on toxicity and environmental concentrations become available, human-health 

benchmarks can be developed, and water-quality assessments can be improved and 

expanded.

Contaminants usually co-occurred with other contaminants in source and 

finished water, and mixtures of two or more contaminants at concentrations 

greater than one-tenth of individual human-health benchmarks were 

dominated by inorganic contaminants (p. 37–41). 

Contaminants detected in source- and finished-water samples from public wells 

usually co-occurred with other contaminants as mixtures. Although few human-health 

benchmarks have been established for mixtures of contaminants, concentrations 

of contaminants in mixtures were compared to individual benchmarks. About 

4 percent of source-water samples contained mixtures of two or more contaminants 

at concentrations greater than individual human-health benchmarks, whereas 

most samples (84 percent) contained mixtures of two or more contaminants at 

concentrations greater than one-tenth of individual benchmarks. All of the most 

common mixtures (those detected in 5 to 33 percent of source-water samples) 

in which contaminant concentrations were greater than one-tenth of individual 

benchmarks were composed of one or more trace elements (arsenic, strontium, 

or uranium were most common), nitrate, and (or) radon (activities greater than 

300 pCi/L). When mixtures of organic contaminants were assessed only on the basis 

of detections and without regard to the availability of human-health benchmarks, 

three-quarters of the organic-contaminant mixtures contained an herbicide (atrazine 

or simazine) or an herbicide degradate (deethylatrazine). Two-thirds of these 

mixtures contained a DBP (chloroform), and 43 percent contained the solvents PCE 

or trichloroethene. Organic-contaminant mixtures were detected somewhat more 

frequently in finished water than in source water because of the formation of DBPs in 

finished water.

 Summary of Major Findings and Implications 9
Implications:

• The widespread and frequent detections of contaminant mixtures in source water is 

a matter of increasing concern and attention because the total combined toxicity of 

contaminants in water may be greater than that of any individual contaminant. Little 

is known about the potential health effects associated with exposure to multiple 

contaminants, and more investigation is needed to evaluate the potential toxicity of 

contaminant mixtures to humans.

• This study identifies which contaminant mixtures may be of most concern in 

groundwater used for public water supply because of their frequency of occurrence 

and comparisons of contaminant concentrations to individual human-health 

benchmarks. This information can help human-health researchers to target and 

prioritize toxicity assessments of contaminant mixtures.

The most complex mixtures in source water—those with the greatest number 

of contaminants—were most often detected in samples from unconfined 

aquifers (p. 42).

Mixtures with the greatest number of contaminants were detected more frequently 

in source-water samples from public wells that withdraw water from shallower 

unconfined aquifers than in samples from deeper confined aquifers. For example, 

about two-thirds of the mixtures containing three contaminants, and nearly all 

mixtures containing 10 contaminants, were detected in samples from unconfined 

aquifers. This finding was observed for mixtures that were assessed relative to 

individual human-health benchmarks and for mixtures of organic contaminants that 

were assessed without regard to the availability of benchmarks. Generally, as the 

number of contaminants in mixtures increased, the proportion of samples containing 

organic contaminants also increased, reflecting the greater vulnerability of unconfined 

aquifers to contaminants from man-made sources.

Implications:

• The frequent occurrence of complex contaminant mixtures, particularly in 

source‐water samples from unconfined aquifers, reinforces the implication derived 

from the occurrence of individual contaminants, that multiple contaminant sources 

and transport pathways may affect the quality of water from public wells.

• Measures to protect groundwater used for public water supply often need to 

take into consideration multiple sources of man-made contaminants and their 

co-occurrence with contaminants from natural sources.

Additional Information

For more information on the quality of water in 

public wells, refer to reports by Toccalino and 

others (2010) available at http://pubs.usgs.gov/

sir/2010/5024/, and by Hopple and others (2009) 

available at http://pubs.usgs.gov/sir/2009/5200/. 

10 Quality of Water from Public-Supply Wells in the United States, 1993–2007
>80

>60 to 80

>40 to 60

>20 to 40

Less than or equal to 20

Figure 1. Nationwide, about 105 million people rely on public wells for drinking water. In many parts of the United States, 

public wells supply drinking water for large percentages of the population. Water-use data are from Solley and others (1998); 

>, greater than.

Population (1995) supplied by 

public wells, in percent of 

total county population

EXPLANATION

Introduction 11

Figure 1. Nationwide, about 105 million people rely on public wells for drinking water. In many parts of the 

United States, public wells supply drinking water for large percentages of the population. Water-use data 

are from Solley and others (1998). >, greater than.

Introduction

Safe drinking water is essential to public health, and the 

quality of the Nation’s drinking-water supply is an issue of 

growing national importance (U.S. General Accounting Office, 

1997). In 2008, about 105 million people in the United States—

more than one-third of the Nation’s population—obtained their 

drinking water from about 140,000 public water systems that 

derive all or part of their source-water supply from groundwater 

(U.S. Environmental Protection Agency, 2008c; U.S. Census 

Bureau, 2009). Public wells are a primary source of drinking 

water in large areas of the country, including much of the 

southwestern, southern, and central United States (fig. 1).

The quality of finished drinking water (after treatment and 

before distribution) from the Nation’s public water systems 

is regulated by the U.S. Environmental Protection Agency 

(USEPA) under the Safe Drinking Water Act (SDWA) (U.S. 

Environmental Protection Agency, 2004c). The SDWA defines 

a public water system as one that serves piped drinking water to 

at least 25 people or 15 service connections for at least 60 days 

a year (U.S. Environmental Protection Agency, 2003f). Public 

water systems range from very small, privately owned systems 

such as those that supply some mobile-home parks, to 

large, publicly owned systems that serve millions of 

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

Most (nearly 90 percent) of the public wells sampled in 

this study were part of community water systems that 

serve the same people year-round (U.S. Environmental 

Protection Agency, 2003f), and about half of the sampled 

public wells were part of large and very large public water 

systems (table 1).

The SDWA, originally passed by Congress in 1974 

and amended in 1986 and 1996, requires many actions 

to protect drinking water and its sources—rivers, lakes, 

reservoirs, springs, and groundwater. For example, the 

SDWA authorizes the USEPA to set national health-based 

standards for drinking water to protect against both natu‐

rally occurring and man-made contaminants that may be 

detected in drinking water (U.S. Environmental Protection 

Agency, 2004c). USEPA oversees the states, localities, 

and water suppliers who implement the drinking-water 

standards. 

Introduction 11
Table 1. About one-half of the public wells sampled in this study 

were part of large and very large public water systems that each 

serve more than 10,000 people.

[System size classifications are from the U.S. Environmental Protection 

Agency (2010b)]

System size Number of 

people served

Percentage of 

public wells 

sampled in 

this study

Very small 25 to 500 11

Small 501 to 3,300 21

Medium 3,301 to 10,000 13

Large 10,001 to 100,000 31

Very large More than 100,000 18

Unknown Unknown 6

12 Quality of Water from Public-Supply Wells in the United States, 1993–2007

Many studies have demonstrated the presence of naturally 

occurring and man-made contaminants in finished drinking 

water (U.S. Environmental Protection Agency, 2001a, 2002b, 

2003e) and in untreated source water (Gilliom and others, 

2006; Zogorski and others, 2006; Ayotte and others, 2007) from 

public wells, providing valuable information about various 

aspects of public-well water quality (see sidebar on page 13). 

The objectives of this study by the National Water-Quality 

Assessment (NAWQA) Program of the U.S. Geological Survey 

(USGS) were to evaluate (1) the occurrence of contaminants in 

source water from public wells and their potential significance 

to human health, (2) whether contaminants that occur in source 

water also occur in finished water after treatment, and (3) the 

occurrence and characteristics of contaminant mixtures. In 

addressing these objectives, this study differs from most previous 

studies in several ways:

• The quality of the source water that supplies groundwater to 

public water systems is evaluated, whereas many previous 

studies examined only the quality of finished drinking 

water delivered to consumers (Westrick and others, 1984; 

Longtin, 1988; U.S. Environmental Protection Agency, 

2001a, 2003e). An evaluation of contaminant occurrence in 

source water provides background information regarding the 

presence of a contaminant in the environment. 

• The occurrence of a diverse suite of organic contaminants 

in source water and in finished water (before and after 

treatment) is compared in a limited subset of public-well 

samples. Many of these organic contaminants are not 

regulated in drinking water under the SDWA.

• Specific sites or areas with known water-quality problems 

are not sampled, whereas some previous studies include, and 

in some instances, focus on areas of known contamination 

(Westrick and others, 1984; Focazio and others, 2001). 

• More contaminants and contaminant groups are assessed 

than in previous public-well studies (Longtin, 1988; 

Westrick, 1990; Grady and Casey, 2001), including many 

inorganic and organic contaminants. 

• Contaminants evaluated include some that are regulated 

in drinking water and many that are not, whereas studies 

of SDWA compliance data commonly focus only on 

regulated contaminants (U.S. Environmental Protection 

Agency, 1999b, 2003e, 2008c). Many unregulated 

contaminants analyzed in this study are not included 

in other source-water and finished-water monitoring 

programs such as the Unregulated Contaminant 

Monitoring program (U.S. Environmental Protection 

Agency, 2010c).

• Analytical reporting levels used in this study typically 

are 2-fold to more than 1,000-fold lower than in many 

previous studies (U.S. Environmental Protection 

Agency, 2001a, 2002b, 2003e) (see sidebar on page 17). 

Contaminant occurrence documented at these low 

reporting levels can help to identify emerging issues and 

to track changes in concentrations over time (Gilliom and 

others, 2006). 

• More human-health benchmarks are available for 

comparisons with contaminant concentrations in this 

study than in pre-2005 studies that considered the 

occurrence of unregulated contaminants in drinking water. 

The USGS, in collaboration with USEPA and others, has 

developed Health-Based Screening Levels (HBSLs) for 

many contaminants that are not regulated in drinking 

water under the SDWA. HBSLs supplement USEPA 

human-health benchmarks and provide a basis for a more 

comprehensive evaluation of contaminant-occurrence 

data in the context of human health (Toccalino, 2007).

• The co-occurrence of contaminants as mixtures and the 

potential significance of such mixtures to human health is 

assessed. Data from this study enhance previous studies 

of contaminant mixtures by examining the co-occurrence 

of a larger number of contaminants in source and finished 

water from public wells, and by assessing the occurrence 

of mixtures relative to individual human-health 

benchmarks. 

• Contaminant occurrence is examined by regional aquifers, 

providing a framework for understanding water-quality 

conditions within similar hydrogeologic and geographic 

regions. The nationally consistent sampling and 

analysis design used by the USGS NAWQA Program 

enables comparisons of water-quality conditions among 

geographic areas and across the United States.

 The water-quality findings from this assessment of 

public wells, together with the accumulation of findings 

from previous studies, provide a foundation for improving 

our understanding and management of this critical source 

of drinking water.
Results are Consistent with Previous

National-Scale Studies

This study complements and expands upon findings reported in previous national-scale 

studies of public wells in several ways, as described in the Introduction, but findings from 

this study generally confirm and reinforce previous conclusions about many contaminants. A 

detailed comparison to findings reported in previous public-well studies is included in Toccalino 

and others (2010).

Detection frequencies and the percentages of samples with contaminant concentrations 

greater than benchmarks determined in this study generally were similar to those observed 

in previous national-scale U.S. Geological Survey (USGS) studies of specific contaminant 

groups (Gilliom and others, 2006; Zogorski and others, 2006). These similarities occur because 

one-quarter to one-third of the public wells sampled in this study also were included in some of 

the previous USGS studies and because of similarities in study designs.

Those contaminants that were most and least frequently detected in this study also 

generally were most and least frequently detected in several previous national-scale U.S. 

Environmental Protection Agency (USEPA) monitoring studies (Westrick and others, 1984; 

Longtin, 1988; U.S. Environmental Protection Agency, 1999b, 2001a, 2003e, 2008c). Many 

individual contaminants were detected more frequently in this study than were reported in 

the USEPA monitoring studies because of factors such as differences in analytical reporting 

levels and sampling points. USGS uses lower analytical reporting levels (see sidebar on 

page 17) than those used in most USEPA monitoring reports. USGS also typically collects 

untreated source-water samples whereas USEPA typically collects treated finished-water 

samples. In this study and in USEPA monitoring studies, many regulated contaminants were 

detected at concentrations greater than one-tenth of Maximum Contaminant Levels (MCLs), but 

concentrations greater than MCLs were not common—the same few contaminants generally 

were most frequently detected at concentrations greater than MCLs. 

The results from this study and from a recent national-scale USGS study of domestic 

wells (DeSimone and others, 2009) also were compared. Overall, the quality of water from 

domestic and public wells in the United States is similar. In both this study and the domesticwell study (DeSimone and others, 2009), about one in five samples contained one or more 

contaminants at concentrations greater than human-health benchmarks, naturally occurring 

trace elements and radionuclides were the contaminants that were detected most frequently 

at concentrations greater than benchmarks, and contaminant mixtures were common. Notable 

differences between samples from public and domestic wells were that pesticide compounds 

and volatile organic compounds (VOCs) were detected more frequently at concentrations 

greater than human-health benchmarks in public-well samples, whereas nitrate was detected 

more than twice as frequently at concentrations greater than the MCL in domestic-well 

samples. The detection frequencies of some individual organic contaminants—mostly VOCS—

were 2-fold to 6-fold greater in samples from public wells than from domestic wells, likely 

because of the proximity of public wells to developed areas and the higher pumping rates used 

for public wells than for domestic wells.

Introduction 13
Source-water samples from 

public wells were collected 

before any treatment 

or blending. These are 

examples of pumps and 

plumbing configurations 

used by public water 

systems.

(Top photograph by Roland Tollett, 

U.S. Geological Survey. 

Bottom photograph by U.S. 

Geological Survey.)

This report presents a national assessment 

of the quality of source (untreated) water from 

932 public wells and a comparison of source 

and finished (treated) water for a subset of 

94 wells. The wells are located in selected parts 

of 41 states and withdraw water from parts of 

30 regionally extensive aquifers used for water 

supply (fig. 2); these aquifers constitute about 

one-half of the principal aquifers in the United 

States (U.S. Geological Survey, 2009a). Each 

well was sampled once during the period of 

1993–2007, and all samples were collected and 

analyzed following a nationally consistent study 

design (Gilliom and others, 1995; Lapham and 

others, 2005).

This report combines and summarizes 

the major findings from two detailed USGS 

public-well assessments (Hopple and others, 

2009; Toccalino and others, 2010). In the study 

by Toccalino and others (2010), as many as 

six water-quality properties and 212 chemical 

contaminants were analyzed in source-water 

samples collected from 932 public wells 

during 1993–2007 (Appendix 1). Source-water 

samples were collected at the wellhead (the 

point at which the groundwater exits the well, 

at the land surface) before any treatment or 

blending that potentially could alter contaminant 

concentrations. As a result, the quality of the 

sampled source water does not necessarily reflect 

the quality of finished water ingested by the 

people served by these wells.

In the study by Hopple and others (2009), 

as many as 272 organic contaminants were 

analyzed in source-water samples collected 

from 221 public wells during 2002–2005, and 

in both source and finished (treated) water 

from a subset of 94 wells. These 221 public 

wells are a subset of the 932 wells included 

in the study by Toccalino and others (2010). 

Finished-water samples were collected after the 

water passed through all treatment processes, 

but prior to distribution (Hopple and others, 

2009). About one-half of the 272 organic 

contaminants (147 pesticide and volatile organic 

compounds) included in the study by Hopple and 

others (2009) also were included in the study 

by Toccalino and others (2010) and are listed 

in Appendix 1, whereas the remaining organic 

contaminants (125) were unique to the Hopple 

study and are listed in Appendix 2. 

All together, as many as six water-quality 

properties and 337 chemical contaminants 

were assessed as part of the combined analysis 

described in this report. The numbers of 

individual contaminants in each contaminant 

group included in this study, along with 

the numbers of samples collected for each 

contaminant group, are summarized in table 2. 

Several additional aspects of the study 

design and data characteristics, as well as the 

approach to data analysis and interpretation, need 

to be considered when interpreting the findings in 

this report.

• Principal aquifers are regionally extensive 

aquifers or aquifer systems that have the 

potential to be used as sources of potable 

water (Miller, 2000; U.S. Geological Survey, 

2009b). Principal aquifers were grouped into 

eight rock-type categories by following the 

classifications used in the National Atlas 

(Miller, 2000; U.S. Geological Survey, 2003, 

2009a) (fig. 2). Aquifers within these rocktype categories are likely to share general 

characteristics of groundwater flow and, in 

some cases, overall geochemistry (Miller, 

2000). Water-quality conditions, however, 

can vary substantially within principal 

aquifers, indicating the diversity of natural 

systems and human influences. 

• The aquifer type for each public well 

was coded as unconfined, confined, or 

mixed using data from a variety of sources 

including well owners, well-construction 

logs, and other records from state or local 

jurisdictions (U.S. Geological Survey, 2006) 

(see sidebar on page 18).

14 Quality of Water from Public-Supply Wells in the United States, 1993–2007

NAWQA’s Approach to Assessing the Quality of 

Water from Public Wells
EXPLANATION

Principal aquifer rock type (Figure 2A)

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Number of public wells (Figure 2B)

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B. Principal aquifer rock types for 932 public wells

A. Principal aquifer rock types of the United States

Figure 2. Samples were collected from 932 public wells that are located in 41 states and withdraw water from about one-half 

of the regionally extensive aquifers used for water supply in the United States. Map (A) shows the areal extent, by rock type, 

of all principal aquifers in the United States, and map (B) shows the locations and the principal aquifer rock types associated 

with the public wells sampled in this study.

Unconsolidated sand and gravel aquifers (non-glacial origin)

Glacial sand and gravel aquifers

Semi-consolidated sand and gravel aquifers

Sandstone aquifers

Sandstone and carbonate-rock aquifers

Carbonate-rock aquifers

Basaltic and other volcanic-rock aquifers

Crystalline-rock aquifers

Other aquifers 

Figure 2. Source-water samples were collected from 932 public wells that are located in parts of 41 states and withdraw 

water from parts of about one-half of the regionally extensive aquifers used for water supply in the United States. Map (A) 

shows the areal extent, by rock type, of all principal aquifers in the United States, and map (B) shows the locations and the 

principal aquifer rock types associated with the public wells sampled in this study.

NAWQA’s Approach to Assessing the Quality of Water from Public Wells 15
• NAWQA studies were designed as integrated waterresource assessments of critical and regionally extensive 

hydrologic systems of the Nation, but were not designed 

as a statistically representative sampling of all public 

wells nationwide. Within the sampled hydrogeologic 

settings, however, the public wells were randomly 

selected to represent typical aquifer conditions. 

• Because each public well was sampled once, percentages 

of samples presented in this study are the same as 

percentages of wells. 

• Trends within the 15-year study period (1993–2007) were 

not assessed, but changes in contaminant occurrence 

over time were not expected to be large compared to 

geographic variability because of the relatively slow 

movement of groundwater. The occurrence of some 

contaminants, however, particularly in unconfined 

aquifers, may have changed over time in some wells 

because of variability in contaminant sources and because 

Table 2. Water-quality properties, inorganic contaminants, and organic contaminants were 

analyzed in source-water samples from 932 public wells. 

Contaminant or contaminant group

Number of 

contaminants 

in group

Number of sourcewater samples 

analyzed 1

Contaminants analyzed in source-water samples evaluated by 

Toccalino and others (2010)—Appendix 1

Water-quality properties 2 6 802 to 896

Major ions 9 787 to 810

Trace elements 23 3 598 to 809

Nutrients 8 201 to 817

Radionuclides

Radon 1 506

Radionuclides other than radon 3 84 to 191

Organic contaminants

Pesticide compounds 83 4 507 to 898

Volatile organic compounds 85 758 to 832

Additional organic contaminants analyzed in a subset of source-water samples 

evaluated by Hopple and others (2009)—Appendix 2

Personal-care and domestic-use products 21 5 203 to 217

Other organic contaminants

Other pesticide compounds 60 6 200 to 221

Herbicide and herbicide degradates 18 43 to 73

Other volatile organic compounds 9 119 to 217

Manufacturing additives, pavement- and combustion-derived

compounds, plant- or animal-derived biochemicals

17 202 to 217

1

 The number of samples analyzed for individual contaminants varies because of factors such as differences in study 

designs. 2

 Alkalinity, dissolved oxygen, pH, specific conductance, temperature, and total dissolved solids.

3

 Five trace elements were analyzed in 437 to 503 samples.

4

 Four pesticide compounds were analyzed in 159 to 165 samples.

5

 Three personal-care and domestic-use products were analyzed in 118 to 187 samples. 6

 Hexazinone was analyzed in 113 samples.

pumping and other human activities can enhance the 

mobility of contaminants (Jurgens and others, 2009).

• Contaminant concentrations were compared to 

Maximum Contaminant Levels (MCLs) or to HBSLs 

when available, to provide an initial perspective on 

the potential signficance of detected contaminants 

to human health (see sidebars on pages 4 and 19). 

Detections of contaminants do not necessarily indicate 

a concern for human health; samples were analyzed 

using methods with detection levels that often are 

100‐fold to 1,000-fold lower than MCLs or HBSLs 

(see sidebar on page 17).

• The occurrence and composition of unique contaminant 

mixtures were assessed in various subsets of samples. 

Unique mixtures are specific combinations of two, 

three, or more contaminants, regardless of the presence 

of other contaminants.

