NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Quality of Water from Domestic Wells 

in Principal Aquifers of the United States, 

1991–2004

Scientific Investigations Report 2008–5227

U.S. Department of the Interior

U.S. Geological Survey
Front cover. Photographs from iStock© Photos (middle photograph) and U.S. Geological Survey.
Quality of Water from Domestic Wells in 

Principal Aquifers of the United States, 

1991–2004

By Leslie A. DeSimone

National Water-Quality Assessment Program

Scientific Investigations Report 2008–5227

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

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

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Suggested citation:

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 online at http://pubs.usgs.gov/

sir/2008/5227.
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 ground water? How are conditions changing over time? How do 

natural features and human activities affect the quality of streams and ground water, and where are 

those effects most pronounced? By combining information on water chemistry, physical characteristics, stream habitat, and aquatic life, the NAWQA Program aims to provide science-based insights for 

current and emerging water issues and priorities. From 1991–2001, 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 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 ground water. 

For example, increased emphasis has been placed on assessing the quality of source water and finished water associated with many of the Nation’s largest community water systems. During the second 

decade, NAWQA is addressing five national priority topics that build an understanding of how natural 

features and human activities affect water quality, and establish links between sources of contaminants, the transport of those contaminants through the hydrologic system, and the potential effects of 

contaminants on humans and aquatic ecosystems. Included are topics on 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, selected trace elements, and aquatic ecology are continuing.

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

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

and will foster increased citizen awareness and involvement in the protection and restoration of our 

Nation’s waters.

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

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

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

groups. Your assistance and suggestions are greatly appreciated.

Matthew C. Larsen

Associate Director for Water
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v

Contents

Foreword ........................................................................................................................................................iii

Abstract ...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Methods...........................................................................................................................................................6

Well Selection .......................................................................................................................................6

Sample Collection and Analysis .........................................................................................................7

Quality Assurance and Quality Control .............................................................................................7

Data Sources and Treatment ..............................................................................................................7

Statistical Methods and Data Presentation .....................................................................................8

Water-Quality Benchmarks for Human Health .........................................................................................8

Domestic Wells Sampled for the NAWQA Program, 1991–2004 ............................................................9

Quality of Water from Domestic Wells .....................................................................................................17

Physical Properties and Dissolved Solids ......................................................................................17

Major Ions ............................................................................................................................................23

Trace Elements ....................................................................................................................................27

Nutrients and Dissolved Organic Carbon .......................................................................................34

Radon and Other Radionuclides .......................................................................................................37

Organic Compounds ...........................................................................................................................42

Fecal Indicator Bacteria ....................................................................................................................53

Summary Comparison of Contaminant Concentrations to Human-Health Benchmarks ........55

Co-occurrence of Contaminants as Mixtures .........................................................................................57

Approach for Assessment of Mixtures ...........................................................................................57

Occurrence and Distribution of Mixtures .......................................................................................58

Composition of Mixtures ....................................................................................................................58

Assessing the Potential Significance of Mixtures to Human Health .........................................63

Comparison with Previous Studies of Domestic Well-Water Quality .................................................64

Summary and Conclusions .........................................................................................................................66

Acknowledgments .......................................................................................................................................69

References ....................................................................................................................................................69

APPENDIXES IN BACK OF REPORT:

Appendix 1. Summary of selected studies of domestic well water quality in the 

United States ...................................................................................................................................83

Appendix 2. Characteristics, principal aquifers, study units, and assessment studies of 

domestic wells sampled for the NAWQA Program, 1991–2004, by aquifer type .................93

Appendix 3. Sampling and analytical methods and summary of field quality-control data 

for chemical contaminants measured in domestic wells sampled for the NAWQA 

Program, 1991–2004 .....................................................................................................................101

Appendix 4. Physical properties and contaminants analyzed in samples collected from 

domestic wells for the NAWQA Program, 2001–2004, and associated human-health 

benchmarks for drinking water ..................................................................................................111
vi

Figures

1. Map showing NAWQA study units and assessment studies in which domestic 

wells were sampled, 1991–2004 .................................................................................................5

2. Maps showing (A) principal aquifers of the United States and (B) domestic wells 

sampled for the NAWQA Program in aquifer studies, 1991–2004.......................................10

3. Boxplots showing (A) well depth, (B) depth to top of open interval, and (C) depth 

to water in domestic wells sampled for the NAWQA Program in aquifer studies, 

1991–2004 .....................................................................................................................................14

4. Maps showing population supplied by domestic wells by county in the United States 

in 1995 by (A) population density and (B) population as percent of total county 

population ....................................................................................................................................15

5. Bar graphs showing water withdrawals for public supply in the United States in 

2000 and numbers of domestic wells sampled for the NAWQA Program in aquifer 

studies, 1991–2004, by principal aquifer .................................................................................16

6. Map showing geographic distribution of dissolved solids concentrations in 

samples collected from domestic wells for the NAWQA Program in aquifer studies, 

1991–2004 .....................................................................................................................................18

7. Boxplots showing (A) dissolved solids, (B) pH, (C) alkalinity, and (D) dissolved 

oxygen in samples collected from domestic wells sampled for the NAWQA Program 

in aquifer studies, 1991–2004, by aquifer rock type ..............................................................20

8. Map showing geographic distribution of pH in samples collected from domestic 

wells for the NAWQA Program in aquifer studies, 1991–2004 ............................................21

9. Map showing geographic distribution of alkalinity in samples collected from 

domestic wells for the NAWQA Program in aquifer studies, 1991–2004...........................22

10. Boxplot showing dissolved oxygen concentrations in samples collected from 

domestic wells for the NAWQA Program in aquifer studies, 1991–2004, by aquifer 

condition.......................................................................................................................................23

11. Trilinear diagrams of ionic composition of water in samples collected from domestic 

wells for the NAWQA Program in aquifer studies, 1991–2004, by principal aquifer rock 

type in (A) basin-fill and other non-glacial sand and gravel aquifers, (B) glacial sand 

and gravel aquifers, (C) coastal plain aquifers in semi-consolidated sand, (D) sandstone aquifers, (E) sandstone and carbonate-rock aquifers, (F) carbonate-rock 

aquifers, (G) basaltic-rock aquifers, and (H) crystalline-rock aquifers.............................24

APPENDIXES ON CD-ROM IN BACK OF REPORT:

Appendix 5. Summary statistics for physical properties and concentrations of major ions, 

trace elements, nutrients, radionuclides, and fecal indicator bacteria in samples 

collected from domestic wells for the NAWQA Program, 1991–2004 ...............[on CD-ROM]

Appendix 6. Percentages of wells in principal aquifers with values of physical properties 

or concentrations of major ions, trace elements, nutrients, radionuclides, or organic 

compounds greater than human-health benchmarks or guidelines for drinking 

water in samples collected from domestic wells for the NAWQA Program, 

1991–2004.....................................................................................................................[on CD-ROM]

Appendix 7. Detection frequencies of organic compounds at any concentration and at 

several common reporting levels in samples collected from domestic wells for 

the NAWQA Program, 1991–2004 ............................................................................[on CD-ROM]
vii

12. Map showing geographic distribution of hardness of water in samples collected 

from domestic wells for the NAWQA Program in aquifer studies, 1991–2004 .................26

13. Map showing geographic distribution of fluoride concentrations in samples 

collected from domestic wells for the NAWQA Program in aquifer studies, 

1991–2004 .....................................................................................................................................27

14. Boxplots showing concentrations of (A) arsenic, (B) boron, (C) manganese, 

(D) strontium, and (E) uranium in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2001, by aquifer rock type..............................31

15. Map showing geographic distribution of arsenic concentrations in samples 

collected from domestic wells for the NAWQA Program in aquifer studies, 

1991–2004 .....................................................................................................................................32

16. Map showing geographic distribution of manganese concentrations in samples 

collected from domestic wells for the NAWQA Program in aquifer studies, 

1991–2004 .....................................................................................................................................33

17. Map showing geographic distribution of uranium concentrations in samples 

collected from domestic wells for the NAWQA Program in aquifer studies, 

1991–2004 .....................................................................................................................................34

18. Map showing geographic distribution of nitrate concentrations in samples collected 

from domestic wells for the NAWQA Program in aquifer studies, 1991–2004 .................37

19. Boxplots showing nitrate concentrations in samples collected from domestic wells 

for the NAWQA Program in aquifer studies, 1991–2004, by (A) aquifer rock type and 

(B) land use..................................................................................................................................38

20. Map showing geographic distribution of radon concentrations in samples collected 

from domestic wells for the NAWQA Program in aquifer studies, 1994–2004 .................40

21. Boxplot showing radon concentrations in samples collected from domestic wells 

for the NAWQA Program in aquifer studies, 1991–2004, by aquifer rock type.................41

22. Scatterplot showing concentrations of frequently detected organic compounds in 

samples collected from domestic wells for the NAWQA Program in aquifer studies, 

1991–2004 .....................................................................................................................................43

23. Bar graph showing detection frequency for frequently occurring organic 

compounds in samples collected from domestic wells for the NAWQA Program in 

aquifer studies, 1991–2004 ........................................................................................................44

24. Bar graph showing percentage of samples in which one or more organic 

compounds were detected, at any concentration and at common reporting levels of 

0.02 and 0.2 micrograms per liter, for domestic wells sampled for the NAWQA 

Program, 1991–2004, in aquifer studies...................................................................................45

25A–C. Maps showing geographic distribution of organic compounds in samples 

collected from domestic wells for the NAWQA Program in aquifer studies, 1991–2004, 

at (A) detections above a common reporting level of 0.2 microgram per liter (μg/L), 

(B) detections above a common reporting level of 0.02 μg/L, and (C) concentrations 

greater than human-health benchmarks................................................................................46

26. Boxplot showing concentrations of nitrate and organic compounds in samples 

collected from domestic wells for the NAWQA Program in aquifer studies, 

1991–2004 .....................................................................................................................................50

27. Map showing geographic distribution of atrazine and deethylatrazine in samples 

collected from domestic wells for the NAWQA Program in aquifer studies, 

1991–2004 .....................................................................................................................................50

28. Map showing geographic distribution of chloroform in samples collected from 

domestic wells for the NAWQA Program in aquifer studies, 1991–2004...........................51
viii

29. Map showing geographic distribution of methyl tert-butyl ether (MTBE) in samples 

collected from domestic wells for the NAWQA Program in aquifer studies, 

1991–2004 .....................................................................................................................................52

30. Map showing geographic distribution of fecal indicator bacteria detected in samples 

collected from domestic wells for the NAWQA Program in aquifer studies, 

1991–2004 .....................................................................................................................................54

31. Bar graph showing fecal indicator bacteria detected in samples collected from 

domestic wells for the NAWQA Program in aquifer studies, 1991–2004, by aquifer 

rock type.......................................................................................................................................54

32. Map showing geographic distribution of wells with at least one contaminant at 

concentrations greater than or greater than one-tenth of human-health benchmarks 

in samples collected from domestic wells for the NAWQA Program in aquifer studies, 

1991–2004 .....................................................................................................................................56

33. Graph showing frequency of occurrence of contaminants in mixtures in samples 

collected from domestic wells for the NAWQA Program in aquifer studies, 

1991–2004 .....................................................................................................................................58

34A–B. Maps showing geographic distribution of wells with multiple contaminants (A) at 

concentrations greater than human-health benchmarks and (B) at concentrations 

greater than one-tenth of human-health benchmarks in samples collected from 

domestic wells for the NAWQA Program in aquifer studies, 1991–2004...........................59

35. Boxplots showing dissolved solids concentrations in relation to the numbers of 

contaminants present at (A) concentrations greater than human-health benchmarks 

and (B) concentrations greater than one-tenth of human-health benchmarks in 

samples collected from domestic wells sampled for the NAWQA Program in aquifer 

studies, 1991–2004 ......................................................................................................................61
ix

Tables

1. Characteristics of domestic wells sampled for the NAWQA Program in aquifer 

studies, 1991–2004, by principal aquifer .................................................................................12

2. Summary statistics for physical properties of, and major ions in, samples collected 

from domestic wells for the NAWQA Program in aquifer studies, 1991–2004 .................17

3. Comparison of physical properties and concentrations of major ions in samples 

collected from domestic wells for the NAWQA Program in aquifer studies, 1991–2004, 

with human-health benchmarks and non-health guidelines for drinking water..............19

4. Summary statistics for trace elements in samples collected from domestic wells 

for the NAWQA Program in aquifer studies, 1991–2004.......................................................28

5. Comparison of concentrations of trace elements in samples collected from domestic wells for the NAWQA Program in aquifer studies, 1991–2004, with human-health 

benchmarks and non-health guidelines for drinking water ................................................30

6. Summary statistics for nutrients, dissolved organic carbon, radionuclides, and 

fecal indicator bacteria in samples collected from domestic wells for the NAWQA 

Program in aquifer studies, 1991–2004....................................................................................36

7. Comparison of concentrations of nitrate and radionuclides in samples collected 

from domestic wells for the NAWQA Program in aquifer studies, 1991–2004, with 

human-health benchmarks and non-health guidelines for drinking water ......................39

8. Land-use classification used to categorize domestic wells sampled for the 

NAWQA Program in aquifer studies, 1991–2004....................................................................39

9. Comparison of concentrations of organic compounds in samples collected from 

domestic wells for the NAWQA Program in aquifer studies, 1991–2004, with humanhealth benchmarks for drinking water....................................................................................49

10. Number and percentage of domestic wells, in a subset of 1,389 domestic wells 

from the NAWQA Program in aquifer studies, 1991–2004, containing one or more 

contaminants at concentrations greater than a human-health benchmark or greater 

than one-tenth of a benchmark................................................................................................55

11. Composition and frequency of occurrence of unique mixtures of contaminants 

individually at concentrations of potential concern for human health in samples collected from domestic wells for the NAWQA Program in aquifer studies, 1991–2004 .....62

12. Composition and frequency of occurrence of unique mixtures of contaminants 

individually at concentrations of potential concern for human health and any detectable concentration for organic compounds in samples collected from domestic 

wells for the NAWQA Program in aquifer studies, 1991–2004 ............................................63
x

Conversion Factors and Abbreviations

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2)

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.00378 milliliter (mL)

Mass

ounce, avoirdupois (oz) 28.35 gram (g) 

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F=(1.8×°C)+32

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius 

(μS/cm at 25°C).

Concentrations of chemical constituents in water are given in milligrams per liter (mg/L) or 

micrograms per liter (μg/L).

Concentrations of fecal indicator bacteria in water are given in colony forming units per 

100 milliliters (CFU/100mL).

Concentrations of radioisotopes in water are given in picocuries per liter (pCi/L).

ABBREVIATIONS USED IN REPORT

a acceptable statistical significance level

AA atomic absorption spectrometry

BTEX benzene, toluene, ethylbenzene, and xylenes

CaCO3 calcium carbonate

CDC U.S. Centers for Disease Control and Prevention

DBCP dibromochloropropane

E. coli Escherichia coli

EDB ethylene dibromide

GCMS gas chromatography/mass spectrometry

HBSL Health-Based Screening Level

HPLC high performance liquid chromatography

ICP inductively-coupled plasma
xi

ICP-AES inductively-coupled plasma–atomic emission spectroscopy

ICP-MS inductively-coupled plasma–mass spectrometry

LRL laboratory reporting level

LT-MDL long-term method-detection level

MTBE methyl tert-butyl ether

MCL Maximum Contaminant Level

MDL method-detection level

MRL minimum reporting level

n number of samples

NAWQA National Water-Quality Assessment

NPS National Survey of Pesticides in Drinking-Water Wells

NSA National Statistical Assessment of Rural Water Conditions

NWIS National Water Information System

NWQL National Water-Quality Laboratory

p attained statistical significance level

QC quality control

PCE perchloroethene

R 2 coefficient of determination, or fraction of variance explained by regression

RPD relative percent difference

RSD relative standard deviation

SD standard deviation

SDWA Safe Drinking Water Act

SMCL Secondary Maximum Contaminant Level

SSMDC sample-specific minimum detectable concentration

TCE trichloroethene

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VOC volatile organic compound
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Abstract

As part of the National Water-Quality Assessment Program of the U.S. Geological Survey (USGS), water samples 

were collected during 1991–2004 from domestic wells (private 

wells used for household drinking water) for analysis of drinking-water contaminants, where contaminants are considered, 

as defined by the Safe Drinking Water Act, to be all substances 

in water. Physical properties and the concentrations of major 

ions, trace elements, nutrients, radon, and organic compounds 

(pesticides and volatile organic compounds) were measured in 

as many as 2,167 wells; fecal indicator bacteria and radionuclides also were measured in some wells. The wells were 

located within major hydrogeologic settings of 30 regionally 

extensive aquifers used for water supply in the United States. 

One sample was collected from each well prior to any in-home 

treatment. Concentrations were compared to water-quality 

benchmarks for human health, either U.S. Environmental 

Protection Agency (USEPA) Maximum Contaminant Levels 

(MCLs) for public water supplies or USGS Health-Based 

Screening Levels (HBSLs). 

No individual contaminant was present in concentrations 

greater than available health benchmarks in more than 

8 percent of the sampled wells. Collectively, however, about 

23 percent of wells had at least 1 contaminant present at concentrations greater than an MCL or HBSL, based on analysis 

of samples from 1,389 wells in which most contaminants were 

measured. Radon, nitrate, several trace elements, fluoride, 

gross alpha- and beta-particle radioactivity, and fecal indicator 

bacteria were found most frequently (in one or more percent of 

wells) at concentrations greater than benchmarks and, thus, are 

of potential concern for human health. Radon concentrations 

were greater than the lower of two proposed MCLs (300 picocuries per liter or pCi/L) in about 65 percent of the wells and 

greater than the higher proposed MCL (4,000 pCi/L) in about 

4 percent of wells. Nitrate, arsenic, manganese, strontium, 

and gross alpha-particle radioactivity (uncorrected) each were 

present at levels greater than MCLs or HBSLs in samples from 

about 5 to 7 percent of the wells; boron, fluoride, uranium, and 

gross beta-particle radioactivity were present at levels greater 

than MCLs or HBSLs in about 1 to 2 percent of the wells. 

Total coliform and Escherichia coli bacteria were detected in 

about 34 and 8 percent, respectively, of sampled wells. Thus, 

with the exception of nitrate and fecal indicator bacteria, the 

contaminants that were present in the sampled wells most 

frequently at concentrations greater than human-health benchmarks were naturally occurring.

Anthropogenic organic compounds were frequently 

detected at low concentrations, using typical analytical 

detection limits of 0.001 to 0.1 micrograms per liter, but 

were seldom present at concentrations greater than MCLs or 

HBSLs. The most frequently detected compounds included 

the pesticide atrazine, its degradate deethylatrazine, and the 

volatile organic compounds chloroform, methyl tert-butyl 

ether, perchloroethene, and dichlorofluoromethane. Only 7 of 

168 organic compounds were present in samples at concentrations greater than MCLs or HBSLs, each in less than 1 percent 

of wells. These were diazinon, dibromochloroprane, dinoseb, 

dieldrin, ethylene dibromide, perchloroethene, and trichloroethene. Overall, concentrations of any organic compound 

greater than MCLs or HBSLs were present in 0.8 percent of 

wells, and concentrations of any organic compound greater 

than one-tenth of MCLs or HBSLs were present in about 

3 percent of wells.

Several other properties and contaminants were measured 

at values or concentrations outside of recommended ranges 

for drinking water for aesthetic quality (for example, taste 

or odor) or other non-health reasons. About 16 percent of 

the sampled wells had pH values less than (14.4 percent) or 

greater than (1.9 percent) the USEPA recommended range 

of 6.5 to 8.5. Total dissolved solids were greater than the 

USEPA Secondary Maximum Contaminant Level (SMCL) 

of 500 milligrams per liter in about 15 percent of wells. Iron 

and manganese concentrations were greater than SMCLs in 

about 19 and 21 percent of wells, respectively. Concentrations 

of fluoride, which can be harmful at high levels but prevents 

tooth decay at lower levels, were less than those recommended 

by the U.S. Centers for Disease Control and Prevention in 

about 90 percent of the sampled wells.

Geographic patterns of occurrence among principal 

aquifers showed that several contaminants and properties 

may be of greater potential concern in certain locations or 

regions than nationally. For example, radon concentrations 

were greater than the proposed MCLs in 30 percent (higher 

proposed MCL) and 90 percent (lower proposed MCL) of 

wells in crystalline-rock aquifers located in the Northeast, 

Quality of Water from Domestic Wells in 

Principal Aquifers of the United States, 1991–2004

By Leslie A. DeSimone
2 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

the central and southern Appalachians, and Colorado. Nitrate 

was present at concentrations greater than the MCL more 

frequently in agricultural areas than in other land-use settings. 

Contaminant concentrations also were related to geochemical 

conditions. For example, uranium concentrations were correlated with concentrations of dissolved oxygen in addition to 

showing regional patterns of occurrence; relatively high iron 

and manganese concentrations occurred everywhere, but were 

inversely correlated with dissolved oxygen concentrations. 

Mixtures of two or more contaminants at concentrations 

greater than human-health benchmarks occurred in only about 

4 percent of wells, but mixtures of two or more contaminants 

with concentrations greater than one-tenth of their benchmarks 

occurred in about 73 percent of wells. The more complex 

mixtures, with the largest numbers of contaminants, were most 

common in several aquifers in the western and south-central 

United States in ground water with high concentrations of 

dissolved solids overall. Two-thirds of the unique mixtures 

of contaminants with concentrations greater than one-tenth 

of their benchmarks that occurred in 5 percent or more of 

wells were composed of two or more of six contaminants—

nitrate, arsenic, radon, and uranium, and to a lesser extent, 

molybdenum and manganese. Organic compounds were 

rarely components of unique mixtures of contaminants at 

concentrations greater than human-health benchmarks or 

greater than one-tenth of their benchmarks. However, mixtures 

of naturally occurring contaminants at concentrations greater 

than one-tenth of their benchmarks and organic compounds 

detected at any concentration were common, occurring in 

about 90 percent of wells. Several combinations of organic 

compounds in mixtures with possible health effects were 

identified—specifically, atrazine and deethylatrazine, atrazine 

or simazine with nitrate, and perchloroethene and three other 

solvents—but combined concentrations either were less than 

the health benchmarks or no benchmarks were available for 

the mixtures. These co-occurrences may be a potential concern 

for human health, but the long-term cumulative effects of low 

concentrations of multiple contaminants on human health 

currently are unknown. 

Introduction

More than 43 million people—about 15 percent of the 

population of the United States—rely on privately owned 

household wells for their drinking water (Hutson and others, 

2004). The quality and safety of these water supplies, known 

as private or domestic wells, are not regulated under Federal 

or, in most cases, state law. Rather, individual homeowners are 

responsible for maintaining their domestic well systems and 

for any routine water-quality monitoring. The Safe Drinking Water Act (SDWA) governs the Federal regulation and 

monitoring of public water supplies. Although the SDWA 

does not include regulation of domestic wells, its approach to 

evaluating the suitability of drinking water for public supplies 

provides a useful approach for evaluating the quality of drinking water obtained from domestic wells. The SDWA defines 

terminology related to water supply and the process by which 

drinking-water standards, called Maximum Contaminant 

Levels (MCLs), are established to ensure safe levels of specific contaminants in public water systems. The SDWA defines 

a contaminant as “any physical, chemical, biological, or 

radiological substance or matter in water” (U.S. Senate, 2002), 

whether potentially harmful or not (see sidebar on page 3).

When the SDWA was passed in 1974, it mandated a 

national study of rural water systems, including domestic 

wells. In that study, which focused on indicator bacteria and 

inorganic contaminants, contaminant concentrations were 

found to be greater than health benchmarks, which included 

available MCLs, in more than 15 percent of the domestic wells 

in the United States (National Statistical Assessment of Rural 

Water Conditions, or NSA; U.S. Environmental Protection 

Agency, 1984). Studies of many geographic areas and contaminants since then have shown that a variety of contaminants 

can be present in domestic wells, although usually at concentrations that are unlikely to have adverse human-health effects. 

Comprehensive and reliable information on the occurrence of contaminants in domestic wells is essential for the 

protection of public health. Although few in number (averaging less than 20 outbreaks per year), about 20 to 40 percent 

of the waterborne-disease outbreaks from drinking water 

that were reported to the U.S. Centers for Disease Control 

and Prevention (CDC) from 1999 to 2002 were attributed to 

contaminants in domestic well water; gastrointestinal illness 

associated with the consumption of domestic well water is 

likely underreported (Blackburn and others, 2004; Liang and 

others, 2006). Health risks associated with contaminants in 

domestic well water also have been noted to include exposure 

to elevated concentrations of nitrate, arsenic, radon, lead, and 

organic compounds (U.S. Centers for Disease Control and 

Prevention, 2003). Information on contaminants in domestic 

wells also is important for the management and regulation of 

chemical use, pesticides for example, and a number of surveys 

have been conducted for these purposes (Brook and others, 

2002; Townsend and others, 1998; Troiano and others, 2001).

Most ground water in the United States is generally considered of good quality and safe to drink (U.S. Environmental 

Protection Agency, 2000a, 2002a). However, the large number 

and widespread presence of potential sources of contaminants 

make water-quality testing fundamental to determining the 

safety of consuming water from domestic wells. State and 

local regulations, where they exist, provide for a minimum 

amount of testing of water from domestic wells. Existing regulations apply primarily at the time of well installation and are 

limited in scope. Fewer than half of the states require testing 

of new domestic wells, typically for bacteria and nitrate only. 

County or other local testing requirements for new wells also 

exist in some states. Water-quality testing at the time of home 

sales is a condition of some home loans and also is required 

by a few states (Oregon Department of Human Services, 

2003; Veterans Benefits Administration, 2005; New Jersey 
Introduction 3

What is a Contaminant?

A contaminant is defined by the SDWA as “any physical, chemical, biological, or radiological 

substance or matter in water” (U.S. Senate, 2002; 40 CFR 141.2). This broad definition of contaminant 

includes every substance that may be found dissolved or suspended in water—everything but the 

water molecule itself. Another term sometimes used to describe a substance in water is “waterquality constituent,” which has a meaning similar to the SDWA definition of contaminant.

The presence of a contaminant in water does not necessarily mean that there is a humanhealth concern. Whether a particular contaminant in water is potentially harmful to human health 

depends on its toxicity and concentration in drinking water. In fact, many contaminants are 

beneficial at certain concentrations. For example, many naturally occurring inorganic contaminants, 

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

higher amounts may cause adverse health effects (Eaton and Klaassen, 2001). On the other hand, 

anthropogenic organic contaminants, such as pesticides, are not required by humans, but may or 

may not have adverse effects on humans, depending on concentrations, exposure, and toxicity. As 

a first step toward evaluating whether a particular contaminant may adversely affect human health, 

its concentration measured in water can be compared to a U.S. Environmental Protection Agency 

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

Screening Level (HBSL). Concentrations greater than these water-quality benchmarks indicate 

the potential for health effects (see discussion in the section, “Water-Quality Benchmarks for 

Human Health”).

The types of contaminants measured in this study are listed below (see “Methods” section for 

a complete description of measurements and methods). There are other types of contaminants, such 

as pathogens and other organic compounds, that were not measured in this study.

Types of Contaminants Measured in this Study

•	 Inorganic

•	 Major ions (such as chloride)

•	 Trace elements (such as arsenic)

•	 Radionuclides (such as radon)

•	 Nutrients (such as nitrate)

•	 Organic

•	 Volatile organic compounds (such as chloroform)

•	 Pesticides (such as atrazine)

•	 Microorganisms

•	 Fecal indicator bacteria (such as Escherichia coli)

These contaminants originate from a wide range of natural and anthropogenic sources. The 

inorganic chemicals, nutrients, and microorganisms measured in this study occur naturally, but 

their concentrations in ground water may be altered by human activities. For example, nitrate from 

natural sources is present in many wells, but concentrations are often increased by contributions 

from anthropogenic sources in agricultural and urban areas. In contrast, the organic contaminants 

measured in this study are anthropogenic, though some also may form in ground water through 

various chemical and biological transformation processes.

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 (6th ed.): New York, McGraw-Hill, p. 1236.

U.S. Senate, 2002, Title XIV of the public health service act; safety of public water systems (Safe Drinking 

Water Act); Part A—Definitions (as amended through P.L. 107–377): U.S. Senate, updated December 31, 2002, 

accessed July 29, 2008, at http://epw.senate.gov/sdwa.pdf.
4 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Department of Environmental Protection, 2008). A few states 

conduct free voluntary testing programs or test high-risk wells 

(Riding and Quilter, 2004; State Water Resources Control 

Board, 2005; Florida Department of Health, 2006). Many 

public health agencies, environmental protection agencies, and 

non-profit organizations advise homeowners on testing procedures and recommend annual testing (U.S. Environmental 

Protection Agency, 2002a; National Ground Water Association, 2006). These programs, regulations, and recommendations all contribute to the safety of drinking water supplied by 

domestic wells. However, the limited number of contaminants 

assessed, the small numbers of wells tested, and the infrequent 

and (or) voluntary nature of testing that results from these 

programs, regulations, and recommendations do not provide 

domestic well users the same level of protection afforded to 

users of public water systems. 

In the absence of comprehensive, routine monitoring, 

information from a wide variety of sources is commonly used 

to assess the occurrence and distribution of contaminants in 

domestic wells. Potential sources of information to address 

specific questions about the quality of water from domestic 

wells include one-time statewide or regional surveys, 

voluntary testing programs, and ambient ground-water 

monitoring programs (Appendix 1). Results of these surveys 

and programs are useful for prioritizing contaminants for 

further study, identifying areas of particular concern for 

specific contaminants, guiding public-health programs and 

local testing recommendations, and identifying factors that 

potentially affect the occurrence of contaminants in wells. 

Differences in study design, methods, and target contaminants, 

however, make available survey studies difficult to compare 

or extrapolate (Ray and Schock, 1996). Many statewide or 

regional studies have focused on vulnerable aquifers or land 

uses that were more likely to be affected by human activities 

than other areas, and voluntary testing programs may be 

biased toward wells with high contaminant concentrations 

(Peters and others, 1999). 

A retrospective analysis of USGS data from 1986–2001 

from domestic wells recently provided information on the 

occurrence of 26 contaminants in a large number of wells in 

every state and Puerto Rico (Focazio and others, 2006). The 

retrospective study included data from the National WaterQuality Assessment (NAWQA) Program (some of which is 

also included in the present study) as well as data from many 

other studies, including those in known problem areas. Findings were that arsenic (11 percent) and nitrate (8 percent) 

exceeded USEPA MCLs most often, while uranium, mercury, 

and fluoride also exceeded MCLs in smaller numbers. Organic 

compounds rarely exceeded drinking-water standards or other 

human-health benchmarks. The present analysis expands 

the scope of the retrospective study by considering a larger 

number of contaminants and properties, by using USGS 

Health-Based Screening Levels (HBSLs) and MCLs to evaluate concentrations of potential health concern, and by assessing the co-occurrence of contaminants. The present study also 

includes only NAWQA Program data, which characterize 

ambient water-quality conditions in major aquifers used for 

water supply.

The present study synthesizes and describes waterquality conditions for 2,167 domestic wells sampled in major 

hydrogeologic settings of 30 regionally extensive aquifers 

used for water supply (principal aquifers) in the United States. 

The domestic wells were sampled for the NAWQA Program 

during 1991–2004 as part of interdisciplinary assessments of 

surface- and ground-water quality in study areas across the 

United States (fig. 1; Gilliom and others, 1995; Lapham and 

others, 2005; http://water.usgs.gov/nawqa). There were three 

primary objectives for the present study:

1. Assess the occurrence and distribution of water-quality 

conditions for domestic wells and evaluate contaminant 

concentrations in relation to human-health benchmarks 

for drinking water. Physical properties (such as temperature and specific conductance), major ions, nutrients, 

radon, trace elements, pesticides, and volatile organic 

compounds were measured; fecal indicator bacteria and 

additional radionuclides also were measured in some 

wells. Measured concentrations of contaminants were 

compared to USEPA MCLs and USGS HBSLs to evaluate 

their potential importance to human health.

2. Describe the variation in water quality among principal 

aquifers and rock types. Water-quality conditions in 

domestic wells were described at the national scale, by 

principal aquifer, and by principal aquifer rock type. 

Principal aquifers provide a framework for regional 

synthesis and extrapolation of water quality; comparisons 

at this and the national scale are made possible by 

consistent sampling design used for the NAWQA aquifer 

assessment studies. 

3. Assess the co-occurrence of contaminants as mixtures. 

The comprehensive suite of contaminants analyzed in 

each well made it possible to investigate the co-occurrence of multiple contaminants. Historically, toxicologists 

have evaluated the potential health effects from exposure 

to individual contaminants, but assessing the potential 

effects of chemical mixtures is an area of active research 

(U.S. Environmental Protection Agency, 2001c; Argonne 

National Laboratory, 2005), and current understanding 

is limited. Information on the occurrence of contaminant mixtures obtained from the NAWQA studies can 

contribute to this research by describing the mixtures 

commonly found in water samples from domestic wells. 

Co-occurrence is investigated in this study using 1,389 

wells, located in 25 principal aquifers and 45 states, with 

uniform and complete chemical analyses.

The NAWQA studies were designed as integrated 

assessments of the water resources within hydrologic systems 

of the Nation, not as a single, national-scale assessment 

of domestic well water quality or of water quality within 
Introduction 5

principal aquifers. Consequently, several characteristics and 

limitations are important to consider when the findings about 

domestic wells presented in this report are interpreted:

• NAWQA assessment studies describe ambient 

conditions within the areas that were targeted for 

sampling, without focus on specific sites or areas with 

known water-quality problems. 

• The targeted sampling areas were defined geographically by the extent of major hydrogeologic settings. 

Thus, the domestic wells sampled in NAWQA assessment studies are not uniformly distributed across the 

United States or within principal aquifers, and they are 

not a statistically representative sample of all domestic 

wells in the United States.

• Water samples were collected prior to any in-home 

distribution plumbing or treatment systems, and thus 

represent the quality of source water from domestic 

wells rather than water consumed by homeowners.

• USGS analytical methods were designed to measure 

concentrations as low as economically and technically 

feasible, which, particularly for organic compounds, 

were well below available standards or guidelines for 

drinking water. Thus, some contaminants may have 

been frequently detected at concentrations that are not 

expected to have adverse effects. 

Figure 1. NAWQA study units and assessment studies in which domestic wells were sampled, 1991–2004.

NAWQA study unit—See Appendix 2 for study- unit names

EXPLANATION

Study of major aquifer with wells used in this study

Study of agricultural land-use setting

High Plains Regional Ground Water Study (HPGW)

Assessment study with domestic wells—Central location

 of well network is shown

PUGT

WILL

COOK

OAHU

SANJ

SACR

NVBR

NVBR

SANA

USNK

NROK

CCPT

YELL

REDN

CAZB

RIOG

GRSL

UCOL

SPLT

SCTX

TRIN

CNBR

OZRK

UMIS

EIWA

WMIC

LIRB

UIRB LERI

ACAD

SOFL

GAFL

MISE

MOBL

UTEN

WHIT

SANT

ALBE

ACFB

ALMN

KANA

DLMV

LSUS

HDSN

DELR

CONN

NECB

LINJ

LTEN

MIAM POTO

YAKI

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

Methods

Methods used in the present study for well selection, 

sample collection and documentation, chemical analysis, and 

quality assurance and quality control that are described here 

are those used by the NAWQA Program for assessment studies 

conducted in major river basins and aquifers throughout the 

United States during 1991–94. Methods of data treatment 

and analysis used in the present study and described here 

are consistent with those used by national-scale syntheses 

of the NAWQA water-quality data on pesticides, volatile 

organic compounds, and nutrients (U.S. Geological Survey, 

1999; Gilliom and others, 2006; Zogorski and others, 2006). 

Methods of data presentation and analysis used in the present 

study also are described in this section.

Well Selection

The wells included in the present study were sampled 

as part of assessment studies that were designed to describe 

the water quality of major hydrogeologic settings or major 

aquifers used for drinking water within NAWQA study units 

(Gilliom and others, 1995; 2006, fig. 1). Each assessment 

study used a sampling network of about 20 to 30 wells, 

randomly selected within a target area that represented the 

extent of the aquifer or hydrogeologic setting (Scott, 1990). 

Most of these were domestic wells, that is, privately owned 

wells that supply water for household use (Zogorski and 

others, 2006). Other types of wells that may have been 

sampled in the original assessment studies were not included 

in the present study. A total of 2,167 domestic wells were 

included in the present study. The wells were part of 93 

assessment studies (fig. 1), and the numbers of domestic 

wells ranged from 1 to 104 domestic wells per study with a 

median of 25. Results of the individual studies are described 

in reports for NAWQA study units (http://water.usgs.gov/

nawqa). Additional information on the assessment studies and 

NAWQA study units is given in Appendix 2. 

In some NAWQA study units, studies were conducted to 

assess the quality of shallow ground water beneath specific 

land uses, such as agriculture, within a major aquifer or 

hydrogeologic setting. Monitoring wells composed large 

proportions of the wells in these studies, but domestic wells 

were included in some cases. Domestic wells or monitoring 

wells were selected by location using a random sampling 

design such as that used for major aquifer assessment studies. 

Water-quality data on some contaminants in domestic wells 

sampled for the NAWQA agricultural land-use assessment 

studies are summarized in this report to aid in describing 

the potential effects of agricultural land-use practices on the 

ground water in the aquifers studied. Data from 436 wells 

in agricultural land-use settings, primarily near the east and 

west coasts of the United States, are summarized. Information 

on these wells also is provided in Appendix 2. In general, 

however, water-quality results described in this report do not 

include results for wells from the agricultural land-use studies 

unless specifically indicated.

Specific criteria for well selection and documentation are 

described by Lapham and others (1995; 1997) and Koterba 

(1998). Criteria included the proximity of the well to the 

randomly selected site, the availability of well-construction 

and aquifer information, and the availability of a suitable 

sampling location. Ancillary data for each well, collected 

during or near the time of sample collection, included latitude 

and longitude; topographic setting; well depth, diameter, 

screened interval, and casing material; lithology and aquifer(s) 

or rock units contributing water to the well; static water level 

(depth to water); and potential sources of contamination and 

land use near the well. 

Wells used in the present study were categorized in terms 

of the principal aquifers from which they withdrew water. 

A principal aquifer is defined as “... a regionally extensive 

aquifer or aquifer system that has the potential to be used as 

a source of potable water” (U.S. Geological Survey, 2003a). 

Principal aquifers were mapped by USGS and are described 

in the National Atlas of the United States by name and by 

rock type (Miller, 2000; U.S. Geological Survey, 2003a, 

2003b, hereafter referred to as the “National Atlas”). The 

principal aquifers associated with individual domestic wells 

in this study were identified using information from NAWQA 

regional aquifer assessments (T.L. Arnold, U.S. Geological 

Survey, written commun., 2005; Lapham and others, 2005) 

and using lithologic information available for the wells in the 

USGS National Water Information System (NWIS) databases 

(Koterba, 1998). Principal aquifer definitions, extent, and 

lithologic groupings followed those used in the National Atlas 

(Miller, 2000; U.S. Geological Survey, 2003a, 2003b), with a 

few modifications. Unconsolidated sand and gravel aquifers 

were divided into two categories: (1) basin-fill and other 

non-glacial sand and gravel aquifers, and (2) glacial sand 

and gravel aquifers. Glacial sand and gravel aquifers were 

further split into eastern, central, west-central, and western 

glacial aquifers, on the basis of geography and differing source 

materials (Warner and Arnold, 2006). Also, the areal extent 

of glacial sand and gravel aquifers used in this study extends 

farther west than shown in the National Atlas, and include 

deposits of both Laurentide and Cordilleran continental 

glaciations (Warner and Arnold, 2006). The Valley and Ridge 

aquifers, which are combined as sandstone/carbonate-rock 

aquifers in the National Atlas, were separately defined as 

Valley and Ridge sandstone aquifers and Valley and Ridge 

carbonate-rock aquifers. Finally, wells in the New England 

and Rocky Mountain Front Range crystalline-rock aquifers 

and in some alluvial aquifers, which are described as local 

aquifers, or are not described, in the National Atlas, were 

sampled. These aquifers are included with principal aquifers in 

this study.
Methods 7

Sample Collection and Analysis

Water samples were collected and processed using 

methods designed to yield samples that were representative of 

environmental conditions, minimally affected by contamination, and comparable among NAWQA study units nationwide. 