16 Quality of Water from Public-Supply Wells in the United States, 1993–2007
Importance of Low-Level Detections

Recent advances in laboratory analytical 

methods have given scientists increasingly refined 

tools to detect a wide range of contaminants in the 

environment at progressively lower concentrations. 

U.S. Geological Survey (USGS) analytical methods 

were designed to detect concentrations as low as 

technically and economically feasible. The analytical 

methods used in this study have detection levels 

that are commonly 100-fold to 1,000-fold lower 

than human-health benchmarks for water quality. 

Detections of contaminants, therefore, do not 

necessarily indicate a concern for human health, 

but rather identify the environmental presence of 

a wide variety of contaminants, many of which are 

not commonly monitored in water resources, and 

U.S. Geological Survey chemist at the National 

Water-Quality Laboratory in Lakewood, Colorado. 

(Photograph by U.S. Geological Survey.)

help to characterize aquifer vulnerability, regardless of well depth or aquifer characteristics. Trends in 

contaminant concentrations were not assessed in this study, but low-level detections can be used in other 

studies to track changes in contaminant occurrence and concentrations over time. The reporting levels for 

the analytical methods used by the U.S. Environmental Protection Agency, water utilities, or commercial 

laboratories for the analysis of finished-water samples for compliance monitoring typically are higher than 

reporting levels for analytical methods used by USGS. As a result, contaminant detection frequencies in USGS

reports may be greater than detection frequencies in annual water-quality reports (consumer confidence 

reports) provided by water utilities.

• Detailed analysis of mixtures focused on a subset of 383 source-water samples in which most 

contaminants—major ions, trace elements, nutrients, radon, pesticide compounds, and volatile organic 

compounds (VOCs)—were analyzed. Mixtures of contaminants were identified in three categories: (1) 

contaminants each detected at concentrations greater than individual human-health benchmarks; in this 

category, radon activities were compared to the higher proposed Alternative MCL (4,000 picocuries per 

liter, or pCi/L); (2) contaminants each detected at concentrations greater than one‐tenth of individual 

benchmarks; radon activities were compared to the lower proposed MCL (300 pCi/L); and (3) same as 

the second category, but with additional consideration for man-made organic contaminants detected at 

any concentration. 

• Mixtures of man-made organic contaminants also were evaluated in a subset of 814 source-water 

samples, and in a subset of 94 paired samples of source and finished water, regardless of the 

availability of human-health benchmarks or the concentrations relative to benchmarks.

Data used in this report previously have been selectively included in national-scale NAWQA

assessments of specific contaminant groups (trace elements, radionuclides, nutrients, pesticide compounds, 

and VOCs) and have been used in detailed investigations of water-quality conditions in individual NAWQA 

assessment areas and principal aquifers. Publications from these previous assessments are available online 

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

NAWQA’s Approach to Assessing the Quality of Water from Public Wells 17
An aquifer is a saturated, permeable geologic formation that can

transmit substantial quantities of water, such as to wells (Freeze and Cherry,

1979). The amount of groundwater that can flow through a geologic formation

primarily depends on the permeability (the size and arrangement of the 

connected spaces in the materials that comprise the formation) and the 

hydraulic gradient. Permeable geologic materials—such as gravel, sand, 

sandstone, and fractured rock—have large or many connected spaces 

through which groundwater can readily flow. Relatively impermeable 

materials—such as clay or shale—have spaces that are not well connected

or are so small that the flow of groundwater is restricted (Heath, 1983).

A confined aquifer is one that is located beneath a relatively 

impermeable (confining) layer, and an unconfined aquifer is one in which 

the water table forms the upper boundary (Heath, 1983) (fig. 3). Confined 

aquifers tend to be deeper than unconfined aquifers, which commonly 

are near the land surface (Freeze and Cherry, 1979). Accordingly, of the 

public wells sampled in this study, the 397 wells that withdraw water from 

confined aquifers generally were deeper than the 502 wells that withdraw 

water from unconfined aquifers (Toccalino and others, 2010). 

The susceptibility of an aquifer to contamination from activities at 

the land surface can be directly related to the degree of aquifer isolation 

from the land surface by the confining layers (Lindsey and others, 2009). In 

general, an aquifer tends to be less susceptible to contamination if it has 

a deep water table (for an unconfined aquifer), a thick, low-permeability 

layer, such as clay-rich material, between the aquifer and the land surface 

(for a confined aquifer), and no highly permeable fractures that can act as 

conduits for fluid flow. By contrast, an aquifer tends to be more susceptible 

to contamination if it has a shallow water table, no low-permeability 

protecting layer, and features that facilitate the movement of water, such 

as fractures and nearby wellbores (Younger, 2007). The vulnerability of an 

aquifer to contamination depends on (1) intrinsic susceptibility (the physical 

aquifer properties that affect groundwater flow), as discussed above, and 

(2) the presence of contaminant sources and additional factors that affect 

contaminant fate and transport (Focazio and others, 2002).

In general, deep wells are presumed to be less vulnerable than 

shallower wells to contaminants that originate at the land surface 

(Zogorski and others, 2006). Large withdrawal rates from public wells, 

however, combined with proximity to developed areas and the possible 

presence of short-circuiting flow paths, make even deep wells vulnerable 

to contamination. Short-circuiting flow paths can be natural (such as 

fractures in a confining layer) or man-made (such as wells) and can allow 

water and contaminants to bypass substantial portions of aquifers that 

would otherwise restrict their movement to deep public wells (Landon 

and others, 2009). Deep public wells, which are commonly pumped for 

sustained periods at high rates, can intercept groundwater flowing along 

extensive flow paths with long residence times (Zogorski and others, 2006). 

Groundwater that flows over long distances to public wells may 

include water that drains from a variety of land-use settings 

and may contain degradation products from parent compounds 

and contaminants with substantial historical use (Gilliom and 

others, 2006). Additionally, public wells in some hydrogeologic 

settings may intercept water with a wide range of ages, including 

relatively young water flowing along short flow paths that make 

such wells vulnerable to contaminants in recently recharged 

groundwater (McMahon and others, 2008).

Aquifer Types and Well Depth

Figure 3. Samples were collected from 502 public wells 

that withdraw water from unconfined aquifers and 397 

public wells that withdraw water from confined aquifers 

(33 sampled wells withdraw water from mixed or unknown 

aquifer types). Unconfined aquifers tend to be shallower, 

and more vulnerable to contamination from human activities 

at the land surface, than confined aquifers. This simplified 

diagram was modified from Alley and others (1999). 

18 Quality of Water from Public-Supply Wells in the United States, 1993–2007

Land surface

Unsaturated

zone

Unconfined

aquifer

Confined

aquifer

Low-permeability

confining

layer

Water table

Shallow well in

unconfined 

aquifer

Deep well in

confined aquifer
Evaluating Results in the Context of Human Health

IP-014233_fig 05

HBSLs

40 percent

(135 contaminants)

MCLs

17 percent

(58 contaminants)

Benchmarks

not available

43 percent

(144 contaminants)

USEPA Maximum Contaminant Levels (MCLs)—

 regulated in drinking water

USGS Health-Based Screening Levels (HBSLs)—

 not regulated in drinking water

Benchmarks not available—

 not regulated in drinking water

EXPLANATION

Human-health benchmarks used in this study

Contaminant concentrations in untreated source water from public wells were 

compared to Maximum Contaminant Levels (MCLs) or Health-Based Screening 

Levels (HBSLs) to provide an initial perspective on the potential significance of 

detected contaminants to human health and to help prioritize further investigations 

(see sidebar on page 4). Such comparisons are not designed to evaluate specific 

effects of contaminants on human health, and are not a substitute for comprehensive 

risk assessments, which generally include many additional factors such as multiple 

avenues of exposure (Toccalino and others, 2006).

Contaminant concentrations greater than human-health benchmarks in source 

water are of potential human-health concern, but do not necessarily indicate that 

adverse effects will occur, because (1) the benchmarks are conservative (protective) 

(Toccalino and others, 2006; Toccalino, 2007), and (2) source-water samples 

were collected prior to any treatment or blending that could alter contaminant 

concentrations in finished drinking water. In this study, contaminant concentrations 

greater than MCLs do not represent MCL violations because MCLs appy to finished 

water and the majority of the samples were collected from source waters. None of 

the source-water or finished-water samples were collected for regulatory compliance 

purposes; further, compliance with some MCLs is based on running annual average 

concentrations, not on concentrations detected in single samples as collected in this 

study.

Exposure to individual contaminants detected at concentrations less than 

benchmarks is unlikely to result in adverse human-health effects because the 

benchmarks typically are concentrations in drinking water that are not anticipated 

to cause adverse effects from a lifetime of exposure (Toccalino, 2007; U.S. 

Environmental Protection Agency, 2009a). The occurrence of contaminants for which 

human-health benchmarks are not yet available were not evaluated in the context of 

potential human-health significance in this study.

Human-health benchmark values used in this report were current as of 

September 2009. MCL values were obtained from the U.S. Environmental Protection 

Agency (USEPA) (2006a), and HBSL values were obtained from the HBSL website 

(Toccalino and others, 2008). USEPA updated their drinking-water standards and 

guidelines document in late 2009 (U.S. Environmental Protection Agency, 2009a), but 

data from the updated USEPA report were not available in time for inclusion in this 

report. MCLs or HBSLs were available for 193 of the 337 contaminants analyzed in 

this study (fig. 4).

Concentrations greater than one-tenth of benchmarks were used in this study 

to provide an early and conservative indication of contaminants that may approach 

concentrations of potential human-health concern, either individually or in mixtures, 

and to identify contaminants that may warrant additional monitoring (Toccalino and 

others, 2010). Various federal and state agencies use the criteria of one-tenth (or 

one-half) of a human-health benchmark for a variety of purposes; examples include: 

(1) reporting contaminant occurrence in groundwater (U.S. Environmental Protection 

Agency, 1999b; New Jersey Department of Environmental Protection, 2003), 

(2) reporting pesticide detections in water to 

USEPA under the Federal Insecticide, Fungicide, 

and Rodenticide Act (Nebraska Department of 

Agriculture, 1997; U.S. Environmental Protection 

Agency, 1998), (3) ranking the susceptibility of 

wells to contamination (New Jersey Department 

of Environmental Protection, 2003, 2004), 

and (4) identifying contaminants of potential 

human-health concern for risk assessment 

evaluations (U.S. Environmental Protection Agency, 

1993, 1994).

Figure 4. Human-health benchmarks were 

available for 57 percent of the contaminants 

analyzed in this study. Most (83 percent) of 

the 337 contaminants analyzed in this study 

are not regulated by the U.S. Environmental 

Protection Agency in drinking water under the 

Safe Drinking Water Act (SDWA); MCLs are 

not available for unregulated contaminants. 

Contaminants that are not regulated in drinking 

water under the SDWA may be regulated in 

other contexts and under other statutes.

NAWQA’s Approach to Assessing the Quality of Water from Public Wells 19
20 Quality of Water from Public-Supply Wells in the United States, 1993–2007

Major Findings

More than one in five source-water samples from public wells contained one or 

more contaminants at concentrations greater than human-health benchmarks

One or more chemical contaminants 

were detected at concentrations greater than 

human‐health benchmarks in more than one in 

five (22 percent) of the 932 source-water samples 

from public wells (fig. 5). Concentrations of at 

least one chemical contaminant were greater 

than an MCL in 15 percent of the source-water 

samples when radon activities were compared to 

the proposed Alternative MCL of 4,000 pCi/L. 

Concentrations of at least one contaminant 

were greater than an HBSL in 10 percent of 

the source-water samples, and contaminant 

concentrations were greater than both MCLs 

and HBSLs in some samples. MCLs or HBSLs 

were not available for 144 (43 percent) of the 

contaminants analyzed in this study (fig. 4). 

Collectively, contaminants regulated in drinking 

water accounted for 60 percent of concentrations 

greater than human-health benchmarks, and 

unregulated contaminants accounted for 

40 percent of concentrations greater than 

benchmarks. 

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! One or more contaminants at concentrations greater than human-health benchmarks (206 wells)

! One or more contaminants at concentrations greater than one-tenth of benchmarks, but less than benchmarks (539 wells)

! No contaminants at concentrations greater than one-tenth of benchmarks, or no contaminants detected (187 wells)

EXPLANATION

Figure 6. Twenty-two percent of 932 source-water samples from public wells contained one or more chemical contaminants at 

concentrations greater than Maximum Contaminant Levels or Health-Based Screening Levels. A total of 80 percent of samples contained one 

or more contaminants at concentrations greater than one-tenth of human-health benchmarks. Public wells yielding water in which

contaminant concentrations were greater than benchmarks, as well as those in which concentrations were greater than one-tenth of 

benchmarks, were distributed throughout the United States and included wells that withdraw water from every principal aquifer rock type 

included in this study. (See fig. 2A for explanation of principal aquifer rock types.)

Figure 5. Twenty-two percent of 932 source-water samples from public wells contained one or more chemical 

contaminants at concentrations greater than Maximum Contaminant Levels or Health-Based Screening Levels. A 

total of 80 percent of samples (206 plus 539 wells) contained one or more contaminants at concentrations greater 

than one-tenth of human-health benchmarks. Public wells yielding water in which contaminant concentrations 

were greater than benchmarks, as well as those in which concentrations were greater than one-tenth of 

benchmarks, were distributed throughout the United States and included wells that withdraw water from every 

principal aquifer rock type included in this study. (See fig. 2A for explanation of principal aquifer rock types.) 
Major Findings 21

A total of 23 contaminants were detected 

at concentrations greater than MCLs or HBSLs 

in at least one source-water sample from public 

wells. Ten of these contaminants—seven 

from natural sources and three primarily from 

man‐made sources—individually were detected 

at concentrations greater than human-health 

benchmarks in at least 1 percent of samples 

(fig. 6) and collectively accounted for most 

concentrations greater than benchmarks. 

Contaminant concentrations greater than 

human-health benchmarks in source water are 

of potential human-health concern, but do not 

necessarily indicate that adverse effects will 

occur (see sidebar on page 19). The potential 

health effects associated with the ingestion of 

water containing any of these 10 contaminants 

at concentrations greater than benchmarks 

include an increased risk of cancer (for six 

of the contaminants); various neurological, 

developmental, and reproductive effects; liver 

problems; and "blue-baby syndrome" (table 3). 

Five of the 10 contaminants that were 

detected at concentrations greater than 

benchmarks in at least 1 percent of source-water 

samples (radium, arsenic, gross alpha-particle 

radioactivity, nitrate, and perchloroethene 

(PCE)) are regulated in drinking water by 

the USEPA under the SDWA, and therefore 

managed, in finished drinking water (fig. 6); 

radon has proposed MCLs. Manganese, boron, 

and dieldrin have undergone USEPA’s regulatory 

determination process under the SDWA, but after 

evaluating available occurrence, exposure, and 

health-effects information, USEPA determined 

that no regulatory action was appropriate for 

these contaminants because they infrequently 

occur at health levels of concern in public water 

systems, and regulating these contaminants in 

drinking water would not “present a meaningful 

opportunity for health risk reduction” (U.S. 

Environmental Protection Agency, 2003a, 

2008b). Strontium is listed on USEPA’s most 

recent (third) Contaminant Candidate List (CCL) 

(U.S. Environmental Protection Agency, 2010a).

A total of 80 percent of source-water 

samples contained at least one contaminant at 

concentrations greater than one-tenth of humanhealth benchmarks. Most individual contaminant 

detections, however, were less than one-tenth of 

human-health benchmarks, and many detections 

were several orders of magnitude less than 

benchmarks. Public wells yielding water in 

which contaminant concentrations were greater 

than benchmarks, as well as those in which 

concentrations were greater than one-tenth of 

benchmarks, were distributed throughout the 

United States and include wells that withdraw 

water from all principal aquifer rock types 

included in this study (fig. 5).

0 10 20 30 40 50 60

PERCENTAGE OF SOURCE-WATER SAMPLES

WITH CONCENTRATIONS GREATER

THAN HUMAN-HEALTH BENCHMARKS

Figure 7. Ten contaminants, including seven from natural sources and three from primarily man-made sources, were each detec

concentrations greater than human-health benchmarks in at least 1 percent of source-water samples from public wells. Radium

alpha-particle radioactivity, nitrate, and perchloroethene are regulated in drinking water under the Safe Drinking Water Act.

Radon*

Radium-226 plus radium-228

Arsenic

Gross alpha-particle radioactivity

Manganese

Strontium

Boron

Dieldrin

Nitrate

Perchloroethene

Contaminant from natural sources

Contaminant primarily from 

 man-made sources

*Radon activities were greater than the proposed 

Maximum Contaminant Level (MCL) of 300 picocuries 

per liter (pCi/L) in 55 percent of samples, and were 

greater than the proposed Alternative MCL of 4,000 

pCi/L in 0.6 percent of samples

Figure 6. Ten contaminants, including seven from natural sources and three from primarily manmade sources, each were detected at concentrations greater than human-health benchmarks in at 

least 1 percent of source-water samples from public wells. Radium, arsenic, gross alpha-particle 

radioactivity, nitrate, and perchloroethene are regulated in drinking water under the Safe Drinking 

Water Act.
22 Quality of Water from Public-Supply Wells in the United States, 1993–2007

1

 Human-health benchmark values were current as of September 2009. MCL values were obtained from the USEPA (2006a) and HBSL values were obtained 

from the HBSL website (Toccalino and others, 2008).

2

 Unless otherwise indicated, references for potential health effects and sources of contaminant are from the USEPA (2003f, 2009b). MCLs and HBSLs 

generally are contaminant concentrations at which adverse effects are not expected over a lifetime of exposure.

3

 Radon activities greater than the proposed AMCL of 4,000 pCi/L were counted.

4

 Radon activities greater than the proposed MCL of 300 pCi/L were counted.

5

 The MCL for gross alpha-particle radioactivity excludes alpha-particle radioactivity from radon and uranium (U.S. Environmental Protection Agency, 2000). 

Gross alpha-particle radioactivity values for public-well samples in this study were not corrected for radon or uranium.

6

 Manganese, dieldrin, and boron have undergone USEPA’s regulatory determination process under the Safe Drinking Water Act, but were not selected for 

regulation in drinking water (U.S. Environmental Protection Agency, 2003a, 2008b).

7

 Strontium is listed on USEPA’s most recent (third) Contaminant Candidate List (U.S. Environmental Protection Agency, 2010a).

8 Low end of HBSL range corresponding to a 10-6

 (one in one million) cancer risk. The HBSL range corresponds to a 10-6 to 10-4 cancer risk range. 

9

 The MCL for perchloroethene is under review by the USEPA.

Table 3. The potential health effects associated with the 10 contaminants that each were detected at concentrations greater than 

developmental, and reproductive effects; liver problems; and "blue-baby syndrome". Contaminants are listed in decreasing percentage

[pCi/L, picocuries per liter; AMCL, Alternative Maximum Contaminant Level; MCL, Maximum Contaminant Level; μg/L, micrograms per liter; HBSL, 

Contaminant Contaminant 

type

Human-health 

benchmark 1

Potential health effects from ingestion of water with contaminant 

concentration greater than the human-health benchmark 2

Value Type

Radon-222 Radionuclide 4,000

(300) pCi/L

Proposed AMCL

(proposed MCL)

Increased risk of lung cancer a,b 

Radium-226 plus

radium-228

Radionuclide 5 pCi/L MCL Increased risk of cancer 

Arsenic Trace element 10 μg/L MCL Skin damage or problems with circulatory system, and may have

increased risk of getting cancer

Gross alpha-particle

radioactivity

Radionuclide 15 pCi/L MCL 5 Increased risk of cancer 

Manganese 6 Trace element 300 μg/L HBSL Neurological effects c,d,e 

Strontium 7 Trace element 4,000 μg/L HBSL Abnormal bone development f,g 

Dieldrin 6 Pesticide

compound,

insecticide

0.002 μg/L HBSL low 8 Liver and central nervous system problems, increased risk of cancer d,h,i,j 

Boron 6 Trace element 1,000 μg/L HBSL Reproductive and developmental effects d,k,l 

Nitrate Nutrient 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 "blue-baby syndrome"

Perchloroethene Volatile organic

compound, solvent

5 μg/L MCL 9 Liver problems, increased risk of cancer 
Major Findings 23

Sources of contaminant

in drinking water 2

Number of 

public wells 

sampled

Percentage of samples with

Principal aquifer rock types most often associated 

with concentrations greater than benchmarks

Concentrations 

greater than 

benchmark

Concentrations 

greater than onetenth of benchmark

Radioactive decay of uranium in aquifer materials a,b 506 3 0.6, 4 55 4 55 Unconsolidated sand and gravel (non-glacial)

Erosion of natural deposits 191 19 62 Sandstone—not all rock types were sampled

Erosion of natural deposits; runoff from orchards; runoff

from industrial wastes

638 10 42 Unconsolidated sand and gravel (non-glacial)

Erosion of natural deposits of certain radioactive

minerals

84 5 54 Semi-consolidated sand and gravel—not all rock

types were sampled

Aquifer materials; some industrial wastes c,d 808 5 20 Glacial sand and gravel and unconsolidated

sand and gravel (non-glacial) 

Aquifer materials; some industrial wastes f 503 3 49 Sandstone aquifers

Historical agricultural applications and insecticidal

use; waste sites; all uses banned in 1987 i,j

896 3 3 Semi-consolidated sand and gravel

Aquifer materials; sewage or septic-system effluent;

fertilizer application; some industrial wastes d,k

501 3 28 Sandstone

Runoff from fertilizer use; leaching from septic tanks;

sewage; erosion of natural deposits

806 2 45 Unconsolidated sand and gravel (non-glacial)

Discharge from factories and dry cleaners 829 1 3 Semi-consolidated sand and gravel

a

 National Research Council (1999).

b

 U.S. Environmental Protection Agency (1999a).

c

 Agency for Toxic Substances and Disease Registry (2008).

d

 World Health Organization (2004).

e

 U.S. Environmental Protection Agency (1995).

f

 Agency for Toxic Substances and Disease Registry (2004).

g

 U.S. Environmental Protection Agency (1992).

h

 U.S. Environmental Protection Agency (1987).

i

 Agency for Toxic Substances and Disease Registry (2002b).

j

 U.S. Environmental Protection Agency (2003c).

k

 U.S. Environmental Protection Agency (2008d).

l

 U.S. Environmental Protection Agency (2004a).

human-health benchmarks in at least 1 percent of source-water samples include an increased risk of cancer; various neurological, 

of source-water samples from public wells with concentrations greater than human-health benchmarks.