Sampling methods are described in Koterba and others (1995), 

U.S. Geological Survey (variously dated), Kolpin and others 

(1998), Embrey and Runkle (2006), and Moran and others 

(2006). The samples were collected directly from the wells 

before any water treatment and, in most instances, before any 

pressure or holding tanks. Because the water samples were 

collected before any treatment systems that may have been 

operating within homes or distribution plumbing, they characterize the quality of the raw source water supplied to households, rather than the finished water consumed by household 

residents in some cases. 

The physical properties of water—pH, temperature, 

specific conductance, and dissolved oxygen—were measured 

in the field during sampling. Samples were collected for 

laboratory analysis of alkalinity, major ions, nutrients, 

dissolved organic carbon, trace elements, pesticides, volatile 

organic compounds (VOCs), and radon, and, at some wells, 

for analysis of fecal indicator bacteria, gross alpha- and 

beta-particle radioactivities, and radium. Water samples were 

analyzed at the USGS National Water Quality Laboratory 

(NWQL) in Lakewood, Colorado. Sampling and analytical 

methods are described in detail in Appendix 3; analytes are 

listed in Appendix 4. 

Quality Assurance and Quality Control

Procedures to ensure and assess data quality in the 

NAWQA Program include written protocols for well selection, sample collection, and chemical analysis; the collection 

of field and laboratory quality-control (QC) samples; and 

field-measurement and laboratory evaluation programs (Moran 

and others, 2006). Procedures and protocols for well selection, sampling, and analysis have been described previously. 

The quality of field measurements made by USGS personnel 

is assessed by the USGS Branch of Quality Systems, National 

Field Quality Assurance Program, which has conducted 

assessments annually since 1979 (Stanley and others, 1998; 

U.S. Geological Survey, 2006b). Quality assurance and QC 

procedures at the NWQL include method QC samples, blindsample programs, and interlaboratory performance-evaluation 

studies (Ludtke and Woodworth, 1997; Pirkey and Glodt, 

1998; U.S. Geological Survey, 2006c). Field QC samples, 

which were an important part of the NAWQA assessment 

study design (Koterba and others, 1995), can be used to assess 

bias and variability in the water-quality analytical data at the 

study unit and national scale. Results of national-scale investigations of field QC data on nutrients, trace elements, pesticides, VOCs, fecal indicator bacteria, and radionuclides and 

summaries of field QC data in study unit reports are described 

in detail in Appendix 3. These assessments indicated that the 

analytical data for samples from domestic wells collected for 

the NAWQA Program are reproducible and representative of 

environmental conditions.

Data Sources and Treatment

Well site information, ancillary data, and most waterquality data were obtained from the USGS NAWQA Data 

Warehouse (Bell and Williamson, 2006; http://water.usgs.gov/

nawqa/data). Data were retrieved from the Data Warehouse 

mostly from December 2004 to May 2005. Data from samples 

collected from 1991 through June 2004 were retrieved. 

Data from one sample per well, the primary sample used to 

characterize water quality at the well and typically the sample 

with greatest number of analytes, were used. Samples from 

the first 10-year monitoring period of the NAWQA Program 

were used for wells that were sampled in both the first and 

second 10-year monitoring periods. Data on fecal indicator 

bacteria were those used in an analysis by Embrey and 

Runkle (2006; S.S. Embrey, U.S. Geological Survey, written 

commun., 2005). The data on radium isotopes were those used 

in a national analysis of radionuclide data collected for the 

NAWQA Program (Zoltan Szabo, U.S. Geological Survey, 

written commun., 2006 and 2008).

Censored water-quality data (non-detects) from the Data 

Warehouse, particularly for organic compounds, incorporated 

the different reporting conventions (minimum reporting levels 

and laboratory reporting levels, or MRLs and LRLs) that were 

used by the NWQL during the study period and are not strictly 

comparable (Helsel, 2005a). Thus, censored values that were 

reported relative to LRLs were redefined relative to their associated long-term method-detection levels (LT-MDLs) using 

information on historical LT-MDLs for analytical methods 

used by the NWQL (see Appendix 3 for further discussion). 

This procedure was used to improve consistency of the reporting conventions in the data set and to avoid any potential bias 

in summary statistics that might result from the inclusion of 

estimated values below LRLs (Helsel, 2005a). 

The numerical values of MRLs and LT-MDLs for many 

individual analytes changed during the study period. MRLs 

and LT-MDLs also differed considerably among analytes 

(Appendix 4). Differing reporting or detection levels cause 

problems in some analyses of water-quality data. Contaminants for which analytical methods provided lower detection 

limits may seem to occur more frequently in the environment 

relative to other contaminants with higher analytical detection limits. Regional patterns of occurrence of a contaminant 

may be distorted by greater numbers of detections in study 

areas where analytical methods with lower detection limits 

were used. Correlation analyses also may be distorted by 

differing reporting or detection levels (Helsel, 2005b). In this 

study, water-quality data that are compared, illustrated on 

maps, or correlated with other variables have been recensored 

to common reporting levels. This recensoring was done by 
8 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

converting measured values that were less than the common 

reporting level to “less-than” values or non-detections relative 

to the common reporting level. Common reporting levels were 

applied to values of trace elements and organic compounds, as 

groups, and to values of major ions, nutrients, and alpha-particle radioactivity, individually. Common reporting levels were 

chosen as the most frequently used MRL or LT-MDL in the 

data set to minimize the number of recensored data values. For 

trace elements, a common reporting level of 1 μg/L was used 

(for all elements except boron, iron, and strontium). Three 

common reporting levels, 0.02, 0.1, and 0.2 μg/L, were used 

for organic compounds that represented frequently used MRLs 

or LT-MDLs for subsets of these data (Gilliom and others, 

2006; Zogorski and others, 2006). For major ions and nutrients, common reporting levels varied by analyte and ranged 

from 0.01 to 0.1 mg/L. A common reporting level of 3 pCi/L 

was used for alpha-particle radioactivity. Differing reporting 

levels were acceptable for major ions and nutrients because 

detection frequencies for these commonly occurring contaminants were not compared. 

When common reporting levels were used, values 

reported as less than a detection limit that was higher than the 

common reporting level were removed from the data set. This 

generally resulted in the elimination of less than 2 percent of 

data values per contaminant for major ions, trace elements, 

and pesticides. These data values usually were isolated cases 

that resulted from problems in the laboratory. During the 

study period, VOC data were reported relative to MDLs that 

differed by an order of magnitude because of improvements 

to laboratory analyses. Before 1997, most VOCs were reported 

relative to MRLs of about 0.2 μg/L. Consequently, when 

VOC data were recensored to a common reporting level of 

0.02 μg/L, about 30 percent of the VOC samples were 

removed from the data set, including all observations, detections and non-detections, that resulted from analyses when the 

MRL or LT-MDL was 0.2 μg/L. This was necessary to avoid 

biasing the calculated detection frequencies in the direction 

of greater detections, which would have resulted if only those 

observations below the method detection limit were removed.

Spatial data such as those for land use, soil properties, 

and other characteristics relevant to water quality were determined by USGS using national-scale data sets that included 

those described in U.S. Geological Survey (1999), Vogelmann 

and others (2001), Nakagaki and Wolock (2005), and Price 

and others (2007) for land use; U.S. Department of Agriculture 

(1994), Wolock (1997), and Schwarz and Alexander (1995) 

for soils; and Ruddy and others (2006) for nitrogen fertilizer 

applications. Land-use characteristics, soil properties, and 

other characteristics were determined as area-weighted averages for circular areas with radii of 500 m (1,640 ft or 0.3 mi) 

around the sampled wells (Nakagaki and Wolock, 2005). The 

soil characteristic used in correlations in this study is the State 

Soil Geographic (STATSGO) database soil drainage-class 

number. Nitrogen fertilizer applications used in correlations 

are the averages of annual estimates for 1992 to 2001 of 

nitrogen to farmland and non-farmland. Population density 

is the average of densities derived from the 1990 and 2000 

censuses.

Statistical Methods and Data Presentation

Statistical methods included the calculation of summary 

statistics (median and percentile concentrations) and correlation analysis. Summary statistics for contaminants with less 

than 10 percent censored data values were calculated using 

the Statview software package (version 5.0, SAS Institute, 

1998). The robust ROS (regression on order statistics) method, 

as implemented in the NADA (Nondetects and Data Analysis 

for Environmental Data) package of the R statistical software 

program, was used to calculate summary statistics for contaminants with more than 10 percent censored data values; 

this method allows for multiple detection limits in the data 

set (Helsel, 2005b; Lee, 2005). Nonparametric, rank-based 

methods were used for comparison of contaminant concentrations or physical property values to one another or to potential 

explanatory variables. The Spearman correlation was used 

for correlation of two continuous variables; and the MannWhitney test was used for analyses with one continuous and 

one categorical value. When these methods were used, all 

observations below the common reporting levels were tied at 

the lowest rank (Helsel, 2005a). 

Water types were determined from concentrations of 

major ions. A single cation or anion was identified as characterizing the water type of a sample when that ion made up 

more than 50 percent of the total concentrations of cations or 

anions, in milliquivalents per liter (Hem, 1985). Two cations 

or ions were identified as characterizing the water type of a 

sample when they together made up more than 80 percent of 

the total concentrations of cations or anions, in milliequivalents per liter. Other combinations of multiple cations or ions 

resulted in samples that were identified as being of mixed 

cation or anion water type.

In map figures showing the geographic distribution of 

contaminants, the large number and close proximity of wells 

resulted in overlapping symbols in many areas. Well symbols 

are layered such that symbols showing higher concentrations 

are on top of symbols showing lower concentrations, and 

symbols showing detections are on top of symbols showing 

non-detections. Graphical presentation of data in boxplots 

required a minimum of 20 data points per category. 

Water-Quality Benchmarks for 

Human Health

Contaminants in drinking-water supplies may be of concern when they approach concentrations that may be harmful 

to human health. Several types of benchmarks are available 

to identify contaminant concentrations of concern. USEPA 

MCLs are legally enforceable standards for drinking water 
Domestic Wells Sampled for the NAWQA Program, 1991–2004 9

that specify the maximum permissable level of a contaminant 

in water that is delivered by public water systems. MCLs are 

set as close as feasible to levels at which there is no known 

or anticipated adverse human-health effects, and they include 

a margin of safety (U.S. Environmental Protection Agency, 

2006a). Currently (2009), MCLs have been established for 

about 90 elements and compounds. 

Information also is available on potential human-health 

effects in drinking water of many elements and compounds 

for which MCLs have not been established (unregulated 

compounds). This information has been used by USEPA 

to calculate Lifetime Health Advisories (concentrations 

not expected to cause adverse effects from a lifetime of 

exposure) for noncarcinogens and Cancer Risk Concentrations 

(concentrations associated with specified lifetime cancer 

risks) for carcinogens (U.S. Environmental Protection 

Agency, 2006a). Every 5 years, USEPA evaluates at least five 

unregulated contaminants for possible regulation in drinking 

water. To date, USEPA has evaluated 19 contaminants, 

including 13 analyzed in this study (including manganese 

and boron), and has determined that no regulatory action 

was appropriate for these contaminants (U.S. Environmental 

Protection Agency, 2003a; 2008a). 

In 1998, the USGS began a collaborative effort with 

USEPA to use standard USEPA methods and the most current 

USEPA toxicity information to calculate HBSLs for some 

compounds and elements for which drinking-water standards 

were not available (Toccalino and others, 2003). HBSLs are 

non-enforceable benchmark concentrations that can be used 

in screening-level assessments to evaluate water-quality data 

within the context of human health; they are available in a 

web-based database (Toccalino and others, 2005; Toccalino 

and Norman, 2006; Toccalino and others, 2006). Because 

HBSLs are calculated using USEPA toxicity information and 

standard USEPA Office of Water equations for establishing 

drinking-water guideline values, HBSLs are equivalent to 

existing USEPA Lifetime Health Advisory and Cancer Risk 

Concentration values (when they exist), except for unregulated 

compounds for which more recent toxicity information has 

become available (Toccalino, 2007). 

Water-quality data from domestic wells in the present 

study were compared to USEPA MCLs and USGS HBSLs 

in a screening-level assessment, which can provide an initial 

perspective on the potential significance of contaminant 

occurrence to human health and can help prioritize future 

studies (Toccalino and others, 2006; Toccalino, 2007). This 

kind of assessment also provides a perspective on the areas 

in which adverse effects of contaminants in domestic well 

water are more likely to occur, based on the NAWQA data, 

and which contaminants may be responsible for those effects 

(Gilliom and others, 2006). A screening-level assessment is 

not designed to evaluate specific effects of contaminants in 

domestic well water on human health, nor is it suitable for 

a comprehensive risk assessment, which generally includes 

additional factors such as multiple avenues of exposure 

(Toccalino and others, 2006). 

Water-quality benchmarks, including MCLs and HBSLs, 

were available for 154 of the 214 contaminants measured in 

this study. MCLs were available for 52 contaminants (U.S. 

Environmental Protection Agency, 2006a), and HBSLs were 

available for 96 contaminants (Toccalino and others, 2006; 

values used in this report were current as of March 2009; see 

http://water.usgs.gov/nawqa/HBSL). USEPA screening or 

action levels also were used as human-health benchmarks 

for a few contaminants (for example, copper and lead; 

Appendix 4). Specific MCLs and HBSLs are presented in sections of this report where water-quality results are described 

and in Appendix 4. Contaminants that occurred in concentrations greater than MCLs and HBSLs were identified, and the 

frequencies at which they occurred at these elevated concentrations were calculated. Contaminant concentrations that are 

less than but approaching MCLs and HBSLs also may be of 

interest, for example, for prioritizing further study or increased 

monitoring. Contaminant concentrations that were less than 

MCLs or HBSLs were evaluated by identifying measured 

concentrations that were greater than one-tenth (or 10 percent) 

of the contaminant’s MCL or HBSL value (Toccalino, 2007). 

These concentration levels can be used to identify contaminants that may warrant additional monitoring in order to 

analyze trends in occurrence and to provide an early indication 

of contaminant concentrations that approach human-health 

benchmarks (U.S. Environmental Protection Agency, 1997a; 

Toccalino and Norman, 2006; Toccalino, 2007). 

Water-quality data from domestic wells in the present study also were compared to non-health guidelines for 

drinking-water quality. Non-health guidelines are recommended maximum concentrations of various contaminants that 

can have adverse cosmetic effects (such as tooth staining) or 

result in water that is unpleasant for use (due to color, taste, or 

odor; U.S. Environmental Protection Agency, 2006a). Nonhealth guidelines for drinking water used in the present study 

are primarily USEPA Secondary Maximum Contaminant 

Levels (SMCLs), which are non-enforceable guidelines 

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

Domestic Wells Sampled for the 

NAWQA Program, 1991–2004

The 2,167 domestic wells sampled for the NAWQA 

Program between 1991 and June 2004 were located in 48 of 

the 50 states (fig. 2). The wells also were distributed among 

30 regionally extensive aquifers (table 1) and thus represent 

about half of the principal aquifers of the United States (U.S. 

Geological Survey, 2003a). The principal aquifers sampled 

with the domestic wells in the present study were grouped 

according to lithology into eight categories following the classification used in the National Atlas (fig. 2). Aquifers within 

these rock-type categories share common characteristics of 

ground-water flow and, in some cases, overall geochemistry. 

The categories are (A1) unconsolidated basin-fill, fluvial, and 
10 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Figure 2. (A) Principal aquifers of the United States and (B) domestic wells sampled for the NAWQA Program in aquifer studies, 

1991–2004. Names of principal aquifers sampled in this study are listed in table 1. The uppermost aquifer is shown, unless otherwise 

indicated; therefore, the full areal extent of underlying aquifers may not be shown. Areal extent of aquifers from U.S. Geological Survey 

(2003a) and U.S. Geological Survey, NAWQA Program, written commun., 2007.

A

B

CV

BR

BR

RG

OP

BR

BR

SPf

M

WL

VRs

ET

NR

HP

F

NA

Gw

CL

MV

CL

TC SC

SD

LT

P

EM

P

SPb

M PBx

M

SD

O

O

Gwc

CO

ET

F

VRs

VRc

VRc

VRs

Gw

NEx

NA

EM

Gc

Ge

SC

Principal aquifers and well locations—Areal extent of aquifers sampled by domestic wells in this study shown 

 by the darker color. Locations of domestic wells shown by colored circles. See table 1 for principal aquifer names, indicated 

 here with 2–3 letter abbreviations.

Coastal plain aquifers in 

 semi-consolidated sand

Sandstone aquifers

Sandstone and carbonate-

 rock aquifers

Carbonate-rock aquifers

Crystalline-rock aquifers—Aquifers

 in New England and New York are present 

 at depth. Rocky Mountain Front Range

 aquifers in Colorado are not shown.

Basin-fill and other non-glacial

 sand and gravel aquifers

Glacial sand and gravel aquifers—

 Aquifers are discontinuous within area shown.

Basaltic-rock aquifers

EXPLANATION
Domestic Wells Sampled for the NAWQA Program, 1991–2004 11

other non-glacial sand and gravel aquifers (basin-fill sand and 

gravel, BFSG; including the National Atlas’s basin-fill and 

blanket aquifers); (A2) unconsolidated glacial sand and gravel 

aquifers (glacial, GLAC), which occur discontinuously in the 

northern states; (B) coastal plain aquifers in semi-consolidated 

sand (coastal plain, CP), consisting of interbedded sediments 

that occur in eastern and southeastern coastal states; (C) sandstone aquifers (SS), widespread across the United States; (D) 

interbedded sandstone and carbonate aquifers (sandstone/carbonate, SS/CARB) in Texas, the Appalachian region, and the 

Midwest; (E) carbonate aquifers (CARB), which occur mostly 

in the central and southeastern United States; (G) basaltic-rock 

aquifers (basaltic, BAS), in the Northwest; and (H) crystallinerock aquifers (crystalline, CRYS), which are igneous and 

metamorphic fractured-rock aquifers that supply water in the 

Northeast, the Appalachian region, and locally in other parts of 

the United States (Miller, 2000). 

Descriptions of wells—well numbers and other 

characteristics—are listed in table 1 by principal aquifer; 

information on wells, grouped by principal aquifer and 

NAWQA assessment study, is given in Appendix 2. Well 

characteristics varied among principal aquifers, with well 

depths and depths to water typically shallower and closer 

to land surface in the glacial, coastal plain, sandstone, and 

carbonate aquifers than in the basin-fill sand and gravel and 

sandstone/carbonate aquifers (fig. 3). The median well depth 

overall was 147 ft (average 174 ft), with most (80 percent) 

well depths between 50 and 335 ft; the median depth to water 

was 38 ft (average 67 ft), with most depths to water between 

9 and 164 ft. Depth to the top of the open interval of wells was 

between 25 and 248 ft below land surface for most wells, with 

a median value of 81 ft (average 114 ft) below land surface 

(table 1). The wells were nearly all (94 percent) drilled wells, 

rather than dug or driven wells, with steel or polyvinylchloride 

casings, 4 to 8 in. in diameter. The wells also were relatively 

young, with an average age of 22 years and with most between 

10 and 45 years in age. 

The NAWQA assessment studies were not specifically 

designed as a single, national-scale assessment of domestic 

well water quality. Thus, the domestic wells sampled for the 

NAWQA Program do not provide a statistically based sampling of all domestic wells in the United States. However, the 

distribution of domestic wells sampled by NAWQA compares 

favorably with the national distribution of domestic water use, 

as indicated by county-based water-use data for 1995 (Solley 

and others, 1998). Many areas of relatively high populations 

served by domestic wells, for example in the eastern half of 

the country and along the west coast, are represented by the 

wells sampled (fig. 4A). The domestic wells sampled for the 

NAWQA Program also represent some, though not all, of the 

areas where the population density is sparse but where users of 

domestic well water account for a large part of the total population, for example in the northern High Plains (fig. 4B). 

The principal aquifers, represented by domestic wells 

sampled for the NAWQA Program, were some of those most 

heavily used for public water supplies in the United States, 

and, therefore, those most likely to be frequently tapped 

by domestic wells. (Information on water withdrawals by 

domestic wells for principal aquifers was not available, but 

the characteristics of aquifers that favor their use for public 

supplies also likely lead to their frequent use for domestic 

supplies). Two-thirds of the 42 principal aquifers that 

supplied 95 percent of public-water withdrawals in 2000 were 

represented by the NAWQA domestic wells (fig. 5; Maupin 

and Barber, 2005). The glacial aquifers, which extend across 

a large part of the northernmost and north-central United 

States (fig. 2), were sampled most frequently, accounting for 

18 percent of the NAWQA domestic wells. These aquifers 

were the largest source of water for public supply in 2000, 

providing 12 percent of public-supply withdrawals. The 

glacial aquifers also are likely to be the largest source of 

domestic supply, and may provide about 25 to 50 percent of 

total self-supplied domestic withdrawals (M.A. Horn, U.S. 

Geological Survey, written commun., 2005; K.L. Warner, 

U.S. Geological Survey, written commun., 2005). 

Most of the domestic wells included in the present study 

were originally selected to characterize the water quality 

within major hydrogeologic settings of more limited extent 

than the regionally extensive principal aquifers. (Wells in the 

High Plains aquifer are an exception.) Consequently, the wells 

are geographically clustered within principal aquifers, leaving 

extensive areas without representation, and do not represent a 

statistically based sample of ground water or domestic wells 

within principal aquifers (fig. 2). Sampling density also was 

variable; most (80 percent) principal aquifers were represented by 19 to 109 wells each. However, the targeted major 

hydrogeologic settings represent important hydrologic systems 

within principal aquifers, and the sampled domestic wells 

were randomly selected within those settings. 

Principal aquifers provide a useful framework for 

regional synthesis of water-quality data within units of broadly 

similar permeability, rock type, geologic or depositional setting, and climate (Lapham and others, 2005). However, waterquality conditions vary substantially within principal aquifers, 

reflecting the heterogeneity of natural systems. In some areas, 

principal aquifers also may overlie or underlie less extensive 

local aquifers that have different water-quality characteristics, 

so that water-quality conditions described for a principal aquifer do not describe the quality of water from all domestic wells 

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

Table 1. Characteristics of domestic wells sampled for the NAWQA Program in aquifer studies, 1991–2004, by principal aquifer.

—Continued

[Map identifier: 2–3 letter abbreviation identifying principal aquifer location in figure 2. Number of wells used for well depth, depth to top of open interval, 

and depth to water may differ. --, not available or not applicable]

Map 

identifier Principal aquifer Number of wells

Median well 

depth, in feet 

below land 

surface

Median depth to 

top of open 

interval, in feet 

below land surface

Median depth to 

water, in feet 

below land 

surface

Basin-fill and other non-glacial sand and gravel aquifers (BFSG)

BR Basin and Range basin-fill aquifers 129 241 200 112

CV Central Valley aquifer system 57 158 118 57

HP High Plains aquifer 298 200 170 93

MV Mississippi River Valley alluvial aquifer 5 80 50 12

NR Northern Rocky Mountains Intermontane 

Basins aquifer system

57 83 114 33

OS1 Other stream valley alluvial aquifers 44 55 43 11

RG Rio Grande aquifer system 25 178 173 12

SPf Snake River Plain basin-fill aquifers 34 135 92 12

WL Willamette Lowland basin-fill aquifers 69 60 50 16

All aquifers -- 718 168 139 70

Glacial sand and gravel aquifers (GLAC)

Ge Eastern glacial aquifers 73 86 80 24

Gc Central glacial aquifers 185 69 63 20

Gwc West central glacial aquifers 80 66 64 15

Gwc Western glacial aquifers 51 75 67 41

All aquifers -- 389 72 67 22

Coastal plain aquifers in semi-consolidated sand (CP)

CL Coastal lowlands aquifer system 85 140 117 39

NA North Atlantic Coastal Plain aquifer system 65 88 75 22

SC Southeastern Coastal Plain aquifer system 26 141 121 53

TC Texas coastal uplands aquifer system 10 102 -- 25

All aquifers -- 186 105 92 32

Sandstone aquifers (SS)

CO Cambrian-Ordovician aquifer system 70 173 126 41

EM Early Mesozoic basin aquifers 69 175 50 25

LT Lower Tertiary and Upper Cretaceous aquifers 24 158 85 29

P Pennsylvanian aquifers 46 98 24 46

VRs Valley and Ridge sandstone and shale aquifers 56 169 62 39

All aquifers -- 265 157 64 38

Sandstone and carbonate-rock aquifers (SS/CARB)

ET Edwards-Trinity aquifer system 69 239 150 72

M Mississippian aquifers 18 100 50 40

All aquifers -- 87 205 98 57
Domestic Wells Sampled for the NAWQA Program, 1991–2004 13

Table 1. Characteristics of domestic wells sampled for the NAWQA Program in aquifer studies, 1991–2004, by principal aquifer.

—Continued

[Map identifier: 2–3 letter abbreviation identifying principal aquifer location in figure 2. Number of wells used for well depth, depth to top of open interval, 

and depth to water may differ. --, not available or not applicable]

Map 

identifier Principal aquifer Number of wells

Median well 

depth, in feet 

below land 

surface

Median depth to 

top of open 

interval, in feet 

below land surface

Median depth to 

water, in feet 

below land 

surface

Carbonate-rock aquifers (CARB)

F Floridan aquifer system 96 120 81 22

O Ordovician aquifers 21 140 20 21

OP Ozark Plateaus aquifer system 49 170 74 51

SD Silurian-Devonian aquifers 32 223 150 59

VRc Valley and Ridge carbonate-rock aquifers 19 265 64 80

All aquifers -- 217 160 79 41

Basaltic-rock aquifers (BAS)

SPb Snake River Plain basaltic-rock aquifers 19 260 85 183

Crystalline-rock aquifers (CRYS)

NEx New England crystalline-rock aquifers 113 225 40 18

PBx Piedmont and Blue Ridge crystalline-rock 

aquifers

100 157 58 33

RMx1 Rocky Mountain Front Range crystalline-rock 

aquifers

27 225 82 --

All aquifers -- 240 200 50 26

Aquifer not determined

-- Aquifer not determined 46 125 91 6

All aquifers

All aquifers -- 2,167 147 81 38

1

Aquifer not shown on figure 2.
14 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Figure 3. (A) Well depth, (B) depth to top of open interval, and (C) depth to water in domestic wells sampled for the NAWQA 

Program in aquifer studies, 1991–2004. BFSG, basin-fill and other non-glacial sand and gravel aquifers; GLAC, glacial sand and 

gravel aquifers; CP, coastal plain aquifers in semi-consolidated sand; SS, sandstone aquifers; SS/CARB, sandstone/carbonate-rock 

aquifers; CARB, carbonate-rock aquifers; BAS, basaltic-rock aquifers; and CRYS, crystalline-rock aquifers.

714

366 185

265

73

210

19

240

182

638

298 157

240

49

19

223

587

235

68

170

47

151

171

52

Median

25th percentile

75th percentile

90th percentile

10th percentile

Number of samples

C Depth to water 

EXPLANATION

A Well depth B Depth to top of open interval 

0

100

200

300

400

0

100

200

300

400

500

600

BFSG GLAC CP SS SS/

CARB

CARB BAS CRYS

0

BFSG GLAC CP SS SS/

CARB

CARB BAS CRYS

BFSG GLAC CP SS SS/

CARB

CARB BAS CRYS

100

200

300

400

FEET BELOW LAND SURFACEFEET BELOW LAND SURFACE

 FEET BELOW LAND SURFACE
Domestic Wells Sampled for the NAWQA Program, 1991–2004 15

Figure 4. Population supplied by domestic wells by county in the United States in 1995 by (A) population density and (B) population as 

percent of total county population. Water use data from Solley and others (1998). Domestic wells sampled for the NAWQA Program in 

aquifer studies, 1991–2004, also are shown. ≥, greater than or equal to; <, less than.

EXPLANATION

Population (1995) supplied by

 domestic wells, in persons per 

 square mile

≥ 100

≥10 and < 100

≥ 1 and < 10

≥ 0.1 and < 1

≥ 0.01 and < 0.1

EXPLANATION

Population (1995) supplied by

 domestic wells, in percent of

 total county population

≥ 80

≥ 60 and < 80

≥ 40 and < 60

≥ 20 and < 40

< 20

Domestic well sampled 

 for the NAWQA Program

Domestic well sampled 

 for the NAWQA Program

B Population as percent of total county population 

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

Figure 5. Water withdrawals for public supply in the United States in 2000 and numbers of domestic wells sampled for 

the NAWQA Program in aquifer studies, 1991–2004, by principal aquifer. Water withdrawals from Maupin and Barber 

(2005); aquifers listed provided 95 percent of withdrawals for public supply in 2000. Shading indicates rock type of 

principal aquifer. Basin-fill and other non-glacial sand and gravel aquifers, blue; glacial sand and gravel aquifers, grey; 

coastal plain aquifers, yellow; sandstone aquifers, green; sandstone/carbonate-rock aquifers, purple; carbonate-rock 

aquifers, brown; and basaltic-rock and crystalline-rock aquifers, orange.

WITHDRAWALS FOR PUBLIC SUPPLY IN 2000,

IN MILLION GALLONS PER DAY

NUMBER OF

DOMESTIC WELLS

IN THIS STUDY

0

1,000

200600

1,200

400800

1,4001,6001,8002,000

0

100200300400

Other (or Unknown)

Lower Cretaceous

North Coast Limestone (Puerto Rico)

Pacific Northwest basaltic-rock

Valley and Ridge sandstone and shale

Florida Intermediate aquifer system

Mississippi River Valley alluvial

Snake River Plain basaltic-rock

Basin and Range carbonate-rock

Pacific Northwest basin-fill

New England crystalline-rock

Pennsylvanian

Northern Rocky Mountains Intermontane Basins

Snake River Plain basin-fill

Piedmont and Blue Ridge crystalline-rock

Willamette Lowland basin-fill 

Colorado Plateaus

Early Mesozoic Basins 

Ozark Plateaus

Texas coastal uplands

Silurian-Devonian

Valley and Ridge carbonate-rock

Puget Sound

Mississippian

Columbia Plateau basaltic-rock

Rio Grande

Volcanic-rock (Hawaii)

Surficial

Southeastern Coastal Plain

Edwards-Trinity

Other stream-valley alluvial

High Plains

Mississippi Embayment

Cambrian-Ordovician

Biscayne

Northern Atlantic Coastal Plain

Central Valley

Coastal Lowlands

Basin and Range basin-fill

Floridan

California Coastal Basin

Glacial

PRINCIPAL AQUIFER
Quality of Water from Domestic Wells 17

Quality of Water from Domestic Wells

Physical properties and contaminant concentrations 

for the domestic wells sampled for the NAWQA Program in 

aquifer studies are summarized and compared to human-health 

benchmarks and non-health guidelines in several tables in this 

section. Physical properties and contaminant concentrations 

are summarized and compared to health benchmarks and nonhealth guidelines, by principal aquifer and by principal aquifer 

rock type, in Appendixes 5 and 6. Detection frequencies for 

organic compounds at several common reporting levels are 

listed in Appendix 7. Appendixes 5, 6, and 7 include data for 

wells sampled for the NAWQA Program in aquifer studies 

and also for wells sampled for agricultural land-use studies 

for reference purposes. Physical properties and contaminant 

concentrations discussed in the text are for wells from aquifer 

studies only, unless otherwise indicated. 

Physical Properties and Dissolved Solids

Specific conductance, pH, alkalinity, dissolved oxygen, 

and water temperature are general indicators of water quality 

and affect how chemical contaminants behave in the water. 

Variability in these properties results from natural sources—

precipitation chemistry, soil properties, and the physical and 

chemical characteristics of aquifer materials—and from the 

effects of human activity. The total dissolved solids content of 

water is another indicator of water quality and of the overall 

suitability of water for drinking and other uses. The dissolved 

solids in water mostly result from the dissolution of soil and 

aquifer materials but can be augmented by the effects of 

human activities, such as irrigation and waste disposal. No 

health guidelines have been established for physical properties or dissolved solids, but extreme values of pH, alkalinity, 

dissolved oxygen, and dissolved solids may cause aesthetic 

problems or may affect the concentrations of other contaminants that are important to human health. Non-health recommendations or guidelines are available for several of these 

properties and contaminants (U.S. Environmental Protection 

Agency, 2006a; World Health Organization, 2004).

Specific conductance is a measure of the ability of the 

water to conduct an electric current; this ability results from 

ions dissolved in the water. Dissolved ions make up a large 

fraction of the total dissolved solids in natural waters, and 

specific conductance and dissolved solids were closely related 

in the domestic wells sampled in this study (linear R2 equal to 

0.92). Specific conductance of most (80 percent) of the wells 

Table 2. Summary statistics for physical properties of, and major ions in, samples collected from domestic wells for the NAWQA 

Program in aquifer studies, 1991–2004.

[Common reporting level: Most common level for data set is shown; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); mg/L, milligrams 

per liter; CaCO3, calcium carbonate; --, not applicable; <, less than]

Property or major ion Number of 

samples

Number of 

detections

Common 

reporting 

level

Concentration or value

Percentile

10th 25th Median 75th 90th

Physical properties

Specific conductance, in μS/cm at 25°C 2,138 2,137 1 116 224 417 625 945

pH, in standard units 2,144 2,144 -- 6.1 6.9 7.3 7.6 7.9

Temperature, in °C 2,140 2,140 -- 10.2 11.9 13.9 17.7 22.1

Dissolved oxygen, in mg/L 2,053 1,972 0.1 <0.1 0.2 3.0 6.3 8.2

Alkalinity as CaCO3, in mg/L 2,033 2,030 1 32.0 78.0 156 241 325

Major ions, in mg/L

Bromide 2,058 1,875 0.01 0.01 0.02 0.04 0.09 0.20

Calcium 2,160 2,160 0.02 7.2 20.6 43.0 70.0 95.3

Chloride 2,157 2,155 0.1 1.3 3.1 8.6 23.0 62.8

Fluoride 2,156 1,509 0.1 <0.1 <0.1 0.2 0.4 1.1

Magnesium 2,160 2,159 0.01 1.7 4.4 11.0 23.0 36.0

Potassium 2,160 2,148 0.1 0.50 0.9 1.7 3.5 6.6

Silica 2,160 2,160 0.01 8.4 12.0 19.0 31.2 50.5

Sodium 2,160 2,159 0.2 2.8 5.2 11.0 29.0 78.7

Sulfate 2,157 2,083 0.1 0.8 4.7 14.4 36.0 94.0

Hardness as CaCO3 2,160 2,160 0.05 30.3 76.2 162 267 370

Total dissolved solids 2,102 2,100 10 81.0 146 254 381 590
18 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

ranged from 116 to 945 μS/cm at 25°C, with a median value 

of 417 μS/cm at 25°C (table 2). Concentrations of dissolved 

solids for most wells ranged from 81.0 to 590 mg/L, with a 

median value of 254 mg/L (fig. 6). 

The USEPA SMCL for dissolved solids is 500 mg/L, and 

dissolved solids concentrations in 14.8 percent of the wells 

were greater than this value (table 3). Wells with dissolved 

solids concentrations greater than the SMCL were distributed 

across the United States and in most aquifer types (fig. 6), 

but concentrations greater than the SMCL occurred most 

frequently in samples from about 40 percent or more of the 

wells in several principal aquifers in the western and southcentral United States. These include the Basin and Range 

and Rio Grande basin-fill aquifers, the Lower Tertiary/Upper 

Cretaceous sandstone aquifers, and the Edwards-Trinity 

sandstone/carbonate aquifer (fig. 7; Appendix 6). Relatively 

high concentrations of dissolved solids in ground water would 

be expected in areas with more soluble and easily eroded rock 

types and sediments, irrigation seepage, and low precipitation 

(Hem, 1985), and high concentrations of dissolved solids 

could be attributed to these conditions in several of the 

sampled aquifers (for example, gypsum dissolution and 

irrigation seepage in the Basin and Range aquifers; Coes and 

others, 2000; Edmonds and Gellenbeck, 2002). Dissolved 

solids typically increase with ground-water age and distance 

along the flow path, as a result of longer contact times between 

the water and the aquifer materials. Thus, old water can have 

relatively high concentrations of dissolved solids (for example, 

in Upper Cretaceous aquifers in the Red River of the North 

Basin, and in the glacial and underlying Lower Tertiary/Upper 

Cretaceous aquifers underlying the Yellowstone River Basin; 

Cowdery, 1998; Bartos and others, 2005). Dissolved solids 

concentrations greater than the SMCL occurred infrequently 

and specific conductance values were relatively low in the 

coastal plain aquifers, in crystalline-rock aquifers of the 

eastern United States, and in some glacial, sandstone, and 

carbonate-rock aquifers.

Figure 6. Geographic distribution of dissolved solids concentrations in samples collected from domestic wells for 

the NAWQA Program in aquifer studies, 1991–2004. >, greater than; ≤, less than or equal to.

Dissolved solids concentration, in milligrams per liter 

> 500 

EXPLANATION

> 250 and ≤ 500 ≤ 250
Quality of Water from Domestic Wells 19

Table 3. Comparison of physical properties and concentrations of major ions in samples collected from domestic wells for the NAWQA Program in aquifer studies, 1991–2004, with human-health benchmarks and non-health guidelines for drinking water.[MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels (SMCLs) for public water supplies; HBSLs are Health-Based Screening Levels developed by the U.S. Geological Survey using USEPA toxicity data and methods (Toccalino and Norman, 2006); μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); mg/L, milligrams per liter; CaCO3, calcium carbonate; --, not applicable]

Property or major ion

Benchmark or guideline Wells exceeding benchmarks or non-health guidelines

Human-health benchmark Non-health guideline Human-health benchmark One-tenth of human health benchmark Non-health guideline

Value Type Value Type Value Percent Value Percent Value Percent

Physical properties

Specific conductance, in μS/cm at 25°C -- -- -- -- -- -- -- -- -- --pH, in standard units -- -- 16.5–8.5 SMCL -- -- -- -- 350 16.3Temperature, in °C -- -- -- -- -- -- -- -- -- --Dissolved oxygen, in mg/L -- -- -- -- -- -- -- -- -- --Alkalinity as CaCO3, in mg/L -- -- -- -- -- -- -- -- -- --

Major ions, in mg/L

Bromide -- -- -- -- -- -- -- -- -- --Calcium -- -- -- -- -- -- -- -- -- --Chloride -- -- 250 SMCL -- -- -- -- 46 2.13Fluoride 4 MCL 2.00 SMCL 25 1.16 -- -- 87 4.04Magnesium -- -- -- -- -- -- -- -- -- --Potassium -- -- -- -- -- -- -- -- -- --Silica -- -- -- -- -- -- -- -- -- --Sodium -- -- -- -- -- -- -- -- -- --Sulfate -- -- 250 SMCL -- -- -- -- 82 3.79Hardness as CaCO3 -- -- -- -- -- -- -- -- -- --Total dissolved solids -- -- 500 SMCL -- -- -- -- 310 14.8

1Guidelines recommend that pH values be within this range.
20 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Figure 7. (A) Dissolved solids, (B) pH, (C) alkalinity, and (D) dissolved oxygen in samples collected from domestic wells sampled 

for the NAWQA Program in aquifer studies, 1991–2004, by aquifer rock type. BFSG, basin-fill and other non-glacial sand and 

gravel aquifers; GLAC, glacial sand and gravel aquifers; CP, coastal plain aquifers in semi-consolidated sand; SS, sandstone 

aquifers; SS/CARB, sandstone/carbonate-rock aquifers; CARB, carbonate-rock aquifers; BAS, basaltic-rock aquifers; CRYS, 

crystalline-rock aquifers; CaCO3, calcium carbonate; and SMCL, U.S. Environmental Protection Agency Secondary Maximum 

Contaminant Level.