Health-Based Screening Level; mg/L as N, milligrams per liter as nitrogen; USEPA, U.S. Environmental Protection Agency]
Trace elements in groundwater originate primarily from the dissolution and weathering of the rocks and minerals that make up aquifers and soils. Example 

sources, from left to right, include arsenopyrite, a sulfide mineral containing arsenic; pyrolusite, a manganese oxide; celestite, a strontium sulfate; and borax, a 

sodium borate compound. 

(Arsenopyrite, pyrolusite, and borax photographs by Andrew Young, Brigham Young University. Celestite photograph by Mary Sutherland, James Madison University.)

24 Quality of Water from Public-Supply Wells in the United States, 1993–2007

Contaminants from natural sources accounted for about three-quarters of contaminant 

concentrations greater than human-health benchmarks in source-water samples

Naturally occurring trace elements and 

radionuclides accounted for about three-quarters 

(74 percent) of all contaminant concentrations greater 

than human-health benchmarks in source-water 

samples from public wells. Four trace elements 

(arsenic, manganese, strontium, and boron) and three 

radionuclides (radon, radium, and gross alpha-particle 

radioactivity) each were detected at concentrations 

greater than benchmarks in at least 1 percent of 

source-water samples (fig. 6). Radon activities were 

greater than the higher proposed Alternative MCL of 

4,000 pCi/L in less than 1 percent of samples, but were 

greater than the lower proposed MCL of 300 pCi/L 

in 55 percent of samples. Each of the remaining 

six trace elements and radionuclides was detected 

at concentrations greater than benchmarks in 3 to 

19 percent of source-water samples (fig. 6). 

Overall, concentrations of one or more trace 

elements were greater than human-health benchmarks 

in 16 percent of source-water samples (fig. 7), and 

were greater than one-tenth of benchmarks in a total 

of 73 percent of samples (Toccalino and others, 2010). 

One or more radionuclide activities were greater 

than benchmarks in 8 percent of samples when radon 

activities were compared to the higher proposed 

Alternative MCL of 4,000 pCi/L (fig. 7), and were 

greater than one-tenth of benchmarks in a total of 

68 percent of samples (Toccalino and others, 2010). All 

together, concentrations of one or more trace elements 

or radionuclides were greater than MCLs or HBSLs 

in 19 percent of samples when radon activities were 

compared to 4,000 pCi/L. 

Many trace elements and radionuclides were 

detected at concentrations greater than benchmarks in 

samples from both confined and unconfined aquifers 

Figure 8. Contaminants from natural sources were detected more 

frequently at concentrations greater than human-health benchmarks 

in source-water samples from public wells than were man-made 

contaminants. Trace elements and radionuclides were detected at 

concentrations greater than benchmarks in samples from both 

unconfined and confined aquifers, consistent with the fact that these 

contaminants originate primarily from aquifer materials. By contrast, 

most man-made contaminants were detected at concentrations 

greater than human-health benchmarks only in samples from 

unconfined aquifers, consistent with the fact that these contaminants 

originate at the land surface.

0

3

6

9

12

15

18

Trace

elements

Radionuclides*Nitrate Pesticide

compounds

Volatile

organic

compounds

PERCENTAGE OF SOURCE-WATER SAMPLES WITH

CONCENTRATIONS GREATER THAN

HUMAN-HEALTH BENCHMARKS 

Mixed or unknown aquifers

Confined aquifers

Unconfined aquifers

*Radon activities were compared to the proposed

Alternative Maximum Contaminant Level of 4,000 

picocuries per liter

CONTAMINANTS 

FROM NATURAL 

SOURCES

CONTAMINANTS 

PRIMARILY FROM 

MAN-MADE SOURCES

Figure 7. Contaminants from natural sources were detected more 

frequently at concentrations greater than human-health benchmarks 

in source-water samples from public wells than were man-made 

contaminants. Trace elements and radionuclides were detected 

at concentrations greater than benchmarks in samples from both 

unconfined and confined aquifers, consistent with the fact that 

these contaminants originate primarily from aquifer materials. By 

contrast, man-made contaminants were detected at concentrations 

greater than human-health benchmarks mainly in samples 

from unconfined aquifers, consistent with the fact that these 

contaminants originate from human activities at the land surface.

(fig. 7), consistent with the fact that these naturally occurring 

contaminants originate primarily from aquifer materials, rather 

than from man-made sources at the land surface. 
The concentrations of naturally occurring inorganic 

contaminants in groundwater reflect, in large part, their 

abundance in aquifer materials, the length of time that the 

water is in contact with aquifer materials, and physical and 

geochemical conditions, such as pH and dissolved oxygen 

concentrations, that govern their solubility and mobility 

(Freeze and Cherry, 1979; Hem, 1985). The chemical 

characteristics of water in natural systems, therefore, vary 

from place to place and with the types of soils, rocks, and 

aquifer materials through which the groundwater moves. 

Human activities such as mining and industrial discharges 

also can have a strong influence on the occurrence of many 

trace elements in water (Hem, 1985). Man-made sources of 

radionuclides include nuclear facilities and power plants, and 

institutional and industrial facilities, such as hospitals and 

laboratories (U.S. Environmental Protection Agency, 2000). 

In areas where development has occurred (such as increased 

irrigation and pumping associated with agricultural and 

urban development), associated changes in groundwater flow 

and aquifer chemistry can enhance the mobility of natural 

contaminants in aquifers, resulting in increased concentrations 

in groundwater withdrawn from public wells (Jurgens and 

others, 2009).

The occurrences of radon and arsenic, the radionuclide 

and trace element detected at concentrations greater than 

benchmarks in the highest percentages of samples, are 

described below. Detailed descriptions of the occurrences of 

the remaining eight contaminants detected at concentrations 

greater than human-health benchmarks in at least 1 percent of 

the public-well samples are included in Toccalino and others 

(2010). 

Radon

Radon is a naturally occurring radioactive gas that is 

soluble in groundwater. Radon activities in groundwater can 

reflect the distribution of uranium-bearing rocks, from which 

radon originates (Ayotte and others, 2007). In this study, 

radon activities were positively correlated with uranium 

concentrations in most principal aquifer rock types. Radon 

dissolved in water was detected in nearly all (98 percent) of 

506 source-water samples analyzed for radon in this study.

USEPA classifies radon as a human carcinogen (U.S. 

Environmental Protection Agency, 2006a). Radon contributes 

to lung-cancer deaths through exposure to radon and its decay 

products in indoor air. Because radon is a gas, many uses 

of water, such as showering, readily release radon from the 

water into indoor air. Ingestion of radon in drinking water may 

contribute to adverse effects in the gastrointestinal tract and 

other organs (National Research Council, 1999). 

USEPA has proposed an MCL of 300 pCi/L and 

an Alternative MCL of 4,000 pCi/L for radon in public 

water systems (U.S. Environmental Protection Agency, 

2006a). Some states also have set their own standards or 

guidelines for radon in drinking water, including New 

Hampshire (2,000 pCi/L), Maine (4,000 pCi/L), Connecticut 

(5,000 pCi/L), and Massachusetts (10,000 pCi/L) (New 

Hampshire Department of Environmental Services, 

2008; Maine Center for Disease Control and Prevention, 

2006; Connecticut Department of Public Health, 2005; 

Commonwealth of Massachusetts, 2009). States may set radon 

guidelines higher than USEPA’s proposed MCL or Alternative 

MCL because USEPA’s proposed benchmarks for radon 

are not final, and are therefore not yet legally enforceable. 

Under the SDWA, individual states may set and enforce 

drinking‐water standards that are more stringent than USEPA’s 

national standards (U.S. Code, 1996).

Radon is formed as part of the normal radioactive decay chain of 

uranium. Most rocks contain some uranium, but some types of 

rocks—such as granite shown here—have higher than average 

uranium contents. These include some volcanic rocks, dark 

shales, sedimentary rocks that contain phosphate, and granites. 

(Photograph from U.S. Geological Survey Photographic Library.)

Major Findings 25
Radon activities were greater than the proposed 

Alternative MCL of 4,000 pCi/L in less than 1 percent of 

source-water samples from public wells (3 samples), but 

55 percent of samples contained radon activities greater 

than the proposed MCL of 300 pCi/L (table 3 and fig. 6). 

Public wells yielding water with radon activities greater than 

300 pCi/L were geographically distributed across the United 

States and included wells that withdraw water from every 

principal aquifer rock type (fig. 8). Radon activities generally 

were lowest in public wells in the central United States and in 

a few smaller sampled areas, such as eastern North Carolina, 

where radon either was not detected or activities were less 

than 300 pCi/L (fig. 8). 

High radon activities in water withdrawn from crystallinerock aquifers, particularly in the northeastern United States, have

been reported in several previous studies. For example, Ayotte 

and others (2007) and DeSimone (2009) each calculated a median

radon activity of greater than 2,000 pCi/L in groundwater from 

observation, domestic, and public wells that withdraw water 

from crystalline-rock aquifers, primarily in the northeastern 

United States. In this study, radon was analyzed in only two

samples from crystalline-rock aquifers, so substantive comparisons

to previous studies could not be made, but radon activities were 

greater than 2,000 pCi/L in each of those two samples.

Figure 8. Radon activities were greater than the proposed Maximum Contaminant Level (MCL) of 300 pCi/L in 

55 percent of source-water samples from public wells. These wells are distributed throughout the United States, 

and include wells that withdraw water from every principal aquifer rock type sampled. Radon activities greater 

than the proposed Alternative MCL of 4,000 pCi/L were detected in only 3 samples. Radon was analyzed in 506 

source-water samples. (See fig. 2A for explanation of principal aquifer rock types; >, greater than.) 

26 Quality of Water from Public-Supply Wells in the United States, 1993–2007

EXPLANATION

Radon activities, in picocuries per liter (pCi/L)

! >4,000 (3 wells)

! >300 to 4,000 (274 wells)

! Less than or equal to 300 or not detected (229 wells)

Figure 9. Radon activities were greater than the proposed Maximum Contaminant Level (MCL) of 300 pCi/L in 55 percent of 

source-water samples from public wells. These wells are distributed throughout the United States, and withdraw water from 

every principal aquifer rock type sampled. Radon activities greater than the proposed Alternative MCL of 4,000 pCi/L were 

detected in only 3 samples. Radon was analyzed in 506 source-water samples. (See fig. 2A for explanation of principal aquifer

rock types; >, greater than.)

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Major Findings 27

Arsenic

Arsenic is a naturally occurring nonmetallic 

trace element in rocks and soils, as well as waters in 

contact with them (Hem, 1985; Focazio and others,

2000). Man-made sources of arsenic include

agricultural pesticides, wood preservatives, 

contaminants from mining activities, and livestock 

feed additives (Welch and others, 2000; Nordstrom, 

2002). Arsenic was detected in 70 percent of 

638 source-water samples analyzed for arsenic in 

this study. 

Recognized as a toxic element for centuries, 

arsenic is of human-health concern because 

it can contribute to a wide variety of adverse 

health effects, including several types of cancers 

(National Research Council, 2001). Concentrations 

of arsenic were greater than the MCL of 10 

micrograms per liter (μg/L) in 10 percent of the 

source‐water samples (table 3). Seven to 10 percent 

of groundwater samples in previous national-scale 

studies (Focazio and others, 2000; Welch and 

others, 2000; DeSimone, 2009) contained arsenic 

at concentrations greater than 10 μg/L, which is 

consistent with the findings from this study, even 

though the earlier studies included more water 

samples from different types of wells.

Almost all of the source-water samples with 

arsenic concentrations greater than the MCL 

were from public wells that withdraw water from 

unconsolidated sand and gravel aquifers (nonglacial origin) and from glacial sand and gravel 

aquifers. Although public wells yielding water 

with arsenic concentrations greater than the MCL 

were distributed across the United States and 

withdraw water from both unconfined and confined 

aquifers, about three-quarters of these wells were 

in the western United States (fig. 9). Arsenic 

concentrations were lowest in samples from public 

wells in the north-central and the southeastern areas 

of the United States (except for Florida). Previous 

national-scale studies also have reported arsenic 

concentrations in groundwater to generally be 

highest in the western United States (Focazio and 

others, 2000; Welch and others, 2000; Ryker, 2001) 

and lowest in the southeastern United States (Welch 

and others, 2000). Regional-scale studies, however, 

have noted arsenic concentrations in groundwater 

greater than 10 μg/L in other areas of the country, 

such as eastern New England (Ayotte and others, 

2003). A total of 42 percent of source-water samples 

from public wells contained arsenic concentrations 

greater than one-tenth of the MCL (table 3); these 

public wells were widely distributed across the 

United States (fig. 9). 

The mobility and solubility of arsenic in 

groundwater is increased by low concentrations 

of dissolved oxygen, high pH (greater than 8.5), 

and the presence of substances such as organic 

carbon and iron oxides (Welch and others, 2000; 

Nordstrom, 2002; Kelly and others, 2005). Arsenic 

concentrations greater than the MCL in groundwater 

are associated with factors such as the release of 

arsenic from iron oxide, on which arsenic may 

be adsorbed (Welch and others, 2000), and old 

groundwater recharged before 1953 (Thomas, 

2007). Additionally, sulfide minerals are both a 

source and sink for arsenic, and geothermal waters 

with high evaporation rates also are associated 

with arsenic concentrations greater than 10 μg/L 

in groundwater, particularly in the western United 

States (Welch and others, 2000).
28 Quality of Water from Public-Supply Wells in the United States, 1993–2007

IP-016007_fig10

Figure 10. Arsenic concentrations were greater than the Maximum Contaminant Level (MCL) of 10 micrograms per liter in 

10 percent of source-water samples from public wells. Most of these wells withdraw water from sand and gravel aquifers 

(glacial and non-glacial origin) in several western states. A total of 42 percent of samples contained arsenic concentrations 

greater than one-tenth of the MCL. Arsenic was analyzed in 638 source-water samples. (See fig. 2A for explanation of 

principal aquifer rock types; >, greater than.)

EXPLANATION

Arsenic concentration, in micrograms per liter

! >10 (63 wells)

! >1 to 10 (206 wells)

! Less than or equal to 1 or not detected (369 wells)

!

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Figure 9. Arsenic concentrations were greater than the Maximum Contaminant Level (MCL) of 10 

micrograms per liter in 10 percent of source-water samples from public wells. Most of these wells 

withdraw water from sand and gravel aquifers (glacial and non-glacial origin) in several western states. 

A total of 42 percent of samples (63 plus 206 wells) contained arsenic at concentrations greater than 

one-tenth of the MCL. Arsenic was analyzed in 638 source-water samples. (See fig. 2A for explanation of 

principal aquifer rock types; >, greater than.)
Perchloroethene (PCE) is the 

most commonly used solvent at 

commercial dry cleaners.

(Photograph from iStock Photos.)

Although nitrate occurs 

naturally in groundwater, 

concentrations often are 

increased by human activities 

in agricultural and urban 

areas. Sources of nitrate to 

groundwater include fertilizers, 

livestock, and wastewater.

(Photograph from iStock Photos.)

Dieldrin is an insecticide 

that is no longer used in the 

United States. Historically, 

dieldrin was used to control 

insects on some crops, 

and to control locusts, 

mosquitoes, and termites.

(Photograph from iStock Photos.)

Major Findings 29

Contaminants that originate entirely or primarily from man-made sources 

accounted for about one-quarter of contaminant concentrations greater than 

human-health benchmarks in source-water samples and were detected in nearly 

two-thirds of the samples, predominantly in samples from unconfined aquifers

Nitrate, pesticide compounds, and VOCs 

accounted for about one-quarter (25 percent) 

of all contaminant concentrations greater than 

MCLs or HBSLs in source-water samples from 

public wells. Nitrate (a nutrient), dieldrin (an 

insecticide), and PCE (a solvent), each were 

detected at concentrations greater than MCLs or 

HBSLs in 1 to 3 percent of source-water samples 

(table 3 and fig. 6). Nitrate occurs naturally, but 

nitrate concentrations greater than 1 milligram 

per liter as nitrogen (which is one-tenth of 

the nitrate MCL) are considered indicative of 

man-made sources such as fertilizers, livestock, 

and wastewater in many parts of the United 

States (Nolan and Hitt, 2003). Dieldrin and PCE 

originate from man-made sources. 

Nitrate was detected at concentrations 

greater than the MCL in 2 percent of the 

source-water samples from public wells, and 

at concentrations greater than one-tenth of the 

MCL in a total of 45 percent of samples (table 

3). All samples with nitrate concentrations 

greater than the MCL were collected from 

unconfined aquifers (fig. 7), consistent with 

the fact that nitrate originates from man-made 

sources at the land surface. Most samples with 

nitrate concentrations greater than the MCL 

were collected from public wells that withdraw 

water from unconsolidated sand and gravel 

aquifers, which is consistent with previous 

studies that indicate that aquifers with high 

effective porosities—such as those consisting 

of unconsolidated sand and gravel—are most 

vulnerable to nitrate contamination (Mueller and 

others, 1995; Nolan and Hitt, 2006; Lindsey and 

others, 2009).

Only 7 of the 168 pesticide compounds 

or VOCs analyzed in most source-water 

samples (Appendix 1) were detected at 

concentrations greater than human-health 

benchmarks in at least one public-well 

sample. These organic contaminants included 

dieldrin, alpha‐hexachlorocyclohexane, PCE, 

trichloroethene (TCE), dibromochloropropane, 

ethylene dibromide, and acrylonitrile. All 

together, one or more pesticide compounds were 

detected at concentrations greater than MCLs 

or HBSLs in 3 percent of source-water samples 

from public wells, and one or more VOCs were 

detected at concentrations greater than MCLs or 

HBSLs in 2 percent of samples (fig. 7). These 

findings are consistent with recent national 

assessments that reported that pesticide and VOC 

concentrations were greater than human-health 

benchmarks in 2 and 1.5 percent of public-well 

samples, respectively (Gilliom and others, 2006; 

Zogorski and others, 2006). 

Detections of dieldrin accounted for almost 

all concentrations of pesticide compounds 

greater than human-health benchmarks. Dieldrin, 

a highly persistent insecticide compound, 

originates from historical applications of dieldrin 

and aldrin, which degrades to dieldrin in the 

environment (table 3). By 1974, all except a few 

uses of dieldrin were banned in the United States 

(U.S. Environmental Protection Agency, 1980), 

and the USEPA banned all uses in 1987 (Agency 

for Toxic Substances and Disease Registry, 

2002a). As a result, detections of dieldrin reflect 

pesticide use practices that are no longer allowed, 

and show that source-water protection strategies 

that rely on changes in human activities and 

practices at the land surface to achieve waterquality objectives can take years to centuries to 

affect the quality of water in some public wells 

(McMahon and others, 2008). 
30 Quality of Water from Public-Supply Wells in the United States, 1993–2007

Collectively, concentrations of one or more pesticide 

compounds or VOCs were greater than human-health 

benchmarks in 4.5 percent of source-water samples and 

were greater than one-tenth of benchmarks in a total of 

10 percent of samples (fig. 10). Almost all (86 to 94 percent) 

of the pesticide compound and VOC concentrations greater 

than human-health benchmarks were detected in samples 

from public wells that withdraw water from unconfined 

aquifers, consistent with the fact that these contaminants 

originate at the land surface (fig. 7). 

About two-thirds of the source-water samples with 

concentrations of pesticide compounds or VOCs greater 

than benchmarks were from public wells in the highly 

populated areas of states bordering the East Coast, primarily 

in semi-consolidated sand and gravel aquifers that underlie 

the Coastal Plains (fig. 10). Public wells yielding water with 

concentrations of pesticide compounds or VOCs greater than 

one-tenth of benchmarks were distributed throughout the 

United States, but nearly three-quarters of these public wells 

withdraw water from unconsolidated and semi-consolidated 

sand and gravel aquifers. Most samples collected from public 

wells in the central and west-central United States contained 

concentrations of pesticide compounds or VOCs less than 

one-tenth of benchmarks, or human-health benchmarks were 

not available for detected organic contaminants (fig. 10).

The occurrence of pesticide compounds and VOCs in 

groundwater is influenced by their sources and pathways 

in the hydrologic system; and their volatility, mobility and 

persistence in soils and groundwater (Gilliom and others, 

2006; Zogorski and others, 2006). Pesticides are released 

into the environment primarily through their application to 

agricultural lands, such as croplands, and to non-agricultural 

EXPLANATION

! Concentrations greater than human-health benchmarks (41 wells)

! Concentrations greater than one-tenth of benchmarks, but less than benchmarks (49 wells)

! Concentrations less than or equal to one-tenth of benchmarks, or no benchmarks available (499 wells)

! Not detected (326 wells)

One or more pesticide compounds or VOCs with:

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Figure 11. Concentrations of pesticide compounds or volatile organic compounds (VOCs) were greater than human-health benchmarks in 4.5 

percent of source-water samples from public wells. About two-thirds of these wells withdraw water from semi-consolidated sand and gravel 

aquifers in the highly populated areas of East-Coast states. A total of 10 percent of samples contained concentrations of pesticide compounds 

or VOCs greater than one-tenth of benchmarks. Pesticide compounds and VOCs were analyzed in 915 source-water samples. (See fig. 2A for 

explanation of principal aquifer rock types.)