52

Median

25th percentile

75th percentile

90th percentile

10th percentile

Number of samples

EXPLANATION

686

182

263

87

215

384

240

0

500

1,000

1,500

BFSG GLAC CP SS SS/

CARB

CARB BAS CRYS

A Dissolved solids

 CONCENTRATION, 

IN MILLIGRAMS PER LITER

 CONCENTRATION, 

IN MILLIGRAMS PER LITER AS CaCO3

717 375

87

18

179

265 217

240

5

6

7

8

9

BFSG GLAC CP SS SS/

CARB

CARB BAS CRYS

B pH

 pH, IN STANDARD UNITS

4

689 149

265 197

370

212

0

200

300

400

500

BFSG GLAC CP SS SS/

CARB

CARB BAS CRYS

100

C Alkalinity

87 18

USEPA SMCL

BFSG GLAC CP SS SS/

CARB

CARB BAS CRYS

681

371 78

158

260

217

236

 CONCENTRATION, 

IN MILLIGRAMS PER LITER

0

4

6

8

10

2

D Dissolved oxygen
Quality of Water from Domestic Wells 21

The pH of most (80 percent) samples ranged from 6.1 

to 7.9, with a median value of 7.3 (table 2). pH is a measure 

of the acidity of water and is an important control on the 

solubility of many metals. Highly acidic (low pH) or highly 

alkaline (high pH) water can be corrosive to pipes and may 

have an unpleasant taste (World Health Organization, 2004). 

The USEPA recommends a pH of drinking water from 6.5 to 

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

samples from most of the domestic wells in this study were 

within the recommended range, the pH of samples from 

16.3 percent of the wells was less than (14.4 percent) or 

greater than (1.9 percent) the recommended range. Low pH 

values (less than 6.5) occurred in the East and in the Pacific 

Northwest (fig. 8). This is similar to geographic patterns in 

the acidity of precipitation across the United States (National 

Atmospheric Deposition Program, 2005). The samples with 

low pH values were from wells in aquifers of relatively 

resistant rock types with less capability to neutralize acid 

precipitation, including some of the rock types and principal 

aquifers with low specific conductance and dissolved solids 

(fig. 7). 

Alkalinity, a measure of the ability of water to neutralize acid, ranged from 32 to 325 mg/L as calcium carbonate 

(CaCO3) in most samples, with a median value of 156 mg/L 

as CaCO3. There are no guidelines for alkalinity in drinking 

water, but alkalinity concentrations often must be considered 

in the operation of water-treatment systems. Alkalinity in 

natural waters is almost entirely from dissolved bicarbonate 

and carbonate, which originate from the atmosphere and other 

sources, such as dissolution of carbonate rocks or organic 

respiration (Hem, 1985). Alkalinity was variable in samples 

from wells in most aquifers, but tended to be relatively high in 

carbonate-rock aquifers and in some sandstone/carbonate-rock 

and glacial aquifers; alkalinity was relatively low in the same 

Figure 8. Geographic distribution of pH in samples collected from domestic wells for the NAWQA Program in 

aquifer studies, 1991–2004. <, less than; ≥, greater than or equal to; ≤, less than or equal to; >, greater than. 

pH, in standard units 

< 6.5 

EXPLANATION

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

aquifer rock types in which specific conductance, dissolved 

solids, and pH were low (fig. 7). Geographically, alkalinity 

concentrations were highest in the east-central United States, 

reflecting in part the general distribution of the sampled 

carbonate-rock aquifers (fig. 9). 

Concentrations of dissolved oxygen in most (80 percent) 

samples from the domestic wells in this study ranged from less 

than 0.1 to 8.2 mg/L. The dissolved oxygen in ground water 

originates primarily from the atmosphere and is depleted along 

a flow path through reaction with organic material and reduced 

minerals. Thus, shallow ground water commonly contains 

more dissolved oxygen than does older, deeper ground water. 

There are no guidelines for dissolved oxygen concentrations 

in drinking water. Like pH, however, the dissolved oxygen 

content of water is an important control on the solubility of 

some metals. Dissolved oxygen concentrations varied widely 

in most aquifer types (fig. 7). However, low concentrations 

were found more frequently in samples from wells in the glacial aquifers and in some sandstone/carbonate-rock aquifers. 

Low concentrations of dissolved oxygen also were found more 

consistently in samples from wells that withdrew water from 

aquifers that were considered, on the basis of available data, 

to be confined, than in wells that were considered to be under 

water-table, or unconfined, conditions (fig. 10). This is consistent with the expectation that, in general, water under confined conditions would be older than water under unconfined 

conditions, although residence times vary among and within 

aquifers. Dissolved oxygen concentrations also were negatively correlated with dissolved organic carbon concentrations 

(Spearman’s rho equal to -0.28, p value less than 0.0001). 

Figure 9. Geographic distribution of alkalinity in samples collected from domestic wells for the NAWQA Program 

in aquifer studies, 1991–2004. >, greater than; ≥, greater than or equal to; ≤, less than or equal to; <, less than.

Alkalinity, in milligrams per liter as calcium carbonate

> 300 

EXPLANATION

≥ 100 and ≤ 300 < 100
Quality of Water from Domestic Wells 23

Major Ions

The major ions in water from the sampled domestic 

wells were the cations calcium, magnesium, sodium, and 

potassium and the anions bicarbonate (reported as alkalinity), 

silica, sulfate, and chloride (table 2). Calcium was the most 

abundant cation (in milliequivalents) and bicarbonate was 

the most abundant anion. Water samples from about one-half 

the wells were calcium-bicarbonate type. Other common 

water types were calcium-magnesium-bicarbonate and mixed 

cation-bicarbonate types. Ionic composition was variable in 

most aquifer rock types, reflecting the range of lithologies and 

geochemical processes in these broadly defined categories 

(fig. 11). Calcium and bicarbonate were especially dominant in 

the carbonate-rock aquifers, in which calcite dissolution exerts 

a strong control on water chemistry (fig. 11F). In the coastal 

plain aquifers, sodium, calcium, and mixed-cation types were 

about equally common (fig. 11C). The relative prominence of 

sodium in ground water may result from cation exchange of 

calcium for sodium on aquifer materials and reflect relatively 

long residence times; other possible explanations are mixing 

with brines, the abundance of sodium-bearing minerals, the 

absence of calcium-bearing minerals, the effects of sea spray, 

and the effects of human activities. In several aquifers, sodium 

was more prominent in water from deeper wells or in older 

water, and cation exchange, long residence times, leakage of 

saline water from confining units, or mixing with brines are 

possible explanations (Chapelle and McMahon, 1991; Szabo 

and others, 1997; Cowdery, 1998; Fong and others, 1998; 

Reutter and Dunn, 2000; Bartos and others, 2005). Calciummagnesium and mixed-ion bicarbonate water types were more 

common than calcium-bicarbonate water types in the basalticrock aquifers, reflecting the abundance of mafic minerals in 

those rock types (fig. 11G). Chloride was the dominant anion 

in water samples from some wells (17 percent) in the coastal 

plain aquifers (fig. 11C), perhaps reflecting the effects of sea 

spray and the non-reactive character (absence of reactive 

carbonate or aluminosilicate minerals) of the coastal plain 

aquifer sediments. 

Calcium and magnesium are the principal contributers 

to hardness, a property of water that is used to describe the 

effectiveness of soap in water and the tendency for scale deposition in water heaters. Hardness is reported here as milligrams 

per liter of CaCO3. Water with less than 60 mg/L as CaCO3 of 

hardness is generally considered “soft,” and water with more 

than 120 mg/L as CaCO3 of hardness is generally considered 

“hard” (Hem, 1985); hard or very hard (more than 180 mg/L 

as CaCO3) water often requires treatment. Hard water used for 

drinking-water supply can be treated with a water softener, 

through which dissolved calcium and magnesium ions are 

exchanged with sodium ions. The hardness of water from most 

wells (80 percent) sampled in this study ranged from 30.3 

to 370 mg/L as CaCO3 (table 2). Hard and very hard water 

occurred in the central United States, with a geographic 

distribution similar to that of alkalinity and dissolved solids 

(fig. 12). Hard and very hard water occurred in principal 

aquifer rock types that are relatively soluble and that contain 

calcium- and magnesium-bearing minerals, such as the some 

glacial, basaltic-rock, sandstone/carbonate-rock, and carbonate-rock aquifers (Appendix 5A).

Concentrations of sulfate in water from most (80 percent) of the sampled wells ranged from 0.8 to 94.0 mg/L 

(table 2), less than half of the USEPA SMCL of 250 mg/L 

(table 3). Drinking water with high concentrations of sulfate 

has a tendency for scale deposition on heating, can cause 

unpleasant taste, and may cause gastrointestinal effects 

(World Health Organization, 2004). Sulfate may be contributed to ground water by gypsum dissolution, oxidation 

of sulfide-bearing minerals, fertilizer application, and other 

sources, and may be reduced to sulfide under low-oxygen 

conditions. Sulfate concentrations were greater than the SMCL 

of 250 mg/L in 3.79 percent of wells (table 3). Concentrations 

greater than the SMCL occurred in most parts of the United 

States and in all aquifer rock types, except in the coastal plain 

aquifers in the Southeast and in the crystalline-rock aquifers, 

which were mostly in the Northeast (Appendix 5A). High concentrations of sulfate were correlated with high concentrations 

of dissolved solids (Spearman’s rho equal to 0.67, p value less 

than 0.0001). 

Chloride concentrations in water from most wells ranged 

from 1.3 to 62.8 mg/L and were greater than the SMCL of 

250 mg/L in 2.1 percent of sampled wells (tables 2 and 3). 

High concentrations of chloride can impart an unpleasant taste 

to drinking water (World Health Organization, 2004). Chloride 

concentrations were positively correlated with concentrations 

Figure 10. Dissolved oxygen concentrations in samples 

collected from domestic wells for the NAWQA Program in 

aquifer studies, 1991–2004, by aquifer condition.

52

Median

25th percentile

75th percentile

90th percentile

10th percentile

Number of samples

EXPLANATION

Uncon-

 fined

CONCENTRATIONS OF DISSOLVED OXYGEN, 

Mixed Confined

IN MILLIGRAMS PER LITER

0

4

6

8

10

2

1,243 75

326

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

Figure 11. Trilinear diagrams of the ionic composition of water in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2004, by principal aquifer rock type in (A) basin-fill and other non-glacial 

sand and gravel aquifers, (B) glacial sand and gravel aquifers, (C) coastal plain aquifers in semi-consolidated sand, 

(D) sandstone aquifers, (E) sandstone and carbonate-rock aquifers, (F) carbonate-rock aquifers, (G) basaltic-rock 

aquifers, and (H) crystalline-rock aquifers.

A Basin-fill and other non-glacial sand and gravel aquifers B Glacial sand and gravel aquifers

C Coastal plain aquifers in semi-consolidated sand D Sandstone aquifers

20

40

60

80

100

0

20

40

60

80

100

Magnesium

Sulfate + Chloride

Sulfate

Calcium + Magnesium

PERCENTAGE OF TOTAL MILLIEQUIVALENTS PER LITER

Calcium Chloride, Fluoride, Nitrite + Nitrate

Sodium +

Potassium

Carbonate + 

Bicarbonate

20

80

100

80

100

80

60

40

20

0

100

20

40

80

60

0

100

0

20

40

60

100

80

60

40

20

0

0

100

80

0

0

20

40

60

80

100

0

20

40

60

80

100

Magnesium

Sulfate + Chloride

Sulfate

Calcium + Magnesium

Calcium Chloride, Fluoride, Nitrite + Nitrate

Sodium +

Potassium

Carbonate + 

Bicarbonate

20

80

100

80

100

80

60

40

20

0

100

20

40

80

60

0

100

0

20

40

60

100

80

60

40

20

0

0

100

80

0

0

PERCENTAGE OF TOTAL MILLIEQUIVALENTS PER LITER

20

40

60

80

100

0

20

40

60

80

100

Magnesium

Sulfate + Chloride

Sulfate

Calcium + Magnesium

Calcium Chloride, Fluoride, Nitrite + Nitrate

Sodium +

Potassium

Carbonate + 

Bicarbonate

20

80

100

80

100

80

60

40

20

0

100

20

40

80

60

0

100

0

20

40

60

100

80

60

40

20

0

0

100

80

0

0

PERCENTAGE OF TOTAL MILLIEQUIVALENTS PER LITER

20

40

60

80

100

0

20

40

60

80

100

Magnesium

Sulfate + Chloride

Sulfate

Calcium + Magnesium

Calcium Chloride, Fluoride, Nitrite + Nitrate

Sodium +

Potassium

Carbonate + 

Bicarbonate

20

80

100

80

100

80

60

40

20

0

100

20

40

80

60

0

100

0

20

40

60

100

80

60

40

20

0

0

100

80

0

0

PERCENTAGE OF TOTAL MILLIEQUIVALENTS PER LITER
Quality of Water from Domestic Wells 25

Figure 11. Trilinear diagrams of the ionic composition of water in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2004, by principal aquifer rock type in (A) basin-fill and other non-glacial 

sand and gravel aquifers, (B) glacial sand and gravel aquifers, (C) coastal plain aquifers in semi-consolidated sand, 

(D) sandstone aquifers, (E) sandstone and carbonate-rock aquifers, (F) carbonate-rock aquifers, (G) basaltic-rock 

aquifers, and (H) crystalline-rock aquifers.—Continued

E Sandstone and carbonate-rock aquifers F Carbonate-rock aquifers

G Basaltic-rock aquifers H Crystalline-rock aquifers

20

40

60

80

100

0

20

40

60

80

100

Magnesium

Sulfate + Chloride

Sulfate

Calcium + Magnesium

Calcium Chloride, Fluoride, Nitrite + Nitrate

Sodium +

Potassium

Carbonate + 

Bicarbonate

20

80

100

80

100

80

60

40

20

0

100

20

40

80

60

0

100

0

20

40

60

100

80

60

40

20

0

0

100

80

0

0

PERCENTAGE OF TOTAL MILLIEQUIVALENTS PER LITER

20

40

60

80

100

0

20

40

60

80

100

Magnesium

Sulfate + Chloride

Sulfate

Calcium + Magnesium

Calcium Chloride, Fluoride, Nitrite + Nitrate

Sodium +

Potassium

Carbonate + 

Bicarbonate

20

80

100

80

100

80

60

40

20

0

100

20

40

80

60

0

100

0

20

40

60

100

80

60

40

20

0

0

100

80

0

0

PERCENTAGE OF TOTAL MILLIEQUIVALENTS PER LITER

20

40

60

80

100

0

20

40

60

80

100

Magnesium

Sulfate + Chloride

Sulfate

Calcium + Magnesium

Calcium Chloride, Fluoride, Nitrite + Nitrate

Sodium +

Potassium

Carbonate + 

Bicarbonate

20

80

100

80

100

80

60

40

20

0

100

20

40

80

60

0

100

0

20

40

60

100

80

60

40

20

0

0

100

80

0

0

PERCENTAGE OF TOTAL MILLIEQUIVALENTS PER LITER

20

40

60

80

100

0

20

40

60

80

100

Magnesium

Sulfate + Chloride

Sulfate

Calcium + Magnesium

Calcium Chloride, Fluoride, Nitrite + Nitrate

Sodium +

Potassium

Carbonate + 

Bicarbonate

20

80

100

80

100

80

60

40

20

0

100

20

40

80

60

0

100

0

20

40

60

100

80

60

40

20

0

0

100

80

0

0

PERCENTAGE OF TOTAL MILLIEQUIVALENTS PER LITER
26 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

of dissolved solids (Spearman’s rho equal to 0.58, p value less 

than 0.0001). Concentrations greater than the SMCL were 

scattered across the United States, and were present most 

frequently in some of the aquifers characterized by high dissolved solids, such as the Basin and Range and Rio Grande 

basin-fill sand and gravel aquifers. 

Fluoride concentrations were less than 1.0 mg/L in the 

samples from 90 percent of wells (table 2). Fluoride in low 

concentrations can prevent tooth decay, but high concentrations in drinking water can lead to staining of teeth, and very 

high concentrations can cause bone damage. The USEPA 

MCL for fluoride in drinking water is 4 mg/L; USEPA also has 

established a SMCL of 2 mg/L. Samples from slightly more 

than 1 percent of the domestic wells in this study had concentrations greater than the MCL for fluoride, and 4.0 percent had 

concentrations greater than the SMCL (fig. 13). A range of 

concentrations, 0.7 to 1.2 mg/L, is recommended in drinking 

water by the CDC to prevent tooth decay (U.S. Centers for 

Disease Control and Prevention, Fluoride Recommendations 

Work Group, 2001). In public drinking-water systems, fluoride 

often is added to attain these concentrations in finished water, 

although fluoridation of public water has been controversial 

in some cases (Institute of Medicine, 1997). Of the domestic 

wells sampled in this study, nearly all (85 to 91 percent) had 

fluoride concentrations less than the recommended concentrations to prevent tooth decay (fig. 13).

Fluoride occurs in many rock types, but can occur in 

higher concentrations in some igneous and sedimentary rocks 

and in volcanic rocks and sediments. In this study, fluoride 

concentrations were greater than the MCL in samples from 

the Basin and Range, High Plains, Rio Grande, and Lower 

Tertiary/Upper Cretaceous aquifers (four aquifers with high 

dissolved solids overall); the Floridan aquifer; and the New 

England crystalline-rock aquifers. Fluoride concentrations also 

exceeded the SMCL in samples from these and several other 

aquifers (Appendix 6A). In most aquifer types, concentrations 

of fluoride were positively correlated with concentrations of 

dissolved solids (Spearman’s rho from 0.29 to 0.65, p value 

Figure 12. Geographic distribution of hardness of water in samples collected from domestic wells for the NAWQA 

Program in aquifer studies, 1991–2004. >, greater than; ≤, less than or equal to.

Hardness, in milligrams per liter as calcium carbonate

> 180 

EXPLANATION

> 120 and ≤ 180 > 60 and ≤ 120 ≤ 60
Quality of Water from Domestic Wells 27

Figure 13. Geographic distribution of fluoride concentrations in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2004. >, greater than; ≤, less than or equal to.

less than 0.0001, excluding sandstone and basaltic-rock 

aquifers) and negatively correlated with dissolved oxygen 

(Spearman’s rho from 0.31 to 0.55, p values less than 0.0001, 

excluding glacial and basaltic-rock aquifers). The inverse 

relation between fluoride and dissolved oxygen may be related 

to ground-water age, in that older water is more likely to have 

higher concentrations of solutes overall, as well as lower 

concentrations of dissolved oxygen in some cases. 

Trace Elements

Trace elements include metals and semi-metallic 

elements that typically are found in natural waters at 

concentrations less than 1 mg/L. These elements originate 

primarily from rock weathering; concentrations of trace 

elements in ground water reflect their abundance in aquifer 

materials, geochemical conditions, concentrations of other 

constituents, and attenuation processes such as adsorption. 

Many trace elements may occur as multiple ionic species in 

natural waters that, depending on redox conditions and pH, 

have different solubility characteristics. Human activities such 

as mining and waste disposal also can affect concentrations of 

trace elements in ground water. At high concentrations, many 

trace elements can have adverse health effects, whereas others 

may present aesthetic or nuisance problems. Trace elements 

were not analyzed in samples from all of the wells sampled 

in this study. The concentrations of 16 trace elements were 

analyzed in about two-thirds (about 1,600) of the sampled 

wells (table 4). Iron and manganese were analyzed in nearly 

all (about 2,160) of the wells, and five trace elements—boron, 

lithium, strontium, thallium, and vanadium—were analyzed in 

about one-quarter (about 500 to 600) of the sampled wells. 

Many trace elements were minor constituents in the 

domestic well water sampled in this study. Aluminum, arsenic, 

barium, boron, chromium, copper, iron, lithium, manganese, 

molybdenum, nickel, strontium, vanadium, uranium, and zinc 

were each detected in about one-half or more of the sampled 

Fluoride concentration, in milligrams per liter

EXPLANATION

> 4 > 2 and ≤ 4 > 0.7 and ≤ 2 ≤ 0.7
28 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Table 4. Summary statistics for trace elements in samples collected from domestic wells for the NAWQA Program in aquifer studies, 

1991–2004.

[μg/L, micrograms per liter; <, less than]

Trace element

No common reporting level Common reporting level of 1 μg/L

Number of 

samples

Number of 

detections

Concentration percentile, in μg/L Number of 

samples

Number of 

detections

Frequency of

detections, 

in percent 10th 25th Median 75th 90th

Aluminum 1,454 842 0.32 0.68 1.89 3.73 5.28 1,408 831 59.0

Antimony 1,552 263 <1 <1 <1 <1 <1 1,535 8 0.52

Arsenic 1,774 976 0.11 0.28 0.92 3.42 7.53 1,774 816 46.0

Barium 1,593 1,556 5.42 19.0 50.0 116 219 1,593 1,556 97.7

Beryllium 1,572 51 <1 <1 <1 <1 <1 1,555 8 0.51

Boron 535 474 6.65 10.9 24.0 63.5 218 535 1334 62.7

Cadmium 1,639 138 <1 <1 <1 <1 <1 1,620 6 0.37

Chromium 1,614 879 0.17 0.37 0.81 2.08 4.00 1,591 744 46.8

Cobalt 1,572 537 0.03 0.06 0.11 0.23 0.52 1,543 56 3.63

Copper 1,619 1,094 0.17 0.42 1.07 3.71 12.3 1,599 825 51.6

Iron 2,157 1,234 0.17 1.24 8.53 110.0 1,110 2,142 2955 44.5

Lead 1,639 492 0.01 0.02 0.08 0.30 1.09 1,605 202 12.6

Lithium 662 642 0.71 1.93 5.87 18.5 43.8 649 558 86.0

Manganese 2,159 1,380 0.05 0.28 2.06 36.0 172 2,105 1,246 59.2

Molybdenum 1,572 921 0.13 0.30 0.92 2.88 6.00 1,545 763 49.4

Nickel 1,572 925 0.10 0.23 0.70 2.00 3.00 1,546 710 45.9

Selenium 1,625 537 0.13 0.28 0.59 1.19 3.02 1,565 390 24.9

Silver 1,573 4 <1 <1 <1 <1 <1 1,551 3 0.19

Strontium 488 485 30.0 85.0 207 679 2,240 488 2485 99.4

Thallium 613 114 <1 <1 <1 <1 <1 611 0 0.0

Uranium 1,725 934 0.01 0.06 0.39 3.00 8.03 1,720 692 40.2

Vanadium 662 452 0.12 0.34 1.29 8.4 20.4 600 326 54.3

Zinc 1,594 1,424 1.00 2.36 8.57 34.1 99.9 1,552 1,414 91.1

1

Common reporting level for boron is 16 μg/L.

2

Common reporting level for iron and strontium is 10 μg/L.
Quality of Water from Domestic Wells 29

wells (using a common reporting level of 1 μg/L for most trace 

elements, table 4). Antimony, beryllium, cadmium, silver, 

and thallium were detected infrequently. These elements were 

detected in less than 1 percent of the sampled wells (using a 

common reporting level of 1 μg/L). Concentrations of most 

trace elements were less than available human-health benchmarks for drinking water in 99 percent or more of wells 

(table 5). Exceptions were arsenic, boron, manganese, strontium, and uranium, all of which exceeded health benchmarks 

in about 1 to 7 percent of wells. Concentrations of aluminum, 

iron, and manganese were greater than USEPA SMCLs in 

about 7 to 21 percent of wells (table 5).

Aluminum is a common constituent of aquifer materials. 

Aluminum produced by rock weathering, however, is readily 

immobilized through precipitation and adsorption reactions. 

Concentrations of aluminum in most wells ranged from less 

than 1 to 5.28 μg/L (table 4) and showed no readily apparent regional patterns of occurrence. High concentrations of 

aluminum in drinking water can result in undesirable color 

and turbidity (World Health Organization, 2004), and the 

USEPA recommends that concentrations do not exceed 50 to 

200 μg/L. Concentrations of aluminum greater than the lower 

end of the recommended range occurred in 1.51 percent of the 

sampled wells (table 5). These wells were completed in several aquifer rock types but typically had water with low pH, 

which is consistent with desorption of aluminum. 

Arsenic concentrations in most wells ranged from less 

than 1 to 7.53 μg/L (table 4), less than the USEPA MCL of 

10 μg/L for drinking water. Arsenic in drinking water can 

contribute to skin, bladder, and lung cancers and has been 

associated with cardiovascular and neurological effects 

(National Research Council, 2001; World Health Organization, 

2004). Concentrations of arsenic were greater than the MCL 

in 6.75 percent of all wells and in more than 10 percent of 

wells in several aquifer types, including crystalline-rock 

aquifers in New England; basin-fill aquifers in California, 

Nevada, Arizona and New Mexico, Texas, and Nebraska; and 

the Snake River basaltic-rock aquifers in Idaho (figs. 14 and 

15 and Appendix 6A). Relatively low arsenic concentrations 

were found in coastal plain, sandstone/carbonate-rock, and 

carbonate-rock aquifers. These findings are consistent with 

previous national-scale studies of arsenic in ground water that 

showed elevated concentrations in the West, Midwest and 

Northeast, and lowest concentrations in the Southeast (Ryker, 

2001; Welch and others, 2000). The percentages of domestic 

wells and public-supply wells with arsenic concentrations 

greater than the MCL appear similar, based on findings of 

this study and the estimate of 7.6 percent for public-supply 

systems in the United States (Focazio and others, 2000). 

Sources of arsenic to ground water include arsenic 

sulfides and other common minerals in which arsenic is an 

accessory; arsenic compounds also have been used as pesticides. Geochemical conditions and the distribution of arsenic 

sources, however, have important roles in controlling arsenic 

in ground water, through processes that differ among aquifers. For example, processes that result in elevated arsenic in 

ground water include arsenic release from iron oxides, which 

is favored by low dissolved oxygen concentrations and high 

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

2003; Kolker and others, 2003; Erickson and Barnes, 2005; 

Kelly and others, 2005; Thomas, 2007). In the present study, 

arsenic concentrations were negatively correlated with dissolved oxygen concentrations in samples from wells in several 

principal aquifers, consistent with this process, including the 

Basin and Range and Central Valley basin-fill aquifers (rho 

equal to -0.333 and -0.494, respectively, and p values less 

than 0.001); the eastern, central, and western glacial aquifers 

(rho from -0.459 to -0.621, p values less than or equal to 

0.004); the Coastal Lowlands coastal plain aquifer (rho equal 

to -0.551, p values less than 0.001), and the New England 

crystalline-rock aquifers (rho equal to -0.315, p value less than 

0.003). Arsenic concentrations also were positively associated 

with pH in these and other aquifers (rho from 0.241 to 0.763, 

p values less than 0.05). Oxidation of sulfide-bearing minerals 

in some sandstone aquifers and mined areas, evaporative concentration in some basin-fill aquifers in the Southwest, and the 

influence of geothermal water locally in the western United 

States are other processes that can result in elevated arsenic 

concentrations in ground water (Bexfield and Plummer, 2003; 

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

2003; Schreiber and others, 2003). Arsenic concentrations also 

can vary considerably over short distances or even within a 

single well over time (Gotkowitz and others, 2004; Kelly and 

others, 2005). 

Boron concentrations were greater than the HBSL of 

1,000 μg/L in 1.30 percent of wells (table 5). In most wells, 

boron concentrations ranged from <16 to 218 μg/L. Boron is 

transported with little attenuation in ground water (Hem, 1985; 

Barber and others, 1988). Thus, high concentrations typically 

reflect the distribution of boron sources in the environment. 

Boron in water may originate from natural sources (for example, evaporite deposits, geothermal water, and saline ground 

water) or from anthropogenic sources (including sewage 

disposal) because it is used in the production of glass, cleaning 

agents, fire retardants, and other products (U.S. Environmental 

Protection Agency, 2008b). Relatively high concentrations of 

boron in the domestic wells sampled in this study were most 

often in the basin-fill aquifers in California, in the southern 

High Plains basin-fill aquifers, and in the Lower Tertiary/

Upper Cretaceous sandstone aquifers underlying the 

Yellowstone River Basin in Wyoming (Appendix 6A); these 

high concentrations may be associated with evaporate 

deposits or saline ground water (Bartos and others, 2005; 

Bruce and others, 2007). Health effects of boron include gastrointestinal, reproductive, and developmental effects (World 

Health Organization, 2004; U.S. Environmental Protection 

Agency, 2008b).

Manganese- and iron-bearing minerals are common 

constituents of unconsolidated sediments and aquifer 

materials; iron and manganese are more soluble under 

conditions of low dissolved oxygen concentrations (reducing 
30 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Table 5. Comparison of concentrations of trace elements in samples collected from domestic wells for the NAWQA Program in aquifer 

studies, 1991–2004, with human-health benchmarks and non-health guidelines for drinking water.

[MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels 

(SMCLs) for public water supplies; HBSLs are Health-Based Screening Levels developed by the U.S. Geological Survey using USEPA toxicity data and methods (Toccalino and Norman, 2006); --, not applicable. Action level is a concentration which, if exceeded, triggers treatment or other requirements in USEPA 

regulations for public water supplies. All benchmarks and guidelines are in micrograms per liter]

Trace 

element

Benchmark or guideline Wells exceeding benchmarks or non-health guidelines

Human-health 

benchmark Non-health guideline Human-health benchmark

One-tenth of humanhealth benchmark Non-health guideline

Value Type Value Type Value Percent Value Percent Value Percent

Aluminum -- -- 50 to 200 SMCL -- -- -- -- 7 to 22 0.48 to 1.51

Antimony 6 MCL -- -- 1 0.06 11 0.71 -- --

Arsenic 10 MCL -- -- 120 6.75 758 42.7 -- --

Barium 2,000 MCL -- -- 0 0.00 185 11.6 -- --

Beryllium 4 MCL -- -- 1 0.06 15 0.95 -- --

Boron 1,000 HBSL -- -- 7 11.30 108 120.2 -- --

Cadmium 5 MCL -- -- 2 0.12 6 0.37 -- --

Chromium 100 MCL -- -- 0 0.00 21 1.30 -- --

Cobalt -- -- -- -- -- -- -- -- -- --

Copper 1,300 Action level 1,000 SMCL 1 0.06 13 0.80 1 0.06

Iron -- -- 300 SMCL -- -- -- 0 412 19.1

Lead 15 Action level -- -- 0 0.00 126 7.69 -- --

Lithium -- -- -- -- -- -- -- -- -- --

Manganese 300 HBSL 50 SMCL 113 5.22 579 26.8 461 21.3

Molybdenum 40 HBSL -- -- 9 0.57 257 16.35 -- --

Nickel 100 HBSL -- -- 1 0.06 22 1.40 -- --

Selenium 50 MCL -- -- 3 0.18 92 5.65 -- --

Silver 100 HBSL 100 SMCL 0 0.00 0 0.00 0 0.00

Strontium 4,000 HBSL -- -- 36 17.32 169 134.6 -- --

Thallium 2 MCL -- -- 0 10.00 8 11.30 -- --

Uranium 30 MCL -- -- 30 1.74 420 24.35 -- --

Vanadium -- -- -- -- -- -- -- -- -- --

Zinc 2,000 HBSL 5,000 SMCL 0 0.00 79 4.94 0 0.00

1

Total numbers of samples were about one-third those of other trace elements; see table 4 for sample numbers.
Quality of Water from Domestic Wells 31

Figure 14. Concentrations of (A) arsenic, (B) boron, (C) manganese, (D) strontium, and (E) uranium in samples collected 

from domestic wells for the NAWQA Program in aquifer studies, 1991–2001, by aquifer rock type. BFSG, basin-fill and other 

non-glacial sand and gravel aquifers; GLAC, glacial sand and gravel aquifers; CP, coastal plain aquifers in semi-consolidated 

sand; SS, sandstone aquifers; SS/CARB, sandstone/carbonate-rock aquifers; CARB, carbonate-rock aquifers; BAS, basalticrock aquifers; CRYS, crystalline-rock aquifers; MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant 

Level; HBSL, Health-Based Screening Level; SMCL, Secondary Maximum Contaminant Level. The common reporting level is 

1 microgram per liter.

Median

25th percentile

75th percentile

90th percentile

10th percentile

Number of samples

EXPLANATION

52

164

111

99

75

100

200

300

400

500

BFSG GLAC CP SS SS/

CARB

CARB BAS CRYS

B Boron

0

86

BFSG GLAC CP SS SS/

CARB

CARB BAS CRYS

0

300

715

185

264

217

387

240

200

100

C Manganese

87

 CONCENTRATION, IN MICROGRAMS PER LITER

0

BFSG GLAC CP SS SS/

CARB

CARB BAS CRYS

132

111

44

64

50

87

5,000

10,000

15,000

D Strontium

0

4

8

12

16

BFSG GLAC CP SS SS/

CARB

CARB BAS CRYS

E Uranium

605

253

154

224

86

163

240

Non-health guideline—

 USEPA SMCL

0

4

8

12

16

BFSG GLAC CP SS SS/

CARB

CARB BAS CRYS

A Arsenic

707

253

184

194

86 163

186

235

18

MCL or HBSL outside of

 range shown

Human-health benchmarks

MCL or HBSL

One-tenth of MCL or HBSL
32 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

conditions) and low pH. Manganese is an essential trace 

element for human health but may have adverse neurological 

effects at high doses; food is the primary source of exposure 

for people rather than drinking water, however (Agency for 

Toxic Substances and Disease Registry, 2008). Iron also is an 

essential trace element, with no human-health benchmarks 

(World Health Organization, 2004). Both iron and manganese 

have recommended SMCLs in order to avoid poor water taste 

and the staining of laundry and plumbing fixtures. Manganese 

concentrations in most wells ranged from <1 to 172 μg/L 

(table 4), and concentrations were greater than the HBSL for 

drinking water of 300 μg/L in 5.22 percent of wells (table 

5). Manganese concentrations greater than the HBSL were 

present in wells completed in nearly every principal aquifer 

rock type, but most frequently in wells in the glacial aquifers, 

in sandstone aquifers in Pennsylvania and West Virginia, and 

in coastal plain aquifers in Louisiana and Texas (figs. 14 and 

16 and Appendixes 5A and 6A). This distribution of relatively 

high manganese concentrations among principal aquifers also 

was documented by McMahon and Chapelle (2007), who 

showed through an analysis of geochemical conditions in 

domestic wells sampled by the NAWQA Program that anoxic 

conditions occurred most frequently in ground water from 

the glacial and sandstone aquifers compared to other aquifer 

types, nationally. The SMCL for manganese of 50 μg/L was 

exceeded in 21.3 percent of the wells, and the SMCL for iron 

of 300 μg/L was exceeded in 19.1 percent of wells (table 5). 

Consistent with the control of dissolved concentrations by 

redox conditions and pH, concentrations of both manganese 

and iron were inversely correlated with those of dissolved 

oxygen for all wells (Spearman’s rho equal to -0.635 and 

-0.620, respectively, and p values less than 0.0001) and for 

most principal aquifers; manganese also was negatively 

correlated with pH for all wells (Spearman’s rho equal to 

-0.096 and p value less than 0.0001). 

Figure 15. Geographic distribution of arsenic concentrations in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2004. >, greater than; ≥, greater than or equal to; ≤, less than or equal to; 

<, less than.

Arsenic, in micrograms per liter

EXPLANATION

> 10 ≥ 1 and ≤ 10 < 1
Quality of Water from Domestic Wells 33

Figure 16. Geographic distribution of manganese concentrations in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2004. >, greater than; ≤, less than or equal to; ≥, greater than or equal to; 

<, less than.

Strontium concentrations in most wells ranged from 30 to 

about 2,200 μg/L. Concentrations of strontium exceeded the 

HBSL of 4,000 μg/L in 7.32 percent of wells. Relatively high 

concentrations occurred most frequently in the southwestern 

and south-central United States, in the Edwards-Trinity 

sandstone/carbonate-rock aquifer, the southern High Plains 

aquifer, and parts of the Basin and Range basin-fill aquifers 

(Appendixes 5A and 6A). Strontium is geochemically similar 

to calcium and is commonly found in carbonate and sulfate 

rocks and deposits. Health effects include abnormal bone 

development at high doses (Agency for Toxic Substances and 

Disease Registry, 2004a). 

Uranium concentrations were greater than the USEPA 

MCL of 30 μg/L in 1.74 percent of wells (table 5). In most 

wells (80 percent), concentrations of uranium were less than 

one-third of the MCL, ranging from less than 1 to 8.03 μg/L 

(table 4). Health effects of uranium in drinking water include 

possible harm to kidneys (World Health Organization, 2004). 

High concentrations and concentrations greater than the MCL 

occurred most frequently in basin-fill aquifers in the West and 

in crystalline-rock aquifers in the Northeast and Colorado 

(figs. 14 and 17). Concentrations were low (less than 2 μg/L) 

or below detection in the coastal plain and carbonate-rock in 

the Southeast and in glacial aquifers in east-central United 

States. Uranium is a common trace element in many rock 

types, but is particularly enriched in certain types of rocks, 

such as granites (Hess and others, 1985). Like arsenic, manganese, and iron, its presence in ground water is affected by 

geochemical conditions, including dissolved oxygen concentrations. The combination of elevated source-rock concentrations and the conditions that favor uranium solubility—

dissolved oxygen and carbonate alkalinity—have led to 

relatively high concentrations of uranium in ground water in 

various hydrogeologic settings, including the Central Valley 

of California, the Newark Basin sediments of northern New 

Jersey, and the crystalline-rock aquifers of New England 

EXPLANATION

Manganese concentration, in micrograms per liter

> 300 > 30 and ≤ 300 ≥ 1 and ≤30 < 1
34 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

(Szabo and Zapecza, 1991; Ayotte and others, 2007; Jurgens 

and others, 2008). In the domestic wells sampled in the present study, uranium concentrations were positively correlated 

with dissolved oxygen concentrations and pH (Spearman’s rho 

equal to 0.112 and 0.331, respectively, and p values less than 

0.0001). This is consistent with the greater solubility of the 

oxidized form of uranium and its greater stablility at higher 

pH (Hem, 1985). The association of uranium with dissolved 

oxygen was not as strong as that for iron or manganese with 

dissolved oxygen, probably because uranium sources are 

not as ubiquitous as sources of iron or manganese in aquifer 

materials and because uranium solubility is more complex. For 

example, dissolved uranium can form complexes with carbonate, phosphate, and other species that increase its solubility 

(Hem, 1985).

Concentrations of arsenic, boron, strontium, and uranium 

all were positively correlated with concentrations of total dissolved solids and with specific conductance (Spearman’s rho 

from 0.304 to 0.739, p values less than 0.0001). This reflects 

the general pattern of relatively high trace-element concentrations in wells with relatively high solute concentrations 

overall, such as occurred in some aquifers in the western and 

south-central United States. Relatively high solute concentrations in this region could result from the effects of aquifer 

characteristics, relatively low precipitation, irrigation water 

use, and (or) ground-water age. 