Figure 10. Concentrations of one or more pesticide compounds or volatile organic compounds (VOCs) 

were greater than human-health benchmarks in 4.5 percent of source-water samples from public wells. 

About two-thirds of these wells withdraw water from semi-consolidated sand and gravel aquifers in the 

highly populated areas of East-Coast states. A total of 10 percent of samples (41 plus 49 wells) contained 

one or more pesticide compounds or VOCs at concentrations greater than one-tenth of benchmarks. 

Pesticide compounds and VOCs were analyzed in 915 source-water samples. (See fig. 2A for explanation of 

principal aquifer rock types.)
Sources of man-made organic contaminants, such as pesticide compounds and volatile organic 

compounds (VOCs), are various and widespread. More than a billion pounds of pesticides are 

applied each year in the United States to control weeds, insects, and other pests in agricultural 

and urban areas. VOCs are used in many industrial, commercial, and domestic activities, and 

sources to groundwater include landfills, leaking storage tanks and spills, leaking water and 

sewer lines, and vehicular exhaust. 

(Photograph by Jeff Vanuga, Natural Resources Conservation Service, U.S. Department of Agriculture.)

(Photograph from EGDigital, iStock photos.)

Major Findings 31

lands, such as lawns, golf courses, and commercial

areas (Gilliom and others, 2006). For some 

pesticides, USEPA may modify pesticide labels 

and resulting use patterns to mitigate exposures 

to those pesticides in areas expected to be most 

vulnerable. USEPA makes these decisions based 

on their assessments of potential exposure to 

pesticides in drinking water by using modeling 

tools and monitoring data (Elizabeth Behl, 

U.S. Environmental Protection Agency, written 

commun., December 2009), such as those data 

included in this study. VOCs are a broad class of 

organic contaminants that have numerous uses in 

industry, commerce, households, and military sites 

(Zogorski and others, 2006), and consequently 

have several transport pathways into groundwater. 

Collectively, one or more man-made pesticide 

compounds or VOCs were detected in nearly 

two-thirds (64 percent) of the source-water 

samples; pesticide compounds and VOCs each 

were detected in 41 and 60 percent of samples, 

respectively (fig. 11). About two-thirds of the 

samples in which nitrate or pesticide compounds 

were detected were from public wells that 

withdraw water from unconfined aquifers and 

one-third of the samples were from public wells 

that withdraw water from confined aquifers 

(fig. 11). This finding is consistent with previous 

studies that show that shallow groundwater—

particularly from unconfined aquifers—is more 

vulnerable to contamination from human activities 

at the land surface than deep groundwater from 

confined aquifers (U.S. Geological Survey, 1999) 

(see sidebar on page 18). Fifty-nine percent of the 

samples in which VOCs were detected were from 

unconfined aquifers. Nitrate, pesticide compounds, 

and VOCs were detected in a significantly greater 

proportion of samples from unconfined aquifers 

than in samples from confined aquifers. Other 

organic contaminants, such as personal-care and 

domestic-use products, were detected in 18 percent 

of samples, about one-half of which were from 

unconfined aquifers (fig. 11).

A total of 293 organic contaminants were 

analyzed in this study; 169 (58 percent) were 

detected in at least one source-water sample 

from public wells, and 124 (42 percent) were not 

detected in any sample (Appendixes 1 and 2). 

Chloroform, a disinfection by-product (DBP), was 

the most frequently detected organic contaminant 

in source-water samples from public wells—it 

was detected in 46 percent of all source-water 

samples. Chloroform also was reported as the most 

frequently detected VOC in the Nation’s public 

wells in other studies (Westrick and others, 1984; Squillace and others, 

1999; U.S. Environmental Protection Agency, 2002b; Zogorski and others, 

2006). Chloroform is used as a solvent, is a by-product of chlorination of 

drinking water and wastewater, and may be introduced into groundwater 

through artificial recharge of aquifers using chlorinated water (Zogorski 

and others, 2006). 

Other frequently detected VOCs—in 11 to 22 percent of source-water 

samples—included solvents such as PCE, TCE, and 1,1,1-trichloroethane, 

the gasoline oxygenate methyl-tert-butyl ether (MTBE), and 

bromodichloromethane, another DBP. Deethylatrazine, a degradation 

product of atrazine, was the most frequently detected pesticide compound 

in source water, and was detected in 30 percent of samples. The herbicides 

atrazine, simazine, and prometon also were frequently detected—in 11 to 

27 percent of all source-water samples. 

Figure 11. The frequent detections of man-made contaminants in 

source-water samples from public wells, particularly those wells that 

withdraw water from unconfined aquifers, indicate the susceptibility 

of aquifers to contamination from human activities at the land surface. 

Nitrate concentrations greater than 1 milligram per liter (>1 mg/L) as 

nitrogen typically originate from man-made sources.

IP-016007_fig 11

0

25

50

75

Nitrate

>1 mg/L

(806 samples)

Volatile organic

compounds

(833 samples)

Pesticide 

compounds

(904 samples)

Personal-care

products and

other organic

contaminants

(217 samples)

PERCENTAGE OF SOURCE-WATER SAMPLES 

WITH DETECTIONS

Mixed or unknown aquifers

Confined aquifers

Unconfined aquifers
(Photograph from iStock Photos.)

32 Quality of Water from Public-Supply Wells in the United States, 1993–2007

Many organic contaminants detected in source water also were detected in 

finished water at similar concentrations

As many as 272 organic contaminants 

were analyzed in a subset of 94 paired source‐ 

and finished-water samples from public wells; 

inorganic contaminants were not analyzed in 

these paired samples. Public water systems 

typically are required to monitor finished water 

for about 90 contaminants that are regulated 

in drinking water under the SDWA (U.S. 

Environmental Protection Agency, 2009a), but 

generally do not monitor finished water for many 

of the 272 organic contaminants assessed by 

Hopple and others (2009). The extent to which 

contaminants detected in source water also were 

detected in finished water was assessed in this 

study, but the efficacy of water treatment used at 

the public water systems was not evaluated. 

Disinfection was the primary water 

treatment used at about three-quarters of the 

sampled systems (J.A. Hopple, U.S. Geological 

Survey, written commun., January 2010), 

which is about the same proportion as for all 

groundwater-supplied public water systems 

nationwide. Nationally, about three-quarters 

(71 percent) of groundwater-supplied public 

water systems use disinfection only or 

disinfection plus chemical addition, such as 

the addition of fluoride (U.S. Environmental 

Protection Agency, 2002a). Disinfection destroys 

harmful organisms, but disinfection is not 

designed to treat organic contaminants, and may 

chemically transform them into different organic 

contaminants. Public water systems that are 

not in compliance with the SDWA (systems not 

meeting MCLs) using disinfection alone must use 

additional treatment—such as granular activated 

carbon and aeration—or change or blend their 

sources of water (U.S. Environmental Protection 

Agency, 2004b, 2004c).

Forty-five organic contaminants (excluding 

DBPs) were detected in both source- and 

corresponding finished-water samples. DBPs 

were excluded from this analysis because, 

during water treatment, DBPs can form when 

disinfectants, such as chlorine, react with organic 

matter in the water (Richardson and others, 

2002). As a result, DBP concentrations were 

greater in finished water than in source water, 

as is generally expected. Twenty-two of the 45 

contaminants—including pesticides, pesticide 

degradates, and VOCs—each were detected in 

more than 10 percent of source-water samples. 

Concentrations of these 22 frequently detected 

contaminants generally were similar in source 

and finished water, especially for nonblended 

finished water (fig. 12). These results are 

consistent with previous analyses by the USEPA 

Office of Pesticide Programs, which reported 

that pesticides detected in source waters often 

are detected in treated finished waters at similar 

concentrations (U.S. Environmental Protection 

Agency, 2001b). As expected, the variability 

in concentrations between source water and 

finished water generally was greater for finished 

water that was blended with water from other 

wells before distribution (fig. 12). 

When contaminant concentrations 

(including DBPs) were compared to humanhealth benchmarks, two of the source-water 

samples contained PCE, TCE, and (or) 

1,1-dichloroethene at concentrations greater 

than MCLs (Hopple and others, 2009). None 

of the finished-water samples contained any 
Major Findings 33

IP-016007_fig 16

Figure 13. When an organic contaminant was detected in both source and finished water, concentrations generally were similar, especially 

for nonblended finished water. As expected, the variability in concentrations between source water and finished water (scatter around the 1:1 

line) was greater for blended finished water, which includes water from other wells that were not sampled. Data are shown for 22 organic 

contaminants that were detected in more than 10 percent of source-water samples; data for disinfection by-products are not shown. Data are 

from 94 public wells from which source and finished water were sampled.

0.001 0.01 0.1 1 10 100

0.001

0.01

0.1

1

10

100

SOURCE-WATER CONCENTRATION (μg/L)

BLENDED FINISHED-WATER CONCENTRATION (μg/L)

Nondetections set to 0.001 μg/L

1:1

Blended finished water

(60 samples)

0.001 0.01 0.1 1 10 100

0.001

0.01

0.1

1

10

100

SOURCE-WATER CONCENTRATION (μg/L)

NONBLENDED FINISHED-WATER CONCENTRATION,

IN MICROGRAMS PER LITER (μg/L)

Nondetections set to 0.001 μg/L

1:1

Nonblended finished water

(34 samples)

Figure 12. When an organic contaminant was detected in both source and finished water, concentrations generally were 

similar, especially for nonblended finished water. As expected, the variability in concentrations between source water and 

finished water (scatter around the 1:1 line) was greater for blended finished water, which includes water from other wells that 

were not sampled. Data are shown for 22 organic contaminants that each were detected in more than 10 percent of sourcewater samples; data for disinfection by-products are not shown. Data are from 94 public wells from which source and finished 

water were sampled.

organic contaminants at concentrations greater 

than benchmarks. At least one contaminant was 

detected at concentrations greater than one-tenth of 

benchmarks in 9 source-water samples (primarily 

solvents), and in 11 finished-water samples 

(primarily DBPs). Dieldrin was not detected in 

any sample from 48 paired source- and finishedwater samples (Hopple and others, 2009). 

Some contaminants were detected at lower 

concentrations in finished water than in source 

water, or were detected in source water only 

(fig. 12). The solvents 1,1,1-trichloroethane, 

PCE, and TCE are examples of such 

contaminants (Hopple and others, 2009). There 

are several possible explanations for these 

results, including a decrease in concentrations 

in finished water as a result of blending source 

water from several wells, the transformation or 

removal of contaminants during water treatment, 

volatilization, accidental introduction of 

contaminants at the wellhead sample collection 

point, or analytical variability for concentrations 

near the detection level. 

Conversely, some contaminants were 

detected at lower concentrations in source 

water than in finished water, or were detected 

in finished water only (fig. 12). Examples 

of such contaminants include xylenes 

(gasoline hydrocarbons), methyl isobutyl 

ketone (a solvent), and several herbicides 

or herbicide degradates (Hopple and others, 

2009). Determining the possible sources of 

contaminants that were detected only in finished 

water is beyond the scope of this study. Blending 

of the finished water with water from other wells 

prior to distribution, however, appears to account 

for many of these detections in finished water 

only (fig. 12). Maintenance and chemical use at 

public water systems also can possibly explain 

some unmatched detections. 
34 Quality of Water from Public-Supply Wells in the United States, 1993–2007

Human-health benchmarks are not yet available for many organic contaminants, including some 

that were frequently detected in source water 

MCLs or HBSLs were not available for 144 (43 percent) 

of the contaminants analyzed in this study (fig. 4) and, 

therefore, the occurrence of these contaminants could not be 

evaluated in the context of human health. Most contaminants 

for which human-health benchmarks were not available are 

organic contaminants (Appendixes 1 and 2).

The SDWA directs USEPA to regulate contaminants in 

drinking water that present the greatest public-health concern. 

As part of determining whether drinking-water regulations 

are needed, the SDWA requires USEPA to publish a list of 

unregulated contaminants called the CCL every 5 years. The 

CCL includes contaminants that currently are not subject 

to any proposed or promulgated national primary drinking 

water regulations, which are known or anticipated to occur 

in public water systems, and which may require regulation 

under the SDWA in the future. Following the publication 

of each CCL, USEPA decides whether to regulate at least 

five contaminants from the CCL in drinking water (called 

regulatory determinations) on the basis of the contaminant’s 

potential for adverse human-health effects and occurrence in 

public water systems, and a meaningful opportunity to protect 

public health (U.S. Environmental Protection Agency, 2010a). 

The USEPA uses USGS data on the occurrence of unregulated 

contaminants in water resources as part of both the CCL and 

regulatory determination processes.

Nine unregulated contaminants for which human-health 

benchmarks are not available were detected in 6 to 35 percent 

of source-water samples from public wells (fig. 13). Of these 

contaminants, the gasoline oxygenate MTBE, the solvent 

1,1-dichloroethane, and the herbicide degradates alachlor 

ethane sulfonic acid, alachlor oxanilic acid, metolachlor ethane 

sulfonic acid, and metolachlor oxanilic acid are listed on 

the most recent (third) CCL (U.S. Environmental Protection 

Agency, 2010a). Deethylatrazine and deisopropylatrazine are 

degradates of atrazine, and are not listed on the third CCL. 

Most herbicide degradates analyzed in this study are not 

currently regulated by the USEPA in drinking water under 

the SDWA, but some herbicide degradates are regulated 

by USEPA under the Federal Insecticide, Fungicide, and 

Rodenticide Act (FIFRA) (see sidebar on page 36). FIFRA 

requires USEPA to register pesticides sold or used in 

the United States to ensure that pesticides will not cause 

unreasonable risks to the environment or human health when 

used according to the label directions (U.S. Environmental 

Protection Agency, 2009d). 

Figure 13. Degradates of the herbicides 38atrazine, alachlor, and 

metolachlor; a gasoline oxygenate (methyl tert-butyl ether); and a 

solvent (1,1-dichloroethane) were the most frequently detected 

organic contaminants in source-water samples from public wells for 

which human-health benchmarks are not available. Concentrations 

of the parent herbicides atrazine, alachlor, and metolachlor were less 

than MCLs or HBSLs in all source-water samples. ESA, ethane 

sulfonic acid.

Analyzed in a subset of as many 

as 73 samples (see sidebar on 

page 36)

Analyzed in as many as 

853 samples

0 5 10 15 20 25 30 35

Alachlor oxanilic acid

Alachlor ESA 2nd amide

Metolachlor oxanilic acid

Metolachlor ESA

Alachlor ESA

1,1-Dichloroethane

Deisopropylatrazine

MTBE

Deethylatrazine

PERCENTAGE OF SOURCE-WATER 

SAMPLES WITH DETECTIONS

EXPLANATION

Figure 13. Degradates of the herbicides atrazine, alachlor, 

and metolachlor; a gasoline oxygenate (methyl tert-butyl ether 

or MTBE); and a solvent (1,1-dichloroethane) were the most 

frequently detected organic contaminants in source-water 

samples from public wells for which human-health benchmarks 

are not available. Concentrations of the parent herbicides 

atrazine, alachlor, and metolachlor were less than humanhealth benchmarks in all source-water samples. ESA, ethane 

sulfonic acid.
Atrazine, alachlor, and metolachlor are herbicides used to 

control weeds in corn fields, although each also has other 

uses. 

(Photograph from iStock Photos.)

Atrazine and deethylatrazine were the most frequently 

detected pesticide compounds of those that were analyzed in 

most of the source-water samples from public wells. Because 

deethylatrazine is a degradation product of atrazine, it tended 

to occur in the same source-water samples as atrazine. 

Atrazine has been one of the most heavily used herbicides 

in the United States for nearly 20 years, on the basis of 

total pounds applied (Thelin and Stone, 2010). Most of the 

agricultural use of atrazine has been associated with corn 

production, but it also has had some nonagricultural use. 

Atrazine is highly soluble, mobile, and moderately persistent 

in water (Gilliom and others, 2006). Atrazine, but none of its 

degradation products, is regulated by USEPA in drinking water 

under the SDWA (U.S. Environmental Protection Agency, 

2006a). Atrazine concentrations were less than the MCL in all 

source-water samples; concentrations were greater than onetenth of the MCL in 4 samples (Toccalino and others, 2010).

The degradates of alachlor and metolachlor were 

analyzed using a separate analytical method in a subset of 

43 to 73 source-water samples from selected areas where the 

parent herbicides likely were used (see sidebar on page 36). 

These degradates were detected in 6 to 35 percent of the 

samples (fig. 13). Alachlor and metolachlor are used on corn 

and other crops (Gilliom and others, 2006), and in the CornBelt region of the United States, more than four times as 

many pounds of metolachlor than alachlor have been applied 

annually since 1996 (Sullivan and others, 2009). Alachlor is 

regulated in drinking water under the SDWA, but metolachlor 

and the degradates of alachlor and metolachlor are not 

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

concentrations were less than the MCL in all source-water 

samples; concentrations were greater than one-tenth of the 

MCL in one sample. All metolachlor detections were less than 

one-tenth of the HBSL (Toccalino and others, 2010).

Major Findings 35
Herbicide degradates are formed by the transformation 

or breakdown of a parent herbicide by chemical, photochemical,

or biologically mediated reactions. Eight herbicides and 23 

of their degradation products were assessed in a subset 

of as many as 73 source-water samples from public wells 

(Hopple and others, 2009). The only herbicides detected in 

more than one sample were from two classes of herbicides, 

the chloroacetanilides (acetochlor, alachlor, and metolachlor) 

and the triazines (atrazine and simazine). Degradates of these 

herbicides were analyzed because the parent herbicides likely 

were used in the areas that were sampled, and because 

degradates may have similar or greater persistence than their 

parent herbicides (Thurman and others, 1992; Kalkhoff and 

others, 1998).

Herbicide degradates may have less, similar, or greater 

toxicity than parent compounds (Hladik and others, 2005). For 

example, in U.S. Environmental Protection Agency (USEPA) risk 

assessments, the toxicities of the chlorotriazine degradates 

of atrazine, including deethylatrazine, are considered to be 

equivalent to that of the parent compound (U.S. Environmental 

Protection Agency, 2003d, 2006b). By contrast, some 

chloroacetanilide degradates have been shown to be less toxic 

than their parent herbicides (Heydens and others, 2000). For 

many pesticide degradates, however, the toxicity to humans is 

not well understood.

Summed concentrations of the chloroacetanilide 

degradates generally were substantially greater than 

the parent herbicide concentrations (fig. 14). Summed 

concentrations of the atrazine degradates generally were 

similar to or greater than the concentrations of the atrazine 

parent compound. In some samples, herbicide degradates—

but not the parent compounds—were detected, pointing to 

the importance of monitoring for both parent and degradate 

compounds. Total degradate concentrations can be similar to or 

greater than parent-compound concentrations when conditions 

in groundwater favor transformations to degradates that are 

chemically persistent. Groundwater moves relatively slowly 

through aquifer materials, providing time for microbial or 

chemical transformations to occur (Gilliom and others, 2006).

As part of the Federal Insecticide, Fungicide, and 

Rodenticide Act reregistration of atrazine in 2003, the 

USEPA developed Drinking Water Levels of Comparison 

(DWLOCs) for the sum of the concentrations of atrazine 

and its three chlorinated degradates (deethylatrazine, 

deisopropylatrazine, and deethyldeisopropyl atrazine), referred 

to as total chlorotriazines (TCT), in public water systems 

(U.S. Environmental Protection Agency, 2003d). DWLOCs are 

the maximum concentrations in drinking water that, when 

considered together with other dietary exposure, do not exceed a 

level of concern.

The USEPA no longer uses the DWLOC approach for 

dietary risk assessments (Elizabeth Behl, U.S. Environmental 

Protection Agency, written commun., December 2009), and is in 

the process of re-evaluating the potential human-health effects of 

atrazine (U.S. Environmental Protection Agency, 2009c). Although 

this re-evaluation of atrazine will change future assessments, 

TCT concentrations in source-water samples from public wells 

in this study were compared to the DWLOC of 12.5 micrograms 

per liter as a preliminary assessment. This DWLOC for TCT 

is a 90-day moving average that applies when source-water 

samples are collected from a water system less frequently 

than weekly during the high-use season (U.S. Environmental 

Protection Agency, 2003d). None of 73 source-water samples 

analyzed in this study contained summed TCT concentrations 

greater than one-tenth of the DWLOC.

Figure 14. Summed concentrations of herbicide degradates 

generally were similar or greater than concentrations of parent 

compounds for two classes of herbicides, the chloroacetanilides 

and the triazines. Symbols above the 1:1 line indicate that 

concentrations of summed degradates were greater than 

concentrations of parent herbicides. Herbicide degradates were 

analyzed in a subset of as many as 73 source-water samples from 

public wells. 

Figure 15. Summed concentrations of herbicide degradates 

generally were similar or greater than that of parent compounds for 

representatives of two classes of herbicides, the chloroacetanilides 

and the triazines. The variability in concentrations between parent 

herbicide and summed degradate concentrations (scatter around the 

1:1 line) was greater for chloroacetanilides than for triazines. These 

herbicide degrades were analyzed in a subset of as many as 73 

source-water samples from public wells. Details on which herbicide 

degradates were analyzed are included in Hopple and others (2009).