Nutrients and Dissolved Organic Carbon

Nitrogen and phosphorus species in ground water may 

originate from a variety of natural sources, including precipitation, leaching of organic debris, and aquifer materials, and 

human activities such as fertilizer application, animal production, and wastewater disposal. Transformations among nitrogen species in ground water are controlled by redox conditions 

and mediated by microorganisms. Nitrate is the most common 

form of nitrogen in ground water and is stable and mobile 

Figure 17. Geographic distribution of uranium concentrations in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2004. >, greater than; ≤, less than or equal to; ≥, greater than or equal to; 

<, less than.

Uranium concentration, in micrograms per liter

> 30 

EXPLANATION

> 3 and ≤ 30 ≥ 1 and ≤ 3 < 1
Quality of Water from Domestic Wells 35

under oxidizing conditions. Nitrate also is the only nutrient for 

which there is a human-health benchmark. Excessive nitrate 

can result in restriction of oxygen transport in the bloodstream, 

which is of particular concern in infants who lack the enzyme 

needed to correct this condition (resulting in methemoglobinemia, commonly known as blue baby syndrome).

Concentrations of nitrite, ammonia, ammonia plus 

organic nitrogen, total phosphorus, and orthophosphate were 

commonly at or near detection limits in the domestic wells 

sampled in this study (table 6). Ninety percent of nitrite 

concentrations were at or below the common reporting level 

of 0.01 mg/L as N, and 75 percent of ammonia plus organic 

nitrogen concentrations were below the common reporting 

level of 0.2 mg/L. Concentrations of ammonia ranged from 

less than 0.02 to 0.33 mg/L as N in most (80 percent) wells, 

with a median value equal to the common reporting level of 

0.02 mg/L as N. Ammonia is a reduced form that is oxidized 

to nitrate when dissolved oxygen is present, and ammonia concentrations were inversely correlated with dissolved oxygen 

concentrations (Spearman’s rho equal to -0.51, p value less 

than 0.0001). Ammonia also may be removed from solution by 

sorption to aquifer materials. Concentrations of total phosphorus and orthophosphate were less than 0.11 mg/L as P in 

90 percent of wells, with median values of 0.01 and 0.02 mg/L 

as P, respectively. Orthophosphate is the most common form 

of phosphorus in ground water, and its mobility is limited in 

soils and ground water because it tends to precipitate and sorb 

onto aquifer materials. 

Nitrate concentrations in most (80 percent) wells ranged 

from less than 0.05 to 5.79 mg/L as N, with a median value of 

0.55 mg/L as N (table 6). It was a common contaminant and 

was detected at concentrations greater than 0.05 mg/L as N 

in 72 percent of the sampled wells (fig. 18). Concentrations 

of nitrate were greater than 1 mg/L as N, a level which is 

considered to result from the effects of human activities 

in many parts of the United States (Nolan and Hitt, 2003), 

in nearly one-half (41.4 percent) of the wells and in all 

principal aquifer rock types (fig. 19A). The USEPA MCL for 

nitrate is 10 mg/L as N. Concentrations in 95.6 percent of 

sampled domestic wells were less than the MCL, but nitrate 

concentrations were greater than 10 mg/L as N in 4.36 percent 

of the wells (table 7). Concentrations were greater than the 

MCL in more than 10 percent of wells in several aquifers, 

including the Basin and Range and Central Valley basin-fill 

aquifers in the Southwest and in California, the west-central 

glacial aquifers in the Upper Midwest, the North Atlantic 

coastal plain aquifers in the central Appalachian region, and 

the Piedmont crystalline-rock aquifers, also in the central 

Appalachian region (fig. 18 and Appendix 5A).

Several previous studies that used NAWQA data 

from domestic and monitoring wells found that nitrate 

concentrations at the national scale were related to a 

number of factors that described nitrate sources and aquifer 

vulnerability (Nolan and Stoner, 2000; Nolan, 2001; Nolan 

and others, 2002; Nolan and Hitt, 2006). These factors 

included nitrogen loading from fertilizer use, the extent of 

agricultural land (cropland or pasture) surrounding a well, 

population density, soil characteristics, depth to seasonally 

high water, and the presence or absence of sand and gravel 

aquifers. Other factors that appeared to affect nitrate 

concentrations in ground water regionally were denitrification 

of nitrate to nitrogen gas in the Southeast, and the presence of 

septic systems and cesspools locally in the Northeast (Nolan 

and Ruddy, 1996; Nolan, 1999). 

In the domestic wells from aquifer studies included in 

the present study, nitrate concentrations were higher in areas 

of agricultural land use than in areas of other land uses and 

were correlated with several variables indicative of nitrogen 

sources, aquifer characteristics, and geochemical conditions 

that were consistent with the findings of previous studies. 

Concentrations were positively correlated with estimates of 

the amount of nitrogen fertilizer applied in areas surrounding the wells (Spearman’s rho equal to 0.189, p value less 

than 0.0001) and were higher in areas with well drained soils 

(Spearman’s rho equal to -0.213, p value less than 0.0001). 

Nitrate concentrations also were positively correlated with 

concentrations of dissolved oxygen (Spearman’s rho equal 

to 0.611, p value less than 0.0001), suggesting the effects of 

denitrification (which occurs in the absence of oxygen) and 

(or) ground-water age. Finally, nitrate concentrations were 

positively correlated with the percentage of agricultural land 

use (Spearman’s rho equal to 0.288, p value less than 0.0001) 

and negatively correlated with the percentage of undeveloped 

land use (Spearman’s rho equal to -0.336, p value less than 

0.0001) in buffer areas surrounding wells. Consequently, 

when wells were categorized according to land use within the 

surrounding 500-meter-radius buffer areas (table 8; Gilliom 

and others, 2006), wells in predominantly agricultural areas 

had nitrate concentrations that were higher and exceeded the 

MCL more frequently than those in wells in areas of other 

land uses (fig. 19B). Nitrate concentrations were greater than 

the MCL in 7.1 percent of wells in areas of predominantly 

agricultural land use (n equal to 843), 3.1 percent of wells in 

areas of urban land use (n equal to 193), 3.7 percent of wells 

in mixed land-use areas (n equal to 651), and 0.7 percent of 

wells in undeveloped areas (n equal to 445); median values for 

wells in agricultural, urban, mixed, and undeveloped land-use 

areas were 1.1, 0.68, 0.43, and 0.19 mg/L as N, respectively 

(fig. 19B). In wells used in NAWQA assessment studies that 

specifically targeted agricultural land-use areas (fig. 1), nitrate 

concentrations were greater than the MCL of 10 mg/L as N 

even more frequently, at 23.4 percent of the sampled wells 

(Appendix 5B). These findings are consistent with the many 

previous studies that have found relatively high nitrate concentrations in some agricultural areas of the Nation (for example: 

Kolpin and others, 1994; Mehnert and others, 1995; Townsend 

and others, 1998).

Dissolved organic carbon was detected in nearly all 

wells sampled in this study (table 6). Concentrations ranged 

from 0.2 to 1.9 mg/L in most wells, with a median value of 

0.5 mg/L. Dissolved organic carbon originates naturally from 

organic material but also may be contributed to ground water 
36 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 Table 6. Summary statistics for nutrients, dissolved organic carbon, radionuclides, and fecal indicator bacteria in samples collected from domestic wells for the NAWQA Program in aquifer studies, 1991–2004. [Radium-226 plus radium-228 data from Zoltan Szabo, U.S. Geological Survey, written communs., 2006 and 2008. Fecal indicator bacteria data from S.S. Embrey, U.S. Geological Survey, written commun., 2005. mg/L, milligrams per liter; pCi/L, picocuries per liter; CFU/100 mL, colony-forming units per 100 milliliters; <, less than; --, not determined] Contaminant No common reporting level Common reporting levels as indicated Number of samples Number of detec- tions Concentration percentile Common reporting level Number of samples Number of detec- tions Frequency of detections, in percent 10th 25th Median 75th 90th Nutrients and dissolved organic carbon (mg/L) Nitrite as N 2,141 281 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 2,141 244 11.4 Nitrate as N 2,132 1,555 <0.05 0.07 0.55 2.40 5.79 0.05 2,132 1,534 72.0 Ammonia as N 2,095 1,129 <0.02 <0.02 0.02 0.05 0.33 0.02 2,095 1,079 51.5 Ammonia plus organic nitrogen as N 2,086 656 <0.2 <0.2 <0.2 <0.2 0.4 0.2 2,086 358 17.2 Phosphorus, total 1,776 1,028 <0.01 <0.01 0.01 0.04 0.11 0.01 1,720 867 50.4 Orthophosphate as P 2,121 1,382 <0.01 <0.01 0.02 0.04 0.11 0.01 1,979 1,369 69.2 Dissolved organic carbon 1,932 1,818 0.2 0.3 0.5 0.9 1.9 0.10 1,895 1,818 95.9 Radionuclides (pCi/L) Radon 1,958 1,936 141 232 434 898 2,150 80 1,958 1,907 97.4 Gross alpha-particle radioactivity 378 158 <3 <3 <3 4.75 8.70 3 378 158 41.8 Gross beta-particle radioactivity 375 157 <4 <4 <4 5.57 10.8 4 375 157 41.9 Radium-226 plus radium-228 438 1304 -- -- -- -- -- -- -- -- -- Fecal indicator bacteria (CFU/100 mL) Total coliform bacteria 397 133 -- -- -- -- -- 1 397 133 33.5 Escherichia coli bacteria 378 30 -- -- -- -- -- 1 378 30 7.9

1Includes detection of only one or both radionuclides.
Quality of Water from Domestic Wells 37

Figure 18. Geographic distribution of nitrate concentrations in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2004. >, greater than; ≤, less than or equal to.

through waste disposal and animal production. It can form 

complexes with metals that alter the solubility of the metals, 

and through its microbially mediated oxidation, dissolved 

organic carbon can affect redox conditions and the transformation of other chemical species in ground water. Dissolved 

organic carbon also can react with chlorine during water 

disinfection to form trihalomethanes, which are harazardous to 

human health. 

Radon and Other Radionuclides

Radon (radon-222) is a water-soluble, radioactive gas that 

originates from radium-226, part of the uranium-238 decay 

series. Because its initial parent uranium occurs ubiquitously 

in trace amounts in the aquifer sediments and rocks, and 

because it is soluble, radon is common in ground water. Radon 

is chemically inert and does not react with aquifer materials 

or other chemical constituents, but it decays through alphaparticle emission and has a short half-life (3.8 days; Wanty and 

Nordstrom, 1993). Radon concentrations in ground water can 

be affected by multiple factors, including the distribution of 

uranium-bearing minerals in aquifer materials, aquifer physical characteristics, and geochemical conditions that affect the 

uranium and radium mobility (Hess and others, 1985; Wanty 

and others, 1992; Otton and others, 1993). 

Radon and other naturally occurring radionuclides emit 

ionizing radiation and consequently are carcinogens. Radon 

can contribute to the risk of developing lung and gastrointestinal cancers (National Academy of Sciences, 1999). Adverse 

health effects from radon in drinking water result primarily 

from inhalation, after the gas is released from solution in the 

home, although the contribution from drinking water usually is 

small compared to other sources of radon in indoor air (Hopke 

and others, 2000). Water with about 10,000 pCi/L of radon 

contributes about 1 pCi/L of radon to indoor air (Otton and 

others, 1993); USEPA recommends that homes with indoor air 

concentrations at or above 4 pCi/L be fixed to reduce concentrations (U.S. Environmental Protection Agency, 1999). Two 

Nitrate, in milligrams per liter as N

EXPLANATION

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

human-health benchmarks, which are regulations proposed by 

USEPA in 1999 for public water systems, are used for comparison with radon concentrations in this study. The higher 

value, 4,000 pCi/L, is an alternative MCL that is proposed for 

public water systems for states or water-system service areas 

that have programs in place to reduce radon risks from all 

sources (U.S. Environmental Protection Agency, 1999; Hopke 

and others, 2000). The lower value, 300 pCi/L, is proposed 

as the MCL for states or service areas that do not have such 

programs. 

Radon concentrations were measured in nearly all of the 

wells sampled in this study. Radon concentrations are reported 

in activity units, picocuries per liter (pCi/L), which describe 

the number of radioactive emissions (nuclear disintegrations) 

over time, rather than in mass concentration units. One picocurie per liter equals 2.2 radioactive disintegrations per minute 

per liter.

Radon concentrations in domestic wells sampled 

in this study ranged from 141 to 2,150 pCi/L in most 

(80 percent) wells (table 6). The median value, 434 pCi/L, 

and concentrations in 64.6 percent of the wells were greater 

than the lower proposed MCL of 300 pCi/L (table 7). 

Concentrations greater than the lower proposed MCL occurred 

in every principal aquifer and were distributed across the 

United States (figs. 20 and 21). Concentrations greater than the 

higher proposed MCL of 4,000 pCi/L occurred in 4.44 percent 

of wells (table 7). Radon concentrations were highest, and 

were present most frequently at concentrations greater than 

the proposed MCLs, in the crystalline-rock aquifers (fig. 21), 

including the New England, Piedmont, and Rocky Mountain 

Front Range crystalline-rock aquifers, which are located in 

the Northeast, the southern and central Appalachians, and 

Colorado (fig. 20 and Appendix 5A). Concentrations in about 

30 percent of the sampled wells in these aquifers were greater 

Figure 19. Nitrate concentrations in samples collected from domestic wells for the NAWQA Program in aquifer studies, 

1991–2004, by (A) aquifer rock type and (B) land use. BFSG, basin-fill and other non-glacial sand and gravel aquifers; GLAC, 

glacial sand and gravel aquifers; CP, coastal plain aquifers in semi-consolidated sand; SS, sandstone aquifers; SS/CARB, 

sandstone/carbonate-rock aquifers; CARB, carbonate-rock aquifers; BAS, basaltic-rock aquifers; CRYS, crystalline-rock 

aquifers. Land uses are uses in circular areas of 500-meter radii surrounding wells; see table 8 for description of land-use 

categories. MCL, U.S. Environmental Protection Agency Maximum Contaminant Level. 

Median

25th percentile

75th percentile

90th percentile

10th percentile

Number of samples

EXPLANATION

52

709

184

264

87 216

370

238

BFSG GLAC CP SS SS/

CARB

CARB BAS CRYS

A Nitrate concentration by aquifer rock type

18

10

0

8

6

4

2

 CONCENTRATION OF NITRATE, 

IN MILLIGRAMS PER LITER AS N

Undeveloped Mixed Agricultural Urban

445

651 193

843

B Nitrate concentration by land use 

10

0

8

6

4

2

Human-health benchmark

MCL

One-tenth of MCL

LAND USE
Quality of Water from Domestic Wells 39

Table 7. Comparison of concentrations of nitrate and radionuclides in samples collected from domestic wells for the NAWQA Program 

in aquifer studies, 1991–2004, with human-health benchmarks and non-health guidelines for drinking water.

[MCLs are U.S. Environmental Protection Agency Maximum Contaminant Levels for public water supplies; mg/L, milligrams per liter; pCi/L, picocuries per 

liter; --, not applicable or not determined]

Contaminant

Benchmark or guideline Wells exceeding benchmarks or non-health guidelines

Human-health benchmark Non-health 

guideline

Human-health 

benchmark

One-tenth of 

human-health 

benchmark

Non-health 

guideline

Value Type Value Type Value Percent Value Percent Value Percent

Nutrients (mg/L)

Nitrate as N 10 MCL -- -- 93 4.36 883 41.4 -- --

Radionuclides (pCi/L)

Radon 4,000 Proposed alternative MCL-- -- 87 4.44 1,044 53.3 -- --

300 Proposed MCL -- -- 1,265 64.6 -- -- -- --

Gross alpha-particle radioactivity15 MCL1 -- -- 20 5.29 -- -- -- --

Gross beta-particle radioactivity50 Screening level2 -- -- 7 1.87 -- -- -- --

Radium-226 plus radium-228 5 MCL -- -- 10 2.28 -- -- -- --

1

The MCL for alpha-particle radioactivity excludes alpha-particle radioactivity from uranium or radon. Values reported for domestic wells in this study are 

not corrected for uranium or radon, but almost all radon is removed from the water sample during the preparation steps for sample analysis (Zoltan Szabo, 

U.S. Geological Survey, written commun., 2008).

2

Water samples from public supplies with gross beta-particle radioactivity greater than 50 pCi/L must have contributing radionuclides determined.

Table 8. Land-use classification used to categorize domestic wells sampled for the NAWQA Program in 

aquifer studies, 1991–2004.

[Modified from Gilliom and others (2006). Each well was classified according to the dominant land uses in a circular buffer area 

of 500-meter radius around the well, using land-use data and area-weighted land-use percentages as described in Nakagaki and 

Wolock (2005)]

Land-use 

category Land-use classification criteria

Agricultural Greater than 50 percent agricultural land and less than or equal to 5 percent urban land.

Urban Greater than 25 percent urban land and less than or equal to 25 percent agricultural land.

Undeveloped Less than or equal to 5 percent urban land and less than or equal to 25 percent agricultural land.

Mixed All other combinations of urban, agricultural, and undeveloped land.
40 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Figure 20. Geographic distribution of radon concentrations in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2004. >, greater than; ≤, less than or equal to.

than 4,000 pCi/L. The presence of high concentrations in 

ground water in these areas is well documented (Brutsaert 

and others, 1981; Hess and others, 1985; Loomis, 1987; 

Longtin, 1988; Lawrence and others, 1991; Senior, 1998; 

Ayotte and others, 2007). Relatively high radon concentrations 

are common in the crystalline-rock aquifers as a result of 

the prevalence of uranium-bearing minerals in granites, 

pegmatites, and their derivative metamorphic rocks and 

sediments (Otton and others, 1993; LeGrand, 1987). 

Enrichment of radium-226 along the surfaces of water-bearing 

fractures in these rocks also is a likely factor contributing to 

the elevated radon in these aquifers (Wood and others, 2004). 

Radon concentrations also were relatively high in several 

sandstone aquifers (fig. 21 and Appendix 5A). Uranium-rich 

reduced shales interbedded with the sandstone beds could 

be the source of radon (or the radium-226 parent) in these 

aquifers (Szabo and Zapecza, 1991; Becker and others, 2002). 

In wells completed in the crystalline-rock and sandstone 

aquifers and, to a lesser extent, in all wells, radon and uranium 

concentrations were positively correlated (Spearman’s rho 

equal to 0.469, 0.305, and 0.180, respectively, p values less 

than 0.0001). This correlation reflects the coincidence of radon 

in water and elevated concentrations of uranium in source 

rocks, and also the occurrence of geochemical conditions 

(alkaline and oxidizing) that favor uranium mobility in some 

crystalline-rock, sandstone, and other aquifers. 

Radionuclides and measures of radioactivity other than 

radon and uranium concentrations were determined for a 

subset of the domestic wells sampled in this study. Alpha- and 

beta-particle radioactivities and concentrations of the isotopes 

of radium were measured in samples from about 17 percent 

of the wells. These wells were completed in about one-half of 

Radon, in picocuries per liter

> 4,000

EXPLANATION

> 300 and ≤ 4,000 ≤ 300
Quality of Water from Domestic Wells 41

Figure 21. Radon concentrations in samples collected 

from domestic wells for the NAWQA Program in aquifer 

studies, 1991–2004, by aquifer rock type. BFSG, basin-fill 

and other non-glacial sand and gravel aquifers; GLAC, 

glacial sand and gravel aquifers; CP, coastal plain aquifers in 

semi-consolidated sand; SS, sandstone aquifers; SS/CARB, 

sandstone/carbonate-rock aquifers; CARB, carbonate-rock 

aquifers; BAS, basaltic-rock aquifers; CRYS, crystalline-rock 

aquifers; USEPA, U.S. Environmental Protection Agency; MCL, 

Maximum Contaminant Level.

the 30 principal aquifers sampled in this study (Appendix 5A). 

Alpha- and beta-particle radioactivities and concentrations of 

radium isotopes, like radon concentrations, are measured in 

activity units (pCi/L). Radioactive isotopes of uranium were 

not measured individually; uranium was measured chemically as concentrations of the total mass of uranium, which is 

consistent with the current mass-based USEPA MCL. Mass 

concentrations of uranium were described previously, along 

with those of other trace elements. The radium radionuclides 

are carcinogens that have adverse health risks upon ingestion 

and affect different body parts (primarily bone) than radon 

(Mays and others, 1985). 

Gross alpha-particle radioactivity is a measure of alphaparticle radiation emissions from all alpha-emitting radioactive 

sources in a water sample. It is a screening measurement that 

is used to determine high levels of alpha-particle radioactivity without identifying the specific radionuclides responsible 

for the activity. In most cases, alpha-particle radioactivity 

in ground water results from naturally occurring radionuclides, including uranium-238, uranium-234, radium-226, 

radium-224, radon-222, and other isotopes (Focazio and 

others, 2001). The USEPA MCL for alpha-particle radioactivity is 15 pCi/L, which applies to alpha-particle radioactivity 

exclusive of activity that is due to uranium or radon (U.S. 

Environmental Protection Agency, 2000b). Gross alphaparticle radioactivity reported here is not corrected for 

contributions for uranium or radon, but almost all radon is 

removed from the water sample during the preparation steps 

for sample analysis (Zolton Szabo, U.S. Geological Survey, 

written commun., 2008). In most (80 percent) of the domestic 

wells sampled in this study, alpha radioactivities ranged from 

less than 3 to 8.7 pCi/L (table 6), and activities were less than 

15 pCi/L in 94.7 percent of the wells (table 7). Wells in which 

alpha radioactivities were greater than 15 pCi/L (5.29 percent) 

were completed in the New England crystalline-rock aquifers; 

in the Early Mesozoic, Lower Tertiary/Upper Cretaceous, and 

Valley and Ridge sandstone aquifers; and in the Rio Grande 

basin-fill aquifer. In these aquifers, 4 to 19 percent of wells in 

which alpha-particle radioactivity was measured had activities 

greater than 15 pCi/L (Appendix 6A). Most of these aquifers 

also had relatively high concentrations of uranium.

Gross beta-particle radioactivity is a measure of betaparticle radiation emissions from all beta-emitting radioactive 

sources in a water sample. Like alpha-particle radioactivity, 

it is a screening measurement. Radionuclides that contribute 

to beta-particle radioactivity in ground water are naturally 

occurring and anthropogenic (Welch and others, 1995; U.S. 

Environmental Protection Agency, 2000b). Naturally occurring radionuclides are primarily potassium-40 and radium-228 

and, less frequently, lead-210 and short-lived progeny of 

uranium-238. The most common anthropogenic contributors 

include tritium (hydrogen-3), strontium-90 and cesium-137. 

The USEPA MCL for beta-particle radioactivity, 4 millirems 

per year, is given in dose-equivalent units, which measure 

the effect of the radiation on the body or organs. Concentrations of individual radionuclide species must be determined 

if measured beta-particle radioactivity, exclusive of naturally 

occurring postassium-40 activity, is greater than 50 pCi/L 

in public water supplies (Environmental Protection Agency, 

2000b, 2001b). In the domestic wells sampled in this study, 

beta-particle radioactivity ranged from less than 4 to 

10.8 pCi/L in most (80 percent) wells (table 6). Beta-particle 

radioactivity in seven wells, or 1.87 percent of all sampled 

wells, was greater than the screening level of 50 pCi/L for 

public water supplies (table 7). These wells were located in 

the New England crystalline-rock aquifers (5 wells) and in the 

Rio Grande basin-fill aquifer (2 wells; Appendix 5A), aquifers 

with relatively high concentrations of uranium.

EXPLANATION

Median

25th percentile

75th percentile

90th percentile

10th percentile

Number of samples 52

Proposed 

 USEPA MCLS

678

114

243

65

203 363

234

BFSG GLAC CP SS SS/

CARB

CARB BAS CRYS

15

10,000

0

8,000

6,000

4,000

2,000

RADON, IN PICOCURIES PER LITER
42 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Alpha- and beta-particle radioactivity were strongly 

correlated with uranium concentrations (Spearman rho equal 

to 0.528 and 0.335, respectively, p value less than 0.0001), and 

relatively high activities occurred in aquifers with relatively 

high uranium concentrations. Alpha-particle radioactivity 

also was correlated with radon concentrations (Spearman rho 

equal to 0.263, p value less than 0.0001). Uranium isotopes 

and radon, along with radium-226 and radium-224, are alphaparticle emitters. The association of beta-particle radioactivity 

with dissolved uranium may be due, in part, to the betaparticle emission from some uranium-238 progeny, but may 

also reflect the relatively high activities of other natural betaparticle emitters, such as radium-228 and potassium-40, in the 

same regional aquifers (Focazio and others, 2001).

Radium isotopes, which originate from the radioactive 

decay of uranium and thorium, occur as dissolved divalent 

cations in ground water (Zapecza and Szabo, 1986). Isotopes 

that occur frequently in ground water include radium-228, 

radium-226, and radium-224. The USEPA has established an 

MCL of 5 pCi/L for radium-226 and radium-228 combined. 

Radium-228 plus radium-226 were measured by several 

methods in the samples collected for the NAWQA Program. 

Concentrations of radium-226 plus radium-228 were less than 

5 pCi/L in all 97.7 percent of the 438 wells for which radium 

isotopes were measured. The wells with activities greater than 

5 pCi/L (2.28 percent) were completed in the North Atlantic 

and Coastal Lowlands coastal plain aquifers, the New England 

crystalline-rock aquifer, and the Early Mesozoic sandstone 

aquifer (Appendix 5A). Concentrations of combined radium 

greater than 5 pCi/L have been noted in these aquifers in previous studies of radionuclide occurrence (Szabo and Zapecza, 

1991; Focazio and others, 2001; Szabo and others, 2005). 

Organic Compounds

Anthropogenic organic compounds may be released 

to the environment from a variety of activities, including 

agriculture; combustion of fossil fuels; disposal of industrial, 

municipal, and domestic waste; and chemical spills. The 

occurrence of anthropogenic organic compounds in ground 

water depends on their use and sources, physical properties 

(solubility, volatility), mobility through soils and sediments, 

and the extent to which they are degraded chemically or 

biologically in the environment. Some organic compounds 

have been determined to be carcinogenic or have other 

adverse health effects (World Health Organization, 2004; 

U.S. Environmental Protection Agency, 2006a); the possible 

health effects of others are not known. Anthropogenic organic 

compounds in the environment commonly are categorized on 

the basis of use and properties as pesticides or volatile organic 

compounds (VOCs). Pesticides are substances used to kill 

or control undesirable plants, animals, or other organisms 

(Gilliom and others, 2006). VOCs are used for a variety of 

purposes but share properties that allow them to move readily 

into the air (Zogorski and others, 2006). Common VOCs 

include solvents, gasoline hydrocarbons, and refrigerants. 

In this study, 83 pesticides or pesticide breakdown products 

and 85 VOCs were analyzed. The complete list of analytes 

for pesticides and VOCs and numbers of wells in which each 

was analyzed are given in Appendix 4. About one-half of the 

pesticides and about two-thirds of the VOCs were measured 

in nearly all (about 90 percent or more, or about 2,000) of the 

sampled wells; other pesticides and VOCs were measured in 

about 60 percent (about 1,400 to 1,500) of the sampled wells. 

Many of the anthropogenic organic compounds that 

were analyzed in this study (henceforth, referred to as organic 

compounds) were detected in samples from domestic wells, 

but infrequently and at low concentrations (with typical analytical detection limits of 0.001 to 0.12 μg/L). If detections at 

any concentration are considered, 69 percent (116) of the 168 

organic compounds that were analyzed were detected. Most 

detections (83 percent) were at concentrations less than 

0.2 μg/L, and nearly all detections (96 percent) were less than 

1 μg/L. Some of these detections were at concentrations as 

low as 0.001 μg/L. Measured concentrations of 15 frequently 

detected compounds are shown in figure 22.

Whether or not a compound is detected depends on the 

laboratory analytical method detection limit as well as on 

the occurrence of the compound in the environment. When 

analytical methods with low detection limits are used, more 

compounds are detected, and detection frequencies are relatively high. In this study, detection limits for organic compounds varied among compounds and over time. Thus, the 

concentration data for organic compounds were reassessed in 

terms of common reporting levels, so that detection frequencies could be validly compared for different compounds and 

for the same compound in all samples. When concentration 

data are recast in terms of common reporting levels (fig. 23), 

some information is lost: when a higher common reporting 

level is used, more compounds and samples can be compared, 

but information about detections of some compounds (present 

at lower concentrations) in some samples is lost. Thus, several 

common reporting levels were used to describe the occurrence of organic compounds. For example, using a common 

reporting level of 0.2 μg/L, 34 percent (54) of the organic 

compounds that were analyzed were detected. At a lower 

common reporting level, 0.02 μg/L, 55 percent (86) of the analyzed compounds were detected (11 and 12 compounds were 

not assessed at 0.2 and 0.02 μg/L, respectively), and as stated 

previously, when no common reporting level was used (that is, 

a detection at any concentration counts, with typical analytical 

detection limits of 0.001 to 0.1 μg/L), 69 percent (116) of the 

analyzed compounds were detected. 

Organic compounds were detected in many of the 

sampled wells (figs. 24 and 25). When detections at any concentration were considered, one or more organic compounds 

were detected in 60.1 percent of wells. Above a common 

reporting level of 0.02 μg/L, one or more organic compounds 

were detected in 41.5 percent of wells; at higher concentrations, above a common reporting level of 0.2 μg/L, one or 

more organic compounds were detected in 14.4 percent of 
Quality of Water from Domestic Wells 43

Chloroform

Atrazine

Methyl tert-butyl ether

Deethylatrazine

Perchloroethane

Carbon disulfide

1,1,1-Trichloroethane

Toluene

Metolachlor

Dichlorodifluoromethane

CONCENTRATION, IN MICROGRAMS PER LITER

Bromodichloromethane

Bentazon

Trichlorofluoromethane

Methylene chloride

Trichloroethene

Measured concentration

Non-detect, actual concentration 

 is less than the value shown

EXPLANATION

Concentrations Human-health benchmarks

MCL or HBSL

One-tenth of MCL or HBSL

USEPA drinking-water advisory range

 for taste and odor

0.0001 0.001 0.01 0.1 1 10 100

Concentration below CRL of 0.02 μg/L

Concentration below CRL of 0.2 μg/L

MCL or HBSL is greater than 100 μg/L

Figure 22. Concentrations of frequently detected organic compounds in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2004. Data points for non-detects overlap and thus represent multiple values. 

Names of pesticides are shown in brown, and names of volatile organic compounds are shown in blue. CRL, common 

reporting level; MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level; HBSL, Health-Based 

Screening Level; μg/L, microgram per liter.
44 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Chloroform

Atrazine

Methyl tert-butyl ether

Deethylatrazine

Perchloroethene

Carbon disulfide

1,1,1-Trichloroethane

Toluene

Metolachlor

Trichlorofluoromethane

1,2-Dichloropropane

Dichlorodifluoromethane

1,2,3-Trichloropropane

Trichloroethene

Methylene chloride

Bromodichloromethane

Bentazon

Prometon

Dibromochloromethane

Bromacil

1,2,4-Trimethylbenzene

Diuron

Bromoform

Simazine

Carbon tetrachloride

Benzene

1,1-Dichloroethane

cis-1,2-Dichloroethene

1,2-Dichloroethane

Napthalene

Ethylene dibromide

1,2,3,4-Tetramethylbenzene

tert-Amyl methyl ether

Terbacil

Diazinon

Aldicarb sulfoxide

Dinoseb

Chloroethane

Trichlorotrifluoroethane

1,1-Dichloroethene

Carbofuran

Alachlor

Diisopropyl ether

Diethyl ether

2-Ethyltoluene

Fenuron

1,2-Dichlorobenzene

Vinyl chloride

1,4-Dichlorobenzene

trans-1,2-Dichloroethene

Chlorobenzene

Carbaryl

Dibromochloropropane

Isopropylbenzene

Common Reporting 

 Level

0.2 μg/L

0.02 μg/L

0

DETECTION FREQUENCY, IN PERCENT

2 4 6 8 10 12 14 16 18

Figure 23. Detection frequency for frequently occurring organic compounds in samples collected 

from domestic wells for the NAWQA Program in aquifer studies, 1991–2004. Detection frequencies are 

shown relative to common reporting levels of 0.2 and 0.02 micrograms per liter (μg/L); all compounds 

with detections above the 0.2 μg/L common reporting level are shown. Names of pesticides are shown in 

brown, and names of volatile organic compounds are shown in blue. Detection frequencies for toluene 

and 1,2,4-trimethylbenzene were evaluated at an assessment level of 0.3 and 0.05 μg/L, respectively, in 

order to limit the estimated probability of false detections due to random sample contamination to less 

than 1 percent, and thus are likely to underestimate the actual occurrence at or greater than 0.02 μg/L.
Quality of Water from Domestic Wells 45

wells. VOCs were detected in more wells than pesticides at all 

common reporting levels (fig. 24). The frequency of detection of organic compounds in the sampled domestic wells is 

consistent with previous studies of low-level concentrations 

of organic compounds in ground water (Shapiro and others, 

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

of pesticides and VOCs from all well types sampled for the 

NAWQA Program (Gilliom and others, 2006; Zogorski and 

others, 2006). Geographic patterns of occurrence or patterns 

among principal aquifers were not apparent for organic compounds overall (figs. 2 and 25; Appendix 7). 

The most frequently detected organic compounds 

included both pesticides and VOCs (fig. 23). Compounds 

detected in 1 percent or more of wells above the common 

reporting level of 0.2 μg/L were chloroform, a trihalomethane or disinfection byproduct; the gasoline oxygenate methyl 

tert-butyl ether (MTBE); dichlorodifluoromethane, a refrigerant; perchloroethene, a solvent; and the agricultural herbicide 

atrazine and its degradate deethylatrazine. These compounds 

were detected in 1.13 to 5.49 percent of wells at concentrations above 0.2 μg/L. Using a lower common reporting level 

of 0.02 μg/L, these six compounds were still among the most 

commonly detected but at higher detection frequencies, which 

ranged from 3.13 percent to 17.9 percent (fig. 23). At the 

lower common reporting level, many other compounds also 

were detected in 1 percent or more of wells. These included 

additional gasoline hydrocarbons (1,2,4-trimethylbenzene and 

toluene), additional solvents (1,1,1-trichloroethane, methylene

chloride, trichloroethene, and 1,1-dichloroethane), additional 

herbicides (simazine, bentazon, prometon, and metolachlor), 

a fumigant (1,2-dichloropropane), another refrigerant (trichlorofluoromethane), VOCs used in organic synthesis (chloromethane, carbon disulfide, and iodomethane) and additional 

disinfection byproducts (bromodichloromethane and dibromochloromethane). Detection frequencies for all compounds 

are given in Appendix 7. These results indicate that a wide 

variety of organic compounds, encompassing many uses, can 

be found in domestic wells.

Concentrations of organic compounds at concentrations 

of potential concern for human health were rarely present in 

samples from the domestic wells in this study. Four VOCs and 

one pesticide—dibromochloropropane (DBCP, a fumigant), 

ethylene dibromide, perchloroethene, trichloroethene, and 

dinoseb—were present in concentrations greater than MCLs, 

and two pesticides—diazinon and dieldrin—were present 

in concentrations greater than HBSLs. Each compound 

was present in these concentrations in less than 1 percent 

of wells (table 9). Samples from 17 wells, or 0.80 percent 

of the sampled domestic wells, had concentrations of one 

or more organic compounds greater than MCLs or HBSLs 

(fig. 25C; 0.33 percent greater than MCLs and 0.47 percent 

greater than HBSLs). Ten additional compounds occurred 

in concentrations less than, but greater than one-tenth of, an 

MCL (9 compounds) or HBSL (1 compound). These included 

the pesticides alachlor, atrazine, and diuron and the VOCs 

benzene, chloroform, 1,2-dichloroethane, 1,1-dichloroethene, 

1,2-dichloropropane, methylene chloride, and vinyl chloride; 

each compound was present in less than 1 percent of wells 

at these concentrations (table 9). Samples from 63 wells, 

or 2.95 percent, contained compounds with concentrations 

greater than one-tenth of MCLs or HBSLs (2.34 percent of 

wells for compounds with MCLs and 0.66 percent of wells for 

compounds with HBSLs). Concentrations greater than onetenth of benchmarks may be of interest for prioritizing further 

study or increased monitoring. 

Although the various pesticides and VOCs measured 

in this study have many different sources and transport 

characteristics in ground water, they all originate from 

anthropogenic sources (though a few also have natural 

sources). Thus, their occurrence in the sampled domestic 

wells was associated with several factors that describe human 

influence or aquifer vulnerability. Several characteristics 

(fig. 3) indicative of the proximity of ground water in the 

wells to land surface were compared for wells in which 

organic compounds were and were not detected. Wells with 

detections of organic compounds at any concentration were 

Figure 24. Percentage of samples in which one or more 

organic compounds were detected, at any concentration and 

at common reporting levels of 0.02 and 0.2 micrograms per liter 

(μg/L), for domestic wells sampled for the NAWQA Program, 

1991–2004, in aquifer studies. A common reporting level is 

a concentration above which a measured concentration is 

considered a detection. Total number of wells for pesticides, 

2,126 wells for all detection criteria; for volatile organic 

compounds, 1,957 wells for any concentration and for 0.2 μg/L, 

and 1,538 wells for 0.02 μg/L; and for pesticides and VOCs, 

2,134 wells for any detection and for 0.2 μg/L, and 2,132 wells for 

0.02 μg/L. DETECTION FREQUENCY, IN PERCENT OF SAMPLES

Any

concentration

0

80

40

100

20

60

Common

reporting

level of

0.02 μg/L

 Common

reporting

level of

0.2 μg/L

Pesticide compounds

VOCS

Pesticides or VOCS

CONCENTRATION CRITERIA FOR DETECTION
46 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Figure 25A. Geographic distribution of organic compounds in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2004, at (A) detections above a common reporting level of 0.2 microgram 

per liter (μg/L), (B) detections above a common reporting level of 0.02 μg/L, and (C) concentrations greater than 

human-health benchmarks. VOCs, volatile organic compounds.

not as deep and had open intervals that were not as far below 

land surface as those of wells in which no organic compounds 

were detected (Mann-Whitney test, p values less than 0.0001); 

these associations also were significant for pesticides and 

VOCs separately (except for VOCs and well depth). Average 

population density in the area surrounding the wells during 

1990–2000 was greater for wells with detections of pesticides, 

VOCs, or any organic compound that for wells with no 

detections of any organic compounds (Mann-Whitney tests, 

p values less than 0.001). The areas surrounding the wells with 

pesticide detections consisted of less undeveloped land and 

more agricultural land than those without pesticide detections 

(Mann-Whitney test, p values less than 0.0001). The areas 

surrounding the wells with VOC detections consisted of 

greater amounts of urban (residential or commercial) land than 

those without VOC detections (Mann-Whitney test, p value 

less than 0.0001). The effects of land use also were apparent 

from the relatively high frequencies of detections of pesticides 

in domestic wells in NAWQA agricultural land-use studies 

(64 percent), which was more than twice as frequent as in 

domestic wells from NAWQA aquifer studies (29 percent).