0.001 0.01 0.1 1 10

0.001

0.01

0.1

1

10

SUMMED DEGRADATE CONCENTRATION IN SOURCE WATER,

IN MICROGRAMS PER LITER (μg/L)

PARENT HERBICIDE CONCENTRATION IN SOURCE WATER (μg/L)

1:1

Nondetections set to 0.001 μg/L

Atrazine

Acetochlor

Alachlor

Metolachlor

Triazine

Chloroacetanilides

Herbicide Degradates were Frequently Detected in Source Water

36 Quality of Water from Public-Supply Wells in the United States, 1993–2007
Major Findings 37

Contaminants usually co-occurred with other contaminants in source and finished water, and 

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

human-health benchmarks were dominated by inorganic contaminants

Analysis of individual contaminants, as described 

above, is needed to identify those that occur at 

concentrations of potential human-health concern, and to 

assess where and why they occur. Most people, however, 

are exposed to mixtures of man-made organic and naturally 

occurring inorganic contaminants at concentrations less than 

those known to cause adverse effects (Carpenter and others, 

2002). Although few human-health benchmarks have been 

established for mixtures of contaminants, concentrations of 

the contaminants in mixtures were compared to individual 

benchmarks in this study. Little is known about the 

potential health effects associated with exposure to multiple 

contaminants at the low concentrations typically detected 

in the environment, yet toxicologic interactions that can 

result in adverse effects are possible at these concentrations 

(Hasegawa and others, 1994; Yang, 1994).

Identifying and characterizing all possible contaminant 

mixtures in drinking water and assessing their toxicity is 

not practical (Squillace and others, 2002; Ryker and Small, 

2008). Many contaminant combinations are possible, and 

which combinations are potential concerns because of their 

combined toxicity often is unknown. Evaluating the toxicity 

of mixtures to humans is complicated by the possibility of 

multiple types of interactions between the contaminants 

(such as additive, synergistic, antagonist, or no interactions), 

and because many individual contaminants can affect 

multiple organs and functions in the body (Carpenter and 

others, 2002). Findings from this study, however, can help to 

identify which contaminant mixtures may be of most concern 

because of their frequency of occurrence in groundwater 

withdrawn by public wells and comparisons to individual 

human-health benchmarks. 

Occurrence and Distribution of Mixtures

Although 22 percent of source-water samples 

from public wells contained at least one contaminant at 

concentrations greater than human-health benchmarks, 

mixtures of two or more contaminants in which the 

concentration of each contaminant was greater than its 

individual benchmark, were detected less frequently—in 

4 percent of source-water samples (fig. 15). On the other 

hand, 84 percent of source-water samples contained two 

or more contaminants in which the concentration of each 

contaminant was greater than one-tenth of its individual 

benchmark. Mixtures that also considered detections of any 

pesticide compound or VOC (without regard to the availability 

of benchmarks or concentrations relative to individual 

benchmarks) were detected in 92 percent of samples (fig. 15).

20

0

40

60

80

100

0 ≥5 ≥10 ≥15 ≥20 ≥25

PERCENTAGE OF SOURCE-WATER SAMPLES

WITH DETECTIONS

NUMBER OF CONTAMINANTS IN SAMPLE

Figure 16. Mixtures of two or more contaminants were detected at 

concentrations greater than one-tenth of individual human-health 

benchmarks in 84 percent of samples, and mixtures of five or more 

contaminants were detected at such concentrations in 17 percent of 

samples. Little is known about the potential health effects 

associated with exposure to contaminant mixtures. Data are from 

383 source-water samples from public wells in which major ions, 

trace elements, nitrate, radon, pesticide compounds, and volatile 

organic compounds were analyzed. ≥, 

At concentrations greater than individual human-health 

benchmarks

At concentrations greater than one-tenth of individual 

human-health benchmarks

At concentrations greater than one-tenth of individual 

human-health benchmarks plus any detection of a pesticide 

compound or volatile organic compound

Mixtures of two or more contaminants

EXPLANATION

Figure 15. Mixtures of two or more contaminants were detected 

at concentrations greater than one-tenth of individual human-health 

benchmarks in 84 percent of samples, and mixtures of five or more 

contaminants were detected at such concentrations in 17 percent of 

samples. Little is known about the potential health effects associated 

with exposure to contaminant mixtures. Data are from 383 sourcewater samples from public wells in which major ions, trace elements, 

nitrate, radon, pesticide compounds, and volatile organic compounds 

were analyzed. ≥, greater than or equal to.
38 Quality of Water from Public-Supply Wells in the United States, 1993–2007

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Figure 17. Mixtures with the largest number of contaminants (5 to 10) detected at concentrations greater than one-tenth of individual 

human-health benchmarks were distributed among a number of sampled areas throughout the United States. Such mixtures were most

common, however, in samples collected from public wells that withdraw water from unconsolidated sand and gravel aquifers in parts of the 

western United States. Data are from 383 source-water samples from public wells in which major ions, trace elements, nitrate, radon, 

pesticide compounds, and volatile organic compounds were analyzed. Radon activities were compared to the proposed MCL of 300 picocuries 

per liter. (See fig. 2A for explanation of principal aquifer rock types.)

EXPLANATION

Number of contaminants detected at concentrations 

greater than one-tenth of individual human-health benchmarks

! 5 to 10

! 3 to 4

! 2

! 1

! 0

Figure 16. Mixtures with the largest number of contaminants (5 to 10) detected at concentrations greater 

than one-tenth of individual human-health benchmarks were distributed among a number of sampled areas 

throughout the United States. Such mixtures were most common, however, in samples collected from 

public wells that withdraw water from unconsolidated sand and gravel aquifers in parts of the western 

United States. Data are from 383 source-water samples from public wells in which major ions, trace 

elements, nitrate, radon, pesticide compounds, and volatile organic compounds were analyzed. Radon 

activities were compared to the proposed MCL of 300 picocuries per liter. (See fig. 2A for explanation of 

principal aquifer rock types.)

Mixtures with the largest number of 

contaminants (5 to 10) detected at concentrations 

greater than one-tenth of individual human-health 

benchmarks were distributed among a number of 

sampled areas throughout the United States, but 

were most common in public wells sampled in parts 

of the western United States. About three-quarters 

of the public wells with five or more contaminants 

detected at concentrations greater than one-tenth 

of individual benchmarks withdraw water from 

unconsolidated sand and gravel aquifers in the 

western United States, mostly in California, Utah, 

Nevada, and Arizona (fig. 16).
Major Findings 39

Composition of Mixtures

The most common contaminants in unique 

mixtures—specific combinations of contaminants 

regardless of others that may be present—

were those that were most frequently detected 

individually. Twenty-one unique mixtures of 

contaminants at concentrations greater than 

individual benchmarks were identified, and 

each of these mixtures was detected in less than 

1 percent of the source-water samples. These 

mixtures most commonly were composed 

of solvents (primarily PCE and TCE), trace 

elements (such as arsenic and strontium), and 

nitrate, indicating a mix of naturally occurring 

and man-made contaminants (table 4). 

Sixty-eight unique mixtures of contaminants 

at concentrations greater than one-tenth of 

individual benchmarks each were detected in 

at least 5 percent of the source-water samples. 

All of these mixtures were composed of a trace 

element (arsenic, strontium, or uranium were 

most common), radon, and (or) nitrate. The 

most common unique mixture of contaminants 

at concentrations greater than one-tenth of 

individual benchmarks was nitrate and radon, 

which was detected in 32 percent of samples 

(table 4). 

When the unique mixtures of contaminants 

at concentrations greater than one-tenth of 

individual benchmarks were expanded to 

include all detections of pesticide compounds 

or VOCs, regardless of their concentrations 

relative to benchmarks or the availability of 

benchmarks, 125 unique mixtures each were 

detected in at least 10 percent of source-water 

samples. Nitrate and radon were among the most 

common inorganic contaminants in these unique 

mixtures, but consideration of all pesticide or 

VOC detections added herbicides and herbicide 

degradates, such as atrazine and deethylatrazine; 

DBPs, such as chloroform; and solvents, such as 

PCE to the most common contaminants in the 

mixtures. The most common unique mixture in 

this category was nitrate and chloroform, which 

was detected in 34 percent of samples (table 4).

Mixtures of man-made pesticide compounds 

and VOCs also were evaluated separately from 

other contaminant groups because there are no 

natural sources for most of these contaminants 

and because the occurrence and toxicity of such 

mixtures are not well understood. Mixtures of 

pesticide compounds and VOCs were assessed 

without regard to the availability of benchmarks 

or the concentrations relative to individual 

benchmarks. Twenty-three unique mixtures of 

organic contaminants each were detected in at 

least 10 percent of the source-water samples 

(table 5). 

Three-quarters of the organic mixtures 

contained an herbicide (atrazine or simazine) 

or an herbicide degradate (deethylatrazine). 

The most common unique mixture of organic 

contaminants was atrazine and deethylatrazine 

(a degradation product of atrazine), which was 

detected in 25 percent of samples. The common 

co-occurrence of atrazine and deethylatrazine 

also was reported in previous regional- and 

national-scale studies (Ayers and others, 2000; 

Squillace and others, 2002; Gilliom and others, 

2006; DeSimone, 2009). Two-thirds of the 

organic mixtures contained chloroform (a DBP), 

and 43 percent contained a solvent (PCE or TCE) 

(table 5). The most frequently detected mixtures 

of VOCs in other national assessments of public 

wells and public water systems also included 

DBPs and solvents (Grady, 2002; Zogorski and 

others, 2006). 

An analysis of mixtures of organic 

contaminants in paired samples from the 

subset of 94 public wells for which source 

and finished water were compared showed 

similar numbers of organic contaminants in 

source- and finished‐water samples. Mixtures 

were detected in a somewhat larger percentage 

of finished‐water samples than source-water 

samples because of the formation of DBPs 

in finished water. This finding was observed 

regardless of the complexity of (number of 

contaminants detected in) the mixtures (Hopple 

and others, 2009). Several recent studies have 

begun to evaluate DBP mixtures in drinking 

water because of widespread exposures to these 

complex mixtures and because epidemiologic 

and toxicologic studies of DBPs have raised 

concerns for human health (Teuschler and 

Simmons, 2003; U.S. Environmental Protection 

Agency, 2003b; Simmons and others, 2004; 

Teuschler and others, 2004).
40 Quality of Water from Public-Supply Wells in the United States, 1993–2007

Table 4. Unique mixtures of contaminants at concentrations greater than individual human-health 

benchmarks each were infrequently detected in source-water samples from public wells. The most 

common unique mixtures of contaminants at concentrations greater than one-tenth of individual 

benchmarks were composed of nitrate, naturally occurring trace elements, or radon. When 

detections of pesticide compounds or volatile organic compounds were included in the mixtures, 

herbicides, disinfection-by products, and solvents were common contaminants in the mixtures. 

[Data are from 383 source-water samples from public wells in which major ions, trace elements, nitrate, radon, 

pesticide compounds, and volatile organic compounds were analyzed]

Composition of the 10 most common unique mixtures detected in 

source-water samples from public wells by selected mixture criteria

Detection 

frequency of 

unique mixtures 

(percent of 

samples)

Mixtures of contaminants at concentrations greater than individual human-health benchmarks 1

Boron Strontium 0.8

Arsenic Manganese 0.8

Nitrate Perchloroethene 0.5

Arsenic Molybdenum 0.5

Boron Radon 0.4

Nitrate Strontium 0.4

Arsenic Strontium 0.4

Perchloroethene Dieldrin 0.3

Nitrate Perchloroethene Trichloroethene 0.3

Nitrate Perchloroethene Dibromochloropropane 0.3

Mixtures of contaminants at concentrations greater than 

one-tenth of individual human-health benchmarks 2

Nitrate Radon 32

Arsenic Radon 29

Boron Strontium 27

Strontium Radon 22

Uranium Radon 20

Nitrate Strontium 19

Nitrate Arsenic 19

Nitrate Uranium 18

Nitrate Arsenic Radon 16

Arsenic Uranium 16

Mixtures of contaminants at concentrations greater than one-tenth of individual

human-health benchmarks, plus the detection of any pesticide compound or volatile 

organic compound regardless of the availability of benchmarks 2

Nitrate Chloroform 34

Chloroform Radon 34

Nitrate Radon 32

Arsenic Radon 29

Nitrate Deethylatrazine 27

Boron Strontium 27

Deethylatrazine Atrazine 26

Nitrate Atrazine 24

Nitrate Chloroform Radon 24

Nitrate Deethylatrazine Atrazine 23

1

 Radon activities were compared to the proposed Alternative Maximum Contaminant Level of 4,000 picocuries 

per liter (pCi/L). 2

 Radon activities were compared to the proposed Maximum Contaminant Level of 300 pCi/L, which is about 

one-tenth of the proposed Alternative Maximum Contaminant Level.
Major Findings 41

Table 5. The most common unique mixtures of pesticide compounds and volatile 

organic compounds in source-water samples from public wells contained herbicides 

(atrazine or simazine), an herbicide degradate (deethylatrazine), a disinfection by-product 

(chloroform), and (or) solvents (perchloroethene or trichloroethene). These mixtures of 

organic contaminants were assessed without regard to the availability of human-health 

benchmarks. 

[Data are from 814 source-water samples from public wells in which pesticide compounds and volatile 

organic compounds were analyzed]

Composition of the most common unique mixtures of organic 

contaminants detected in source-water samples from public wells

Detection 

frequency of 

unique mixtures 

(percent of 

samples)

Deethylatrazine Atrazine 25

Deethylatrazine Chloroform 24

Chloroform Atrazine 21

Deethylatrazine Chloroform Atrazine 20

Chloroform Perchloroethene 20

Deethylatrazine Perchloroethene 14

Simazine Chloroform 13

Simazine Deethylatrazine 13

Deethylatrazine Chloroform Perchloroethene 13

Perchloroethene Atrazine 13

Simazine Atrazine 13

Simazine Deethylatrazine Atrazine 13

Chloroform Methyl tert-butyl ether 13

Deethylatrazine Perchloroethene Atrazine 12

Chloroform Perchloroethene Atrazine 12

Chloroform Trichloroethene 12

Deethylatrazine Chloroform Perchloroethene Atrazine 12

Simazine Deethylatrazine Chloroform 12

Simazine Chloroform Atrazine 11

Chloroform 1,1,1-Trichloroethane 11

Simazine Deethylatrazine Chloroform Atrazine 11

Chloroform Bromodichloromethane 11

Perchloroethene Trichloroethene 11
42 Quality of Water from Public-Supply Wells in the United States, 1993–2007

The most complex mixtures in source water—those with the greatest number of 

contaminants—were most often detected in samples from unconfined aquifers

The most complex mixtures of contaminants in 

source‐water samples from public wells—those mixtures with 

the greatest number of contaminants—were detected more often 

in samples from shallower unconfined aquifers than in samples 

from deeper confined aquifers. For mixtures of contaminants 

at concentrations greater than one-tenth of individual 

human‐health benchmarks, the proportion of samples from the 

more vulnerable unconfined aquifers increased as the complexity 

of the mixtures increased (fig. 17). 

About one-half of the samples containing two or more 

contaminants at concentrations greater than one-tenth of 

individual benchmarks were from unconfined aquifers, and 

about one-half were from confined aquifers. This finding was 

expected because simple mixtures containing two or three 

contaminants were dominated by inorganic contaminants 

(table 4), which occur in both confined and unconfined aquifers. 

By contrast, about two-thirds of samples containing four to 

six contaminants at concentrations greater than one‐tenth 

of benchmarks, and more than three-quarters of samples 

containing seven to 10 contaminants at concentrations greater 

than one-tenth of benchmarks, were from unconfined aquifers 

(fig. 17). Generally, as the number of contaminants in 

mixtures increased, the proportion of samples containing 

organic contaminants also increased, reflecting the greater 

vulnerability of unconfined aquifers to contaminants from 

man-made sources (fig 11).

Similarly, mixtures of pesticide compounds and (or) 

VOCs that were evaluated without regard to the availability 

of benchmarks or the concentrations relative to benchmarks 

were frequently detected and complex. One-half of the 

source-water samples from public wells contained mixtures 

of two or more pesticide compounds or VOCs, and 

27 percent of the samples contained five or more pesticide 

compounds or VOCs (fig. 18). For mixtures of pesticide 

compounds and VOCs, the proportion of samples from the 

more vulnerable unconfined aquifers also increased as the 

complexity of the mixtures increased. About two-thirds of 

the samples containing mixtures of two or more pesticide 

compounds or VOCs were from unconfined aquifers, 

whereas 90 percent of the samples containing mixtures of 10 

or more pesticide compounds or VOCs were from unconfined 

aquifers (fig. 18).

Figure 18. As the complexity of mixtures of contaminants at 

concentrations greater than one-tenth of individual human-health 

benchmarks increased, the proportion of samples from more vulnerable unconfined aquifers also increased. Data are from 383 samples 

in which major ions, trace elements, nitrate, radon, pesticide 

compounds, and volatile organic compounds were analyzed. Radon 

activities were compared to the proposed MCL of 300 picocuries per 

liter.

10 20 30 40 50 60 70 80 90

10 or more

9 or more

8 or more

7 or more

6 or more

5 or more

4 or more

3 or more

2 or more

NUMBER OF CONTAMINANTS IN MIXTURES AT

CONCENTRATIONS GREATER THAN ONE-TENTH

OF HUMAN-HEALTH BENCHMARKS 

PERCENTAGE OF SOURCE-WATER SAMPLES

Mixed or unknown aquifers

Confined aquifers

Unconfined aquifers

Figure 17. As the complexity of mixtures of contaminants at 

concentrations greater than one-tenth of individual humanhealth benchmarks increased, the proportion of samples from 

more vulnerable unconfined aquifers also increased. Data are 

from 383 source-water samples from public wells in which major 

ions, trace elements, nitrate, radon, pesticide compounds, and 

volatile organic compounds were analyzed. Radon activities 

were compared to the proposed MCL of 300 picocuries per liter.

Figure 19. Mixtures of pesticide compounds and volatile organic 

compounds (VOCs) were detected more often in samples from 

shallower unconfined aquifers than from deeper confined aquifers. 

As the complexity of these mixtures increased, the proportion of 

samples from more vulnerable unconfined aquifers also increased. 

Data are from 814 source-water samples from public wells in which 

as many as 47 pesticide compounds and 85 VOCs were analyzed.

10 or more

9 or more

8 or more

7 or more

6 or more

5 or more

4 or more

3 or more

2 or more

0 10 20 30 40 50 60

NUMBER OF PESTICIDE COMPOUNDS 

OR VOCS IN MIXTURES

PERCENTAGE OF SOURCE-WATER SAMPLES

Mixed or unknown aquifers

Confined aquifers

Unconfined aquifers

Figure 18. Mixtures of pesticide compounds and volatile 

organic compounds (VOCs) were detected more often in 

samples from shallower unconfined aquifers than from 

deeper confined aquifers. As the complexity of these mixtures 

increased, the proportion of samples from more vulnerable 

unconfined aquifers also increased. Data are from 814 

source-water samples from public wells in which pesticide 

compounds and VOCs were analyzed. 
More Information for People Served by Public Water Utilities

The best source of specific information 

about drinking-water quality for people 

served by public water systems is their water 

supplier. Water suppliers that serve the same 

people year-round are required by the U.S. 

Environmental Protection Agency (USEPA) to 

provide their customers an annual water-quality 

report (also called a consumer confidence report, 

or CCR). CCRs may be obtained directly from 

water utilities and some are available online 

at http://www.epa.gov/safewater/ccr/

index.html. Each CCR provides consumers with 

fundamental information about their drinking 

water, including (1) the source of the water, (2) a 

brief summary of the susceptibility of the local 

drinking-water source to contamination, (3) the 

concentrations of any contaminants detected in 

local drinking water, as well as their Maximum 

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 

reliable, 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 

Contaminant Levels, and (4) resources for 

additional information. Many other agencies 

and organizations also provide consumers 

and homeowners with information about 

their drinking-water quality, including: 

USEPA’s Safe Drinking Water Hotline (http://

www.epa.gov/safewater/hotline), the U.S. 

Centers for Disease Control and Prevention 

(ToxFAQs provide summaries about chemical 

exposure and the effects of exposure on 

human health at http://www.atsdr.cdc.

gov/toxfaqs/index.asp), local and state 

environmental and public-health agencies, 

and non-governmental organizations such 

as the American Water Works Association 

(http://www.awwa.org or http://www.

drinktap.org) and the National Ground 

Water Association (http://www.ngwa.org).

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.

Major Findings 43
References Cited

Agency for Toxic Substances and Disease Registry, 2002a, 

ToxFAQs for aldrin/dieldrin: U.S. Department of Health and 

Human Services, Agency for Toxic Substances and Disease 

Registry, Updated February 18, 2010, Accessed February 24, 

2010, at http://www.atsdr.cdc.gov/tfacts1.html

Agency for Toxic Substances and Disease Registry, 2002b, 

Toxicological profile for aldrin/dieldrin: U.S. Department of 

Health and Human Services, Agency for Toxic Substances 

and Disease Registry, September 2002, 354 p. Available at 

http://www.atsdr.cdc.gov/toxprofiles/tp1.html

Agency for Toxic Substances and Disease Registry, 2004, 

Toxicological profile for strontium: U.S. Department of 

Health and Human Services, Agency for Toxic Substances 

and Disease Registry, April 2004, 445 p. Available at http://

www.atsdr.cdc.gov/toxprofiles/tp159.html

Agency for Toxic Substances and Disease Registry, 2008, Draft 

toxicological profile for manganese: U.S. Department of 

Health and Human Services, Agency for Toxic Substances 

and Disease Registry, September 2008, 539 p. Available at 

http://www.atsdr.cdc.gov/toxprofiles/tp151.html

Alley, W.M., Reilly, T.E., and Franke, O.L., 1999, Sustainability 

of ground-water resources: U.S. Geological Survey Circular 

1186, 79 p. Available at http://pubs.usgs.gov/circ/circ1186/

Ayers, M.A., Kennen, J.G., and Stackelberg, P.E., 2000, Water 

quality in the Long Island–New Jersey coastal drainages, 

New York and New Jersey, 1996–98: U.S. Geological Survey 

Circular 1201, 40 p. Available at http://water.usgs.gov/pubs/

circ/circ1201/

Ayotte, J.D., Flanagan, S.M., and Morrow, W.S., 2007, 

Occurrence of uranium and 222radon in glacial and bedrock 

aquifers in the northern United States, 1993–2003: U.S. 

Geological Survey Scientific Investigations Report 20075037, 84 p. Available at http://pubs.usgs.gov/sir/2007/5037/

Ayotte, J.D., Montgomery, D.L., Flanagan, S.M., and Robinson, 

K.W., 2003, Arsenic in groundwater in eastern New England: 

occurrence, controls, and human health implications: 

Environmental Science and Technology, v. 37, no. 10, 

p. 2075–2083.

Carpenter, D.O., Arcaro, K., and Spink, D.C., 2002, 

Understanding the human health effects of chemical 

mixtures: Environmental Health Perspectives, v. 110, 

Supplement 1, p. 25–42.

Commonwealth of Massachusetts, 2009, Standards and 

guidelines for contaminants in Massachusetts drinking 

waters: Executive Office of Energy and Environmental 

Affairs, Department of Environmental Protection; Office 

of Research and Standards, Spring 2009, 14 p. Available 

at http://www.mass.gov/dep/water/dwstand.pdf

Connecticut Department of Public Health, 2005, Radon in 

your water: Division of Environmental Health, Radon 

Program Fact Sheet, March 2005, 2 p. Available at http://

humanservices.townofmanchester.org/health/documents/

RadonProgram-waterfactsheet.pdf

DeSimone, L.A., 2009, Quality of water from domestic 

wells in principal aquifers of the United States, 1991–

2004: U.S. Geological Survey Scientific Investigations 

Report 2008-5227, 139 p. Available at http://pubs.usgs.

gov/sir/2008/5227/

DeSimone, L.A., Hamilton, P.A., and Gilliom, R.J., 2009, 

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. 

Available at http://pubs.usgs.gov/circ/circ1332/

Eaton, D.L., and Klaassen, C.D., 2001, Principles of 

toxicology, in Klaassen, C.D., ed., Casarett and Doull’s 

toxicology: the basic science of poisons (Sixth ed.): New 

York, McGraw-Hill, p. 1236.

Focazio, M.J., Reilly, T.E., Rupert, M.G., and Helsel, 

D.R., 2002, Assessing ground-water vulnerability to 

contamination: providing scientifically defensible 

information for decision makers: U.S. Geological Survey 

Circular 1224, 33 p. Available at http://pubs.usgs.gov/

circ/2002/circ1224/

Focazio, M.J., Szabo, Z., Kraemer, T.F., Mullin, A.H., 

Barringer, T.H., and dePaul, V.T., 2001, Occurrence of 

selected radionuclides in ground water used for drinking 

water in the United States; a reconnaissance survey, 1998: 

U.S. Geological Survey Water-Resources Investigations 

Report 00-4273, 39 p. Available at http://pubs.usgs.gov/

wri/wri004273/

Focazio, M.J., Welch, A.H., Watkins, S.A., Helsel, D.R., 

and Horn, M.A., 2000, A retrospective analysis on the 

occurrence of arsenic in ground-water resources of 

the United States and limitations in drinking-watersupply characterization: U.S. Geological Survey WaterResources Investigations Report 99-4279, 21 p. Available 

at http://pubs.usgs.gov/wri/wri994279/

44 Quality of Water from Public-Supply Wells in the United States, 1993–2007
Freeze, R.A., and Cherry, J.A., 1979, Groundwater: 

Englewood Cliffs, NJ, Prentice Hall, Inc., 604 p.

Gilliom, R.J., Alley, W.M., and Gurtz, M.E., 1995, Design 

of the National Water-Quality Assessment Program: 

occurrence and distribution of water-quality conditions: 

U.S. Geological Survey Circular 1112, 33 p. Available at 

http://pubs.usgs.gov/circ/circ1112/

Gilliom, R.J., Barbash, J.E., Crawford, C.G., Hamilton, 

P.A., Martin, J.D., Nakagaki, N., Nowell, L.H., Scott, 

J.C., Stackelberg, P.E., Thelin, G.P., and Wolock, D.M., 

2006, The quality of our Nation’s waters—pesticides in 

the Nation’s streams and ground water, 1992–2001: U.S. 

Geological Survey Circular 1291, 172 p. Available at

http://pubs.usgs.gov/circ/2005/1291/

Grady, S.J., 2002, A national survey of methyl tert-butyl 

ether and other volatile organic compounds in drinkingwater sources: results of the random survey: U.S. 

Geological Survey Water-Resources Investigations Report 

02-4079, 85 p. Available at http://sd.water.usgs.gov/

nawqa/pubs/wrir/wrir02_4079.html

Grady, S.J., and Casey, G.D., 2001, Occurence and 

distribution of methyl tert-butyl ether and other volatile 

organic compounds in drinking water in the northeast 

and mid-Atlantic regions of the United States, 1993–98: 

U.S. Geological Survey Water-Resources Investigations 

Report 00-4228, 123 p. Available at http://sd.water.usgs.

gov/nawqa/pubs/wrir/wrir00.4228.pdf

Hasegawa, R., Takayama, S., and Ito, N., 1994, Effects 

of low level exposure to multiple carcinogens in 

combination, in Yang, R.S.H., ed., Toxicology of 

chemical mixtures: San Diego, CA, Academic Press, 

p. 361–382.

Heath, R.C., 1983, Basic ground-water hydrology: U.S. 

Geological Survey Water-Supply Paper 2220, 84 p. 

Available at http://pubs.er.usgs.gov/djvu/WSP/wsp_2220.

pdf

Hem, J.D., 1985, Study and interpretation of the chemical 

characteristics of natural water: U.S. Geological Survey 

Water-Supply Paper 2254, 272 p. Available at http://pubs.

usgs.gov/wsp/wsp2254/pdf/wsp2254a.pdf

Heydens, W.F., Wilson, A.G.E., Kraus, L.J., Hopkins, W.E., 

and Hotz, K.J., 2000, Ethane sulfonate metabolite of 

alachlor: assessment of oncogenic potential based on 

metabolic and mechanistic considerations: Toxicological 

Sciences, v. 55, no. 1, p. 36–43.

Hladik, M.L., Hsiao, J.J., and Roberts, A.L., 2005, Are 

neutral chloroacetamide herbicide degradates of potential 

environmental concern? Analysis and occurrence in 

the upper Chesapeake Bay: Environmental Science and 

Technology, v. 39, no. 17, p. 6561–6574.

Hopple, J.A., Delzer, G.C., and Kingsbury, J.A., 2009, 

Anthropogenic organic compounds in source water of 

selected community water systems that use groundwater, 

2002–05: U.S. Geological Survey Scientific Investigations 

Report 2009-5200, 74 p. Available at http://pubs.usgs.gov/

sir/2009/5200/

Jurgens, B.C., Fram, M.S., Belitz, K., Burow, K.R., and 

Landon, M.K., 2009, Effects of groundwater development 

on uranium: Central Valley, California, USA: Ground Water, 

online only in advance of print, 16 p. Available at http://

oh.water.usgs.gov/tanc/pubs/Jurgens&Others_2009.pdf

Kalkhoff, S.J., Kolpin, D.W., Thurman, E.M., Ferrer, I., 

and Barcelo, D., 1998, Degradation of chloroacetanilide 

herbicides: the prevalence of sulfonic and oxanilic acid 

metabolites in Iowa groundwaters and surface waters: 

Environmental Science and Technology, v. 32, no. 11, 

p. 1738–1740.

Kelly, W.R., Holm, T.R., Wilson, S.D., and Roadcap, G.S., 

2005, Arsenic in glacial aquifers: sources and geochemical 

controls: Ground Water, v. 43, no. 4, p. 500–510.

Landon, M.K., Jurgens, B.C., Katz, B.G., Eberts, S.M., Burow, 

K.R., and Crandall, C.A., 2009, Depth-dependent sampling 

to identify short-circuit pathways to public-supply wells in 

multiple aquifer settings in the United States: Hydrogeology 

Journal, Online October 20, 2009, 17 p. Available at http://

www.springerlink.com/content/0m70737w621mkv65/

fulltext.pdf

Lapham, W.W., Hamilton, P.A., and Myers, D.N., 2005, 

National Water-Quality Assessment Program—Cycle II, 

regional assessments of aquifers: U.S. Geological Survey 

Fact Sheet 2005-3013, 4 p. Available at http://pubs.usgs.gov/

fs/2005/3013/

Lindsey, B.D., Berndt, M.P., Katz, B.G., Ardis, A.F., and Skach, 

K.A., 2009, Factors affecting water quality in selected 

carbonate aquifers in the United States, 1993–2005: U.S. 

Geological Survey Scientific Investigations Report 20085240, 117 p. Available at http://pubs.usgs.gov/sir/2008/5240/

Longtin, J.P., 1988, Occurrence of radon, radium, and uranium 

in groundwater: Journal American Water Works Association, 

v. 80, no. 7, p. 84–93.

References Cited 45
Maine Center for Disease Control and Prevention, 2006, 

Frequently asked questions about the 4,000 pCi/L MEG 

for radon in well water: Department of Health and Human 

Services, Division of Environmental Health, Updated 

2006, Accessed March 9, 2009, at http://www.maine.gov/

dhhs/eohp/RealEstate/RadonMEGFAQ.htm

McMahon, P.B., Burow, K.R., Kauffman, L.J., Eberts, S.M., 

Böhlke, J.K., and Gurdak, J.J., 2008, Simulated response 

of water quality in public supply wells to land use change: 

Water Resources Research, v. 44, W00A06. Available at 

http://dx.doi.org/10.1029/2007wr006731

Miller, J.A., 2000, Ground water atlas of the United States: 

U.S. Geological Survey, Chapters archived online 

1990–1999, Accessed June 24, 2009, at http://capp.water.

usgs.gov/gwa/gwa.html

Mueller, D.K., Hamilton, P.A., Helsel, D.R., Hitt, K.J., 

and Ruddy, B.C., 1995, Nutrients in ground water and 

surface water of the United States; an analysis of data 

through 1992: U.S. Geological Survey Water-Resources 

Investigations Report 95-4031, 74 p. Available at http://

pubs.er.usgs.gov/usgspubs/wri/wri954031

National Research Council, 1999, Risk assessment of radon 

in drinking water: Washington, DC, National Academy 

Press, 296 p.

National Research Council, 2001, Arsenic in drinking water: 

2001 update: Washington, DC, National Academy Press, 

244 p.

Nebraska Department of Agriculture, 1997, Nebraska 

pesticides and groundwater generic state management 

plan: Pesticide Program, March 18, 1997, variously 

paged. Available at http://www.agr.state.ne.us/division/

bpi/pes/gsmp.pdf

New Hampshire Department of Environmental Services, 

2008, Radon program frequently asked questions: Air 

Resources Division, Radon Program, Updated 2008, 

Accessed February 24, 2010, at http://des.nh.gov/

organization/divisions/air/pehb/ehs/radon/categories/faq.

htm

New Jersey Department of Environmental Protection, 2003, 

Susceptibility of source water to community water-supply 

wells in New Jersey to contamination by volatile organic 

compounds: New Jersey Department of Environmental 

Protection, 21 p. Available at http://www.state.nj.us/dep/

swap/reports/gw_voc.pdf

New Jersey Department of Environmental Protection, 2004, 

New Jersey source water assessment program statewide 

summary: New Jersey Department of Environmental 

Protection, December 2004, 30 p. Available at http://www.

state.nj.us/dep/swap/reports/swap_sum200412.pdf

Nolan, B.T., and Hitt, K.J., 2003, Nutrients in shallow 

ground waters beneath relatively undeveloped areas in the 

conterminous United States: U.S. Geological Survey WaterResources Investigations Report 2002-4289, 17 p. Available 

at http://pubs.usgs.gov/wri/wri024289/

Nolan, B.T., and Hitt, K.J., 2006, Vulnerability of shallow 

groundwater and drinking-water wells to nitrate in the 

United States: Environmental Science and Technology, 

v. 40, no. 24, p. 7834–7840.

Nordstrom, D.K., 2002, Worldwide occurrences of arsenic in 

ground water: Science, v. 296, no. 5576, p. 2143–2145.

Richardson, S.D., Simmons, J.E., and Rice, G., 2002, 

Disinfection byproducts: the next generation: 

Environmental Science and Technology, v. 36, no. 9, 

p. 198A–205A.

Ryker, S.J., 2001, Mapping arsenic in groundwater: a real 

need, but a hard problem: Geotimes, November, p. 34–36, 

Accessed February 24, 2010 at http://www.agiweb.org/

geotimes/nov01/feature_Asmap.html

Ryker, S.J., and Small, M.J., 2008, Combining occurrence and 

toxicity information to identify priorities for drinking-water 

mixture research: Risk Analysis, v. 28, no. 3, p. 653–666.

Simmons, J.E., Teuschler, L.K., Gennings, C., Speth, 

T.F., Richardson, S.D., Miltner, R.J., Nartosky, M.G., 

Schneck, K.D., Hunter III, E.S., Hertzberg, R.C., and 

Rice, G., 2004, Component-based and whole-mixture 

techniques for addressing the toxicity of drinking-water 

disinfection by-product mixtures: Journal of Toxicology & 

Environmental Health: Part A, v. 67, no. 8–10, p. 741–754.

Solley, W.B., Pierce, R.R., and Perlman, H.A., 1998, 

Estimated use of water in the United States in 1995: U.S. 

Geological Survey Circular 1200, 71 p. Available at http://

water.usgs.gov/watuse/pdf1995/html/, data accessed from 

http://water.usgs.gov/watuse/

Squillace, P.J., Moran, M.J., Lapham, W.W., Price, C.V., 

Clawges, R.M., and Zogorski, J.S., 1999, Volatile organic 

compounds in untreated ambient groundwater of the United 

States, 1985–1995: Environmental Science and Technology, 

v. 33, no. 23, p. 4176–4187.

46 Quality of Water from Public-Supply Wells in the United States, 1993–2007
Squillace, P.J., Scott, J.C., Moran, M.J., Nolan, B.T., and 

Kolpin, D.W., 2002, VOCs, pesticides, nitrate, and their 

mixtures in groundwater used for drinking water in the 

United States: Environmental Science and Technology, 

v. 36, no. 9, p. 1923–1930.

Sullivan, D.J., Vecchia, A.V., Lorenz, D.L., Gilliom, R.J., 

and Martin, J.D., 2009, Trends in pesticide concentrations 

in corn-belt streams, 1996–2006: U.S. Geological 

Survey Scientific Investigations Report 2009-5132, 75 p. 

Available at http://pubs.usgs.gov/sir/2009/5132/

Teuschler, L.K., Rice, G.E., Wilkes, C.R., Lipscomb, 

J.C., and Power, F.W., 2004, A feasibility study of 

cumulative risk assessment methods for drinking water 

disinfection by-product mixtures: Journal of Toxicology 

& Environmental Health: Part A, v. 67, no. 8–10, 

p. 755–777.

Teuschler, L.K., and Simmons, J.E., 2003, Approaching 

DBP toxicity as a mixtures problem: Journal American 

Water Works Association, v. 95, no. 6, p. 131–138.

Thelin, G.P., and Stone, W.W., 2010, Method for estimating 

annual atrazine use for counties in the conterminous 

United States, 1992–2007: U.S. Geological Survey 

Scientific Investigations Report 2010-5034, 129 p. 

Available at http://pubs.usgs.gov/sir/2010/5034/.

Thomas, M.A., 2007, The association of arsenic with 

redox conditions, depth, and ground-water age in the 

glacial aquifer system of the northern United States: 

U.S. Geological Survey Scientific Investigations Report 

2007–5036, 26 p. Available at http://pubs.usgs.gov/

sir/2007/5036/

Thurman, E.M., Goolsby, D.A., Meyer, M.T., Mills, M.S., 

Pomes, M.L., and Kolpin, D.W., 1992, A reconnaissance 

study of herbicides and their metabolites in surface water 

of the Midwestern United States using immunoassay and 

gas chromatography/mass-spectrometry: Environmental 

Science and Technology, v. 26, no. 12, p. 2440–2447.

Toccalino, P.L., 2007, Development and application of 

health-based screening levels for use in water-quality 

assessments: U.S. Geological Survey Scientific 

Investigations Report 2007-5106, 12 p. Available at http://

pubs.usgs.gov/sir/2007/5106/

Toccalino, P.L., Norman, J.E., Booth, N.L., and Zogorski, 

J.S., 2008, Health-based screening levels: a tool for 

evaluating what water-quality data may mean to human 

health: U.S. Geological Survey, National Water-Quality 

Assessment Program, Updated April 10, 2008, Accessed 

February 24, 2010, at http://water.usgs.gov/nawqa/HBSL

Toccalino, P.L., Norman, J.E., and Hitt, K.J., 2010, Quality of 

source water from public-supply wells in the United States, 

1993–2007: U.S. Geological Survey Scientific Investigations 

Report 2010-5024, 206 p. Available at http://pubs.usgs.gov/

sir/2010/5024/

Toccalino, P.L., Rowe, B.L., and Norman, J.E., 2006, Volatile 

organic compounds in the Nation’s drinking-water supply 

wells—what findings may mean to human health: U.S. 

Geological Survey Fact Sheet 2006-3043, 4 p. Available at 

http://pubs.usgs.gov/fs/2006/3043/

U.S. Census Bureau, 2009, Population estimates: Updated 

March 20, 2009, Accessed April 23, 2009, at http://www.

census.gov/popest/estimates.html

U.S. Code, 1996, Safe Drinking Water Act amendments of 

1996: Title 42 The Public Health and Welfare, Chapter 6A 

Public Health Service, SubChapter XII Safety of Public 

Water Systems, Public Law 104-182, Enacted August 6, 

1996, Accessed January 6, 2010, at http://uscode.house.gov/

download/pls/42C6A.txt

U.S. Environmental Protection Agency, 1980, Ambient water 

quality criteria for aldrin/dieldrin: U.S. Environmental 

Protection Agency, Office of Water, Regulations and 

Standards, Criteria and Standards Division EPA 440/5-80019, variously paged. Available at http://www.epa.gov/

waterscience/criteria/library/ambientwqc/aldrindieldrin.pdf

U.S. Environmental Protection Agency, 1987, Integrated 

risk information system: dieldrin (CASRN 60-57-1): U.S. 

Environmental Protection Agency, Office of Research 

and Development, National Center for Environmental 

Assessment, Verification dates March 5, 1987 and April 16, 

1987, Accessed February 24, 2010, at http://www.epa.gov/

ncea/iris/subst/0225.htm

U.S. Environmental Protection Agency, 1992, Integrated risk 

information system: strontium (CASRN 7440-24-6): U.S. 

Environmental Protection Agency, Office of Research 

and Development, National Center for Environmental 

Assessment, Verification date June 23, 1992, Accessed 

February 24, 2010, at http://www.epa.gov/ncea/iris/

subst/0550.htm

U.S. Environmental Protection Agency, 1993, Risk assessment: 

technical guidance manual, selecting exposure routes and 

contaminants of concern by risk-based screening: U.S. 

Environmental Protection Agency, Region 3, Hazardous 

Waste Management Division EPA/903/R-93-001, January 

1993, 5 p. Available at http://www.epa.gov/reg3hwmd/risk/

human/info/guide2.htm

References Cited 47
U.S. Environmental Protection Agency, 1994, Evaluating and 

identifying contaminants of concern for human health: U.S. 

Environmental Protection Agency, Region 8, Hazardous 

Waste Management Division Region 8 Superfund Technical 

Guidance No. RA-03, September 1994, 10 p. Available at 

http://www.epa.gov/region8/r8risk/pdf/r8_ra03-cocs.pdf

U.S. Environmental Protection Agency, 1995, Integrated risk 

information system: manganese (CASRN 7439-96-5): 

U.S. Environmental Protection Agency, Office of Research 

and Development, National Center for Environmental 

Assessment, Verification date May 12, 1995, Accessed 

February 24, 2010, at http://www.epa.gov/ncea/iris/

subst/0373.htm

U.S. Environmental Protection Agency, 1998, Code of Federal 

Regulations, title 40—protection of environment, chapter 

1—Environmental Protection Agency, subchapter E—

pesticide programs, part 159—statements of policies and 

interpretations, subpart D—reporting requirements for risk/

benefit information, 40 CFR 159.184: Washington, DC, 

National Archives and Records Administration, September 

19, 1997; amended June 19, 1998, Accessed February 24, 

2010, at http://www.gpoaccess.gov/ecfr/index.html

U.S. Environmental Protection Agency, 1999a, Proposed 

radon in drinking water rule, technical fact sheet: U.S. 