The occurrence of organic compounds, particularly at 

concentrations approaching levels of potential concern for 

human health, also may be associated with relatively high concentrations of nitrate in some of the domestic wells sampled 

in the present study. The presence of nitrate, which at elevated 

concentrations (greater than 1 mg/L as N) typically results 

from anthropogenic sources, was widespread and, like organic 

compounds, was associated with developed land; concurrent 

sources may have included wastewater disposal and the use 

of fertilizer and pesticides together. Median concentrations 

of nitrate, 3.7 mg/L as N, in wells with organic compounds 

A

Detections of organic compounds at concentrations greater than 0.2 μg/L

Pesticide(s) only

EXPLANATION

VOC(s) only Neither pesticides nor VOC(s)

Both pesticide(s) and VOC(s) 
Quality of Water from Domestic Wells 47

at concentrations greater than one-tenth of benchmarks were 

significantly higher than concentrations of nitrate in wells with 

organic compounds at lower concentrations or not detected 

(0.52 mg/L as N, p value < 0.0001, Mann-Whitney test). The 

differences in concentrations of nitrate among wells also 

were apparent when pesticides and VOCs were considered 

separately (fig. 26). Nitrate concentrations also were higher 

in all wells in which organic compounds were detected at 

any concentration (median 0.72 mg/L as N, including those 

at concentrations greater than one-tenth of benchmarks and 

below benchmarks) than in wells without detectable organic 

compounds (median 0.34 mg/L as N). Similar patterns were 

apparent in the 424 domestic wells sampled for the NAWQA 

Program in agricultural land-use studies (fig. 1). Nitrate 

concentrations were higher overall but were significantly 

greater in wells with organic compounds at concentrations 

greater than one-tenth of benchmarks (11 mg/L as N) than in 

wells in which organic compounds were not present at these 

levels (3.9 mg/L as N). Although nitrate concentrations alone 

would not be a strong predictor of the presence of organic 

compounds—only 4.6 percent of wells in aquifer studies with 

nitrate concentrations greater than 1 mg/L as N had organic 

compounds at concentrations greater than one-tenth of benchmarks—the tendency for organic compounds to be associated 

with relatively high concentrations of nitrate suggests that, in 

some cases, nitrate concentrations may be a useful screening 

tool for the presence of organic compounds at concentrations 

potentially of concern for human health. 

Atrazine is an agricultural herbicide used primarily on 

corn and soybeans. It also is used to control vegetation in 

non-agricultural areas (U.S. Environmental Protection Agency, 

1990a; Gilliom and others, 2006). Atrazine was the most 

B

Detections of organic compounds at concentrations greater than 0.02 μg/L

Pesticide(s) only

EXPLANATION

VOC(s) only Neither pesticides nor VOC(s)

Both pesticide(s) and VOC(s) 

Figure 25B. Geographic distribution of organic compounds in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2004, at (A) detections above a common reporting level of 0.2 microgram 

per liter (μg/L), (B) detections above a common reporting level of 0.02 μg/L, and (C) concentrations greater than 

human-health benchmarks. VOCs, volatile organic compounds.—Continued
48 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

heavily applied agricultural pesticide in the United States in 

recent years. It was used most intensively in the Midwest but 

also was used at various rates throughout the eastern half 

of the United States and in scattered areas of the West 

(Gilliom and others, 2006). Atrazine is relatively persistent in 

the environment, compared to other heavily used pesticides, 

and is mobile in water. Atrazine and its degradate deethylatrazine were the most frequently detected pesticides in the 

domestic wells sampled in this study, and were the fifth and 

sixth most frequently detected organic compounds overall 

(fig. 23). At detection limits as low as 0.003 to 0.004 μg/L, 

atrazine and deethylatrazine were detected in 15.1 and 

17.9 percent of sampled wells, respectively. At higher common 

reporting levels of 0.02 and 0.2 μg/L, each was detected in 

about 6 percent (0.02 μg/L) and in about 1 percent (0.2 μg/L) 

of wells, respectively. Nearly all concentrations (98 percent) 

were less than 1 μg/L (fig. 22). The USEPA MCL for atrazine 

is 3 μg/L. No measured concentration of atrazine was greater 

than 3 μg/L, but concentrations were greater than 0.3 μg/L 

(one-tenth of the MCL) in 0.85 percent of all sampled wells. 

Geographically, atrazine and deethylatrazine were 

detected in wells throughout the United States (fig. 27), but 

detections were more frequent in wells in areas of agricultural and urban land use. Wells in predominantly agricultural, 

urban, or mixed land-use areas (table 8) that were sampled 

for aquifer studies had detection frequencies for atrazine and 

deethlyatrazine (9.9 to 26 percent) that were 2 to 4 times 

higher than detection frequencies (5.2 to 7 percent) for wells 

in areas of undeveloped land (table 8). In wells sampled for 

NAWQA studies targeted at agricultural areas (fig. 1, wells not 

Figure 25C. Geographic distribution of organic compounds in samples collected from domestic wells for the NAWQA 

Program in aquifer studies, 1991–2004, at (A) detections above a common reporting level of 0.2 microgram per liter (μg/L), 

(B) detections above a common reporting level of 0.02 μg/L, and (C) concentrations greater than human-health benchmarks. 

VOCs, volatile organic compounds.—Continued

Concentrations greater than 

 human-health benchmarks

One or more pesticide(s)

EXPLANATION

Concentrations of pesticides and 

 VOCs less than one-tenth 

 of human-health benchmarks

Concentrations greater than one-tenth 

 of human-health benchmark

One or more pesticide(s)

One or more VOC(s) One or more VOC(s)

C
Quality of Water from Domestic Wells 49

Table 9. Comparison of concentrations of organic compounds in samples collected from domestic wells for the NAWQA Program in 

aquifer studies, 1991–2004, with human-health benchmarks for drinking water.

[MCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels for public water supplies; HBSLs are Health-Based Screening 

Levels developed by the U.S. Geological Survey using USEPA toxicity data and methods (Toccalino and Norman, 2006); μg/L, microgram per liter. Only those 

compounds with concentrations greater than benchmarks or greater than one-tenth of benchmarks are shown]

Wells with concentrations greater than human-health benchmarks or 

greater than one-tenth of benchmarks Human-health benchmark

Organic compound One-tenth of human- Human-health benchmark health benchmark

Value (μg/L) Type Number of wells Percent of wells Number of wells Percent of wells

Pesticides

Alachlor 2 MCL 0 0.00 2 0.09

Atrazine 3 MCL 0 0.00 18 0.85

Diazinon 1 HBSL 1 0.05 3 0.14

Dieldrin 1

0.002 HBSL 9 0.43 9 0.43

Dinoseb 7 MCL 1 0.07 1 0.07

Diuron 1

2 HBSL 0 0.00 2 0.15

Volatile organic compounds

Benzene 5 MCL 0 0.00 1 0.05

Chloroform 2

80 MCL 0 0.00 3 0.15

Dibromochloropropane (DBCP) 0.2 MCL 1 0.05 1 0.05

1,2-Dichloroethane 5 MCL 0 0.00 2 0.10

1,1-Dichloroethene 7 MCL 0 0.00 1 0.05

1,2-Dichloropropane 5 MCL 0 0.00 2 0.10

Ethylene dibromide 0.05 MCL 2 0.10 2 0.10

Methylene chloride 5 MCL 0 0.00 5 0.26

Perchloroethene 5 MCL 1 0.05 10 0.52

Trichloroethene 5 MCL 2 0.10 6 0.31

Vinyl chloride 2 MCL 0 0.00 1 0.05

1

Value is low end of range, associated with 10-6 cancer risk (Toccalino and others, 2006).

2

MCL is for sum of four trihalomethanes.

shown in fig. 27), atrazine and deethylatrazine were detected chlorpyrifos were detected most frequently in areas of wheat 

even more frequently, in about 50 percent of the sampled wells and alfalfa agriculture. Detection frequencies of individual 

(Appendix 7B); concentrations of atrazine greater than pesticides also were found to be related to their water solubil0.3 μg/L were present in 9.2 percent of the agricultural land- ity, tendency to sorb onto aquifer materials, and time needed 

use wells. for degradation (Kolpin and others, 1998; Barbash and others, Several studies of pesticides in ground water investigated 1999, 2001; Gilliom and others, 2006). For example, meto- the distribution of individual pesticides such as atrazine; these lachlor degrades more quickly in soils than atrazine and was studies used data from all well types sampled for the NAWQA detected less frequently in areas of corn and soybean agricul- Program. The areas where, and rates at which, the compounds ture, although both compounds were used in about the same were used were found to greatly affect the occurrence and amounts in these areas. Metolachlor also was detected much concentration of the compounds (Kolpin and others, 1998; 

Gilliom and others, 2006). For example, atrazine, metalachlor, less frequently than atrazine in the domestic wells included in 

and acetochlor were detected most frequently in areas of corn the present study (fig. 23 and Appendix 7). A detailed invesand soybean agriculture; molinate, an insecticide, was detected tigation at the national scale found that soil permeability, artifimost frequently in areas of rice agriculture; and 2,4-D and cial subsurface drainage (tile drains), and irrigated agriculture 
50 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Figure 26. Concentrations of nitrate and organic compounds in samples collected from domestic wells 

for the NAWQA Program in aquifer studies, 1991–2004. VOC, volatile organic compound.

Figure 27. Geographic distribution of atrazine and deethylatrazine in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2004. CRL, common reporting level; μg/L, microgram per liter. 

EXPLANATION

Atrazine or deethylatrazine detection

Detected at concentration above CRL of 0.2 μg/L

Detected at any concentration

Not detected at any concentration

Detected at concentration above CRL of 0.02 μg/L

Atrazine concentration compared 

 to human-health benchmark 

Concentration greater than

 one-tenth of human-health 

 benchmark

EXPLANATION

Median

25th percentile

75th percentile

90th percentile

10th percentile

Number of samples 52

Pesticides

or VOCs

34

2,055

63

2,084 2,089

NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER AS N

Pesticides VOCs

29

Pesticide and (or) VOC

 concentrations—

Less than one-tenth of

 human-health benchmarks

 or not detected

Greater than one-tenth of

 human-health benchmarks

14

12

10

8

0

4

6

2

16

ORGANIC COMPOUNDS
Quality of Water from Domestic Wells 51

were important factors affecting the distribution of atrazine in 

wells (Kolpin, 1997).

Chloroform was the most frequently detected organic 

compound in the domestic wells sampled in this study 

(fig. 23). Chloroform, along with bromodichloromethane, 

dibromochloromethane, and bromoform, is a byproduct 

of the disinfection of water with chlorine (Zogorski and 

others, 2006); chloroform occurs naturally as well. Chlorine 

and organic material in the water react to form these 

trihalomethane compounds. Chloroform is used in industry 

and in the synthesis of other organic compounds. It may be 

present in ground water as a result of infiltration of treated 

wastewater or chlorinated, publicly supplied drinking water; 

shock chlorination of domestic wells with chlorine bleach; 

the release of chlorine-containing products through septic 

tanks; or inadvertent spills (Ivahnenko and Zogorski, 2006). 

Chloroform was detected in about 20 percent of the wells 

at any concentration and at concentrations greater than a 

common reporting level of 0.02 μg/L; at concentrations 

greater than a common reporting level of 0.2 μg/L, it was 

detected in 5.5 percent of wells. Chloroform concentrations, 

however, rarely approached levels of concern for human 

health (fig. 22). Concentrations of chloroform either alone, 

or combined with concentrations of other trihalomethanes, 

were greater than one-tenth of the USEPA MCL of 80 μg/L 

for all trihalomethanes in only three wells (0.15 percent). 

Geographically, chloroform was detected throughout 

the United States (fig. 28). The other trihalomethanes—

bromodichloromethane, dibromochloromethane, and 

bromoform—were detected much less frequently and at 

lower concentrations than chloroform; these were detected 

in less than 2 percent of wells at any concentration and 

in less than 0.5 percent of wells at concentrations greater 

than 0.2 μg/L (Appendix 5). Bromodichloromethane, 

EXPLANATION

Chloroform detection

Detected at concentration above CRL of 0.2 μg/L

Detected at any concentration

Not detected at any concentration

Detected at concentration above CRL of 0.02 μg/L

Chloroform concentration compared 

 to human-health benchmark 

Concentration greater than one-tenth of

 human-health benchmark

Figure 28. Geographic distribution of chloroform in samples collected from domestic wells for the NAWQA 

Program in aquifer studies, 1991–2004. CRL, common reporting level; μg/L, microgram per liter. 
52 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Figure 29. Geographic distribution of methyl tert-butyl ether (MTBE) in samples collected from domestic wells for 

the NAWQA Program in aquifer studies, 1991–2004. CRL, common reporting level; μg/L, microgram per liter. 

dibromochloromethane, and bromoform are formed at lower 

concentrations than chloroform during water chlorination 

and are produced in much lower quantities in industry than 

chloroform (Zogorski and others, 2006).

MTBE is an oxygen-containing compound (oxygenate) 

that has been added to gasoline to improve combustion, reduce 

emissions, and increase octane. Oxygenated gasoline was 

required in the late 1990s in some parts of the United States 

where air-quality standards were not met; MTBE was the 

most common oxygenate additive (Moran and others, 2005; 

Zogorski and others, 2006). MTBE is more soluble, less likely 

to sorb to aquifer materials, and less readily degraded than 

most other components of gasoline and, thus, is relatively 

mobile in ground water. MTBE was detected in about 

5 percent of wells at any concentration and at concentrations 

greater than the common reporting level of 0.02 μg/L; at 

concentrations greater than 0.2 μg/L, it was detected in 

2.7 percent of wells. It was the second most frequently 

detected organic compound at concentrations greater than 

0.2 μg/L (fig. 23). MTBE detections were concentrated in the 

Northeast and mid-Atlantic regions (fig. 29), areas of dense 

population and high use of MTBE in gasoline. There is no 

MCL or HBSL for MTBE, but the USEPA Drinking Water 

Advisory recommends that concentrations not exceed 20 to 

40 μg/L to avoid unpleasant odor or taste (U.S. Environmental 

Protection Agency, 1997b). Concentrations in only one well 

were greater than the lower limit of this range (figs. 22 and 

29). MTBE was detected much more frequently than other 

gasoline oxygenates (for example, tert-Amyl methyl ether 

or ethyl tert-butyl ether, Appendix 4), which were detected 

in less than 0.5 percent of wells at all reporting levels 

(Appendix 7) and were less commonly used in gasoline 

EXPLANATION

Methyltert-butyl ether detection

Detected at concentration above CRL of 0.2 μg/L

Detected at any concentration

Not detected at any concentration

Detected at concentration above CRL of 0.02 μg/L

Methyl tert-butyl ether concentration

 compared to water-quality recommendation

Concentration greater than U.S. Environmental 

 Protection Agency drinking-water advisory

 for taste and odor (greater than the lower limit

 of the range, 20 μg/L)
Quality of Water from Domestic Wells 53

(Moran and others, 2005). MTBE also was more frequently 

detected than any gasoline hydrocarbon, such as benzene, 

toluene, ethylbenzene, or xylenes (“BTEX” compounds). 

BTEX compounds were detected in less than 2 percent of 

wells at all reporting levels (Appendix 7). In studies of MTBE 

in ground water that included domestic wells and wells 

of other types sampled for the NAWQA Program, MTBE 

occurrence was primarily related to population density, 

MTBE use in gasoline, and recharge; soil permeability, 

aquifer consolidation, and the number of leaking underground 

gasoline storage tanks also were related but less strongly than 

the other three factors (Ayotte and others, 2005; Moran and 

others, 2005). At a local scale, MTBE occurrence also has 

been shown to be related to the proximity of gasoline stations 

(Lince and others, 2001).

Solvents were among the most frequently detected 

organic compounds in the domestic wells sampled in this 

study. Solvents have many uses in industry, business, and the 

home and can enter ground water through accidental spills, 

improper disposal, wastewater discharge, or septic-tank 

leachate. Perchloroethene, 1,1,1-trichloroethane, methylene 

chloride, trichloroethene, and chloromethane were the most 

frequently detected solvents. These solvents were detected 

in about 2.5 to 8 percent of wells at any concentration and at 

concentrations greater than the common reporting level of 

0.02 μg/L; at concentrations greater than 0.2 μg/L, they 

were detected in 1.2 percent of wells or less (fig. 23 and 

Appendix 7). Detections occurred throughout the United 

States. Concentrations were greater than MCLs or HBSLs 

(MCLs of 5 μg/L for perchloroethene and trichloroethene) 

in only three wells. In 21 wells, or 1 percent of all wells, 

concentrations were greater than one-tenth of MCLs or 

HBSLs (table 9 and fig. 22). In an analysis of four of these five 

solvents in ground water sampled for the NAWQA Program, 

population density and dissolved oxygen concentrations 

were important factors related to their occurrence. Dissolved 

oxygen concentrations were relevant because the compounds 

can be degraded under anoxic conditions. Other variables 

that were less strongly associated with the occurrence of the 

four solvents were the sand content of the soil in the area 

surrounding the well, the depth to the top of the screened 

interval, and the number of hazardous waste sites or septic 

tanks near the well (Moran and others, 2007). 

Fecal Indicator Bacteria

Fecal indicator bacteria live in the intestines of warmblooded animals; their presence in water can signal fecal or 

sewage contamination (World Health Organization, 2004). 

These bacteria typically are not harmful but are more easily 

monitored than the pathogenic bacteria or viruses that also can 

occur in feces and may cause illness. Fecal indicator bacteria 

may be introduced into domestic wells from surface sources 

near the wellhead, especially if the well is not sealed or its 

casing is not intact. Indicator bacteria also may be transported 

through the subsurface from sources such as septic tanks or 

recharge contaminated with livestock wastes.

Fecal indictor bacteria measured in the domestic wells 

sampled in this study were total coliform bacteria and 

Escherichia coli (E. coli). Coliform bacteria are a broad group 

that can live in water and soils and may be present in the environment from sources other than fecal or sewage contamination. E. coli is a member of the total coliform group of bacteria 

that is almost always found only in feces, and consequently is 

a specific indicator of fecal or sewage contamination (World 

Health Organization, 2004; Embrey and Runkle, 2006). E. coli

is the dominant species of the fecal coliform group. Federal 

and many state drinking-water standards use total coliform 

bacteria and fecal coliform bacteria and (or) E. coli bacteria as 

indicators of microbiological water quality. USEPA drinkingwater standards for public-supply systems require sampling 

for total coliform bacteria at frequencies that differ with the 

size of the system. No more than 5 percent of samples per 

month may have detections of total coliform bacteria (U.S. 

Environmental Protection Agency, 2006a). Repeat sampling 

and analysis for fecal coliform or E. coli are required when 

total coliform bacteria are detected; no fecal coliform or 

E. coli bacteria are allowed under the USEPA MCL. 

Total coliform bacteria were measured in samples from 

397 wells, and E. coli were measured in samples from 378 

wells in aquifer studies (table 6); the wells were completed in 

15 principal aquifers (Appendix 5–1). Fecal indicator bacteria are measured as the number of colony-forming units in 

100-milliliter water samples. A complete description of the 

microbiological sampling program and results for NAWQA 

ground-water studies, including the data used in the present 

study, was provided by Embrey and Runkle (2006). Total coliform and E. coli bacteria also were measured in samples from 

86 and 55 wells, respectively, in agricultural land-use studies; 

these wells were located in two carbonate-rock aquifers in the 

Appalachian region (Appendix 5–2). 

Total coliform bacteria were detected in 33.5 percent 

of the domestic wells sampled wells in aquifer studies, and 

E. coli bacteria were detected in 7.9 percent of the wells. 

Geographically, indicator bacteria were detected in wells from 

each of the principal aquifers from which wells were sampled 

for bacteria (figs. 30 and 31). Detections were more frequent 

in samples from the carbonate- and crystalline-rock aquifers 

than in those from other aquifers (fig. 31), consistent with 

findings of Embrey and Runkle (2006) for all well types. Total 

coliform bacteria were detected in more than one-half of the 

sampled wells in several aquifers, including the Central Valley 

basin-fill aquifer, the North Atlantic coastal plain aquifer, 

the Valley and Ridge sandstone aquifer, and the Piedmont 

crystalline-rock aquifer. Wells in which total coliform and 

E. coli bacteria were detected tended to have open intervals 

that were closer to land surface than the open intervals of 

wells in which the bacteria were not detected (Mann Whitney 

test, p values equal to 0.010 and 0.005); wells with detections 

of total coliform bacteria also were older and had higher 

percentages of agricultural land in surrounding areas (p values 
54 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Figure 30. Geographic distribution of fecal indicator bacteria detected in samples collected from domestic wells 

for the NAWQA Program in aquifer studies, 1991–2004.

Figure 31. Fecal indicator bacteria detected in samples 

collected from domestic wells for the NAWQA Program 

in aquifer studies, 1991–2004, by aquifer rock type. BFSG, 

basin-fill and other non-glacial sand and gravel aquifers; 

GLAC, glacial sand and gravel aquifers; CP, coastal plain 

aquifers in semi-consolidated sand; SS, sandstone aquifers; 

SS/CARB, sandstone/carbonate-rock aquifers; CARB, 

carbonate-rock aquifers; BAS, basaltic-rock aquifers; CRYS, 

crystalline-rock aquifers. Numbers above bars are numbers 

of samples.

Detections of fecal indicator bacteria

EXPLANATION

Total coliform bacteria detected Both total coliform and Escherichia coli bacteria detected 

Escherichia coli bacteria detected No detections of either total coliform or Escherichia coli bacteria 

Total coliform bacteria

Escherichia coli

BFSG GLAC CP SS SS/

CARB

BAS CRYS

DETECTION FREQUENCY,

IN PERCENT OF WELLS

100

80

60

40

20

0

101

64

57

72

5

19 88 43 43 16

50

75

0

37

56

48

CARB
Quality of Water from Domestic Wells 55

less than 0.0001). The effects of land use also were apparent 

in the detection frequencies of total coliform and E. coli

bacteria in wells included in the NAWQA agricultural landuse studies, which at 74.4 and 47.3 percent of sampled wells 

(Appendix 5–2), respectively, were higher than detection 

frequencies in the wells sampled as part of aquifer studies.

Summary Comparison of Contaminant 

Concentrations to Human-Health Benchmarks

Concentrations of nearly all contaminants in water 

from domestic wells sampled in this study, when considered 

individually, were less than available MCLs and HBSLs in 

at least 90 percent of wells. Contaminants that occurred most 

frequently (in more than 1 percent of wells) at concentrations 

greater than MCLs or HBSLs were: radon (in 64.6 and 

4.36 percent of wells for the lower and higher proposed 

MCLs of 300 and 4,000 pCi/L, respectively); nitrate 

(4.44 percent, MCL of 10 mg/L as N); the trace elements 

arsenic (6.75 percent, MCL of 10 μg/L), boron (1.30 percent, 

HBSL of 1,000 μg/L), strontium (7.32 percent, HBSL of 

4,000 μg/L), manganese (5.22 percent, HBSL of 300 μg/L) 

and uranium (1.74 percent, MCL of 30 μg/L); fluoride 

(1.16 percent, MCL of 4 μg/L), gross alpha- (uncorrected) and 

beta-particle radioactivity (5.29 and 1.87 percent; MCL of 

15 pCi/L corrected for radon and uranium and screening 

level MCL of 50 pCi/L, respectively) (tables 3, 5, and 7). The 

fecal indicator bacteria, total coliform bacteria and E. coli, 

were detected in 33.5 and 7.9 percent of wells, respectively 

(table 6). Anthropogenic organic compounds were frequently 

detected, but were present at concentrations greater than 

MCLs or HBSLs in only 0.8 percent of wells for all compounds (0.3 percent of wells for MCLs and 0.5 percent of 

wells for HBSLs). Thus, with the exception of nitrate and 

fecal indicator bacteria, the contaminants that occurred most 

frequently at concentrations greater than human-health benchmarks in the sampled domestic wells were naturally occurring.

Collectively, at least one contaminant was present at 

concentrations greater than MCLs or HBSLs in 22.6 percent 

of wells, using the higher proposed MCL for radon (table 10). 

In 19.2 percent of wells, these contaminants were naturally 

occurring. Relative to specific types of benchmarks, concentrations of at least one contaminant were greater than an MCL 

(using action levels for copper and lead) or an HBSL in 13.2 

and 8.14 percent of wells, respectively (table 10). Nearly all 

wells (93.9 percent) had at least one contaminant at concentrations greater than one-tenth of benchmarks (using the lower 

proposed MCL for radon; table 10). These results were based 

Table 10. Number and percentage of domestic wells, in a subset of 1,389 domestic wells from the NAWQA Program in 

aquifer studies, 1991–2004, containing one or more contaminants at concentrations greater than a human-health benchmark 

or greater than one-tenth of a benchmark.

[MCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels for public water supplies and include action levels for 

copper and lead; HBSLs are Health-Based Screening Levels developed by the U.S. Geological Survey using USEPA toxicity data and methods 

(Toccalino and Norman, 2006). Numbers for MCLs, proposed MCLs, and HBSLs do not add up to totals because some wells have concentrations greater than more than one type of benchmark. Data are from 1,389 domestic wells in which major ions, trace elements, nitrate, and organic 

compounds were measured. pCi/L, picocuries per liter; --, not determined]

Human-health benchmark

Wells with concentrations greater than human-health benchmarks or 

greater than one-tenth of benchmarks

Human-health benchmark One-tenth of humanhealth benchmark

Number of wells Percentage of wells Number of wells Percentage of wells

MCLs 184 13.2 837 60.3

Proposed MCL

Higher value of 4,000 pCi/L for radon 59 4.2 -- --

Lower value of 300 pCi/L for radon 902 64.9 -- --

HBSLs 113 8.14 632 45.5

All human-health benchmarks 

(MCLs, proposed MCLs1, and HBSLs)

314 22.6 1,304 93.9

1

Including radon relative to 4,000 pCi/L for concentrations greater than benchmarks and radon relative to 300 pCi/L for concentrations greater 

than one-tenth of benchmarks.
56 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

on a subset of 1,389 wells from 25 principal aquifers in 

45 states in which samples were analyzed for major ions, trace 

elements, nutrients, radon, and organic compounds (fig. 32). 

(The entire data set of 2,167 wells could not be used for this 

analysis primarily because trace elements were measured in 

only about two-thirds of the wells.) Fecal indicator bacteria, 

gross alpha- and beta-particle radioactivity, radium, and rarely 

detected pesticides measured by high-performance liquid 

chromatography/mass spectrometry (HPLC) were excluded 

from this comparison because they were analyzed in samples 

from a limited numbers of wells. Fluoride was excluded from 

the determination of wells with at least one contaminant at 

concentrations greater than one-tenth of benchmarks because a 

fluoride concentration equal to one-tenth of the MCL 

(0.4 mg/L) is less than the range of concentrations in drinking 

water recommended for the prevention of tooth decay (U.S. 

Department of Health and Human Services, 1991; Institute of 

Medicine, 1997; U.S. Centers for Disease Control and Prevention, Fluoride Recommendations Work Group, 2001). Given 

the relatively frequent occurrence of fecal indicator bacteria 

and the benchmark of no bacteria detections, the actual number of wells with concentrations of any contaminant greater 

than a benchmark likely would have been higher than the 

percentage values given in table 10 if fecal indicator bacteria and the other excluded contaminants had been included 

in the summary analysis. Organic compounds for which no 

Figure 32. Geographic distribution of wells with at least one contaminant at concentrations greater than or greater than 

one-tenth of human-health benchmarks in samples collected from domestic wells for the NAWQA Program in aquifer 

studies, 1991–2004. Data are for 1,389 wells in which major ions, trace elements, nutrients, radon, and organic compounds 

were measured. Human-health benchmarks are given in Appendix 4. >, greater than; pCi/L, picocuries per liter.

Concentrations of contaminants compared to human-health benchmarks

EXPLANATION

At least one contaminant > one-tenth of a human-health benchmark

No contaminant > one-tenth of a human-health benchmark

At least one contaminant > a human-health benchmark, using 4,000 pCi/L for the radon benchmark

At least one contaminant > a human-health benchmark, using 300 pCi/L for the radon benchmark
Co-occurrence of Contaminants as Mixtures 57

human-health benchmarks were available were detected in 

many other wells also but typically at concentrations orders of 

magnitude below the MCLs or HBSLs. 

Properties or contaminants affecting the aesthetic or other 

non-health quality of water were more frequently outside 

recommended ranges than were contaminants of potential 

health concern. These properties and contaminants included 

pH (16.3 percent of wells), chloride (2.13 percent of wells), 

fluoride (4.04 percent of wells), iron (19.1 percent of wells), 

manganese (21.3 percent of wells), and total dissolved solids 

(14.8 percent of wells). About 62 percent of all the sampled 

wells, mostly in central United States, had water that generally 

is considered to be “hard.” Collectively, about half (48 percent) of the 1,389 wells in which co-occurrence was assessed 

had one or more property or contaminant at levels outside the 

recommended ranges for aesthetic or other non-health quality. 

Undesirable effects of elevated concentrations of these properties and contaminants include scale deposition or corrosion of 

pipes and plumbing fixtures, staining of laundry or plumbing 

fixtures, unpleasant water color or taste, and teeth staining. 

The water samples were collected prior to any in-home treatment systems, however, that may have been in place in some 

homes to treat these properties and contaminants. 

Co-occurrence of Contaminants as 

Mixtures

Contaminants generally do not occur in isolation in 

ground water but usually co-occur with other contaminants as 

mixtures. Analysis of individual contaminants, as presented 

above, is essential for identifying those of potential concern 

for human health and for assessing where and why they occur. 

However, little is known about the potential health effects of 

most mixtures of contaminants in drinking water. Mixtures of 

multiple contaminants—even at concentrations below benchmarks—may present potential concerns for human health and 

warrant investigation. 

Contaminants often co-occur as mixtures in domestic 

wells because well water itself is a mixture of ground water 

from multiple sources, and each source may be affected by 

different and multiple contaminants. Sources of water may 

include recharge from precipitation or irrigation in different 

types of land use, infiltration of surface water, or inflow from 

adjacent aquifers. Along the flow paths by which ground 

water reaches a well, naturally occurring contaminants, such 

as trace elements, may be contributed by the various minerals 

that compose the aquifer sediments and rocks. Anthropogenic 

contaminants may be introduced from a variety of sources, 

such as wastewater disposal, recharge in agricultural or 

urban areas, or spills of gasoline or other chemicals. The 

composition of contaminant mixtures that are present in 

domestic wells depends on the unique sources and flow paths 

that affect each well.

Approach for Assessment of Mixtures

Because of the uncertainties about the health effects 

of contaminant mixtures in drinking water and the potential 

that some mixtures could have greater effects than the 

effects of their individual components, the co-occurrences 

of contaminants that occur individually at concentrations 

above or approaching their respective human-health 

benchmarks could be of potential concern for human health. 

For example, health effects may be additive for contaminants 

that act by similar mechanisms, and some combinations of 

contaminants may synergistically interact, resulting in greater 

than additive effects. However, with few exceptions (for 

example, the triazine herbicides, as discussed below), there 

are no benchmarks for mixtures as such. The co-occurrence 

of multiple contaminants with individual concentrations at 

greater than one-tenth of benchmarks, thus, was used as a 

simple indicator to identify mixtures of potential concern. 

This analysis necessarily included only those contaminants 

with health benchmarks. In addition, the analysis was 

expanded to include all detections of anthropogenic organic 

contaminants because some do not have human-health 

benchmarks. This expanded approach provides a perspective 

on the patterns in co-occurrences that may be important as 

more is learned about the sources and the potential health 

effects of contaminant mixtures.

The overall occurrence of mixtures was assessed by 

determining the frequency and geographic distribution of 

mixtures of various levels of complexity, as represented by the 

number of co-occurring contaminants. This approach provides 

a perspective on general patterns of co-occurrence of contaminants in domestic wells.

The composition of mixtures, in terms of specific 

contaminants, was assessed by evaluating the occurrence 

of unique mixtures of two or more contaminants. Unique 

mixtures were determined using the methods of Squillace and 

others (2002) and J.C. Scott (U.S. Geological Survey, written 

commun., 2008). Unique mixtures are specific combinations 

of any two, three, or more contaminants, regardless of the 

presence of additional contaminants. A single sample can 

contain many unique mixtures, each of which may pose 

different health effects. For example, a mixture of three 

contaminants in a sample, contaminants A, B, and C, contains 

the unique mixtures AB, AC, BC, and ABC (three unique 

two-compound mixtures and one three-compound mixture). 

Each of these unique combinations of contaminants may have 

different health effects. Evaluation of frequently occurring 

unique mixtures, particularly of contaminants at individual 

concentrations that approach or exceed their respective 

human-health benchmarks, can support prioritization of 

certain unique mixtures for investigation of potential health 

effects. Few unique mixtures have been investigated in terms 

of potential health effects relative to the large number of 

mixtures to which people may be exposed (National Research 

Council, 1989; Carpenter and others, 2002). 
58 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

The occurrence and composition of contaminant mixtures 

was assessed for the 1,389 domestic wells from aquifer studies in which major ions, trace elements, nutrients, radon, and 

organic compounds were measured. (Fecal indicator bacteria, 

gross alpha- and beta-particle radioactivity, radium, and rarely 

detected pesticides measured by HPLC were not included 

because they were measured in too few wells, as described 

previously.) Because the two proposed MCLs for radon differ 

by an order of magnitude and thus produce substantially different results when compared to concentrations, co-occurrence 

was evaluated using both low and high benchmark values for 

radon: radon concentrations were compared to the higher 

proposed MCL of 4,000 pCi/L for mixtures of contaminants 

that exceed human-health benchmarks and to the lower 

proposed MCL of 300 pCi/L for mixtures of contaminants at 

concentrations greater than one-tenth of benchmarks. Fluoride 

was excluded from the determination of wells with at least one 

contaminant at concentrations greater than one-tenth of benchmarks, because fluoride at this level (0.4 mg/L) is less than 

the range of concentrations in drinking water recommended to 

prevent tooth decay.

Occurrence and Distribution of Mixtures

Samples from 72.8 percent of domestic wells contained 

two or more contaminants with concentrations greater than 

one-tenth of benchmarks, but only 4.03 percent of the sampled 

wells had two or more of these contaminants at concentrations greater than human-health benchmarks (fig. 33). With the 

inclusion of all organic contaminant detections in the analysis, 

mixtures were present in 88.9 percent of the sampled wells. 

Mixtures of contaminants at concentrations greater than onetenth of benchmarks occurred in most of the principal aquifers 

sampled. The more complex mixtures (those with the largest numbers of contaminants), however, were most common 

in several aquifers in the western and south-central United 

States, including the Basin and Range, Central Valley, and 

High Plains basin-fill aquifers and the Lower Tertiary/Upper 

Cretaceous sandstone aquifers (fig. 34). Mixtures of two contaminants at concentrations greater than benchmarks, although 

much less frequent, were similarly distributed.

Overall, complex mixtures were most common in wells 

with the highest concentrations of total dissolved solids 

(fig. 35). This reflects the finding that the components of these 

mixtures were naturally occurring trace elements and other 

inorganic contaminants. These contaminants largely originate 

from soil and aquifer minerals, and their concentrations in 

ground water are increased by such factors as longer residence 

times, greater degrees of geochemical weathering, or evaporative concentration—factors that result in relatively high dissolved solids overall.

Composition of Mixtures

There were almost 5,000 unique mixtures of contaminants at concentrations greater than one-tenth of their benchmarks, but most (72 percent) were present in only one well. 

About 40 of these mixtures, however, were found in 

5 percent or more of wells. The compositions of these relatively frequent combinations of contaminants are listed in 

table 11. Two-thirds of these mixtures were composed of two 

or more of only six contaminants—all inorganic and, except 

for nitrate, primarily from natural sources. These contaminants 

Figure 33. Frequency of occurrence of contaminants 

in mixtures in samples collected from domestic wells for 

the NAWQA Program in aquifer studies, 1991–2004. Data 

are from 1,389 wells in which major ions, trace elements, 

nitrate, radon, and organic compounds were measured. 

Criterion for radon in mixtures of contaminants at 

concentrations greater than human-health benchmarks 

is 4,000 picocuries per liter (pCi/L) and criterion for 

contaminants at concentrations greater than one-tenth of 

benchmarks is 300 pCi/L. ≥, greater than or equal to.

Mixtures of contaminants

At concentrations greater than

 human-health benchmarks

EXPLANATION

At concentrations greater than

 one-tenth of human-health benchmarks

At concentrations greater than

 one-tenth of human-health benchmarks

 plus any detectable concentration of

 an organic compound

≥1 ≥2 ≥3 ≥4 ≥5

100

0

80

60

40

20

≥6

NUMBER OF CONTAMINANTS

FREQUENCY OF OCCURRENCE,

IN PERCENT OF WELLS
Co-occurrence of Contaminants as Mixtures 59

Figure 34A. Geographic distribution of wells with multiple contaminants (A) at concentrations greater than 

the human-health benchmarks and (B) at concentrations greater than one-tenth of human-health benchmarks 

in samples collected from domestic wells for the NAWQA Program in aquifer studies, 1991–2004. Data are from 

1,389 wells in which major ions, trace elements, nutrients, radon, and organic compounds were measured. 

Human-health benchmarks are given in Appendix 4; criterion for radon is 4,000 picocuries per liter (pCi/L) for 

concentrations greater than human-health benchmarks and 300 pCi/L for concentrations greater than one-tenth of 

human-health benchmarks. >, greater than.

Number of contaminants for which concentrations were greater than

human-health benchmarks

4 to 6 2 None

> 6 3 1

EXPLANATION

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

Figure 34B. Geographic distribution of wells with multiple contaminants (A) at concentrations greater than 

the human-health benchmarks and (B) at concentrations greater than one-tenth of human-health benchmarks 

in samples collected from domestic wells for the NAWQA Program in aquifer studies, 1991–2004. Data are from 

1,389 wells in which major ions, trace elements, nutrients, radon, and organic compounds were measured. 

Human-health benchmarks are given in Appendix 4; criterion for radon is 4,000 picocuries per liter (pCi/L) for 

concentrations greater than human-health benchmarks and 300 pCi/L for concentrations greater than one-tenth 

of human-health benchmarks. >, greater than.—Continued

EXPLANATION

B

Number of contaminants for which concentrations were greater than 

one-tenth of human-health benchmarks

4 to 6 2 None

> 6 3 1
Co-occurrence of Contaminants as Mixtures 61

were nitrate, arsenic, radon, uranium, and to a lesser extent, 

molybdenum and manganese. Mixtures of two or more of 

these six contaminants with concentrations greater than onetenth of benchmarks were present in as many as 32 percent 

of the wells (table 11). These contaminants also occurred in 

mixtures at concentrations greater than benchmarks but never 

in more than 1 percent of the wells. (Not shown in table 11 is 

the mixture of arsenic and fluoride at concentrations greater 

than MCLs, in 1 percent of wells.)

Mixtures containing nitrate or arsenic were among the 

most frequently occurring unique mixtures of contaminants. 

For example, mixtures of uranium with either nitrate or arsenic 

at concentrations greater than one-tenth of benchmarks were 

found in about 20 percent of wells, and mixtures of uranium 

with either nitrate or arsenic at concentrations greater than 

benchmarks were found in 0.29 to 0.58 percent of wells. 

Nitrate and arsenic were frequent components of mixtures 

defined relative to human-health benchmarks because of their 

broad distribution and relatively high frequency of occurrence 

at concentrations greater than benchmarks (figs. 15 and 18; 

tables 5 and 7). Radon was the most frequent component of 

unique mixtures of contaminants with concentrations greater 

than one-tenth of benchmarks, because of its widespread and 

frequent occurrence at concentrations greater than the criterion 

for radon used to define these mixtures (the lower proposed 

MCL of 300 pCi/L). Radon occurred less frequently as a component of mixtures at concentrations greater than benchmarks, 

because occurrences of radon at levels greater than its higher 

proposed MCL of 4,000 pCi/L were strongly regional and not 

associated with ground water containing high concentrations 

of dissolved solids.