Environmental Protection Agency, Office of Water EPA 

815-F-99-006, October 1999, 6 p. Available at http://www.

epa.gov/safewater/radon/fact.html and http://www.epa.gov/

ogwdw000/radon/fact10.pdf

U.S. Environmental Protection Agency, 1999b, A review 

of contaminant occurrence in public water systems: U.S. 

Environmental Protection Agency, Office of Water EPA 

816-R-99-006, November 1999, 78 p. Available at http://

www.epa.gov/safewater/occur/occur.html

U.S. Environmental Protection Agency, 2000, Radionuclides 

notice of data availability technical support document: 

U.S. Environmental Protection Agency, Office of Ground 

Water and Drinking Water, Standards and Risk Management 

Division, March 2000, 164 p. Available at http://www.epa.

gov/safewater/radionuclides/pdfs/regulation_radionuclides_

rulemaking_techsupportdoc.pdf

U.S. Environmental Protection Agency, 2001a, Occurrence 

of unregulated contaminants in public water systems—an 

initial assessment: U.S. Environmental Protection Agency, 

Office of Water EPA 815-P-00-001, May 2001, 508 p. 

Available at http://www.epa.gov/ogwdw000/ucmr/data/

report_ucm1-2_no_uc.pdf

U.S. Environmental Protection Agency, 2001b, Session 

VI—A set of scientific issues being considered by the 

Environmental Protection Agency regarding: Progress 

report on estimating pesticide concentrations in drinking 

water and assessing water treatment effects on pesticide 

removal and transformation: A consultation: U.S. 

Environmental Protection Agency, FIFRA Scientific 

Advisory Panel Meeting, September 29, 2000 SAP Report 

No. 2001-02, February 12, 2001, 22 p. Available at http://

www.epa.gov/scipoly/sap/meetings/2000/september/

draftwaterdocfinal.pdf

U.S. Environmental Protection Agency, 2002a, Community 

water system survey 2000, volume 1: overview: U.S. 

Environmental Protection Agency, Office of Water EPA 

815-R-02-005A, December 2002, 58 p. Available at 

http://www.epa.gov/safewater/consumer/pdf/cwss_2000_

volume_i.pdf

U.S. Environmental Protection Agency, 2002b, Occurrence 

data, accessing unregulated contaminant monitoring data: 

UCM rounds 1 and 2 and the six-year review data: U.S. 

Environmental Protection Agency, Office of Water, Updated 

November 20, 2002, Accessed February 24, 2010, at http://

www.epa.gov/safewater/ucmr/data.html

U.S. Environmental Protection Agency, 2003a, Fact sheet: 

announcement of regulatory determinations for priority 

contaminants on the drinking water contaminant candidate 

list: U.S. Environmental Protection Agency, Office of Water 

EPA 815-F-03-007, July 2003, 3 p. Available at http://

www.epa.gov/safewater/ccl/pdfs/reg_determine1/fs_ccl1_

regdetermine_july03.pdf

U.S. Environmental Protection Agency, 2003b, The 

feasibility of performing cumulative risk assessments for 

mixtures of disinfection by-products in drinking water: 

U.S. Environmental Protection Agency, National Center 

for Environmental Assessment, Office of Research and 

Development EPA/600/R-03/051, June 2003, 435 p. 

Available at http://cfpub.epa.gov/ncea/cfm/recordisplay.

cfm?deid=56834

U.S. Environmental Protection Agency, 2003c, Health effects 

support document for aldrin/dieldrin: U.S. Environmental 

Protection Agency, Office of Water, Health and Ecological 

Criteria Division EPA 822-R-03-001, February 2003, 

variously paged. Available at http://www.epa.gov/

OGWDW/ccl/pdfs/reg_determine1/support_cc1_aldrindieldrin_healtheffects.pdf

U.S. Environmental Protection Agency, 2003d, Interim 

reregistration eligibility decision for atrazine, case no. 

0062: U.S. Environmental Protection Agency, Office of 

Pesticide Programs, 304 p. Available at http://www.epa.gov/

pesticides/reregistration/REDs/atrazine_ired.pdf

48 Quality of Water from Public-Supply Wells in the United States, 1993–2007
U.S. Environmental Protection Agency, 2003e, Occurrence 

estimation methodology and occurrence findings report 

for the six-year review of existing national primary 

drinking water regulations: U.S. Environmental Protection 

Agency, Office of Water EPA-815-R-03-006, June 

2003, 874 p. Available at http://www.epa.gov/safewater/

standard/review/pdfs/support_6yr_occurancemethods_

final.pdf

U.S. Environmental Protection Agency, 2003f, Water on tap: 

what you need to know: U.S. Environmental Protection 

Agency, Office of Water EPA 816-K-03-007, October 

2003, 36 p. 

U.S. Environmental Protection Agency, 2004a, Integrated 

risk information system: boron and compounds (CASRN 

7440-42-8): U.S. Environmental Protection Agency, 

Office of Research and Development, National Center 

for Environmental Assessment, Agency completion date 

May 26, 2004, Accessed February 24, 2010, at http://

www.epa.gov/ncea/iris/subst/0410.htm

U.S. Environmental Protection Agency, 2004b, The 

standardized monitoring framework: a quick reference 

guide: U.S. Environmental Protection Agency, Office of 

Water EPA 816-F-04-010, March 2004, 3 p. Available 

at http://www.epa.gov/ogwdw000/pws/pdfs/qrg_

smonitoringframework.pdf

U.S. Environmental Protection Agency, 2004c, 

Understanding the safe drinking water act: U.S. 

Environmental Protection Agency, Office of Water EPA 

816-F-04-030, June 2004, 4 p. Available at http://www.

epa.gov/safewater/sdwa/pdfs/fs_30ann_sdwa_web.pdf

U.S. Environmental Protection Agency, 2006a, 2006 Edition 

of the drinking water standards and health advisories: 

U.S. Environmental Protection Agency, Office of Water 

EPA 822-R-06-013, August 2006, 18 p. Available at http://

www.epa.gov/waterscience/criteria/drinking/dwstandards.

pdf

U.S. Environmental Protection Agency, 2006b, Triazine 

cumulative risk assessment: U.S. Environmental 

Protection Agency, Office of Pesticide Programs, Health 

Effects Division, March 28, 2006, 67 p. Available at 

http://www.epa.gov/oppsrrd1/REDs/triazine_cumulative_

risk.pdf

U.S. Environmental Protection Agency, 2008a, EPA’s 2008 

report on the environment: U.S. Environmental Protection 

Agency, National Center for Environmental Assessment 

EPA/600/R-07/045F, May 2008, 366 p. Available at http://

www.epa.gov/roe/

U.S. Environmental Protection Agency, 2008b, Fact sheet: 

final regulatory determinations for the second drinking water 

contaminant candidate list (CCL 2): U.S. Environmental 

Protection Agency, Office of Water EPA 815-F-08-005, July 

2008, 4 p. Available at http://www.epa.gov/safewater/ccl/

pdfs/reg_determine2/fs_ccl2-reg2.pdf

U.S. Environmental Protection Agency, 2008c, Factoids: 

drinking water and ground water statistics for 2008: U.S. 

Environmental Protection Agency, Office of Water EPA 816K-08-004, November 2008, 16 p. Available at http://www.

epa.gov/safewater/databases/pdfs/data_factoids_2008.pdf

U.S. Environmental Protection Agency, 2008d, Regulatory 

determinations support document for selected contaminants 

from the second drinking water contaminant candidate list 

(CCL 2), chapter 3 boron: U.S. Environmental Protection 

Agency, Office of Water EPA 815-R-08-012, June 2008, 

variously paged. Available at http://www.epa.gov/safewater/

ccl/pdfs/reg_determine2/report_ccl2-reg2_supportdocument_

full.pdf

U.S. Environmental Protection Agency, 2009a, 2009 Edition 

of the drinking water standards and health advisories: U.S. 

Environmental Protection Agency, Office of Water EPA 822R-09-011, October 2009, 18 p. Available at http://www.epa.

gov/waterscience/criteria/drinking/dwstandards2009.pdf

U.S. Environmental Protection Agency, 2009b, Drinking 

water contaminants: U.S. Environmental Protection Agency, 

Office of Water, Updated September 11, 2009, Accessed 

February 24, 2010, at http://www.epa.gov/safewater/

contaminants/index.html

U.S. Environmental Protection Agency, 2009c, FIFRA scientific 

advisory panel; notice of public meeting: U.S. Environmental 

Protection Agency, Federal Register, v. 74, no. 193, p. 51593–

51596, Updated October 7, 2009, Accessed January 5, 2010, 

at http://edocket.access.gpo.gov/2009/pdf/E9-24229.pdf

U.S. Environmental Protection Agency, 2009d, Pesticides: 

regulating pesticides: U.S. Environmental Protection Agency, 

Office of Pesticide Programs, Updated October 19, 2009, 

Accessed January 14, 2010, at http://www.epa.gov/pesticides/

regulating/index.htm

U.S. Environmental Protection Agency, 2009e, Proposed 

radon in drinking water rule: U.S. Environmental Protection 

Agency, Office of Water, Updated September 3, 2009, 

Accessed February 24, 2010, at http://www.epa.gov/

safewater/radon/proposal.html

References Cited 49
U.S. Environmental Protection Agency, 2010a, Drinking 

water contaminant candidate list and regulatory 

determinations: U.S. Environmental Protection Agency, 

Office of Water, Updated February 22, 2010, Accessed 

February 24, 2010, at http://www.epa.gov/safewater/ccl/

U.S. Environmental Protection Agency, 2010b, Public 

drinking water systems: facts and figures: U.S. 

Environmental Protection Agency, Office of Water, 

Updated February 24, 2010, Accessed February 24, 2010, 

at http://www.epa.gov/safewater/pws/factoids.html

U.S. Environmental Protection Agency, 2010c, Unregulated 

contaminant monitoring program: U.S. Environmental 

Protection Agency, Office of Ground Water and Drinking 

Water, Updated February 23, 2010, Accessed February 24, 

2010, at http://www.epa.gov/safewater/ucmr/index.html

U.S. General Accounting Office, 1997, Drinking 

water—information on the quality of water found at 

community water systems and private wells: U.S. 

General Accounting Office, Resources, Community, and 

Economic Development Division GAO/RCED-97-123, 

June 12, 1997, 51 p. Available at http://www.gao.gov/

archive/1997/rc97123.pdf

U.S. Geological Survey, 1999, The quality of our Nation’s 

waters—nutrients and pesticides: U.S. Geological Survey 

Circular 1225, 82 p. Available at http://water.usgs.gov/

pubs/circ/circ1225/

U.S. Geological Survey, 2003, Principal aquifers of the 48 

conterminous United States, Hawaii, Puerto Rico, and the 

U.S. Virgin Islands: U.S. Geological Survey, Accessed 

February 24, 2010, at http://www.nationalatlas.gov/mld/

aquifrp.html

U.S. Geological Survey, 2006, Collection of water samples 

(ver. 2.0): U.S. Geological Survey techniques of waterresources investigations, book 9, chap. A4, Updated 

September 2006, Accessed February 24, 2010, at http://

water.usgs.gov/owq/FieldManual/chapter4/html/Ch4_

contents.html

U.S. Geological Survey, 2009a, Aquifer basics: U.S. Geological 

Survey, Office of Ground Water, Updated April 29, 2009, 

Accessed February 24, 2010, at http://capp.water.usgs.gov/

aquiferBasics/index.html

U.S. Geological Survey, 2009b, Water-quality assessments 

of principal aquifers: Updated October 20, 2009, Accessed 

February 24, 2010, at http://water.usgs.gov/nawqa/studies/

praq/

Welch, A.H., Westjohn, D.B., Helsel, D.R., and Wanty, 

R.B., 2000, Arsenic in ground water of the United States: 

occurrence and geochemistry: Ground Water, v. 38, no. 4, 

p. 589–604.

Westrick, J.J., 1990, National surveys of volatile organic 

compounds in ground and surface waters, in Ram, N.M., 

Christman, R.F., and Cantor, K.P., eds., Significance and 

treatment of volatile organic compounds in water supplies: 

Chelsea, MI, Lewis Publishers, p. 103–125.

Westrick, J.J., Mello, J.W., and Thomas, R.F., 1984, The 

groundwater supply survey: Journal American Water Works 

Association, v. 76, no. 5, p. 52–59.

World Health Organization, 2004, Guidelines for drinking water 

quality (Third ed.): Geneva, World Health Organization v. 1 

recommendations, 515 p. Available at http://www.who.int/

water_sanitation_health/dwq/GDWQ2004web.pdf

Yang, R.S.H., 1994, Introduction to the toxicology of chemical 

mixtures, in Yang, R.S.H., ed., Toxicology of chemical 

mixtures: San Diego, CA, Academic Press, p. 1–10.

Younger, P.L., 2007, Groundwater in the environment: an 

introduction: Malden, MA and Oxford, UK, Blackwell 

Publishing, 318 p.

Zogorski, J.S., Carter, J.M., Ivahnenko, T., Lapham, W.W., 

Moran, M.J., Rowe, B.L., Squillace, P.J., and Toccalino, P.L., 

2006, The quality of our Nation’s waters—volatile organic 

compounds in the Nation’s ground water and drinking-water 

supply wells: U.S. Geological Survey Circular 1292, 101 p. 

Available at http://pubs.usgs.gov/circ/circ1292/

50 Quality of Water from Public-Supply Wells in the United States, 1993–2007
Appendix 1. As many as six water-quality properties and 212 chemical 

contaminants were analyzed in source-water samples from 932 public wells 

during 1993–2007. Data are from Toccalino and others (2010). 

[CAS, Chemical Abstract Service; MCL, Maximum Contaminant Level; HBSL, Health-Based Screening Level; –, not available; N, nitrogen; P, phosphorus; 

AMCL, Alternative Maximum Contaminant Level. Property or contaminant column: bold text, contaminant was detected in at least one source-water sample; 

bold italic text, property or contaminant was detected in at least 10 percent of source-water samples; unbolded text, contaminant was not detected in any sourcewater sample]

Property or contaminant CAS registry 

number(s) 1

Number of 

samples

Availability 

of MCL 

or HBSL 

values 2

Trace elements—Continued

Nickel 7440-02-0 629 HBSL

Selenium 7782-49-2 632 MCL

Silver 7440-22-4 606 HBSL

Strontium 7440-24-6 503 HBSL

Thallium 7440-28-0 437 MCL

Uranium 7440-61-1 650 MCL

Vanadium 7440-62-2 457 –

Zinc 7440-66-6 613 HBSL

Nutrients and dissolved organic carbon

Ammonia as N 7664-41-7 806 –

Ammonia plus organic 

nitrogen as N

17778-88-0 603 –

Dissolved organic carbon – 817 –

Nitrate plus nitrite as N – 806 MCL

Nitrite as N 14797-65-0 807 MCL

Orthophosphate as P 14265-44-2 804 –

Phosphorus, dissolved as P 7723-14-0 454 –

Total nitrogen as N (nitrate 

+ nitrite + ammonia + 

organic N)

– 201 –

Radionuclides

Gross alpha-particle 

radioactivity

12587-46-1 84 MCL

Gross beta-particle 

radioactivity

12587-47-2 86 MCL

Radium-226 plus radium-228 13982-63-3, 

15262-20-1

191 MCL

Radon-222 14859-67-7 506 Proposed

MCL and

AMCL

Pesticide compounds

Acetochlor 34256-82-1 800 HBSL

Acifluorfen 50594-66-6 590 HBSL

Property or contaminant CAS registry 

number(s) 1

Number of 

samples

Availability 

of MCL 

or HBSL 

values 2

Water-quality properties

Alkalinity – 840 –

Dissolved oxygen – 865 –

pH – 878 –

Specific conductance – 896 –

Temperature – 893 –

Total dissolved solids – 802 –

Major ions

Bromide 24959-67-9 787 –

Calcium 7740-70-2 809 –

Chloride 16887-00-6 809 –

Fluoride 16984-48-8 808 MCL

Magnesium 7439-95-4 809 –

Potassium 7440-09-7 810 –

Silica 7631-86-9 809 –

Sodium 7440-23-5 809 –

Sulfate 14808-79-8 810 –

Trace elements

Aluminum 7429-90-5 598 –

Antimony 7440-36-0 619 MCL

Arsenic 7440-38-2 638 MCL

Barium 7440-39-3 630 MCL

Beryllium 7440-41-7 622 MCL

Boron 7440-42-8 501 HBSL

Cadmium 7440-43-9 631 MCL

Chromium 7440-47-3 626 MCL

Cobalt 7440-48-4 627 –

Copper 7440-50-8 625 MCL

Iron 7439-89-6 809 –

Lead 7439-92-1 630 MCL

Lithium 7439-93-2 458 –

Manganese 7439-96-5 808 HBSL

Molybdenum 7439-98-7 628 HBSL

Appendix 1 51
Appendix 1. As many as six water-quality properties and 212 chemical 

contaminants were analyzed in source-water samples from 932 public wells

during 1993–2007. Data are from Toccalino and others (2010).—Continued

[CAS, Chemical Abstract Service; MCL, Maximum Contaminant Level; HBSL, Health-Based Screening Level; –, not available; N, nitrogen; P, phosphorus; 

AMCL, Alternative Maximum Contaminant Level. Property or contaminant column: bold text, contaminant was detected in at least one source-water sample; 

bold italic text, property or contaminant was detected in at least 10 percent of source-water samples; unbolded text, contaminant was not detected in any sourcewater sample]

Property or contaminant CAS registry 

number(s) 1

Number of 

samples

Availability 

of MCL 

or HBSL 

values 2

Pesticide compounds—Continued

Alachlor 15972-60-8 869 MCL

Aldicarb 116-06-3 584 HBSL

Aldicarb sulfone 1646-88-4 569 HBSL

Aldicarb sulfoxide 1646-87-3 569 HBSL

Atrazine 1912-24-9 853 MCL

Azinphos-methyl (Guthion) 86-50-0 894 HBSL

Benfluralin 1861-40-1 896 HBSL

Bentazon 25057-89-0 589 HBSL

Bromacil 314-40-9 590 HBSL

Bromoxynil 1689-84-5 590 HBSL

Butylate 2008-41-5 511 HBSL

Carbaryl 63-25-2 898 HBSL

Carbofuran 1563-66-2 644 MCL

Chloramben methyl ester 7286-84-2 580 –

Chlorothalonil 1897-45-6 507 HBSL

Chlorpyrifos 2921-88-2 898 HBSL

Clopyralid 1702-17-6 586 –

Cyanazine 21725-46-2 634 HBSL

2,4-D (2,4-Dichlorophenoxyacetic acid)

94-75-7 590 MCL

Dacthal (DCPA) 1861-32-1 898 HBSL

Dacthal monoacid 887-54-7 590 –

2,4-DB 

(4-(2,4-Dichlorophenoxy) 

butyric acid)

94-82-6 580 HBSL

p,p'-DDE (1-Chloro-4-

[2,2-dichloro-1-(4-

chlorophenyl)ethenyl]

benzene)

72-55-9 512 HBSL

Deethylatrazine 6190-65-4 853 –

Diazinon 333-41-5 897 HBSL

Dicamba 1918-00-9 584 HBSL

Dichlobenil 1194-65-6 159 HBSL

Property or contaminant CAS registry 

number(s) 1

Number of 

samples

Availability 

of MCL 

or HBSL 

values 2

Pesticide compounds—Continued

Dichlorprop 120-36-5 590 HBSL

Dieldrin 60-57-1 896 HBSL

2,6-Diethylaniline 579-66-8 898 –

Dinoseb 88-85-7 590 MCL

Disulfoton 298-04-4 647 HBSL

Diuron 330-54-1 587 HBSL

DNOC (2-Methyl-4,6dinitrophenol)534-52-1 165 –

EPTC (Eptam) 759-94-4 646 HBSL

Ethalfluralin 55283-68-6 512 HBSL

Ethoprop (Ethoprophos) 13194-48-4 647 HBSL

Fenuron 101-42-8 587 –

Fluometuron 2164-17-2 590 HBSL

Fonofos 944-22-9 898 HBSL

alpha-HCH (alphaHexachlorocyclohexane)

319-84-6 512 HBSL

gamma-HCH (Lindane) 58-89-9 512 MCL

3-Hydroxycarbofuran 16655-82-6 580 –

Linuron 330-55-2 512 HBSL

Malathion 121-75-5 898 HBSL

MCPA (4-Chloro-2methylphenoxy acetic acid)94-74-6 582 HBSL

MCPB (4-(2-Methyl-4chlorophenoxy)butyric acid)94-81-5 590 HBSL

Methiocarb 2032-65-7 579 HBSL

Methomyl 16752-77-5 569 HBSL

Metolachlor 51218-45-2 870 HBSL

Metribuzin 21087-64-9 898 HBSL

Molinate 2212-67-1 635 HBSL

Napropamide 15299-99-7 511 HBSL

Neburon 555-37-3 584 –

Norflurazon 27314-13-2 585 HBSL

Oryzalin 19044-88-3 582 HBSL

52 Quality of Water from Public-Supply Wells in the United States , 1993–2007
Appendix 1. As many as six water-quality properties and 212 chemical 

contaminants were analyzed in source-water samples from 932 public wells 

during 1993–2007. Data are from Toccalino and others (2010).—Continued

[CAS, Chemical Abstract Service; MCL, Maximum Contaminant Level; HBSL, Health-Based Screening Level; –, not available; N, nitrogen; P, phosphorus; 

AMCL, Alternative Maximum Contaminant Level. Property or contaminant column: bold text, contaminant was detected in at least one source-water sample; 

bold italic text, property or contaminant was detected in at least 10 percent of source-water samples; unbolded text, contaminant was not detected in any sourcewater sample]