Mixtures that contained organic compounds at concentrations greater than benchmarks or with concentrations greater 

than one-tenth of benchmarks were rare. The frequency of 

occurrence of any such mixture was less than 0.01 percent for 

mixtures of contaminants at concentrations greater than benchmarks, and the frequency was only 0.79 percent for mixtures 

of contaminants with concentrations greater than one-tenth of 

benchmarks. This resulted because concentrations of organic 

compounds seldom approached or exceeded benchmarks; 

also, some organic compounds were excluded when mixtures 

were defined relative to benchmarks because they did not have 

benchmarks. In contrast, eight mixtures of naturally occurring contaminants at concentrations greater than one-tenth of 

benchmarks and organic contaminants at any concentration 

were among the most frequently occurring, being present 

in more than 10 percent of wells (table 12). These mixtures 

included two- and three-component mixtures of chloroform, 

atrazine, or deethylatrazine in combination with either radon 

or nitrate. Co-occurrences of nitrate with the frequently 

Median

25th percentile

75th percentile

90th percentile

10th percentile

Number of samples

EXPLANATION

52

A B

13

3–5 2 1 0

3,000

0

2,500

2,000

1,500

1,000

500

42

258

NUMBER OF CONTAMINANTS

DISSOLVED SOLIDS,

IN MILLIGRAMS PER LITER

1,069 84 291

596

362

49

>6 4–6 2–3 1 0

Figure 35. Dissolved solids concentrations in relation to the numbers of contaminants present at (A) concentrations 

greater than human-health benchmarks and (B) concentrations greater than one-tenth of human-health benchmarks in 

samples collected from domestic wells sampled for the NAWQA Program in aquifer studies, 1991–2004. Data are from 

1,389 wells in which major ions, trace elements, nutrients, radon, and organic compounds were measured. >, greater than.
62 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Table 11. Composition and frequency of occurrence of unique mixtures of contaminants individually at concentrations of potential 

concern for human health in samples collected from domestic wells for the NAWQA Program in aquifer studies, 1991–2004.

[Data are for 1,389 wells in which major ions, trace elements, nitrate, radon, and organic compounds were measured. Criterion for radon in mixtures of 

contaminants at concentrations greater than human-health benchmarks is 4,000 picocuries per liter (pCi/L), and criterion for contaminants at concentrations 

greater than one-tenth of benchmarks is 300 pCi/L. All unique mixtures of contaminants with concentrations greater than one-tenth of benchmarks that occur 

in 5 percent or more of wells are listed.]

Contaminants in the most frequently

occurring unique mixtures

Percentage of wells in which unique mixture occurs

Mixtures of contaminants with 

concentrations greater than 

one-tenth of human-health benchmarks

Mixtures of contaminants at 

concentrations greater than 

human-health benchmarks

Nitrate Radon 32.4 0.07

Arsenic Radon 29.9 0.07

Nitrate Arsenic 24.4 0.22

Uranium Radon 21.0 0.50

Arsenic Uranium 20.8 0.29

Nitrate Uranium 19.4 0.58

Nitrate Arsenic Radon 17.6 0.00

Nitrate Arsenic Uranium 16.4 0.14

Arsenic Uranium Radon 15.9 0.00

Nitrate Uranium Radon 15.2 0.00

Arsenic Molybdenum 13.1 0.22

Nitrate Arsenic Uranium Radon 12.3 0.00

Radon Molybdenum 12.0 0.00

Manganese Radon 10.8 0.07

Uranium Molybdenum 10.6 0.07

Arsenic Molybdenum Radon 9.50 0.00

Nitrate Molybdenum 9.29 0.14

Arsenic Molybdenum Uranium 8.93 0.07

Arsenic Manganese 8.78 0.58

Uranium Molybdenum Radon 8.42 0.00

Radon Strontium 8.28 0.00

Nitrate Arsenic Molybdenum 7.92 0.07

Nitrate Uranium Molybdenum 7.92 0.07

Nitrate Molybdenum Radon 7.27 0.00

Arsenic Strontium 7.20 0.50

Arsenic Molybdenum Uranium Radon 6.98 0.00

Nitrate Arsenic Molybdenum Uranium 6.91 0.07

Arsenic Barium 6.77 0.00

Nitrate Strontium 6.48 0.58

Nitrate Molybdenum Uranium Radon 6.26 0.00

Nitrate Arsenic Molybdenum Radon 6.12 0.00

Arsenic Strontium Radon 5.90 0.00

Arsenic Selenium 5.69 0.00

Radon Barium 5.62 0.00

Radon Lead 5.62 0.00

Nitrate Selenium 5.47 0.07

Uranium Strontium 5.47 0.14

Nitrate Arsenic Molybdenum Uranium Radon 5.33 0.00

Nitrate Barium 5.33 0.00

Nitrate Strontium Radon 5.33 0.00

Nitrate Arsenic Selenium 5.26 0.00

Nitrate Arsenic Strontium 5.18 0.14

Uranium Selenium 5.11 0.00
Co-occurrence of Contaminants as Mixtures 63

occurring pesticide atrazine and (or) deethylatrazine, its 

degradation product, likely reflect the proximity of sources of 

these contaminants, such as fertilizer or manure with pesticide 

use in agricultural areas. Co-occurrences of radon and these 

pesticides likely reflect the ubiquitous distribution of radon at 

concentrations greater than the lower proposed MCL of 

300 pCi/L. No mixture containing only organic compounds 

was present in more than 10 percent of wells except for the 

combination of atrazine and deethylatrazine, which had an 

occurrence frequency of 12.3 percent. The common presence 

of the degradate deethylatrazine with atrazine (in 84 percent 

of wells with atrazine detections) is consistent with previous 

findings on the prevalence of degradates of some pesticides in 

ground water (Gilliom and others, 2006).

Assessing the Potential Significance of 

Mixtures to Human Health

Depending on the specific compounds, the toxicity of a 

mixture may result from additive effects of the components, 

independent effects, antagonistic effects (less than additive), or 

synergistic effects (greater than additive). Each of these toxicity effects, except for antagonistic, usually results in a toxicity 

of the mixture that is greater than that of any of its original 

components (Gilliom and others, 2006). Given the likelihood that many mixtures have combined toxicities greater 

than those of their individual components, and the paucity 

of studies that address specific mixtures, the most frequently 

occurring mixtures and existing knowledge of potential health 

effects (tables 11 and 12) can be used as a starting point to 

prioritize unique mixtures for additional study. 

A few specific mixtures of organic contaminants have 

been identified as having the potential to cause health effects 

because of combined toxicities. Three such mixtures that were 

found in the domestic wells sampled in this study are mixtures 

of triazine herbicides, mixtures of triazine herbicides and 

nitrate, and mixtures of several solvents. Only one of these 

mixtures, however, has a benchmark value available with 

which to compare concentrations quantitatively.

The combined concentrations of triazine herbicides, 

which include atrazine, deethylatrazine, and simazine, are 

considered additive in their toxic effect on human health 

by the Agency for Toxic Substances and Disease Registry 

(2006) and U.S. Environmental Protection Agency (2006b). 

In domestic wells, atrazine frequently co-occurred with its 

degradation product deethylatrazine (84 percent of samples 

with atrazine detections), and simazine (17 percent). All 

concentrations of total triazines in the 457 wells that had 

detections of one or more of these herbicides were less than 

the screening value of 12.5 μg/L, a value developed by 

USEPA for community public-water systems for the sum 

of atrazine and its three chlorinated degradates (including 

deethylatrazine and two others not measured in the present 

study). The maximum combined concentration was 4.6 μg/L, 

and the 90th percentile was 0.23 μg/L. Total chlorinated 

triazine concentrations greater than 12.5 μg/L in public water 

supplies indicate the need for further analysis and monitoring 

(U.S. Environmental Protection Agency, 2003b, 2006b).

Table 12. Composition and frequency of occurrence of unique mixtures of contaminants individually at concentrations of potential 

concern for human health and any detectable concentration for organic compounds in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2004.

[Data are for 1,389 wells in which major ions, trace elements, nitrate, radon, and organic compounds were measured. Criterion for radon is 300 picocuries per 

liter. All unique mixtures of contaminants with concentrations greater than one-tenth of benchmarks and detectable organic compounds that occur in 10 percent 

or more of wells are listed.]

Contaminants in the most frequently

occurring unique mixtures

Percentage of wells in which unique mixture occurs

Mixtures of contaminants with concentrations greater than one-tenth of 

benchmarks for all naturally occurring inorganic constituents and any 

detectable concentration for organic compounds

Radon Chloroform 14.9

Nitrate Deethylatrazine 14.3

Radon Deethylatrazine 13.0

Atrazine Deethylatrazine 12.3

Nitrate Atrazine 11.3

Nitrate Chloroform 11.3

Nitrate Radon Deethylatrazine 10.5

Nitrate Atrazine Deethylatrazine 10.4
64 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

The co-occurrence of nitrate and atrazine or simazine 

may be a health concern because atrazine and simazine can 

react with nitrite, which is formed in the human body from 

nitrate, to form nitrosamine compounds. The structure of these 

compounds suggests that they could be carcinogenic, although 

such effects have not been demonstrated to date (Agency 

for Toxic Substances and Disease Registry, 2006). For the 

domestic wells included in the NAWQA Program aquifer 

studies, the median concentration of nitrate in samples in 

which atrazine and (or) simazine were detected was 

2.9 mg/L as N, greater than background values in many 

areas of the United States. Two-, three-, and four-component 

mixtures with nitrate greater than 1.0 mg/L as N and 

detections of atrazine (11.3 percent of wells; table 12), 

deethylatrazine (14.3 percent of wells), and (or) simazine also 

were among the most frequently occurring unique mixtures 

in wells in the aquifer studies. In domestic wells that were 

sampled as part of targeted studies of ground-water quality 

beneath agricultural land, two- and three-component mixtures 

with nitrate at concentrations greater than 1 mg/L as N and 

atrazine, deethylatrazine, and (or) simazine were present in 

30 to 50 percent of wells; the median nitrate concentration 

was 7.1 mg/L as N in wells in which triazine herbicides 

were detected. Although there are no numerical benchmarks 

available for these mixtures, this co-occurrence of triazine 

herbicides and elevated nitrate suggests the need for further 

study, especially in agricultural areas.

Perchloroethene, a commonly detected solvent, frequently occurred with one or more of three other solvents—

trichloroethene, 1,1-dichloroethane, and 1,1,1,-trichloroethane—for which the toxic effects are considered additive 

(Agency for Toxic Substances and Disease Registry, 2004c). 

Perchloroethene co-occurred with 1,1,1-trichloroethane, its 

degradation product, most frequently, in 30 percent of samples 

in which perchloroethene was detected. The combined concentrations of the four solvents were usually low, however. The 

median value of combined concentrations greater than detection levels was 0.031 μg/L, and the 90th percentile was 0.038, 

for the 240 wells in which any one of the four solvents was 

detected. The maximum combined concentration, 29 μg/L, 

was relatively high, but as with nitrate and triazines, there are 

currently no benchmark values for these mixtures.

Comparison with Previous Studies of 

Domestic Well-Water Quality

Results of the present study, which examined a broad 

suite of contaminants in domestic wells across the United 

States, are generally consistent with the findings of many 

regional, statewide, and national studies that investigated 

various aspects of domestic well water quality or of specific 

subsets of contaminants in domestic wells. Overall, results of 

the present study are similar to findings of the USEPA NSA 

(U.S. Environmental Protection Agency, 1984), which was 

conducted nearly 30 years ago. The NSA was a survey of 

water supplies used by about 2,600 rural households, mostly 

individual domestic wells, that were not regulated by the 

SDWA; the survey used random sampling and included a 

broad range (though limited number) of contaminants. Results 

of the present study also were similar to those compiled in a 

report on the quality of water from domestic wells in six states 

by the U.S. General Accounting Office (1997).

The microbiological quality of domestic well water was 

found to be a common problem in the NSA and the present 

study, with total coliform bacteria detected in 42.1 percent 

of domestic wells in the NSA and in 33.5 percent of domestic wells sampled in the present study. Detections of fecal 

coliform bacteria in 19.8 percent of wells in the NSA were 

somewhat higher than the detection frequency of 7.9 percent 

for the fecal indicator bacteria E. coli. Detection frequencies 

for fecal indicator bacteria in the present study are similar to 

detection frequencies found in a study of randomly selected 

domestic wells in the Midwest by the CDC (U.S. Centers 

for Disease Control and Prevention, 1998) for total coliform 

bacteria (41.3 percent) and E. coli (11.1 percent); in the U.S. 

General Accounting Office (1997) compilation of data for 

California, Illinois, Nebraska, North Carolina, and Wisconsin 

for total coliform bacteria (14 to 45 percent); and in several 

other statewide or regional-scale surveys (Appendix 1). Detections of fecal indicator bacteria in the NSA were more frequent 

in samples from wells that were dug or augered (rather than 

drilled using another method), shallow, or in poor condition, 

or were owned by low-income or low-education households. 

Well construction also was significant in the CDC study. 

Similarly in the present study, samples in which total coliform 

bacteria were detected were from wells that were shallower 

than wells with no detections of total coliform bacteria; data 

for the other factors tested in the NSA were not available for 

the wells in the present study. 

The prevalence of common nuisance contaminants, total 

dissolved solids, iron, and manganese, was similar in the NSA 

and in the present study. Total dissolved solids were greater 

than the SMCL of 500 mg/L in 14.7 percent of domestic wells 

in the NSA and in 14.8 percent of the domestic wells in the 

present study. Concentrations of iron were greater than the 

SMCL of 300 μg/L in 29.9 percent of domestic wells in the 

NSA and in 19.1 percent of wells in the present study; manganese concentrations were greater than the SMCL of 50 μg/L in 

20.7 percent of domestic wells in the NSA and in 21.3 percent 

of the wells of the present study. Elevated concentrations of 

manganese can be of concern for human health, and manganese was present in concentrations greater than the HBSL 

in 5.22 percent of the domestic wells sampled in the present 

study; this aspect of manganese concentrations was not investigated in the NSA. The occurrence of sulfate at concentrations 

greater than the SMCL of 250 μg/L also was similar in the two 

studies, 4.1 percent in the NSA and 3.79 in the present study. 

Nitrate was one of the contaminants that occurred most 

frequently in concentrations greater than human-health 

benchmarks in both the NSA and the present study. Nitrate 
Comparison with Previous Studies of Domestic Well-Water Quality 65

concentrations greater than the USEPA MCL of 10 mg/L as 

N were present in 4.1 percent of domestic wells in the NSA 

and in 4.36 percent of wells from aquifer studies in the present study. For the present study, the percentage of wells from 

aquifer studies with concentrations of nitrate greater than the 

MCL is similar to the range of 2 to 9.4 percent (median value 

3.5) reported for a number of statewide or regional voluntary 

testing programs (programs in Alabama, California, Georgia, 

Kentucky, Idaho, Indiana, Illinois, Minnesota, New Jersey, 

Ohio, Pennsylvania, Virginia, and West Virginia, including 

one statewide mandatory testing program) (see Appendix 1 for 

references).

In the present study, nitrate concentrations were found to 

be higher in agricultural areas than in other areas. This finding 

is consistent with the results of many studies at statewide and 

regional scales and with the results of other studies by the 

NAWQA Program. Nitrate concentrations in the present study 

were greater than 10 mg/L as N in 7.1 percent of wells from 

aquifer studies with substantial amounts of agricultural land 

in their immediate vicinities, and concentrations were greater 

than 10 mg/L as N in 23.4 percent of wells from assessment 

studies that specifically targeted regional areas of agricultural 

land use. These relatively large percentages are similar to 

the percentages of sampled wells with nitrate concentrations 

greater than the MCL in statewide or regional studies that targeted domestic wells from farming households or near active 

agriculture across the United States—2 to 32 percent (median 

value 19 percent) of domestic wells primarily in the Midwest 

(Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, 

Tennessee, and Wisconsin); 25 percent in the Northwest 

(Oregon); and 1.7 to 10 percent in the Southeast (North 

Carolina and Georgia) (see Appendix 1 for references).

Pesticides have been the subject of many previous 

regional and statewide studies, particularly in the Midwest, 

and were included to a limited extent in the NSA. In many 

cases, comparisons among these earlier studies and the present 

study are complicated by differences in the contaminants analyzed and their detection limits. In the NSA, six organic compounds (all herbicides) were analyzed, and two were detected. 

The USEPA National Pesticide Study (NPS), conducted in 

the late 1980s, was more comprehensive and included 127 

pesticides, with detection limits that ranged from about 0.1 to 

0.7 μg/L for most compounds (the median detection limit was 

about 0.2 μg/L; U.S. Environmental Protection Agency, 1990a, 

1990d). The NPS, which sampled about 730 wells, primarily 

used random sampling methods, but also some targeted sampling of high pesticide-use areas. In the NPS, the estimated 

detection frequency for at least one pesticide was 4.2 percent 

of all domestic wells in the United States and 5.5 percent of 

domestic wells in vulnerable areas of row-crop agriculture 

(U.S. Environmental Protection Agency, 1990a). These results 

are comparable to the finding of the present study of at least 

one pesticide present in 3.0 percent of the sampled domestic 

wells at concentrations greater than the common reporting 

level of 0.2 μg/L. The present study and the NPS also found 

similar percentages for wells in which pesticides were present 

at concentrations of concern for human health—0.6 percent 

(relative to USEPA MCLs or lifetime health advisories) in the 

NPS study and 0.5 percent (relative to MCLs or HBSLs) in the 

present study. 

Atrazine was the most frequently detected pesticide in 

the NPS study and in the present study. Detection frequencies 

were comparable; atrazine was detected in an estimated 

0.7 percent of domestic wells nationwide in the NPS (minimum reporting limit of 0.12 μg/L) and in 2.1 percent of 

domestic wells from aquifer studies at concentrations greater 

than 0.1 μg/L in the present study. In regional and statewide 

studies encompassing areas of the United States in which agriculture is dominant, detection frequencies for atrazine were 

higher than those in domestic wells in the NPS or in wells 

from aquifer studies used in the present study. In the CDC 

study in nine Midwestern states, atrazine was detected in 

13.6 percent of wells at concentrations greater than a minimum reporting level of 0.05 μg/L (U.S. Centers for Disease 

Control and Prevention, 1998). In a study that included all 

counties in the United States in which the pesticide alachlor 

was sold, atrazine was detected in about 12 percent of wells 

at concentrations greater than a minimum reporting level of 

0.03 μg/L (Holden and others, 1992). In most of the statewide 

or regional studies in the Midwest described previously for 

nitrate, atrazine was the most frequently detected pesticide, 

with detection frequencies ranging from 4 to 23 percent of 

sampled wells in these studies. Similarly, detection frequencies for atrazine in NAWQA assessment studies that targeted 

regional agricultural land-use areas—32 and 37 percent of 

wells with atrazine concentrations greater than 0.02 and 

0.01 μg/L, respectively—were considerably higher than the 

detection frequencies for atrazine in NAWQA domestic wells 

from aquifer studies, which were not targeted at regional 

agricultural areas. 

The occurrence of selected trace elements and radionuclides in domestic wells also has been studied at regional, 

statewide, and, to a limited extent, national scales. In the 

national-scale NSA, trace elements were analyzed, and 

several, including lead, cadmium, and silver, were found at 

concentrations greater than reference levels for human health 

(some of which were the MCLs used in the present study) 

in relatively large percentages of sampled domestic wells. 

However, contamination from sampling procedures occurred 

for some trace elements in that study (U.S. Environmental 

Protection Agency, 1984). In the present study, concentrations 

of lead, cadmium, silver, and most other trace elements were 

found to exceed human-health benchmarks (several of which 

were lower than the reference levels used in the NSA) in less 

than 1 percent of the sampled domestic wells. Arsenic was 

compared to a reference value of 50 μg/L in the NSA, which 

is greater than the current USEPA MCL. Concentrations of 

arsenic were greater than the reference value in 0.8 percent 

of domestic wells in the NSA; this is similar to the value of 

0.6 percent of wells with concentrations greater than 50 μg/L 

for the present study. A number of studies of arsenic, primarily in public water-supply systems, were associated with the 
66 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

recent reduction in the USEPA MCL for arsenic to 10 μg/L 

(ISSI Consulting Group and others, 2000; Focazio and others, 

2000). Concentrations greater than 10 μg/L were estimated 

to occur in 7.6 percent of public water-supply systems in the 

United States (Focazio and others, 2000); this is comparable 

to the 6.8 percent of domestic wells with arsenic concentrations greater than the MCL in the present study. Similar or, in 

some cases, larger percentages of wells with arsenic concentrations greater than the MCL have been found for regional 

or local-scale study areas in other parts of the country (Peters 

and others, 1999; Riewe and others, 2000; Montgomery and 

others, 2003; Hagan, 2004; Shiber, 2005; Steinmaus and 

others, 2005). A nationwide survey of radionuclides in public 

wells (Longtin, 1988) found concentrations of radium-226 

plus radium-228 greater than the USEPA MCL of 5 pCi/L at 

frequencies (about 1 percent of wells) similar to that in the 

present study; the frequencies are not nearly as great as those 

determined for public water supplies by Focazio and others (21 percent; 2001), where sampling was targeted at areas 

known or suspected to contain radium-rich ground water. 

Finally, findings of the present study may be compared to 

those of the recent national-scale study that used data from all 

domestic wells in USGS databases for arsenic, radon, uranium, 

and fluoride, nitrate, and selected organic compounds (Focazio 

and others, 2006). That study included much of the data used 

in the present study. Overall in the present study, the frequencies at which arsenic, radon, nitrate, and uranium occurred 

in the domestic wells at concentrations greater than humanhealth benchmarks (USEPA MCLs or proposed MCLs) were 

in most cases slightly lower than the frequencies at which 

these contaminants were present at concentrations greater than 

benchmarks in all domestic wells in USGS databases (Focazio 

and others, 2006). The frequencies for the present study and 

for Focazio and others (2006) are, respectively: 6.75 and 

10.6 percent for arsenic; 64.6 and 75 percent for radon greater 

than 300 pCi/L; 4.44 and 9.0 percent for radon greater than 

4,000 pCi/L; 1.74 and 3.7 percent for uranium; 4.36 and 

8.4 percent for nitrate; and 1.16 and 0.81 for fluoride. These 

differences may result, in part, from the inclusion of wells 

near contamination sites (for example, landfills or wastewater 

disposal areas) and, for nitrate, the inclusion of wells from 

NAWQA agricultural land-use studies in the dataset of all 

domestic wells in USGS databases that was used in Focazio 

and others (2006). Differences between the two datasets in 

sampling densities among geographic areas in the United 

States also may have affected study results. 

Summary and Conclusions

As part of the National Water-Quality Assessment 

Program (NAWQA) of the U.S. Geological Survey (USGS), 

water samples collected from 2,167 domestic wells across the 

United States during 1991–2004 were analyzed for physical 

properties and the concentrations of as many as 214 chemical 

contaminants. The wells were located within major hydrogeologic settings in 30 regionally extensive aquifers used for 

water supply (principal aquifers) in the United States. Water 

samples were collected prior to any treatment systems that 

may have been in place, and thus do not represent water consumed by homeowners with in-home treatment. Each well was 

sampled once during the study period. 

The findings of the study are based on a synthesis of data 

for domestic wells from NAWQA ground-water studies that 

were parts of integrated assessments of water resources, with 

study areas that represented typical hydrogeologic settings 

within the regionally extensive principal aquifers. The studies 

were not designed as a single, national-scale assessment of 

domestic well water quality. The wells in the study are clustered geographically and are not a statistically representative 

sampling of all domestic wells in the United States or within 

principal aquifers. Within the representative hydrogeologic 

settings that were studied, however, the wells were randomly 

selected to represent typical aquifer conditions and were not 

focused in areas of known contamination. For this reason, and 

because of the use of consistent sampling and analytical methods, the well-documented quality-assurance procedures, and 

the broad suite of analytes in the data set, the NAWQA data 

provide a valuable description of domestic well water quality 

in the Nation.

Concentrations of water-quality contaminants were 

compared to human-health benchmarks that included U.S. 

Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) for public water supplies and USGS 

Health-Based Screening Levels (HBSLs). No individual 

water-quality contaminant was present in concentrations 

greater than the available benchmarks in more than 8 percent 

of the sampled wells. Collectively, however, 22.6 percent of a 

subset of 1,389 wells had at least one contaminant at concentrations greater than benchmarks; the subset consisted of wells 

in which most contaminants were measured. Concentrations of 

at least one contaminant were greater than an established MCL 

(using action levels for copper and lead) or HBSL in 13.2 and 

8.14 percent of the wells, respectively.

Radon, nitrate, several trace elements, fluoride, gross 

alpha- and beta-particle radioactivity, and fecal indicator 

bacteria occurred most frequently at concentrations of 

potential concern for human health. Radon concentrations 

were greater than the proposed MCL of 300 picocuries per 

liter (pCi/L) in 64.6 percent of the wells and greater than the 

proposed alternative MCL of 4,000 pCi/L in 4.44 percent of 

wells. Other contaminants that occurred in concentrations 

greater than MCLs or HBSLs in more than 1 percent of wells 

were: nitrate, greater than the MCL of 10 micrograms per 

liter (μg/L) as N in 4.36 percent of wells; arsenic, greater than 

the MCL of 10 micrograms per liter (μg/L) in 6.75 percent of 

wells; manganese, greater than the HBSL of 300 μg/L in 

5.22 percent of wells; strontium, greater than the HBSL of 

4,000 μg/L in 7.32 percent of wells; uranium, greater than the 

MCL of 30 μg/L in 1.74 percent of wells; and fluoride, greater 

than the MCL of 4 mg/L in 1.16 percent of wells. Gross 
Summary and Conclusions 67

alpha- (uncorrected) and gross beta-particle radioactivity, 

measured in samples from about 400 wells, were greater than 

the MCLs in 5.29 and 1.87 percent of wells, respectively. 

Contaminants other than those listed were present in 

concentrations less than the available MCLs or HBSLs for 

at least 99 percent of the sampled wells. Total coliform and 

Escherichia coli bacteria, measured in about 400 wells, were 

detected in 33.5 and 7.9 percent of wells, respectively. Thus, 

with the exceptions of nitrate and fecal indicator bacteria, the 

contaminants that occurred most frequently at concentrations 

greater than human-health benchmarks were naturally 

occurring. Collectively, 19.2 percent of wells contained at least 

one naturally occurring contaminant (radon, a trace element, 

or fluoride) at a concentration greater than its benchmark. 

Anthropogenic organic compounds were frequently 

detected—with typical analytical detection limits of 0.001 

to 0.1 μg/L—but seldom at concentrations greater than 

human-health benchmarks. The most frequently detected 

compounds included the pesticide atrazine, its degradate 

deethylatrazine, and the volatile organic compounds 

chloroform, methyl tert-butyl ether (MTBE), perchloroethene, 

and dichlorofluoromethane. These compounds were detected 

in 1 to 5.5 percent of wells at concentrations greater than 

0.2 μg/L and in 3.13 to 17.9 percent of wells at concentrations 

greater than 0.02 μg/L. Only 7 of the 168 measured organic 

compounds—diazinon, dibromochloroprane, dieldrin, dinoseb, 

ethylene dibromide, perchloroethene, and trichloroethene—

occurred at concentrations greater than MCLs or HBSLs, 

each in less than 1 percent of wells. Collectively, only 

0.80 percent of wells contained an organic compound at 

concentrations greater than MCLs or HBSLs. Concentrations 

of any compound greater than 10 percent of MCLs or HBSLs 

occurred in 2.95 percent of wells.

Values or concentrations of several other properties 

and contaminants were outside the recommended ranges for 

drinking water for aesthetic or other non-health purposes. 

About 16 percent of the sampled wells had pH values less 

than (14.4 percent) or greater than (1.9 percent) the USEPA 

recommended range of 6.5 to 8.5. Total dissolved solids were 

greater than the USEPA Secondary Maximum Contaminant 

Level (SMCL) of 500 mg/L in 14.8 percent of wells. Iron and 

manganese concentrations were greater than SMCLs in about 

19.1 and 21.3 percent of wells, respectively. Concentrations 

of fluoride, which can be harmful at high concentrations but 

prevents tooth decay at low concentrations, were less than 

those recommended by the U.S. Centers for Disease Control 

and Prevention in about 90 percent of the sampled wells.

The occurrence and concentrations or values of some 

contaminants and properties varied regionally and by principal 

aquifer and therefore may be of greater potential concern for 

human health or aesthetic water quality in some locations or 

regions than nationally. For example, radon concentrations 

were greater than the proposed MCLs in about 30 percent 

(higher proposed MCL) and about 90 percent (lower proposed 

MCL) of the wells in crystalline-rock aquifers, which are in 

the Northeast, the central and southern Appalachians, and 

Colorado. Nitrate concentrations were more frequently greater 

than the MCL in areas of agricultural land use (7.1 percent) 

than in areas of urban (3.1 percent), mixed (3.7 percent), or 

undeveloped (0.7 percent) land use. Nitrate concentrations 

were higher in agricultural areas than in other areas, occurring 

at concentrations greater than the MCL of 10 mg/L as N in 

7.1 percent of wells from aquifer studies with substantial 

amounts of agricultural land in their immediate vicinities, and 

in 23.4 percent of wells from NAWQA assessment studies that 

specifically targeted regional areas of agricultural land use. 

Contaminant concentrations also were related to geochemical conditions. Arsenic concentrations were inversely 

related to dissolved oxygen concentrations in several principal 

aquifers; uranium concentrations were positively correlated 

with dissolved oxygen concentrations, in addition to showing 

regional patterns of occurrence. Relatively high concentrations of iron and manganese occurred everywhere but were 

inversely correlated with dissolved oxygen and (for manganese) pH. Relations of these contaminants with dissolved 

oxygen and pH result from differences in the solubility of 

redox-sensitive species. Differences in geochemical conditions, such as redox and pH, can result in spatial variability 

of contaminant concentrations within aquifers with similar 

source-rock characteristics. 

Mixtures of contaminants were assessed for individual 

contaminants present at concentrations of potential concern for 

human health and for organic compounds detected at any concentration in a subset of 1,389 wells. Mixtures of two or more 

contaminants at concentrations greater than human-health 

benchmarks occurred in only 4.03 percent of wells, but mixtures of two or more contaminants with concentrations greater 

than one-tenth of their benchmarks occurred in 72.8 percent 

of wells. Mixtures with the largest numbers of contaminants 

were most common in the western and south-central United 

States in ground water with relatively high concentrations of 

dissolved solids. 

The composition of mixtures was described in terms of 

unique mixtures, or specific combinations of any two, three, 

or more contaminants, regardless of the presence of other 

contaminants. Two-thirds of the unique mixtures of contaminants with concentrations greater than one-tenth of individual 

benchmarks that occurred in 5 percent or more of wells were 

made up of two or more of only six contaminants—nitrate, 

arsenic, radon, uranium, and to a lesser extent, molybdenum 

and manganese. Organic compounds were rarely components 

of these unique mixtures, but mixtures of naturally occurring contaminants at concentrations greater than one-tenth of 

benchmarks and organic compounds detected at any concentration were common, occurring in 88.9 percent of wells. Eight 

of these mixtures were each present in more than 10 percent 

of wells and included two- and three-component mixtures of 

chloroform, atrazine, or deethylatrazine in combination with 

radon and nitrate. 

Several combinations of organic compounds in mixtures 

with possible health effects were identified in the sampled 

wells—specifically, atrazine, deethylatrazine, and simazine; 
68 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

atrazine or simazine with nitrate; and perchloroethene and 

three other solvents—but combined concentrations were less 

than available benchmarks, or no benchmarks were available 

for the mixtures. Atrazine commonly co-occurred with its 

degradate deethylatrazine (84 percent of atrazine detections) 

and with simazine (17 percent of atrazine detections), but 

combined concentrations were all less than the USEPA 

screening value of 12.5 μg/L for community public water 

systems for atrazine and its chlorinated degredates. Nitrate 

(at concentrations greater than 1 mg/L as N) co-occurred 

with atrazine, deethylatrazine, and (or) simazine in as many 

as 14 percent of all wells from aquifer studies and even more 

frequently in wells from assessment studies that targeted 

regional agricultural land-use areas, but there are currently 

(2009) no benchmarks for these mixtures. Similarly, there 

are currently no benchmarks available for mixtures of 

perchlorethene with trichloroethene, 1,1-dichloroethane, or 

1,1,1-trichloroethane, which are considered to have additive 

toxic effects with one another. 

Organic compounds typically co-occurred with relatively high concentrations of nitrate, especially for organic 

compounds at concentrations approaching health benchmark 

values. Median nitrate concentrations were 2 to 8 times higher 

in wells that also contained organic compounds detected at 

any concentration, organic compounds present at concentrations greater than one-tenth of MCLs or HBSLs, or organic 

compounds present at concentrations greater than MCLs or 

HBSLs than in wells with no detections of organic compounds 

or without organic compounds present at these elevated concentrations. Although nitrate concentrations were not a strong 

predictor of organic compound occurrence, the small but 

significantly higher likelihood that organic compounds might 

be present at concentrations approaching health benchmarks 

when nitrate concentrations were high suggests that elevated 

nitrate concentrations could be a useful screening tool, in some 

cases, for the occurrence of organic compounds at concentrations of potential health concern. 

Results of this study are similar to findings of a USEPA 

statistical sampling of domestic wells in rural areas, the 

National Statistical Assessment of Rural Water Conditions 

(NSA), that was conducted nearly 30 years ago and was mandated by the Safe Drinking Water Act of 1975. In both studies, 

the microbial quality of domestic well water was found to be 

a common problem, and nitrate was one of the most common 

contaminants to occur in concentrations greater than humanhealth benchmarks. Detection frequencies for total coliform 

and nitrate concentrations greater than the MCL were similar 

in the two studies. The frequencies at which concentrations 

of dissolved solids, iron, manganese, and sulfate were greater 

than SMCLs in the present study were similar to those for 

domestic wells in the NSA study. Several trace elements were 

found to be potentially of concern nationally and regionally 

in the present study, but unlike findings of the NSA study, 

the present study found that most trace elements were not a 

substantial concern for human health at the national scale. 

The present study also differed from the NSA study in its 

comprehensive approach and use of low-detection limits for 

organic compounds, which identified the widespread presence 

of anthropogenic organic compounds in domestic well water, 

although nearly always at concentrations less than humanhealth benchmarks. 

The present study provides the most comprehensive 

national analysis to date of the quality of water from domestic 

wells and provides a foundation for improving our understanding and management of this critical source of drinking 

water. Information about the distribution of naturally occurring contaminants in domestic wells among principal aquifers 

and geographically can help determine the appropriate levels 

of monitoring in aquifers and areas of the most concern for 

specific contaminants. Information about the distribution 

of contaminants that originate from anthropogenic sources 

similarly can be used in monitoring, education, and protection 

programs in vulnerable areas. For example, the findings on 

nitrate and bacteria indicate that domestic wells in or adjacent to areas of agricultural and urban land use, or completed 

in aquifers that allow rapid infiltration, may require special 

consideration. Information from the present study can be used 

to inform homeowners and those concerned with public health 

about the potential risks to domestic-well water quality that 

are associated with some current or past land uses or with 

natural sources. The persistent nature of some contaminants 

and relatively slow movement of ground water, which may be 

affected by past land uses, make understanding of these risks 

of particular concern for residents and water managers in areas 

where land use is changing.

Mixtures of contaminants at concentrations approaching human-health benchmarks and of organic compounds 

at detectable concentrations were prevalent in the domestic 

wells, suggesting a need for further investigation of mixtures 

in drinking water. Existing standards and other human-health 

benchmarks for drinking water were developed for individual 

chemicals and may not address all potential effects of drinking-water contaminants, because exposure is often to mixtures of multiple contaminants. Additional research is needed 

regarding the possible toxicities of mixtures to humans, 

including mixtures with degradation products. A total of 43 

unique mixtures were identified of contaminants with concentrations greater than one-tenth of individual human-health 

benchmarks in more than 5 percent of the domestic wells 

sampled in this study. Such information on the occurrences 

and characteristics of mixtures can be used to aid in prioritizing toxicity assessments. 

The occurrence of anthropogenic organic compounds is 

widespread in principal aquifers, although at concentrations 

well below human-health benchmarks. The widespread 

occurrence demonstrates that all aquifers require some 

level of consideration to prevent or mitigate contamination. 

Long-term, consistent data are essential for assessing trends 

in domestic-well water quality, particularly in response to 

changes in chemical use, well use, and management practices, 

because of the slow rate of ground-water flow and the time 
References 69

lag between the adoption of management practices and 

improvement in water quality. 

Acknowledgments

The author thanks the USGS personnel providing guidance to this study: Marion Berndt, Gregory Delzer, Neil 

Dubrovsky, Michael Focazio, Robert Gilliom, Pixie Hamilton, 

Wayne Lapham, Jeffrey Martin, Thomas Nolan, Gary Rowe, 

Paul Stackelberg, Patty Toccalino, Kelly Warner, William 

Wilber, and Michael Yurewicz. The author also thanks USGS 

colleagues Jonathan Scott, for conducting the analyses of 

unique multi-component combinations of nitrate and organic 

compounds; Zoltan Szabo, for radium data and assistance with 

interpretation of data for radium and other radionuclides; 

Sandra Embrey, for microbiological data; and Stacey 

Archfield, for assistance with statistical methods and analysis. 

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Appendix 1 83

Appendix 1. Summary of selected studies 

on domestic well water quality in the 

United States

Table

1–1. Summary of selected studies on domestic well water quality in the United States ...............84
84 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Table 1–1. Summary of selected studies on domestic well water quality in the United States.—Continued

[Geographic scale: Number in parentheses is number of states or counties in study area. U.S., United States. Targeted contaminants: Bact, fecal indicator bacteria; 

compounds. Study findings: Percentage of wells with concentrations greater than USEPA MCLs: Value for total coliform bacteria is any detection, MCL for 

maximum contaminant level; mg/L, milligram per liter; μg/L, microgram per liter; CDC, U.S. Centers for Disease Control and Prevention; USGS, U.S. Geological 

State Geographic scale Targeted contaminants Sampling design Target population or area(s) 

Multiple states National Bact, NO3, Majors, TE, Rad, 

Pest

Random stratified Rural households 

National NO3, Pest Random and 

random stratified

Rural households 

Multiple states Pest Random stratifed Counties where alachlor was sold 

Multiple states (12) in 

the midwestern U.S.

NO3, Pest Non-random Agriculturual areas and shallow aquifers 

Multiple states (6) All available data Compilation of 

existing data

-- 

Multiple states (5) in 

the midwestern U.S.

NO3, Pest Voluntary testing -- 

Multiple states (9) in 

the midwestern U.S.

Bact, NO3, Pest Random and 

random stratified

-- 

Multiple states (4) in 

the southeastern U.S.

As Voluntary testing -- 

National NO3, Majors, TE, Pest, VOCs, 

Rn, Rad

Non-random -- 

Alabama Statewide NO3 Voluntary testing -- 

County Bact Non-random -- 

California County Bact, NO3, TE, VOCs, Rad Voluntary testing Specific focus areas 

Georgia Multiple counties (61) NO3, Majors Non-random Counties with intensive agriculture 

Statewide Bact, NO3 Voluntary testing -- 

Idaho Statewide NO3 Unknown -- 

Statewide As Stratified random -- 

Statewide NO3 Stratified random -- 

Illinois Multiple counties (5) NO3, Pest Stratified random Agricultural areas with various aquifer 

depths

Multiple counties (74) NO3, Pest Random -- 

Statewide F Voluntary testing -- 

Iowa Statewide Bact, NO3, Majors, Pest Stratified random Rural households 

Statewide Rn Random 

Statewide Bact, NO3, Pest, As Random Wells serving multiple households 

Kansas Statewide NO3, Majors, Rad Non-random -- 

Multiple counties (49) NO3, Majors, TE, Pest, VOCs Non-random Farm households 

Multiple counties (33) NO3, Pest Non-random Agricultural areas with shallow water table 

and permeable sediments

Kentucky Statewide NO3, Pest Voluntary testing -- 

Lousiana Statewide NO3, Majors, TE, Pest, VOCs, 

others

Non-random 

stratified

-- 
Appendix 1 85

 NO3, nitrate; Majors, major ions; TE, trace elements; Rn, radon; Rad, radioactivity or radionuclides other than radon; Pest, pesticides; VOCs, volatile organic 

 nitrate is 10 mg/L as N, and MCL for arsenic is 10 μg/L; numbers have been rounded to two significant digits. USEPA, U.S. Environmental Protection Agency; MCL, 

 Survey; Coop. Ext., University Cooperative Extension Program; --, not available or not applicable; ~, approximately]

Study findings: Percentage of 

wells with concentrations greater 

Number of than USEPA MCLs

Agency or agency type Reference wells Total 

coliform Nitrate Arsenic

bacteria

1,057 USEPA 42 4.1 -- U.S. Environmental Protection Agency (1984)

734 USEPA -- 2.4 -- U.S. Environmental Protection Agency (1990a, 

1990b, 1992)

1,430 Industry -- 4.9 -- Holden and others (1992)

303 USGS -- 6.0 -- Kolpin and others (1994)

-- U.S. General Accounting Office 14–46 3.2–28.2 -- U.S. General Accounting Office (1997)

34,759 University -- 3.4 -- Richards and others (1996)

5,520 CDC 41 13 -- U.S. Centers for Disease Control and Prevention (1998)

1

179 University -- -- 6.0 Shiber (2005)

~2,000–15,000 USGS -- 8.4 11 Focazio and others (2006)

1,021 Coop. Ext. -- 1.0–2.0 -- Liu and others (2005)

50 University 78 8.0 -- Conine and others (1989)

398 State 28 1.8 3.8 State Water Resources Control Board (2005)

2,588 State and University -- 1.7 -- Stuart and others (1995)

2

1,413 Coop. Ext. 41 3.6 -- Sonan and others (2005)

2,524 University and other -- 6.0 -- Mahler and Loeffelman (1999)

3

~1,900 State and USGS -- 5.0 ~15 Hagan (2004)

4

1,868 State and USGS -- 5.0 -- Neely (2005)

240 State -- 18 -- Schock and others (1992); Mehnert and others (1995)

337 State -- 11 -- Goetsch and others (1992) in Ray and Schock (1996)

8,519 State -- -- Mallatt and others (2003)

686 State and University 45 18 -- Hallberg and others (1992)

352 University -- -- Field and Kross (1998) and Field (1996)

103 University and USGS 30 23 2.9 Iowa Department of Natural Resources (2004)

5

766 USGS and State -- 14 -- Spruill (1983)

103 University -- 28 -- Steichen and others (1988)

78 State -- 32 -- Townsend and others (1998)

4,859 Coop. Ext., State, and other -- 4.6 -- Carey and others (1993)

6

194 State -- 0 1.0 Louisiana Department of Environmental Quality (2003)
86 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Table 1–1. Summary of selected studies on domestic well water quality in the United States.—Continued

[Geographic scale: Number in parentheses is number of states or counties in study area. U.S., United States. Targeted contaminants: Bact, fecal indicator bacteria; 

compounds. Study findings: Percentage of wells with concentrations greater than USEPA MCLs: Value for total coliform bacteria is any detection, MCL for 

maximum contaminant level; mg/L, milligram per liter; μg/L, microgram per liter; CDC, U.S. Centers for Disease Control and Prevention; USGS, U.S. Geological 

State Geographic scale Targeted contaminants Sampling design Target population or area(s) 

Maine Statewide VOCs Random -- 

Maryland County Bact, NO3 Required testing -- 

Minnesota Statewide NO3, Pest Non-random Agricultural areas with shallow water table 

and susceptible aquifers

Missouri Multiple counties 

(~17)

NO3 Non-random 

stratifed

Agricultural areas with various soil and 

aquifer types

Montana Multiple counties (38) Bact, NO3 Voluntary testing -- 

Nebraska Statewide VOCs Non-random Wells near waste-disposal sites 

Statewide Bact, NO3, F, Pest Stratified random -- 

New Hampshire Statewide As Random -- 

Multiple counties (3) As Stratified random Bedrock wells 

New Jersey Statewide Bact, NO3, TE, VOCs Required testing -- 

New Mexico Multiple counties (4) Bact, NO3, F, As, others Non-random Wells near potential sources, high population 

areas, and in vulnerable aquifers

New York Multiple counties (2) Pest Non-random Wells near potential sources and other wells 

North Carolina Multiple counties (12) NO3, Pest Non-random Wells near row-crop farming 

Statewide NO3 Voluntary testing Wells near intensive livestock operations 

Ohio Statewide NO3, Pest Voluntary testing -- 

Oregon Multiple counties (2) NO3 Non-random Intensive agricultural areas 

Pennsylvania County Rn Non-random -- 

Statewide NO3, Rn, Pb Unknown -- 

South Carolina Multiple counties (21) NO3, VOCs Non-random Wells near toxic release sites 

Tennesee Statewide Bact, NO3, others Non-random Farm households 

Utah Statewide Bact, NO3, Pest Voluntary testing -- 

Virginia Multiple counties (65) Bact, NO3 Voluntary testing -- 

West Virginia County Bact Random Rural households 

Wisconsin Statewide NO3, Pest Stratified random Dairy farm households 

Statewide NO3, Pest Stratified random -- 

Statewide Bact, NO3, others Non-random Wells near areas of septage disposal or 

densely located septic tanks

Wyoming Statewide Pest Unknown -- 

1

 Number is for bacteria samples only.

2

 88 percent from domestic wells.

3

 67 percent from domestic wells.

4

 About 2/3 from domestic wells.

5

 Percent of samples from domestic wells unspecified.

6

 22 percent from domestic wells.

7

 Number of samples was 3,342.

8

 Not all wells were sampled for arsenic.

9

 At least 30 percent from domestic wells.
Appendix 1 87

 NO , nitrate; Majors, major ions; TE, trace elements; Rn, radon; Rad, radioactivity or radionuclides other than radon; Pest, pesticides; VOCs, volatile organic 3

 nitrate is 10 mg/L as N, and MCL for arsenic is 10 μg/L; numbers have been rounded to two significant digits. USEPA, U.S. Environmental Protection Agency; MCL, 

 Survey; Coop. Ext., University Cooperative Extension Program; --, not available or not applicable; ~, approximately]

Number of 

wells Agency or agency type

Study findings: Percentage of 

wells with concentrations greater 

than USEPA MCLs

Total 

coliform Nitrate Arsenic

bacteria

Reference

946

832

100

226

1,300

63

1,808

992

353

51,028

99

1,111

171

1,595

14,478

89

534

~1,600

70

150

445

9,697

9

155

534

336

50

286

State

County

State

Coop. Ext.

Coop. Ext.

State

University and State

University

USGS, USEPA and State

State

Regional

County

University

State

University

University and State

USGS and State

Coop. Ext.

University

USGS and State

State

Coop. Ext.

University

State

State

University

USGS and State

--

36

--

--

40

--

13

--

--

--

19

--

--

--

--

--

--

--

--

--

33

40

68

--

--

28

--

--

--

23

19

7

5.3

19

2.7

19

--

5.8

10

2.7

25

--

9.0

0.0

2.0

--

1.9

--

10

14

8.0

--

--

--

--

--

--

--

--

13

19

8

2.8

1

--

--

--

--

--

--

--

--

--

--

--

--

--

--

--

--

Maine Department of Human Services (1998)

Tuthill and others (1998)

Klaseus and others (1988)

Sievers and Fulhage (1992)

Bauder and others (1991) and Bauder (1993)

Goodenkauf and Atkinson (1986)

Gosselin and others (1996, 1997, 1999)

Peters and others (1999)

Montgomery and others (2003)

New Jersey Department of Environmental Protection 

(2008)

New Mexico Department of Health (1998)

Suffolk County Department of Health Services (1998)

Maas and others (1995)

North Carolina Department of Health and Human 

Services (2003)

Baker and others (1989)

Mitchell and Harding (1996)

Sloto and Senior (1998)

Swistock and others (1993)

Aelion and Conte (2004)

Carmichael and Bennett (1993)

Riding and Quilter (2004) and others

Boune (2001); Poff and Ross (2000)

Sworobuk and others (1987)

LeMasters and Doyle (1989)

Brook and others (2002)

Borchardt and others (2003)

U.S. Geological Survey (2006a)
88 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

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Appendix 1 91

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THIS PAGE INTENTIONALLY LEFT BLANK
Appendix 2 93

Appendix 2. Characteristics, principal 

aquifers, study units, and assessment studies 

of domestic wells sampled for the NAWQA 

Program, 1991–2004, by aquifer type

Tables

2–1. Characteristics, principal aquifers, study units, and assessment studies of domestic 

wells sampled for the NAWQA Program in aquifer studies, 1991–2004, by 

aquifer rock type.........................................................................................................................94

2–2. Characteristics, principal aquifers, study units, and assessment studies of domestic 

wells sampled for the NAWQA Program in agricultural land-use studies, 1991–2004, 

by aquifer rock type....................................................................................................................99
94 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 Table 2–1. Characteristics, principal aquifers, study units, and assessment studies of domestic wells sampled for the NAWQA Program in aquifer studies, 1991–2004, by aquifer rock type.—Continued [Well and water depths in parentheses are single values only. Numbers of wells for well depth, depth to top of open interval, and depth to water may differ. --, not applicable or not available] Principal aquifer NAWQA study unit Study code Number of wells Median well depth, in feet below land surface Median depth to top of open interval, in feet below land surface Median depth to water, in feet below land surface Sampling period Basin-fill and other non-glacial sand and gravel aquifers (BFSG) Basin and Range basin-fill aquifers (BR) Central Arizona Basins (CAZB) cazbsus1a 17 377 320 251 1996–97 cazbsus1b 18 215 217 65 1996–97 cazbsus2 25 340 230 199 1998 cazbsus3 17 264 222 107 1996 Great Salt Lake Basins (GRSL) grslsus1a 17 238 142 126 1998 grslsus1b 17 202 180 75 1998 Nevada Basin and Range (NVBR) nvbrsus1 4 300 145 -- 1995 nvbrsus2 3 185 120 -- 1995 nvbrsus3 11 125 120 -- 1995 Central Valley aquifer system (CV) Sacramento River Basin (SACR) sacrsus1 27 155 120 22 1996 San Joaquin–Tulare Basins (SANJ) sanjsus1 30 182 133 84 1995, 2002 High Plains aquifer (HP) High Plains aquifer (HPGW) hpgwsus1a 74 263 215 158 1999 hpgwsus1b 46 204 163 137 2001 hpgwsus1c 104 205 183 83 2002–03 hpgwsus2 20 74 64 30 1999 hpgwsus4 27 190 170 89 2003 hpgwsus5 27 113 100 15 2004 Mississippi River Valley alluvial aquifer 

(MV)

Mississippi Embayment (MISE) misesus1 2 70 (30) (12) 1998

misesus3 3 130 105 11 1998

Northern Rocky Mountains Intermontane Basins aquifer system (NR) Northern Rockies Intermontane Basins (NROK) nroksus1 29 137 130 86 1999 nroksus2 28 58 50 26 2001Other stream valley alluvial aquifers 

(OS)

Central Nebraska Basins (CNBR) cnbrsus2 9 50 37 8 1997Upper Colorado River Basin (UCOL) ucolsus1 22 60 48 33 1997

ucolsus2 12 49 43 (5) 1997

Yellowstone River Basin (YELL) yellsus1 1 (19) (6) (11) 1999

Rio Grande aquifer system (RG) Rio Grande Valley (RIOG) riogsus1 25 178 173 12 1995
Appendix 2 95

Table 2–1. Characteristics, principal aquifers, study units, and assessment studies of domestic wells sampled for the NAWQA Program in aquifer studies, 1991–2004, by aquifer rock type.—Continued[Well and water depths in parentheses are single values only. Numbers of wells for well depth, depth to top of open interval, and depth to water may differ. --, not applicable or not available]

Principal aquifer NAWQA study unit Study code Number of wells

Median well 

depth, in 

feet below 

land surface

Median depth 

to top of open 

interval, in 

feet below 

land surface

Median 

depth to 

water, in feet 

below land 

surface

Sampling 

period

Basin-fill and other non-glacial sand and gravel aquifers (BFSG)—Continued

Snake River Plain basin-fill aquifers 

(SPf)

Upper Snake River Basin (USNK) usnksus1 9 67 64 -- 1994

usnksus2 19 145 128 17 1995usnksus3 6 166 112 7 1996

Willamette Lowland basin-fill aquifers 

(WL)

Willamette Basin (WILL) willsus1 69 60 50 16 1993

Glacial sand and gravel aquifers (GLAC)

Eastern glacial aquifers (Ge) Allegheny and Monongahela River Basins (ALMN) almnsus2 30 73 68 -- 1996

Conneticut, Housatonic, and Thames River Basins 

(CONN)

connsus2 28 113 105 25 2002

Delaware River Basin (DELR) delrsus3 12 86 61 21 2001Hudson River Basin (HDSN) hdsnsus1 2 108 -- -- 1994Long Island–New Jersey Coastal Drainages (LINJ) linjsus1 1 (152) (146) -- 1997

Central glacial aquifers (Gc) Lake Erie-Lake Saint Clair Drainages (LERI) lerisus1 27 78 74 30 1998

Lower Illinois River Basin (LIRB) lirbsus1 26 239 235 -- 1996

lirbsus2 28 50 15 41 1996

Great and Little Miami River Basins (MIAM) miamsus1 30 60 60 25 1999Upper Illinois River Basin (UIRB) uirbsus1 27 72 68 22 2001

uirbsus2 23 45 40 10 2000

Western Lake Michigan Drainages (WMIC) wmicsus2 24 82 72 27 2003

West central glacial aquifers (Gwc) Eastern Iowa Basins (EIWA) eiwasus2 32 52 51 11 1998

Red River of the North Basin (REDN) rednsus1 5 62 56 -- 1992, 1995

rednsus2 15 65 58 7 1993rednsus3 3 60 -- (24) 1995rednsus5 25 109 111 19 1991–94

Western glacial aquifers (Gw) Cook Inlet Basin (COOK) cooksus1a 22 105 98 64 1999

Puget Sound Basin (PUGT) pugtsus1 29 68 64 22 1996
96 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 Table 2–1. Characteristics, principal aquifers, study units, and assessment studies of domestic wells sampled for the NAWQA Program in aquifer studies, 1991–2004, by aquifer rock type.—Continued [Well and water depths in parentheses are single values only. Numbers of wells for well depth, depth to top of open interval, and depth to water may differ. --, not applicable or not available] Principal aquifer NAWQA study unit Study code Number of wells Median well depth, in feet below land surface Median depth to top of open interval, in feet below land surface Median depth to water, in feet below land surface Sampling period Coastal plain aquifers in semi-consolidated sand (CP) Coastal lowlands aquifer system (CL) Acadian–Pontchartrain Drainages (ACAD) acadsus1 29 140 128 27 2000–01 acadsus2 27 120 113 (90) 2001 Trinity River Basin (TRIN) trinsus3 29 180 155 39 1994, 2002 North Atlantic Coastal Plain aquifer system (NA) Albemarle–Pamlico Drainage Basin (ALBE) albesus1 1 (20) -- (11) 1994 albesus7 6 90 75 17 2002–2003 Delmarva Peninsula (DLMV) dlmvsus1 13 49 40 8 2001 Long Island–New Jersey Coastal Drainages (LINJ) linjsus2 30 100 90 5 1998 Potomac River Basin and Delmarva Peninsula (PODL) podlsus2 15 81 73 36 2003 Southeastern Coastal Plain aquifer system (SC) Mobile River Basin (MOBL) moblsus1 24 144 121 53 1999 Santee River Basin and Coastal Drainages (SANT) santsus1 2 125 105 57 1997 Texas coastal uplands aquifer system (TC) Trinity River Basin (TRIN) trinsus2 10 102 (20) 25 1994 Sandstone aquifers (SS) Cambrian-Ordovician aquifer system (CO) Upper Mississippi River Basin (UMIS) umissus3 25 180 126 31 1996 umissus4 25 200 175 88 1996 Western Lake Michigan Drainages (WMIC) wmicsus1 20 128 97 35 1995

Early Mesozoic basin aquifers (EM) Delaware River Basin (DELR) delrsus1 25 160 40 25 1999

Long Island–New Jersey Coastal Drainages (LINJ) linjsus3 22 193 50 (14) 1998–99Potomac River Basin (POTO) potosus2 22 155 45 29 1994

Lower Tertiary/Upper Cretaceous 

aquifers (LT)

Red River of the North Basin (REDN) rednsus5 1 (127) (123) (48) 1994Yellowstone River Basin (YELL) yellsus2 23 160 80 27 2000–01
Appendix 2 97

Table 2–1. Characteristics, principal aquifers, study units, and assessment studies of domestic wells sampled for the NAWQA Program in aquifer studies, 1991–2004, by aquifer rock type.—Continued[Well and water depths in parentheses are single values only. Numbers of wells for well depth, depth to top of open interval, and depth to water may differ. --, not applicable or not available]

Principal aquifer NAWQA study unit Study code Number of wells

Median well 

depth, in 

feet below 

land surface

Median depth 

to top of open 

interval, in 

feet below 

land surface

Median 

depth to 

water, in feet 

below land 

surface

Sampling 

period

Sandstone aquifers (SS)—Continued

Pennsylvanian aquifers (P) Alleheny and Monongahela River Basin (ALMN) almnsus1 30 105 21 45 1996

Kanawha–New River Basins (KANA) kanasus1 16 92 29 63 1997

Valley and Ridge sandstone and shale 

aquifers (VRs)

Delaware River Basin (DELR) delrsus2 27 185 82 41 2000Lower Susquehanna River Basin (LSUS) lsussus1 29 155 60 39 1993

Sandstone and carbonate-rock aquifers (SS/CARB)

Edwards-Trinity aquifer system (ET) South-Central Texas (SCTX) sctxsus1 21 295 180 130 1996

sctxsus2 25 400 123 170 1996sctxsus3 1 (1,500) (250) -- 1997

Trinity River Basin (TRIN) trinsus1 22 162 125 26 1994

Mississippian aquifers (M) Kanawha–New River Basins (KANA) kanasus1 5 205 42 101 1997

Lake Erie–Lake Saint Clair Drainages (LERI) lerisus1 1 -- -- -- 1998Lower Tennessee River Basin (LTEN) ltensus1 8 86 56 38 1999

ltensus2 4 99 58 35 2000

Carbonate-rock aquifers (CARB)

Floridan aquifer system (F) Apalachicola–Chattahoochee-Flint River Basin 

(ACFB)

acfbsus1 7 100 69 34 1995

Georgia-Florida Coastal Plain (GAFL) gaflsus2 30 93 65 26 2002–03

gaflsus3 30 150 95 28 2002

Santee River Basin and Coastal Drainages (SANT) santsus2 29 170 90 18 1998

Ordovician aquifers (O) Lower Tennessee River Basin (LTEN) ltensus2 21 140 20 21 2000Ozark Plateaus aquifer system (OP) Ozark Plateaus (OZRK) ozrksus2a 33 196 74 54 1993

ozrksus3a 16 140 77 49 1993

Silurian-Devonian aquifers (SD) Eastern Iowa Basins (EIWA) eiwasus1 32 223 150 59 1996Valley and Ridge carbonate-rock 

aquifers (VRc)

Kanawha–New River Basins (KANA) kanasus2 1 (325) (183) (157) 1997Upper Tennessee River Basin (UTEN) utensus1 18 245 64 73 1998–99
98 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 Table 2–1. Characteristics, principal aquifers, study units, and assessment studies of domestic wells sampled for the NAWQA Program in aquifer studies, 1991–2004, by aquifer rock type.—Continued [Well and water depths in parentheses are single values only. Numbers of wells for well depth, depth to top of open interval, and depth to water may differ. --, not applicable or not available] Principal aquifer NAWQA study unit Study code Number of wells Median well depth, in feet below land surface Median depth to top of open interval, in feet below land surface Median depth to water, in feet below land surface Sampling period Basaltic-rock aquifers (BAS) Snake River Plain basaltic-rock aquifers (SPb) Upper Snake River Basin (USNK) usnksus1 14 280 117 183 1992–95 usnksus2 5 210 50 -- 1995 Crystalline-rock aquifers (CRYS) New England crystalline-rock aquifers (NEx) Connecticut, Housatonic, and Thames River Basins (CONN) connsus1 27 225 35 18 1995 Long Island–New Jersey Coastal Drainages (LINJ) linjsus1 28 201 59 9 1997 New England Coastal Basins (NECB) necbsus1 28 188 25 12 1999–2000 necbsus2 30 303 35 23 1999–2000 Piedmont and Blue Ridge crystalline- rock aquifers (PBx) Kanawha–New River Basins (KANA) kanasus2 18 159 49 44 1995 Lower Susquehanna River Basins (LSUS) lsussus2 30 147 69 42 1994 Potomac River Basin (POTO) potosus1 23 142 44 36 1997 Santee River Basin and Coastal Drainages (SANT) santsus3 29 200 61 26 1997 Rocky Mountain Front Range crystalline-rock aquifers (RMx) South Platte River Basin (SPLT) spltsus1 27 225 82 -- 1995 Aquifer not determined Aquifer not determined Hudson River Basin (HDSN) hdsnsus1 44 125 (91) 6 1994 Upper Snake River Basin (USNK) usnksus2 1 (615) (504) -- 1995 usnksus3 1 (75) (52) (5) 1995
Appendix 2 99

Table 2–2. Characteristics, principal aquifers, study units, and assessment studies of domestic wells sampled for the NAWQA Program in agricultural land-use studies, 1991–2004, by aquifer rock type.—Continued[Well and water depths in parentheses are single values only. Numbers of wells for well depth, depth to top of open interval, and depth to water may differ. --, not applicable or not available]

Principal aquifer NAWQA study unit Study code

Primary 

agricultural 

land use

Number 

of wells

Median 

well depth, 

in feet 

below land 

surface

Median depth 

to top of open 

interval, in 

feet below 

land surface

Median 

depth to 

water, in feet 

below land 

surface

Sampling 

period

Basin-fill and other non-glacial sand and gravel aquifers (BFSG)

Central Valley aquifer system (CV) San Joaquin–Tulare Basins (SANJ) sanjluscr1a Row crops 23 147 110 76 1995, 2002

sanjlusor1a Vineyards 27 155 120 47 1993, 2001sanjlusor2a Almonds 26 155 118 39 1994, 2001

Columbia Plateau basin-fill aquifers 

(CPf)

Central Columbia Plateau (CCPT) ccptlusag2a Row crops 12 126 109 (30) 1993–94

ccptlusor1a Orchards 9 160 141 121 1994–95

Other stream valley alluvial aquifers 

(OS)

Mobile River Basin (MOBL) moblluscr1 -- 1 (40) (30) (25) 2001Potomac River Basin (POTO) potolusag2 -- 1 (71) -- -- 1995

Snake River Plain basin-fill aquifers 

(SPf)

Upper Snake River Basin (USNK) usnkluscr1 Row crops 25 35 32 9 1993

usnkluscr3 Row crops 1 (180) (121) (77) 1994

Glacial sand and gravel aquifers (GLAC)

West central glacial aquifers (Gwc) Red River of the North Basin (REDN) rednlusag1 -- 2 54 47 -- 1994

Upper Mississippi River Basin (UMIS) umisluscr1 Row crops 3 28 (53) (27) 1998

Western glacial aquifers (Gw) Puget Sound Basin (PUGT) pugtluscr1 Raspberries 3 41 35 18 1997–98

Coastal plain aquifers in semi-consolidated sand (CP)

North Atlantic Coastal Plain aquifer 

system (NA)

Albemarle–Pamlico Drainage Basin 

(ALBE)

albelusag2 Row crops 10 118 75 (3) 1994

Delmarva Peninsula (DLMV) dlmvluscr1 Row crops 2 37 31 21 2001

Sandstone aquifers (SS)

Valley and Ridge sandstone and shale 

aquifers (VRs)

Potomac River Basin (POTO) potolusag2 -- 23 122 31 29 1996

Carbonate-rock aquifers (CARB)

Ozark Plateaus aquifer system (OP) Ozark Plateaus (OZRK) ozrklusag1a Cattle pasture 19 165 -- 45 1994

ozrklusag2a Poultry pasture 20 180 28 34 1995

Piedmont and Blue Ridge carbonaterock aquifers (PBc) Lower Susquehanna River Basin (LSUS) lsuslusag1 -- 29 160 102 33 1993
100 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 Table 2–2. Characteristics, principal aquifers, study units, and assessment studies of domestic wells sampled for the NAWQA Program in agricultural land-use studies, 1991–2004, by aquifer rock type.—Continued [Well and water depths in parentheses are single values only. Numbers of wells for well depth, depth to top of open interval, and depth to water may differ. --, not applicable or not available] Principal aquifer NAWQA study unit Study code Primary agricultural land use Number of wells Median well depth, in feet below land surface Median depth to top of open interval, in feet below land surface Median depth to water, in feet below land surface Sampling period Carbonate-rock aquifers (CARB)—Continued Valley and Ridge carbonate-rock aquifers (VRc) Lower Susquehanna River Basin (LSUS) lsuslusag2 -- 29 170 100 64 1994 lsuslusag3 -- 29 158 109 56 1995 Potomac River Basin and Delmarva Peninsula (PODL, POTO) potolusag1 -- 29 144 57 43 1993 potolusag2 -- 1 (280) (42) -- 1995 Basaltic-rock aquifers (BAS) Columbia Plateau basaltic-rock aquifers (CPb) Central Columbia Plateau (CCPT) ccptlusag1a Row crops 16 102 32 32 1993–94 ccptlusag2a Row crops 17 157 74 (96) 1993–94 ccptlusor1a Orchards and vineyards

9 170 98 30 1994–95

Snake River Plain basaltic-rock 

aquifers (SPb)

Upper Snake River Basin (USNK) usnkluscr2 Row crops 27 228 63 180 1993

usnkluscr3 Row crops 26 205 20 157 1994usnkluscr4 Row crops 14 357 19 292 1995

Aquifer not determined

Aquifer not determined Central Columbia Plateau (CCPT) ccptlusag1a -- 1 (60) -- -- 1994

Upper Snake River Basin (USNK) usnkluscr2 Row crops 1 -- -- (132) 1995

usnkluscr4 Row crops 1 (420) (18) -- 1995
Appendix 3 101

Appendix 3. Sampling and analytical methods 

and summary of field quality control data for 

chemical contaminants measured in domestic 

wells sampled for the NAWQA Program, 

1991–2004

Contents

Sampling and Analytical Methods..........................................................................................................102

Summary of Field Quality Control Data ..................................................................................................103

Field Quality Control Data Overview..............................................................................................103

Major Ions..........................................................................................................................................104

Trace Elements..................................................................................................................................104

Nutrients.............................................................................................................................................104

Radon and Other Radionuclides.....................................................................................................105

Organic Compounds.........................................................................................................................105

Pesticides..................................................................................................................................105

Volatile Organic Compounds..................................................................................................106

Fecal Indicator Bacteria..................................................................................................................106

References Cited........................................................................................................................................106
102 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

titration with sulfuric acid (Koterba and others, 1995; U.S. 

Geological Survey, variously dated). For about 25 percent 

of samples in this study, alkalinity [in mg/L as calcium 

carbonate (CaCO3)] was calculated from reported bicarbonate 

concentrations. Samples collected for analysis of the fecal 

indicator bacteria, total coliform bacteria and Escherichia 

coli (E. coli), were processed on site within 6 hours of 

collection (U.S. Geological Survey, variously dated; Embrey 

and Runkle, 2006). Total coliform bacteria were enumerated 

using mENDO medium (method 9222B, American Public 

Health Association and others, 1992) or MI medium (method 

1604, U.S. Environmental Protection Agency, 2002c); E. coli

were enumerated using NA-MUG medium (method 9222G, 

American Public Health Association and others, 1992), mTEC 

medium (method 1103.1, U.S. Environmental Protection 

Agency, 2002b), or MI medium.

Water samples were analyzed at the USGS National 

Water Quality Laboratory (NWQL) in Lakewood, Colorado, 

for inorganic and organic analytes. The inorganic analytes 

and methods used were: major ions, by atomic absorption 

spectrometry (AA), colorimetry, or inductively-coupled 

plasma (ICP) (Fishman and Friedman, 1989; Fishman, 1993); 

nutrients, by various methods (Fishman, 1993; Patton and 

Truitt, 2000); trace elements, by ICP-atomic emission spectroscopy (ICP-AES), ICP-mass spectrometry (ICP-MS), 

graphite-furnace AA, or hydride generation AA (Fishman and 

Friedman, 1989; Faires, 1993; McClain, 1993; Garbarino, 

1999); and radon, by liquid scintillation (American Society for 

Testing and Materials, 1996). Dissolved organic carbon was 

analyzed by ultraviolet-light-promoted persulfate oxidation 

and infrared spectrometry (Brenton and Arnett, 1993). Iron 

and manganese were analyzed using methods for major ions 

and trace elements; data used in this study were the results of 

the trace-element methods because these had lower reporting 

limits than the methods for major ions. Nitrate was analyzed 

as nitrite plus nitrate; however, nitrite concentrations were 

very low, averaging less than 0.1 percent of nitrite plus nitrate 

concentrations. Consequently, nitrite plus nitrate is referred to 

as nitrate in this report.

Pesticides were analyzed by two methods at the NWQL: 

47 pesticides were analyzed by solid-phase extraction using 

a C-18 cartridge and gas chromatography/mass spectrometry 

(GCMS) (Zaugg and others, 1995), and 36 pesticides and 

pesticide degradates were analyzed by graphitized carbonbased solid-phase extraction and high-performance liquid 

chromatography/mass spectrometry (HPLC) (Werner and 

others, 1996; Furlong and others, 2001). Data in this study 

for carbaryl, carbofuran, and linuron, which were analyzed by 

both methods, were the results of analysis by GCMS. Results 

from HPLC analyses during March 1999 to March 2000, in 

which sample holding times were exceeded (E.T. Furlong 

and others, U.S. Geological Survey, written commun., 2003), 

were excluded from the study. Nearly all VOCs were analyzed using purge and trap capillary column GCMS (Rose and 

Schroeder, 1995; Connor and others, 1998). Another method 

(USGS 0-3120-90; Fishman, 1993) was sometimes used 

Sampling and Analytical Methods

The sampling methods used in the NAWQA assessment 

studies are described in Koterba and others (1995), U.S. Geological Survey (variously dated), Kolpin and others (1998), 

Embrey and Runkle (2006), and Moran and others (2006). 

Wells were pumped until a minimum volume of water was 

removed (at least three well-casing volumes in most cases) 

and the field measurements of pH, water temperature, specific 

conductance, and dissolved oxygen had stabilized. After diversion from the well, the water contacted only Teflon, stainless 

steel, glass, or aluminum materials and was processed in an 

environmental chamber to minimize contamination. A low 

flow rate (about 0.1 gallon or 500 milliliters per minute) was 

used for sampling. Equipment was cleaned after sampling 

following procedures described in Koterba and others (1995). 

Samples were analyzed for alkalinity, major ions, nutrients, 

dissolved organic carbon, trace elements, pesticides, VOCs, 

and radon; for some wells, the samples were analyzed for fecal 

indicator bacteria, gross alpha- and beta-particle radioactivities, and radium. 

Samples collected for analysis of alkalinity, major ions, 

dissolved nutrients, trace elements, gross alpha- and beta-particle radioactivities, and radium were filtered on site through 

0.45-micron capsule or plate filters. Samples for analysis of 

major cations and trace elements were preserved with nitric 

acid to pH less than 2; samples for analysis of major anions 

were not preserved. Samples for analysis of nutrients were 

collected in brown polyethylene bottles and were immediately 

chilled on ice for shipment to the laboratory. Before 1995, 

nutrient samples were preserved using mercuric chloride for 

all analytes; thereafter, samples were not preserved except for 

those for analysis of total concentrations, which, after 1998, 

were preserved with sulfuric acid to pH less than 2. Samples 

collected for analysis of pesticides were filtered through 

0.7-micron baked glass-fiber filters into amber-colored glass 

bottles and were immediately chilled. Samples for analysis of 

VOCs, radon, and fecal indicator bacteria were not filtered. 

VOC samples were collected into 40-milliliter septum vials 

with no headspace, preserved with hydrochloric acid to pH 

less than 2, and immediately chilled. Radon samples were 

collected using a pressurized in-line sampling assembly and a 

gas-tight syringe and were injected into glass scintillation vials 

below mineral oil. Samples for analysis of dissolved organic 

carbon were filtered through 0.45-micron silver membrane 

filters, and silver from the filters served as a preservative; 

samples were immediately chilled on ice. Samples for analysis 

of fecal indicator bacteria were collected in sterile, ambercolored bottles and immediately chilled. 

The physical properties pH, water temperature, specific 

conductance, and dissolved oxygen were measured at the 

time of sampling in a flow-through chamber using methods 

described in the U.S. Geological Survey National Field 

Manual (U.S. Geological Survey, variously dated). Alkalinity 

(carbonate alkalinity) was determined on site with incremental 
Appendix 3 103

for ethylene dibromide (EDB) and dibromochloropropane 

(DBCP) to achieve lower detection limits for these compounds 

(Moran and others, 2005). Gross alpha- and beta-particle 

radioactivity were analyzed at the NWQL by low background 

Planchet counting post-evaporation (USEPA method 900.0, 

U.S. Environmental Protection Agency, 1980). Radium isotopes (radium-226 and radium-228) were analyzed by coprecipitation and alpha planchet counting, radon de-emanation, 

or beta counting after progeny ingrowth (U.S. Environmental 

Protection Agency methods 903.0, 903.1, and 904.0, respectively; U.S. Environmental Protection Agency, 1980) or by 

alpha and gamma spectral methods (Focazio and others, 2001; 

Szabo and others, 2005; Zoltan Szabo, U.S. Geological Survey, written commun., 2007).

Analytical results from the NWQL for the analytes 

measured during the study period were given using several 

reporting conventions. Major ions, nutrients, trace elements, 

and radon were reported primarily in terms of minimum 

reporting levels (MRLs). MRLs were defined for the NWQL 

as the minimum concentration of a constituent that can be 

reliably measured with an analytical method (Childress and 

others, 1999). MRLs for individual analytes were established 

using various methods that may have included statistical 

analysis of quality-control samples. Pesticides and VOCs were 

reported relative to MRLs that were statistically determined 

and, towards the end of the study period, relative to long-term 

method-detection levels (LT-MDLs). The LT-MDL is similar to the method detection level (MDL) defined by the U.S. 

Environmental Protection Agency in that it is statistically 

determined as the minimum concentration of an analyte that 

can be measured and reported with 99-percent confidence that 

the concentration is greater than zero (Childress and others, 

1999). In other words, for analytical results at or above the 

MDL or LT-MDL, there is at most a 1-percent chance that an 

analyte is reported as detected when it is not there (false positive). The LT-MDL differs from the USEPA-defined MDL in 

that it accounts for more potential sources of variability, for 

example, from multiple analytical instruments and operators; 

the LT-MDL is recalculated each year. For methods that incorporated the use of LT-MDLs, analytical results were reported 

in terms of LT-MDLs and laboratory reporting levels (LRLs). 

The LRL is the concentration at which there is no more than a 

1-percent chance that an analyte will not be detected when it is 

actually present (false negative; the chance of false negatives 

occurring at the LT-MDL is 50 percent). The LRL is about 

two times higher than the LT-MDL. For methods that use LTMDLs, non-detections were reported relative to the LRL by 

the NWQL and were stored in this way in the NAWQA Data 

Warehouse; positive detections at concentrations less than the 

LRL were not censored but were reported as estimated values 

(Childress and others, 1999). Analytical results less than the 

lowest calibration standard or greater than the highest calibration standard also were reported as estimated. The uncertainty 

associated with estimated values is expected to be higher than 

that of other, unqualified concentrations (Childress and others, 1999). In this study, estimated values were used without 

qualification, and non-detections that were reported relative 

to LRLs were redefined relative to their associated LT-MDLs 

using information on historical LT-MDLs for analytical 

methods from the NWQL. The LT-MDL and LRL reporting 

conventions were phased into use at the NWQL beginning in 

October 1998. Radon, near the end of the study period, and 

radium isotopes, throughout the study period, were reported 

relative to sample-specific minimum detectable concentrations 

(SSMDC; Zoltan Szabo, U.S. Geological Survey, written 

commun., 2006).

Summary of Field Quality Control Data

Field Quality Control Data Overview

The collection of field quality-control (QC) samples 

was an integral part of NAWQA assessment studies. For each 

assessment study of about 25 to 30 wells, the study design 

included routine collection of at least 2 to 3 field blanks for 

all analytes except radionuclides; 2 to 3 replicates for major 

ions, nutrients, trace elements, and radon; 2 to 3 field-spiked 

samples and field-spike replicates for pesticides and VOCs; 

1 source-solution blank for DOC, and trip blanks and (or) 

source-solution blanks for VOCs (Koterba and others, 1995). 

Field blanks were collected by passing water that did not 

contain the analyte(s) of interest (except for some VOCs) 

through sampling equipment in the field; these can be used to 

assess the introduction of contamination into environmental 

samples during sampling and analysis. Trip blanks can be used 

to assess contamination from sample shipping and handling. 

Replicate samples are sequentially collected ground-water 

samples, which can be used to assess the effects of sampling 

and analysis procedures on measurement variability. Fieldspiked samples are environmental samples to which a known 

amount of the analyte(s) of interest was added in the field; 

these can be used to assess bias from matrix interferences or 

analyte loss during sample holding and processing. Spiked 

samples also were routinely prepared in the laboratory using 

environmental samples or other solutions. The USGS Branch 

of Quality Systems also routinely submitted and analyzed the 

results from blind samples for inorganic and organic analytes 

to the USGS NWQL during the study period (http://bqs.usgs.

gov). Field QC sample data have been evaluated in reports 

by individual study units (http://water.usgs.gov/nawqa) and 

also have been evaluated at the national scale for nutrients, 

trace elements, pesticides, VOCs, fecal-indicator bacteria, and 

radionuclides. QC results for various contaminants in ground 

water sampled in the NAWQA Program are described in this 

section; the discussion is based on summaries at the national 

scale, where available.

Environmental data determined to be affected by contamination are specifically identified in the NAWQA Data 

Warehouse (“V-coded” data); these data were not used in the 
104 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

present study of domestic-well water quality. Criteria for identifying these data were as follows: there was direct evidence 

of contamination, concentrations were significant compared 

to environmental concentrations, and the contamination was 

well understood, for example in terms of source and magnitude (U.S. Geological Survey, 1997). Analyses of field blanks, 

of other QC samples, and of sampling processes were used 

by study unit and national synthesis personnel to identify 

data that were affected by contamination. Environmental data 

were rarely V-coded; about 0.1 percent of analytical results 

(excluding radionuclides or fecal indicator bacteria) for the 

domestic wells in the present study received the V-code designation. Analytes with the most V-coded analyses included (in 

descending order) aluminum, chloroform, 1,2,4-trimethylbenzene, zinc, ammonia, tetrachloroethene, copper, orthophosphate, 1,2-dichloropropane, toluene, chromium, nitrite plus 

nitrate, and several other VOCs. 