Property or contaminant CAS registry 

number(s) 1

Number of 

samples

Availability 

of MCL 

or HBSL 

values 2

Pesticide compounds—Continued

Oxamyl 23135-22-0 569 MCL

Parathion (Ethyl parathion) 56-38-2 512 HBSL

Parathion-methyl (Methyl 

parathion)

298-00-0 889 HBSL

Pebulate 1114-71-2 512 HBSL

Pendimethalin 40487-42-1 898 HBSL

cis-Permethrin 54774-45-7 898 HBSL

Phorate 298-02-2 898 HBSL

Picloram 1918-02-1 585 MCL

Prometon 1610-18-0 885 HBSL

Pronamide (Propyzamide) 23950-58-5 898 HBSL

Propachlor 1918-16-7 512 HBSL

Propanil 709-98-8 647 HBSL

Propargite 2312-35-8 644 HBSL

Propham 122-42-9 589 HBSL

Propoxur (Baygon) 114-26-1 586 HBSL

Simazine 122-34-9 884 MCL

2,4,5-T (2-(2,4,5Trichlorophenoxy)acetic 

acid)

93-76-5 165 HBSL

Tebuthiuron 34014-18-1 859 HBSL

Terbacil 5902-51-2 508 HBSL

Terbufos 13071-79-9 898 HBSL

Thiobencarb 28249-77-6 647 HBSL

2,4,5-TP (Silvex) 93-72-1 165 MCL

Triallate 2303-17-5 512 HBSL

Triclopyr 55335-06-3 590 HBSL

Trifluralin 1582-09-8 898 HBSL

Volatile organic compounds

Acetone 67-64-1 771 HBSL

Acrylonitrile 107-13-1 771 HBSL

tert-Amyl methyl ether 

(Methyl tert-pentyl ether)

994-05-8 771 –

Benzene 71-43-2 831 MCL

Bromobenzene 108-86-1 832 –

Bromochloromethane 74-97-5 832 HBSL

Bromodichloromethane 75-27-4 831 MCL

Property or contaminant CAS registry 

number(s) 1

Number of 

samples

Availability 

of MCL 

or HBSL 

values 2

Volatile organic compounds—Continued

Bromoform 

(Tribromomethane)

75-25-2 832 MCL

Bromomethane (Methyl 

bromide)

74-83-9 832 HBSL

n-Butylbenzene 104-51-8 831 –

sec-Butylbenzene 135-98-8 831 –

tert-Butylbenzene 98-06-6 832 –

Carbon disulfide 75-15-0 765 HBSL

Carbon tetrachloride 

(Tetrachloromethane)

56-23-5 832 MCL

Chlorobenzene 

(Monochlorobenzene)

108-90-7 832 MCL

Chloroethane 75-00-3 830 –

Chloroform 

(Trichloromethane)

67-66-3 831 MCL

Chloromethane (Methyl 

chloride)

74-87-3 806 HBSL

3-Chloropropene 107-05-1 771 –

2-Chlorotoluene (o-) 

(1-Chloro-2-methylbenzene)

95-49-8 832 HBSL

4-Chlorotoluene (p-) 106-43-4 832 HBSL

Dibromochloromethane 124-48-1 832 MCL

Dibromochloropropane 

(DBCP)

96-12-8 832 MCL

Dibromomethane 74-95-3 832 –

1,2-Dichlorobenzene (o-) 95-50-1 831 MCL

1,3-Dichlorobenzene (m-) 541-73-1 832 HBSL

1,4-Dichlorobenzene (p-) 106-46-7 831 MCL

trans-1,4-Dichloro-2-butene 110-57-6 771 –

Dichlorodifluoromethane 75-71-8 832 HBSL

1,1-Dichloroethane 75-34-3 832 –

1,2-Dichloroethane 107-06-2 825 MCL

1,1-Dichloroethene 75-35-4 832 MCL

cis-1,2-Dichloroethene 156-59-2 832 MCL

trans-1,2-Dichloroethene 156-60-5 832 MCL

1,2-Dichloropropane 78-87-5 830 MCL

1,3-Dichloropropane 142-28-9 832 –

2,2-Dichloropropane 594-20-7 832 –

Appendix 1 53
Appendix 1. As many as six water-quality properties and 212 chemical 

contaminants were analyzed in source-water samples from 932 public wells

during 1993–2007. Data are from Toccalino and others (2010).—Continued

[CAS, Chemical Abstract Service; MCL, Maximum Contaminant Level; HBSL, Health-Based Screening Level; –, not available; N, nitrogen; P, phosphorus; 

AMCL, Alternative Maximum Contaminant Level. Property or contaminant column: bold text, contaminant was detected in at least one source-water sample; 

bold italic text, property or contaminant was detected in at least 10 percent of source-water samples; unbolded text, contaminant was not detected in any sourcewater sample]

1

 This study contains CAS Registry Numbers®, which is a Registered Trademark of the American Chemical Society. CAS recommends the verification of the 

CASRNs through CAS Client ServicesSM.

2

 MCL or HBSL values were current as of September 2009. MCL values were obtained from U.S. Environmental Protection Agency (2006a) and HBSL 

values were obtained from the HBSL website (Toccalino and others, 2008). See Toccalino and others (2010) for additional information about these benchmarks.

Property or contaminant CAS registry 

number(s) 1

Number of 

samples

Availability 

of MCL 

or HBSL 

values 2

Volatile organic compounds—Continued

1,1-Dichloropropene 563-58-6 832 –

cis-1,3-Dichloropropene 10061-01-5 832 HBSL

trans-1,3-Dichloropropene 10061-02-6 832 HBSL

Diethyl ether (Ethyl ether) 60-29-7 771 HBSL

Diisopropyl ether 108-20-3 758 –

Ethyl methacrylate 97-63-2 771 –

Ethyl tert-butyl ether (tertButyl ethyl ether)

637-92-3 771 –

Ethylbenzene 100-41-4 830 MCL

Ethylene dibromide 

(1,2-Dibromoethane)

106-93-4 832 MCL

2-Ethyltoluene (1-Ethyl-2-

methylbenzene)

611-14-3 770 –

Hexachlorobutadiene 87-68-3 831 HBSL

Hexachloroethane 67-72-1 771 HBSL

Iodomethane (Methyl iodide) 74-88-4 771 –

Isopropylbenzene 98-82-8 830 HBSL

n-Isopropyltoluene 

(1-Methyl-4-

isopropylbenzene)

99-87-6 831 –

Methyl acrylate 96-33-3 771 –

Methyl acrylonitrile 

(Methacrylonitrile)

126-98-7 771 HBSL

Methyl butyl ketone 

(2-Hexanone)

591-78-6 771 –

Methyl ethyl ketone (Ethyl 

methyl ketone)

78-93-3 769 HBSL

Methyl isobutyl ketone 

(4-Methyl-2-pentanone)

108-10-1 769 –

Methyl methacrylate 80-62-6 771 HBSL

Methyl tert-butyl ether 1634-04-4 832 –

Methylene chloride 

(Dichloromethane)

75-09-2 832 MCL

Property or contaminant CAS registry 

number(s) 1

Number of 

samples

Availability 

of MCL 

or HBSL 

values 2

Volatile organic compounds—Continued

Naphthalene 91-20-3 831 HBSL

Perchloroethene 

(Tetrachloroethene)

127-18-4 829 MCL

n-Propylbenzene 103-65-1 831 –

Styrene 100-42-5 829 MCL

1,1,1.2-Tetrachloroethane 630-20-6 832 HBSL

1,1,2,2-Tetrachloroethane 79-34-5 832 HBSL

Tetrahydrofuran 109-99-9 771 –

1,2,3,4-Tetramethylbenzene 488-23-3 770 –

1,2,3,5-Tetramethylbenzene 527-53-7 770 –

Toluene 108-88-3 818 MCL

1,2,3-Trichlorobenzene 87-61-6 832 –

1,2,4-Trichlorobenzene 120-82-1 831 MCL

1,1,1-Trichloroethane 71-55-6 832 MCL

1,1,2-Trichloroethane 79-00-5 829 MCL

Trichloroethene 79-01-6 832 MCL

Trichlorofluoromethane 75-69-4 832 HBSL

1,2,3-Trichloropropane 96-18-4 832 HBSL

Trichlorotrifluoroethane 

(1,1,2-Trichloro-1,2,2-

trifluoroethane)

76-13-1 832 HBSL

1,2,3-Trimethylbenzene 526-73-8 770 –

1,2,4-Trimethylbenzene 95-63-6 828 –

1,3,5-Trimethylbenzene 108-67-8 831 –

Vinyl bromide (Bromoethene) 593-60-2 771 –

Vinyl chloride 

(Chloroethene)

75-01-4 832 MCL

o-Xylene 95-47-6 769 MCL

m- and p- Xylenes 108-38-3, 

106-42-3

769 MCL

54 Quality of Water from Public-Supply Wells in the United States , 1993–2007
Appendix 2. As many as 125 additional organic contaminants were analyzed in 

source-water samples from a subset of 221 public wells during 2002–2005. Data 

are from Hopple and others (2009). 

[CAS, Chemical Abstract Service; MCL, Maximum Contaminant Level; HBSL, Health-Based Screening Level; –, not available. Contaminant column: bold 

text, contaminant was detected in at least one source-water sample; bold italic text, contaminant was detected in at least 10 percent of source-water samples; 

unbolded text, contaminant was not detected in any source-water sample]

Contaminant CAS registry 

number1

Number of 

samples

Availability 

of MCL 

or HBSL 

values 2

Personal-care and domestic-use products

Acetophenone 98-86-2 187 HBSL

Acetyl hexamethyl 

tetrahydronaphthalene 

(AHTN)

21145-77-7 217 –

3-tert-Butyl-4-hydroxy anisole 

(BHA)

25013-16-5 217 –

Caffeine 58-08-2 168 –

Camphor 76-22-2 217 –

Cotinine 486-56-6 118 –

4-Cumylphenol 599-64-4 217 –

Hexahydrohexamethylcyclopentabenzopyran 

(HHCB)

1222-05-5 217 –

Indole 120-72-9 217 –

Isoborneol 124-76-5 217 –

Isoquinoline 119-65-3 217 –

d-Limonene 5989-27-5 217 –

Menthol 89-78-1 203 –

Methyl salicylate 119-36-8 217 HBSL

Nonylphenol, diethoxy- (total) 26027-38-2 217 –

4-n-Octylphenol 1806-26-4 217 –

4-tert-Octylphenol 140-66-9 217 –

Octylphenol, diethoxy- – 217 –

Octylphenol, monoethoxy- – 217 –

Triclosan 3380-34-5 217 –

Triethyl citrate (ethyl citrate) 77-93-0 217 –

Other pesticide compounds

Azinphos-methyl-oxon 961-22-8 221 –

Bendiocarb 22781-23-3 215 HBSL

Benomyl 17804-35-2 215 HBSL

Bensulfuron-methyl 83055-99-6 215 HBSL

Contaminant CAS registry 

number1

Number of 

samples

Availability 

of MCL 

or HBSL 

values 2

Other pesticide compounds—Continued

Chlorimuron-ethyl 90982-32-4 215 HBSL

2-Chloro-2,6-diethylacetanilide 6967-29-9 221 –

4-Chloro-2-methylphenol 1570-64-5 221 –

3(4-Chlorophenyl)-1-methyl 

urea

5352-88-5 215 –

Chlorpyrofos, oxygen analog 5598-15-2 221 –

Cycloate 1134-23-2 215 HBSL

Cyfluthrin 68359-37-5 221 HBSL

Cypermethrin 52315-07-8 221 HBSL

2,4-D methyl ester 1928-38-7 215 –

Deethyldeisopropyl atrazine 3397-62-4 213 –

Deisopropylatrazine 1007-28-9 215 –

Desulfinylfipronil – 221 –

Desulfinylfipronil amide – 221 –

Diazinon, oxygen analog 962-58-3 221 –

3,4-Dichloroaniline 95-76-1 221 –

Dichlorvos 62-73-7 221 HBSL

Dicrotophos 141-66-2 221 HBSL

Dimethoate 60-51-5 221 HBSL

Diphenamid 957-51-7 215 HBSL

Ethion 563-12-2 221 HBSL

Ethion monoxon 17356-42-2 221 –

2-Ethyl-6-methylaniline 24549-06-2 221 –

Fenamiphos 22224-92-6 221 HBSL

Fenamiphos sulfone 31972-44-8 221 –

Fenamiphos sulfoxide 31972-43-7 200 –

Fipronil 120068-37-3 221 –

Fipronil sulfide 120067-83-6 221 –

Fipronil sulfone 120068-36-2 221 –

Flumetsulam 98967-40-9 215 HBSL

Appendix 2 55
Appendix 2. As many as 125 additional organic contaminants were analyzed in 

source-water samples from a subset of 221 public wells during 2002–2005. Data 

are from Hopple and others (2009).—Continued

[CAS, Chemical Abstract Service; MCL, Maximum Contaminant Level; HBSL, Health-Based Screening Level; –, not available. Contaminant column: bold 

text, contaminant was detected in at least one source-water sample; bold italic text, contaminant was detected in at least 10 percent of source-water samples; 

unbolded text, contaminant was not detected in any source-water sample]

Contaminant CAS registry 

number1

Number of 

samples

Availability 

of MCL 

or HBSL 

values 2

Other pesticide compounds—Continued

Fonofos, oxygen analog 944-21-8 206 –

Hexazinone 51235-04-2 113 HBSL

2-Hydroxy atrazine 2163-68-0 215 HBSL

Imazaquin 81335-37-7 215 HBSL

Imazethapyr 81335-77-5 215 HBSL

Imidacloprid 138261-41-3 215 HBSL

Iprodione 36734-19-7 221 HBSL

Isofenphos 25311-71-1 221 HBSL

3-Ketocarbofuran 16709-30-1 213 –

Malaoxon 1634-78-2 221 –

Metalaxyl 57837-19-1 221 HBSL

Methidathion 950-37-8 221 HBSL

Metsulfuron methyl 74223-64-6 214 HBSL

Myclobutanil 88671-89-0 221 HBSL

1-Naphthol 90-15-3 221 –

Nicosulfuron 111991-09-4 215 HBSL

Paraoxon-methyl 950-35-6 221 –

Pentachlorophenol 87-86-5 211 MCL

Phorate oxon 2600-69-3 221 –

Phosmet 732-11-6 211 HBSL

Phosmet oxon 3735-33-9 201 –

Prometryn 7287-19-6 221 HBSL

Propiconazole 60207-90-1 215 HBSL

Siduron 1982-49-6 215 HBSL

Sulfometuron-methyl 74222-97-2 215 HBSL

Terbufos-O-analogue sulfone 56070-15-6 221 –

Terbuthylazine 5915-41-3 221 HBSL

Other herbicide and herbicide degradates

Acetochlor ethane sulfonic 

acid

187022-11-3 73 –

Acetochlor oxanilic acid 184992-44-4 73 –

Contaminant CAS registry 

number1

Number of 

samples

Availability 

of MCL 

or HBSL 

values 2

Other herbicide and herbicide degradates—Continued

Acetochlor sulfynilacetic acid – 73 –

Acetochlor/Metolachlor 2nd 

Amide ESA

– 43 –

Alachlor ESA 2nd amide – 43 –

Alachlor ethane sulfonic acid 142363-53-9 58 –

Alachlor oxanilic acid 171262-17-2 73 –

Alachlor sulfynilacetic acid 140939-16-8 73 –

Dimethenamid 87674-68-8 43 –

Dimethenamid ethane sulfonic 

acid

205939-58-8 73 –

Dimethenamid oxanilic acid – 73 –

Flufenacet 142459-58-3 43 –

Flufenacet ethane sulfonic acid – 73 –

Flufenacet oxanilic acid – 73 –

Metolachlor ethane sulfonic 

acid

171118-09-5 73 –

Metolachlor oxanilic acid 152019-73-3 73 –

Propachlor ethane sulfonic acid – 73 –

Propachlor oxanilic acid – 73 –

Other volatile organic compounds

Anthraquinone 84-65-1 217 –

tert-Butyl alcohol 75-65-0 119 –

Carbazole 86-74-8 217 –

p-Cresol 106-44-5 217 –

2,6-Dimethylnaphthalene 581-42-0 217 –

Methyl acetate 79-20-9 119 –

2-Methyl-2-butanol 75-85-4 119 –

1-Methylnaphthalene 90-12-0 217 –

2-Methylnaphthalene 91-57-6 217 HBSL

56 Quality of Water from Public-Supply Wells in the United States , 1993–2007
1

 This study contains CAS Registry Numbers®, which is a Registered Trademark of the American Chemical Society. CAS recommends the verification of the 

CASRNs through CAS Client ServicesSM.

2

 MCL or HBSL values were current as of September 2009. MCL values were obtained from U.S. Environmental Protection Agency (2006a) and HBSL 

values were obtained from the HBSL website (Toccalino and others, 2008). See Hopple and others (2009) for additional information about these benchmarks.

Contaminant CAS registry 

number1

Number of 

samples

Availability 

of MCL 

or HBSL 

values 2

Manufacturing additives, pavement- and combustion-derived 

compounds, plant- or animal-derived biochemicals

Anthracene 120-12-7 217 HBSL

Benzo[a]pyrene 50-32-8 217 MCL

Bisphenol A 80-05-7 212 HBSL

Cholesterol 57-88-5 217 –

3-beta-Coprostanol 360-68-9 217 –

Fluoranthene 206-44-0 217 HBSL

5-Methyl-1H-benzotriazole 136-85-6 215 –

3-Methyl-1(H)-indole 

(Skatole)

83-34-1 217 –

Phenanthrene 85-01-8 217 –

Contaminant CAS registry 

number1

Number of 

samples

Availability 

of MCL 

or HBSL 

values 2

Manufacturing additives, pavement- and combustion-derived 

compounds, plant- or animal-derived biochemicals—Continued

Pyrene 129-00-0 216 HBSL

beta-Sitosterol 83-46-5 217 –

beta-Stigmastanol 19466-47-8 217 –

Tri(2-butoxyethyl)phosphate 78-51-3 203 –

Tri(2-chloroethyl)phosphate 115-96-8 217 –

Tributyl phosphate 126-73-8 217 –

Triphenyl phosphate 115-86-6 202 –

Tris(dichlorisopropyl)

phosphate

13674-87-8 217 –

Appendix 2. As many as 125 additional organic contaminants were analyzed in 

source-water samples from a subset of 221 public wells during 2002–2005. Data 

are from Hopple and others (2009).—Continued

[CAS, Chemical Abstract Service; MCL, Maximum Contaminant Level; HBSL, Health-Based Screening Level; –, not available. Contaminant column: bold 

text, contaminant was detected in at least one source-water sample; bold italic text, contaminant was detected in at least 10 percent of source-water samples; 

unbolded text, contaminant was not detected in any source-water sample]

Appendix 2 57
Abbreviations, Acronyms, and Units of Measure

CCL Contaminant Candidate List

CCR consumer confidence report

DBP disinfection by-product

DWLOC Drinking Water Level of Comparison

FIFRA Federal Insecticide, Fungicide, and Rodenticide Act

HBSL Health-Based Screening Level

MCL Maximum Contaminant Level

mg/L milligrams per liter

MTBE methyl tert-butyl ether

NAWQA National Water-Quality Assessment Program (USGS)

PCE perchloroethene

pCi/L picocuries per liter

SDWA Safe Drinking Water Act

TCE trichloroethene

TCT total chlorotriazines

μg/L micrograms per liter

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VOC volatile organic compound

58 Quality of Water from Public-Supply Wells in the United States , 1993–2007
Acknowledgments

Special thanks to the following individuals and organizations for their contributions:

NAWQA personnel in study areas across the Nation for their contributions of data and 

research results.

NAWQA’s many partners in the governmental and non-governmental sectors who have 

helped guide scientific efforts and ensure that NAWQA information meets the needs of 

local, state, tribal, regional, and national stakeholders.

USGS reviewers:

• Gregory Delzer

• Leslie DeSimone

• Sandra Eberts

• Michael Focazio

• Robert Gilliom

• Pixie Hamilton

• Thomas Reilly (aquifer-type sidebar on page 18)

External reviewers:

• Rebecca Allen, USEPA, Office of Ground Water and Drinking Water

• Randy Beavers, Des Moines Water Works

• Elizabeth Behl, USEPA, Office of Pesticide Programs

• Meredith Benedict, USEPA, Office of Ground Water and Drinking Water

• Judy Campbell-Bird, Environmental Policy and Communications Consulting

• Thomas Carpenter, USEPA, Office of Ground Water and Drinking Water

• Joyce Donohue, USEPA, Office of Science and Technology

• Debra Gutenson, USEPA, Office of Ground Water and Drinking Water

• Darrell Osterhoudt, Association of State Drinking Water Administrators

• Christine Reimer, National Ground Water Association

• Alan Roberson, American Water Works Association

Technical editor:

• Chester Zenone, Editorial Consultant

Graphic design and layout:

• Yvonne Roque, Bill Gibbs, and Sharon Wahlstrom, USGS

Publishing support provided by the U.S. Geological Survey

Publishing Network, Sacramento and Tacoma Publishing Service Centers

For more information concerning the research in this report, contact:

Chief, National Water-Quality Assessment Program

U.S. Geological Survey

413 National Center

12201 Sunrise Valley Drive

Reston, VA 20192

or visit our Web site at:

http://water.usgs.gov/nawqa/studies/public_wells/
Toccalinl and others—Quality of Water from Public-Supply Wells in the United States, 1993–2007—Overview of Major Findings—Circular 1346