Major Ions

Concentrations of major ions in water samples from 

domestic wells appeared unaffected by contamination 

and were reproducible based on a review of field QC data 

described in reports from 20 NAWQA study units. Most 

major ions were either not detected in blank samples or were 

detected at concentrations much lower than those in environmental samples (for example, see Menheer and Brigham, 

1997; Hamlin and others, 2002; Bruce and McMahon, 1998; 

Reutter and Dunn, 2000; Fong and others, 1998; Apodaca and 

Bails, 2000). Silica was detected in blanks in a few study units 

at concentrations near those in environmental samples (Savoca 

and others, 1999; Inkpen and others, 2000), but these findings 

were not widespread. The variability of measured concentrations of major ions was low, with relative percent differences 

(RPDs) of replicate pairs nearly always less than 10 percent; 

larger RPDs sometimes occurred when concentrations were 

near MRLs (for example, see Coes and others, 2000; Glass, 

2001; Pope and others, 2001; Robinson, 2003; Bexfield and 

Anderholm, 1997; Milby-Dawson, 2001). In many cases, 

RPDs of replicate pairs were less than 5 percent. 

Trace Elements

QC data for 23 trace elements were evaluated at the 

national scale to estimate bias and variability in all water 

samples collected by the NAWQA Program from 1991 to 

2002 (Apodaca and others, 2006). The number of field blanks 

and replicates varied by trace element, but there were about 

350 blanks and about 275 to 350 replicate-sample pairs for 

most trace elements. Potential contamination in ground-water 

samples was estimated from field blanks with 95-percent 

confidence to be less or near 1 mg/L in 95 percent of samples 

for many trace elements; this value (1 mg/L) was the MRL for 

many trace elements. Potential contamination for antimony, 

arsenic, beryllium, cadmium, cobalt, lead, molybdenum, 

nickel, selenium, silver, thallium, and uranium was estimated 

to be less than 1 mg/L in 95 percent of samples; for barium, 

chromium, and manganese, potential contamination was 

estimated to be at or near 1 mg/L in 95 percent of samples. 

The potential for contamination was relatively large for 

aluminum, boron, and zinc. Potential contamination for 

these three trace elements was estimated with 95-percent 

confidence to be at or greater than 1 mg/L in at least 

50 percent of samples, and concentrations greater than 9 mg/L 

for aluminum, 15 mg/L for boron, and 18 mg/L for zinc were 

estimated to occur in at least 5 percent of samples. Potential 

contamination for copper, iron, and strontium was estimated 

to be at or greater than 1 mg/L in 75 to 85 percent of samples. 

Too few field blanks were collected for lithium or vanadium 

to estimate potential contamination for these trace elements. 

Except for aluminum, the concentration levels for potential 

contamination in samples analyzed for trace elements were 

less than one-tenth of any drinking-water standards at the 

95- or 99-percent confidence levels (Apodaca and others, 

2006). Variability, represented by the mean relative standard 

deviation (RSD) for all concentration ranges, was estimated to 

be less than 10 percent for antimony, arsenic, barium, boron, 

cobalt, iron, lithium, manganese, molybdenum, strontium, and 

uranium; variability was less than or equal to 15 percent for 

the remaining trace elements aluminum, cadmium, chromium, 

copper, lead, nickel, selenium, vanadium, and zinc. Variability 

could not be estimated for beryllium, silver, or thallium 

because there were too few replicate pairs. This analysis 

indicates that potential contamination and variability are 

unlikely to affect interpretations in this report for most trace 

elements at the national scale.

Nutrients

A national-scale evaluation of field QC data indicated 

that little significant contamination had occurred from field 

or laboratory procedures and low variability for nutrients in 

all ground-water samples collected for the NAWQA Program 

(Mueller and Titus, 2005). The analysis was based on 541 field 

blanks and 520 replicate sample pairs collected along with 

environmental samples from 1991 through 2001. For nitrite 

plus nitrate, ammonia plus organic nitrogen, orthophosphate, 

and total phosphorus, potential contamination was estimated to 

be near the associated MRLs in at least 80 percent of environmental samples, and near the MRLs in at least 75 percent of 

samples for ammonia. Potential contamination in 95 percent of 

samples was estimated with 99-percent confidence to be less 

than 0.09 mg/L as N for nitrite plus nitrate, less than 

0.044 mg/L as N for ammonia, less than the MRL of 

0.2 mg/L as N for ammonia plus organic nitrogen, and less 

than 0.02 mg/L as P for orthophosphate. Variability was estimated separately for low and high concentration ranges using 

the standard deviation (SD; low concentrations) or RSD (high 

concentrations) of replicate-sample pairs. For high concentrations, average variability was estimated to be 2.9 percent for 
Appendix 3 105

nitrite plus nitrate (concentrations greater than 1 mg/L as N), 

1.3 percent for ammonia (concentrations greater than 0.3 mg/L 

as N), 7.8 percent for ammonia plus organic nitrogen (concentrations greater than 0.5 mg/L as N), and 10 percent for 

orthophosphate (concentrations greater than 0.1 mg/L as P). 

For low concentrations, variability was estimated to be 

0.043 mg/L as N for nitrite plus nitrate, 0.0047 mg/L as N for 

ammonia, 0.022 mg/L as N for ammonia plus organic nitrogen, and 0.0039 mg/L as P for orthophosphate.

Radon and Other Radionuclides

The variability of radon was estimated from 343 replicate 

ground-water sample pairs collected by the NAWQA Program 

from 1991 to 2002 (Apodaca and others, 2006). The mean 

SD for low concentrations of radon (about 45 to 700 pCi/L) 

was 19.9 pCi/L, and the mean RSD for high concentrations 

(about 700 pCi/L to about 200,000 pCi/L) was 5.9 percent, 

indicating that radon results were reproducible with low variability at most concentrations. The quality of radium-226 and 

radium-228 data is described in Zoltan Szabo, U.S. Geological 

Survey, written commun., 2006.

Organic Compounds

Pesticides

Potential contamination in pesticide samples collected 

by NAWQA was investigated by Martin (1999a and 1999b) 

using field blanks collected during 1992–95. Most groundwater field blanks during this time had no detections of any 

pesticide compound (89 and 98 percent of blanks analyzed by 

GCMS and HPLC, respectively), and most compounds were 

not detected in any blank. Overall, pesticides of any kind were 

detected in 0.3 percent of the analyses of pesticides in blank 

samples (33 detections in 10,458 analyses). Thirty-two of 47 

pesticides analyzed using GCMS were not detected in any 

field blanks (n equal to 145), and 33 of 37 pesticides analyzed using HPLC were not detected in field blanks (n equal 

to 98–104). The most frequently detected pesticides in field 

blanks were p,p’-DDE (4.1 percent), atrazine (2.8 percent), 

and benfluralin, chlorpyrifos, metolachlor, simazine, and triallate (1.4 percent each); all others were detected in 1 percent or 

less of field blanks. Greater use in the environment and lower 

detection limits for pesticides determined using the GCMS 

method, compared to the HPLC method, likely explained 

the higher frequency of detections in blanks with the GCMS 

method (Martin, 1999a). Detected concentrations were very 

low; maximum concentrations of individual pesticides in 

ground-water field blanks ranged from 0.001 μg/L (estimated) 

to 0.02 μg/L. The potential contamination indicated by these 

low detection frequencies and concentrations of pesticides 

in field blanks could affect interpretations of environmental 

data, however, for compounds that occur with comparable low 

frequency and concentrations, such as bromacil, benfluralin, 

chlorphyrifos, dachtal, p,p ́-DDE, diazonon, diuron, fenuron, 

cis-permethrin, pronamide, propanil, triallate, and triflualin 

(Martin, 1999a). Maximum concentrations of any pesticides 

detected in field blanks were two to four orders of magnitude 

less than drinking-water standards or other human-healthbased screening levels (except for p,p ́-DDE, for which the 

health standard was 50 times the maximum concentration in 

blanks), indicating that potential contamination was negligible when evaluating concentrations of potential concern for 

human health.

The variability of detections and concentrations in pesticide samples collected for the NAWQA Program was investigated using field replicate samples collected during 1992–97 

by Martin (2002). The assessment was based on 335 replicate 

sets of samples for GCMS analysis and 253 replicate sets for 

HPLC analysis, and included surface- and ground-water samples. The variability of pesticide detections was described by 

calculating the percentage of replicate sets with inconsistent 

detections for each pesticide compound (that is, sets in which 

a pesticide compound is detected in one replicate sample but 

not in the other), using all detections, including those less than 

the laboratory MRLs or MDLs. The percentage of inconsistent 

replicate sets generally decreased with increasing pesticide 

concentration; it was more likely that compounds would have 

been detected if they were present in the sample at higher concentrations. For all replicate sets analyzed for all pesticides, 

the overall rates of inconsistent replicates were 60 percent for 

pesticide concentrations less than MRLs or MDLs, 14 percent 

for concentrations at or within an order of magnitude of MRLs 

or MDLs, and 1 percent for concentrations greater than 10 

times the MRLs or MDLs. The variability of measured pesticide concentrations also decreased with increasing concentrations. The typical variability of pesticide concentrations in the 

data set studied, measured by the median pooled RSD of field 

replicates with consistent detections, was 11 to 15 percent for 

concentrations less than 1 μg/L and less than 10 percent for 

concentrations greater than 1 μg/L (Martin, 2002).

The bias from matrix interferences or analyte loss during sample shipment or holding was investigated using field 

and laboratory spiked samples and other laboratory QC data. 

Median recovery for most pesticides analyzed by GSMS and 

HPLC ranged from 70 to 106 percent for field matrix spiked 

samples (61 pesticides) and laboratory reagent spikes (70 

pesticides) collected during 1992–96 (Martin, 1999b). For 

several pesticides, laboratory QC data indicated that analytical 

results were considered to be negatively biased or unusually 

variable, and were qualified as “estimated” (Zaugg and others, 

1995; Werner and others, 1996; U.S. Geological Survey, 1998; 

Gilliom and others, 2006). These pesticides were azinphosmethyl, carbaryl, carbofuran, deethylatrazine and terbacil 

(analyzed by GCMS) and aldicarb, aldicarb sulfone, aldicarb 

sulfoxide, chlorothalonil, dichlobenil, and 2-methyl-4,6-dinitrophenol (analyzed by HPLC).
106 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004

Volatile Organic Compounds

Potential contamination of VOC samples collected for the 

NAWQA Program was investigated using analyses of blank 

samples collected during three periods during 1997–2005. 

About 70 percent (64) of all VOCs analyzed were detected 

infrequently (less than 1 percent) or not at all in field blanks 

(total n equal to 519; J.S. Zogorski and D.A. Bender, U.S. 

Geological Survey, written commun., 2008). Of the remaining VOCs, 11 were detected in 1 to 5 percent of field blanks 

and 11 were detected in more than 5 percent of field blanks. 

The VOCs detected in more than 5 percent of field blanks 

were (in decreasing order) toluene, 1,2,4-trimethylbenzene, 

m- and p-xylenes, chloroform, ethylbenzene, acetone, dichloromethane, benzene, styrene, carbon disulfide, and o-xylene. 

Because of several factors, including the presence on occasion 

of some VOCs in the blank water used to process field blanks 

(Taglioli and others, 2000) and the effectiveness of samplingequipment rinsing with well water prior to the collection of a 

ground-water sample in eliminating this and other potential 

contamination sources, the results for the field blanks overstate 

the potential for random contamination associated with field 

protocols in ground-water samples. Analysis of field blank and 

environmental sample pairs was used to determine whether 

assessment levels were required to address the potential random contamination associated with field protocols for those 

VOCs detected in 1 or more percent of field blanks 

(J.S. Zogorski and D.A. Bender, U.S. Geological Survey, 

written commun., 2008). Assessment levels of 0.03 μg/L for 

toluene and 0.05 μg/L for 1,2,4-trimethylbenzene thus were 

determined to be appropriate. These assessment levels are 

adequate to limit the estimated probability of false detections 

of these compounds due to random sample contamination to 

less than 1 percent, and were used in the present study. 

The bias from matrix interferences or analyte loss for 

VOC compounds during sample shipment or holding was 

investigated using field and laboratory spiked samples collected during 1997–2001 by Rowe and others (2005). The 

assessment was based on 428 spiked samples, including field 

spikes, field spike replicates, laboratory matrix spikes, and 

laboratory reagent spikes, for 85 VOCs in ground water and 

surface water. The median recoveries for all 85 VOCs ranged 

from 64 to 102 percent in field matrix spikes and ranged from 

90 to 120 percent in laboratory reagent spikes; these were 

acceptable levels for all VOCs (Rowe and others, 2005).

Fecal Indicator Bacteria

Data on fecal indicator bacteria in most (95 percent) 

samples in this study were collected following standard 

USGS quality-assurance procedures, which at a minimum 

included the collection of filter-blank QC samples before and 

after filtration of each water sample (U.S. Geological Survey, 

variously dated; Embrey and Runkle, 2006). After October 

2002, field blanks and positive and negative control samples 

also were collected (Embrey and Runkle, 2006). QC data for 

bacteria samples collected prior to October 2002 were not 

readily available at the national scale at the time this report 

was written (2007). However, analysis of available QC data 

associated with microbiological data after October 2002 indicated that target organisms were infrequently detected in filter 

blanks (3.4 percent or 13 of 379 blanks; Embry and Runkle, 

2006). Environmental data associated with filter blanks in 

which target bacteria colonies were greater than 5 percent of 

the environmental bacteria colony concentrations (“V”-coded 

data) were not used in this study. Replicate QC samples were 

collected after October 2002 for microbiological data; RPDs 

for replicate pairs (n equal to 402) ranged from 0 to 200 percent, with average and median values of 13 and 0, respectively 

(Embry and Runkle, 2006).

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Appendix 4 111

Appendix 4. Physical properties and 

contaminants analyzed in samples collected 

from domestic wells for the NAWQA Program, 

2001–2004, and associated human-health 

benchmarks for drinking water

Table

4–1. Physical properties and contaminants analyzed in samples collected from 

domestic wells for the NAWQA Program, 2001–2004, and associated human-health 

benchmarks for drinking water..............................................................................................112
112 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 Table 4–1. Physical properties and contaminants analyzed in samples collected from domestic wells for the NAWQA Program, 2001–2004, and associated human-health benchmarks for drinking water.—Continued [USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Common method reporting levels (MRLs) and long-term method detection levels (LT-MDLs) are those used for at least about 10 percent of samples. Predominant use group is from Gilliom and others (2006) and Moran and others (2005) and includes compound class for pesticides (AMD, amide; CAB, carbamate; CBE, chlorobenzoic acid esters; CPA, chlorophenoxy acids; DNA, dinitroanalines; MSA, miscellaneous acids; MSC, miscellaneous; NPH, nitrophenols; OCL, organochlorines; OPH, organophosphates; PHN, phenols; PYR, pyrethroids; SFE, sulfite esters; TRZ, triazines, URC, uracils; URA, ureas). Pesticide degradation products are designated by use group “DP” with parent compound indicated in parentheses. MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public water supplies; AMCL, USEPA proposed Alternative Maximum Contaminant Level for public water supplies; HBSL, Health-Based Screening Level developed by USGS using USEPA toxicity information and methods (Toccalino and Norman, 2006); °C, degrees Celsius; μS/cm, microsiemens per centimeter; CaCO3, calcium carbonate; mg/L, milligrams per liter; ROE, residue on evaporation; μg/L, micrograms per liter; pCi/L, picocuries per liter; CFU/100 mL, colony-forming units per 100 milliliters; --, not available or not applicable] Property or contaminant Units USGS parameter code CAS number Predominant use group Common MRLs and LT-MDLs Human-health benchmark Number of analyses Value Type Aquifer studies Agri- cultural land-use studies Aquifer and agricultural land-use studies Physical properties Temperature °C 00010 -- -- -- -- -- 2,140 428 2,568 Specific conductance μS/cm at 25°C 00095 -- -- 1 -- -- 2,138 432 2,570 Dissolved oxygen mg/L 00300 -- -- 0.1 -- -- 2,053 431 2,484 pH Standard units 00400 -- -- -- -- -- 2,144 432 2,576 Alkalinity as CaCO3 mg/L -- -- -- 1 -- -- 2,038 379 2,412 Major ions Bromide mg/L 71870 24959-67-9 -- 0.01 -- -- 2,058 418 2,476 Calcium mg/L 00915 7440-70-2 -- 0.01–0.02 -- -- 2,160 424 2,584 Chloride mg/L 00940 16887-00-6 -- 0.1 -- -- 2,157 422 2,579 Fluoride mg/L 00950 16984-48-8 -- 0.1 4 MCL 2,156 422 2,578 Magnesium mg/L 00925 7439-95-4 -- 0.01 -- -- 2,160 424 2,584 Potassium mg/L 00935 7440-70-2 -- 0.1 -- -- 2,160 424 2,584 Silica mg/L 00955 7631-86-9 -- 0.01 -- -- 2,160 424 2,584 Sodium mg/L 00930 7440-23-5 -- 0.2 -- -- 2,160 424 2,584 Sulfate mg/L 00945 14808-79-8 -- 0.1 -- -- 2,157 422 2,579

Dissolved solids (ROE) mg/L 70300 -- -- 1–10 -- -- 2,102 422 2,524

Trace elements

Aluminum μg/L 01106 7429-90-5 -- 0.8–1 -- -- 1,454 98 1,552Antimony μg/L 01095 7440-36-0 -- 0.05–1 6 MCL 1,552 78 1,630Arsenic μg/L 01000 7440-38-2 -- 0.2–1 10 MCL 1,774 116 1,890Barium μg/L 01005 7440-39-3 -- 1 2,000 MCL 1,593 78 1,671Beryllium μg/L 01010 7440-41-7 -- 0.03–1 4 MCL 1,572 78 1,650Boron μg/L 01020 7440-42-8 -- 7–16 1,000 HBSL 535 26 561
Appendix 4 113

Table 4–1. Physical properties and contaminants analyzed in samples collected from domestic wells for the NAWQA Program, 2001–2004, and associated human-health benchmarks for drinking water.—Continued[USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Common method reporting levels (MRLs) and long-term method detection levels (LT-MDLs) are those used for at least about 10 percent of samples. Predominant use group is from Gilliom and others (2006) and Moran and others (2005) and includes compound class for pesticides (AMD, amide; CAB, carbamate; CBE, chlorobenzoic acid esters; CPA, chlorophenoxy acids; DNA, dinitroanalines; MSA, miscellaneous acids; MSC, miscellaneous; NPH, nitrophenols; OCL, organochlorines; OPH, organophosphates; PHN, phenols; PYR, pyrethroids; SFE, sulfite esters; TRZ, triazines, URC, uracils; URA, ureas). Pesticide degradation products are designated by use group “DP” with parent compound indicated in parentheses. MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public water supplies; AMCL, USEPA proposed Alternative Maximum Contaminant Level for public water supplies; HBSL, Health-Based Screening Level developed by USGS using USEPA toxicity information and methods (Toccalino and Norman, 2006); °C, degrees Celsius; μS/cm, microsiemens per centimeter; CaCO3, calcium carbonate; mg/L, milligrams per liter; ROE, residue on evaporation; μg/L, micrograms per liter; pCi/L, picocuries per liter; CFU/100 mL, colony-forming units per 100 milliliters; --, not available or not applicable]

Property or contaminant Units

USGS 

parameter 

code

CAS number Predominant use 

group

Common 

MRLs and 

LT-MDLs

Human-health benchmark Number of analyses

Value Type Aquifer studies

Agricultural 

land-use 

studies

Aquifer and 

agricultural 

land-use 

studies

Trace elements—Continued

Cadmium μg/L 01025 7440-43-9 -- 0.02–1 5 MCL 1,639 78 1,717Chromium μg/L 01030 7440-47-3 -- 0.4–1 100 MCL 1,614 78 1,692Cobalt μg/L 01035 7440-48-4 -- 1 -- -- 1,572 78 1,650Copper μg/L 01040 7440-50-8 -- 0.2–1 1,300 Action level 1,619 78 1,697Iron μg/L 01046 7439-89-6 -- 3–10 -- -- 2,157 424 2,581Lead μg/L 01049 7439-92-1 -- 0.04–1 15 Action level 1,639 78 1,717Lithium μg/L 01130 7439-93-2 -- 0.3–3 -- -- 662 36 698Manganese μg/L 01056 7439-96-5 -- 0.1–1 300 HBSL 2,159 424 2,583Molybdenum μg/L 01060 7439-98-7 -- 0.03–1 40 HBSL 1,572 78 1,650Nickel μg/L 01065 7440-02-0 -- 0.2–1 100 HBSL 1,572 78 1,650Selenium μg/L 01145 7782-49-2 -- 0.7–1.2 50 MCL 1,625 78 1,703Silver μg/L 01075 7440-22-4 -- 0.1–1 100 HBSL 1,573 77 1,650Strontium μg/L 01080 7440-24-6 -- 10 4,000 HBSL 488 82 570Thallium μg/L 01057 7440-28-0 -- 0.02–0.9 2 MCL 613 36 649Uranium μg/L 22703 7440-61-1 -- 0.01–1 30 MCL 1,725 190 1,915Vanadium μg/L 01085 7440-62-2 -- 0.2–1 -- -- 662 36 698Zinc μg/L 01090 7440-66-6 -- 0.5–1 2,000 HBSL 1,594 78 1,672

Nutrients and dissolved organic carbon

Nitrite as N mg/L 00613 14797-65-0 -- 0.003–0.01 1 MCL 2,141 424 2,565Nitrate as N mg/L 00631 114797-55-8 -- 0.02–0.05 10 MCL 2,132 424 2,556Ammonia as N mg/L 00608 7664-41-7 -- 0.02 -- -- 2,095 424 2,519Ammonia plus organic 

nitrogen as N

mg/L 00623 -- -- 0.05–0.2 -- -- 2,086 424 2,510

Phosphorus, dissolved as P mg/L 00666 7723-14-0 -- 0.002–0.01 -- -- 1,776 399 2,175Orthophosphate as P mg/L 00671 14265-44-2 -- 0.01 -- -- 2,121 419 2,540Dissolved organic carbon mg/L 00681 -- -- 0.1, 0.3 -- -- 1,932 414 2,346
114 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 Table 4–1. Physical properties and contaminants analyzed in samples collected from domestic wells for the NAWQA Program, 2001–2004, and associated human-health benchmarks for drinking water.—Continued [USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Common method reporting levels (MRLs) and long-term method detection levels (LT-MDLs) are those used for at least about 10 percent of samples. Predominant use group is from Gilliom and others (2006) and Moran and others (2005) and includes compound class for pesticides (AMD, amide; CAB, carbamate; CBE, chlorobenzoic acid esters; CPA, chlorophenoxy acids; DNA, dinitroanalines; MSA, miscellaneous acids; MSC, miscellaneous; NPH, nitrophenols; OCL, organochlorines; OPH, organophosphates; PHN, phenols; PYR, pyrethroids; SFE, sulfite esters; TRZ, triazines, URC, uracils; URA, ureas). Pesticide degradation products are designated by use group “DP” with parent compound indicated in parentheses. MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public water supplies; AMCL, USEPA proposed Alternative Maximum Contaminant Level for public water supplies; HBSL, Health-Based Screening Level developed by USGS using USEPA toxicity information and methods (Toccalino and Norman, 2006); °C, degrees Celsius; μS/cm, microsiemens per centimeter; CaCO3, calcium carbonate; mg/L, milligrams per liter; ROE, residue on evaporation; μg/L, micrograms per liter; pCi/L, picocuries per liter; CFU/100 mL, colony-forming units per 100 milliliters; --, not available or not applicable] Property or contaminant Units USGS parameter code CAS number Predominant use group Common MRLs and LT-MDLs Human-health benchmark Number of analyses Value Type Aquifer studies Agri- cultural land-use studies Aquifer and agricultural land-use studies Radon and other radionuclides Radon pCi/L 82303 14859-67-7 -- 26–80 4,000 and 300 Proposed AMCL and MCL 1,958 323 2,281 Gross alpha-particle radio- activity pCi/L 04126 12587-46-1 -- 3 15 MCL 378 0 378 Gross beta-particle radio- activity pCi/L 03515 12587-47-2 -- 4 50 Screening level 375 0 375 Radium-226 plus radium-228 pCi/L Various -- -- -- 5 MCL 438 6 444 Pesticides Acetochlor μg/L 49260 34256-82-1 Herbicide (AMD) 0.002, 0.003 2

1 HBSL 1,822 224 2,046

Acifluorfen μg/L 49315 50594-66-6 Herbicide (MCA) 0.007–0.062 90 HBSL 1,372 397 1,769Alachlor μg/L 46342 15972-60-8 Herbicide (AMD) 0.001, 0.002 2 MCL 2,115 425 2,540Aldicarb μg/L 49312 116-06-3 Insecticide (CAB) 0.016–0.55 9 HBSL 1,374 398 1,772Aldicarb sulfone μg/L 49313 1646-88-4 DP (Aldicarb) 

(CAB)

0.016–0.16 7 HBSL 1,366 392 1,758

Aldicarb sulfoxide μg/L 49314 1646-87-3 DP (Aldicarb) 

(CAB)

0.008–0.027 7 HBSL 1,369 392 1,761

Atrazine μg/L 39632 1912-24-9 Herbicide (TRZ) 0.001, 0.004 3 MCL 2,116 425 2,541Azinphos-methyl (Guthion) μg/L 82686 86-50-0 Insecticide (OPH) 0.001, 0.02 10 HBSL 2,115 420 2,535Benfluralin μg/L 82673 1861-40-1 Herbicide (DNA) 0.002, 0.005 4 HBSL 2,113 424 2,537Bentazon μg/L 38711 25057-89-0 Herbicide (MSC) 0.011–0.019 200 HBSL 1,370 397 1,767Bromacil μg/L 04029 314-40-9 Herbicide (URA) 0.033–0.081 70 HBSL 1,379 398 1,777Bromoxynil μg/L 49311 1689-84-5 Herbicide (PHN) 0.017–0.057 10 HBSL 1,371 397 1,768Butylate μg/L 04028 2008-41-5 Herbicide (CAB) 0.001, 0.002 400 HBSL 2,115 424 2,539Carbaryl μg/L 82680 63-25-2 Insecticide (CAB) 0.003, 0.021 240 HBSL 2,115 425 2,540Carbofuran μg/L 82674 1563-66-2 Insecticide (CAB) 0.003, 0.01 40 MCL 2,114 425 2,539Chloramben methyl ester μg/L 61188 7286-84-2 Herbicide (MCA) 0.011–0.42 -- -- 1,379 398 1,777
Appendix 4 115

Table 4–1. Physical properties and contaminants analyzed in samples collected from domestic wells for the NAWQA Program, 2001–2004, and associated human-health benchmarks for drinking water.—Continued[USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Common method reporting levels (MRLs) and long-term method detection levels (LT-MDLs) are those used for at least about 10 percent of samples. Predominant use group is from Gilliom and others (2006) and Moran and others (2005) and includes compound class for pesticides (AMD, amide; CAB, carbamate; CBE, chlorobenzoic acid esters; CPA, chlorophenoxy acids; DNA, dinitroanalines; MSA, miscellaneous acids; MSC, miscellaneous; NPH, nitrophenols; OCL, organochlorines; OPH, organophosphates; PHN, phenols; PYR, pyrethroids; SFE, sulfite esters; TRZ, triazines, URC, uracils; URA, ureas). Pesticide degradation products are designated by use group “DP” with parent compound indicated in parentheses. MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public water supplies; AMCL, USEPA proposed Alternative Maximum Contaminant Level for public water supplies; HBSL, Health-Based Screening Level developed by USGS using USEPA toxicity information and methods (Toccalino and Norman, 2006); °C, degrees Celsius; μS/cm, microsiemens per centimeter; CaCO3, calcium carbonate; mg/L, milligrams per liter; ROE, residue on evaporation; μg/L, micrograms per liter; pCi/L, picocuries per liter; CFU/100 mL, colony-forming units per 100 milliliters; --, not available or not applicable]

Property or contaminant Units

USGS 

parameter 

code

CAS number Predominant use 

group

Common 

MRLs and 

LT-MDLs

Human-health benchmark Number of analyses

Value Type Aquifer studies

Agricultural 

land-use 

studies

Aquifer and 

agricultural 

land-use 

studies

Pesticides—Continued

Chlorothalonil μg/L 49306 1897-45-6 Fungicide (OCL) 0.035–0.48 25 HBSL 1,377 398 1,775Chlorpyrifos μg/L 38933 2921-88-2 Insecticide (OPH) 0.003, 0.004 2 HBSL 2,113 424 2,537Clopyralid μg/L 49305 1702-17-6 Herbicide (MCA) 0.014–0.23 -- -- 1,376 394 1,770Cyanazine μg/L 04041 21725-46-2 Herbicide (TRZ) 0.004, 0.009 1 HBSL 2,115 425 2,5402,4-D μg/L 39732 94-75-7 Herbicide (CPA) 0.022–0.15 70 MCL 1,368 397 1,765Dacthal monoacid μg/L 49304 887-54-7 Herbicide (CBE) 0.012–0.072 -- -- 1,367 395 1,7622,4-DB μg/L 38746 94-82-6 Herbicide (CPA) 0.016–0.24 200 HBSL 1,377 396 1,773DCPA (Dacthal) μg/L 82682 1861-32-1 Herbicide (CBE) 0.002 70 HBSL 2,115 424 2,539p,p

‘-DDE μg/L 34653 72-55-9 DP (p,p‘-DDT) 

(OCL)

0.001, 0.006 20.1 HBSL 2,114 424 2,538

Deethylatrazine μg/L 04040 6190-65-4 DP (Atrazine) 

(TRZ)

0.002, 0.003 -- -- 2,116 425 2,541

Diazinon μg/L 39572 333-41-5 Insecticide (OPH) 0.002, 0.003 1 HBSL 2,115 424 2,539Dicamba μg/L 38442 1918-00-9 Herbicide (MSA) 0.013–0.096 3,000 HBSL 1,376 397 1,773Dichlobenil μg/L 49303 1194-65-6 Herbicide (OCL) 0.020, 1.2 9 HBSL 924 363 1,287Dichlorprop μg/L 49302 120-36-5 Herbicide (CPA) 0.014–0.05 300 HBSL 1,377 396 1,773Dieldrin μg/L 39381 60-57-1 Insecticide, DP 

(Aldrin) (OCL)

0.001, 0.002 20.002 HBSL 2,115 424 2,539

2,6-Diethylaniline μg/L 82660 579-66-8 DP (Alachlor) 

(AMD)

0.001, 0.003 -- -- 2,115 424 2,539

Dinoseb μg/L 49301 88-85-7 Herbicide (NPH) 0.012–0.043 7 MCL 1,371 397 1,768Disulfoton μg/L 82677 298-04-4 Insecticide (OPH) 0.011, 0.017 0.9 HBSL 2,115 424 2,539Diuron μg/L 49300 330-54-1 Herbicide (URA) 0.015–0.079 22 HBSL 1,374 398 1,772DNOC (2-Methyl-4,6-

dinitrophenol)

μg/L 49299 534-52-1 Herbicide (NPH) 0.035–0.42 -- -- 919 362 1,281

EPTC μg/L 82668 759-94-4 Herbicide (CAB) 0.001, 0.002 200 HBSL 2,115 424 2,539
116 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 Table 4–1. Physical properties and contaminants analyzed in samples collected from domestic wells for the NAWQA Program, 2001–2004, and associated human-health benchmarks for drinking water.—Continued [USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Common method reporting levels (MRLs) and long-term method detection levels (LT-MDLs) are those used for at least about 10 percent of samples. Predominant use group is from Gilliom and others (2006) and Moran and others (2005) and includes compound class for pesticides (AMD, amide; CAB, carbamate; CBE, chlorobenzoic acid esters; CPA, chlorophenoxy acids; DNA, dinitroanalines; MSA, miscellaneous acids; MSC, miscellaneous; NPH, nitrophenols; OCL, organochlorines; OPH, organophosphates; PHN, phenols; PYR, pyrethroids; SFE, sulfite esters; TRZ, triazines, URC, uracils; URA, ureas). Pesticide degradation products are designated by use group “DP” with parent compound indicated in parentheses. MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public water supplies; AMCL, USEPA proposed Alternative Maximum Contaminant Level for public water supplies; HBSL, Health-Based Screening Level developed by USGS using USEPA toxicity information and methods (Toccalino and Norman, 2006); °C, degrees Celsius; μS/cm, microsiemens per centimeter; CaCO3, calcium carbonate; mg/L, milligrams per liter; ROE, residue on evaporation; μg/L, micrograms per liter; pCi/L, picocuries per liter; CFU/100 mL, colony-forming units per 100 milliliters; --, not available or not applicable] Property or contaminant Units USGS parameter code CAS number Predominant use group Common MRLs and LT-MDLs Human-health benchmark Number of analyses Value Type Aquifer studies Agri- cultural land-use studies Aquifer and agricultural land-use studies Pesticides—Continued Ethalfluralin μg/L 82663 55283-68-6 Herbicide (DNA) 0.004, 0.005 30 HBSL 2,115 424 2,539 Ethoprop (Ethoprophos) μg/L 82672 13194-48-4 Herbicide (OPH) 0.002, 0.003 2

1 HBSL 2,115 425 2,540

Fenuron μg/L 49297 101-42-8 Fenuron (URA) 0.013–0.074 -- -- 1,371 398 1,769Fluometuron μg/L 38811 2164-17-2 Herbicide (URA) 0.031–0.062 4 HBSL 1,378 398 1,776Fonofos μg/L 04095 944-22-9 Insecticide (OPH) 0.001, 0.003 10 HBSL 2,115 424 2,539alpha-HCH μg/L 34253 319-84-6 DP (gamma-HCH) 

(OCL)

0.002 20.006 HBSL 2,115 424 2,539

gamma-HCH (Lindane) μg/L 39341 58-89-9 Insecticide (OCL) 0.002, 0.004 0.2 MCL 2,115 424 2,5393-Hydroxycarbofuran μg/L 49308 16655-82-6 DP (Carbofuran) 

(CAB)

0.006–0.062 -- -- 1,371 398 1,769

Linuron μg/L 82666 330-55-2 Herbicide (URA) 0.002, 0.018 5 HBSL 2,115 424 2,539Malathion μg/L 39532 121-75-5 Insecticide (OPH) 0.005, 0.014 50 HBSL 2,115 424 2,539MCPA μg/L 38482 94-74-6 Herbicide (CPA) 0.016–0.17 30 HBSL 1,377 397 1,774MCPB μg/L 38487 94-81-5 Herbicide (CPA) 0.015–0.14 100 HBSL 1,377 396 1,773Methiocarb μg/L 38501 2032-65-7 Insecticide (CAB) 0.008–0.08 40 HBSL 1,378 398 1,776Methomyl μg/L 49296 16752-77-5 Insecticide (CAB) 0.004–0.077 200 HBSL 1,366 392 1,758Metolachlor μg/L 39415 51218-45-2 Herbicide (AMD) 0.002, 0.006 700 HBSL 2,115 425 2,540Metribuzin μg/L 82630 21087-64-9 Herbicide (TRZ) 0.003, 0.004 90 HBSL 2,115 424 2,539Molinate μg/L 82671 2212-67-1 Herbicide (CAB) 0.001, 0.004 0.7 HBSL 2,115 424 2,539Napropamide μg/L 82684 15299-99-7 Herbicide (AMD) 0.003 800 HBSL 2,115 425 2,540Neburon μg/L 49294 555-37-3 Herbicide (URA) 0.012–0.075 -- -- 1,378 398 1,776Norflurazon μg/L 49293 27314-13-2 Herbicide (MSC) 0.016–0.078 10 HBSL 1,378 398 1,776Oryzalin μg/L 49292 19044-88-3 Herbicide (DNA) 0.018–0.31 2

4 HBSL 1,376 398 1,774

Oxamyl μg/L 38866 23135-22-0 Insecticide (CAB) 0.012–0.018 200 MCL 1,365 392 1,757Parathion (Ethyl parathion) μg/L 39542 56-38-2 Insecticide (OPH) 0.003–0.005 0.02 HBSL 2,115 424 2,539
Appendix 4 117

Table 4–1. Physical properties and contaminants analyzed in samples collected from domestic wells for the NAWQA Program, 2001–2004, and associated human-health benchmarks for drinking water.—Continued[USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Common method reporting levels (MRLs) and long-term method detection levels (LT-MDLs) are those used for at least about 10 percent of samples. Predominant use group is from Gilliom and others (2006) and Moran and others (2005) and includes compound class for pesticides (AMD, amide; CAB, carbamate; CBE, chlorobenzoic acid esters; CPA, chlorophenoxy acids; DNA, dinitroanalines; MSA, miscellaneous acids; MSC, miscellaneous; NPH, nitrophenols; OCL, organochlorines; OPH, organophosphates; PHN, phenols; PYR, pyrethroids; SFE, sulfite esters; TRZ, triazines, URC, uracils; URA, ureas). Pesticide degradation products are designated by use group “DP” with parent compound indicated in parentheses. MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public water supplies; AMCL, USEPA proposed Alternative Maximum Contaminant Level for public water supplies; HBSL, Health-Based Screening Level developed by USGS using USEPA toxicity information and methods (Toccalino and Norman, 2006); °C, degrees Celsius; μS/cm, microsiemens per centimeter; CaCO3, calcium carbonate; mg/L, milligrams per liter; ROE, residue on evaporation; μg/L, micrograms per liter; pCi/L, picocuries per liter; CFU/100 mL, colony-forming units per 100 milliliters; --, not available or not applicable]

Property or contaminant Units

USGS 

parameter 

code

CAS number Predominant use 

group

Common 

MRLs and 

LT-MDLs

Human-health benchmark Number of analyses

Value Type Aquifer studies

Agricultural 

land-use 

studies

Aquifer and 

agricultural 

land-use 

studies

Pesticides—Continued

Parathion-methyl 

(Methyl parathion)

μg/L 82667 298-00-0 Insecticide (OPH) 0.003, 0.006 1 HBSL 2,115 424 2,539

Pebulate μg/L 82669 1114-71-2 Herbicide (CAB) 0.001–0.004 50 HBSL 2,115 424 2,539Pendimethalin μg/L 82683 40487-42-1 Herbicide (DNA) 0.004–0.011 70 HBSL 2,115 424 2,539cis-Permethrin μg/L 82687 54774-45-7 Insecticide (PYR) 0.003, 0.005 24 HBSL 2,115 424 2,539Phorate μg/L 82664 298-02-2 Insecticide (OPH) 0.002, 0.006 4 HBSL 2,115 424 2,539Picloram μg/L 49291 1918-02-1 Herbicide (MSA) 0.02–0.071 500 MCL 1,361 387 1,748Prometon μg/L 04037 1610-18-0 Herbicide (TRZ) 0.007, 0.018 400 HBSL 2,115 425 2,540Pronamide (Propyzamide) μg/L 82676 23950-58-5 Herbicide (AMD) 0.002, 0.003 21 HBSL 2,115 425 2,540Propachlor μg/L 04024 1918-16-7 Herbicide (AMD) 0.005, 0.007 21 HBSL 2,115 425 2,540Propanil μg/L 82679 709-98-8 Herbicide (AMD) 0.004, 0.005 6 HBSL 2,115 425 2,540Propargite μg/L 82685 2312-35-8 Acaricide (SFE) 0.011, 0.013 21 HBSL 2,107 424 2,531Propham μg/L 49236 122-42-9 Herbicide (CAB) 0.01–0.0072 100 HBSL 1,378 398 1,776Propoxur (Baygon) μg/L 38538 114-26-1 Insecticide (CAB) 0.008–0.06 29 HBSL 1,369 398 1,767Simazine μg/L 04035 122-34-9 Herbicide (TRZ) 0.002–0.006 4 MCL 2,114 425 2,5392,4,5-T μg/L 39742 93-76-5 Herbicide (CPA) 0.035 70 HBSL 919 361 1,280Tebuthiuron μg/L 82670 34014-18-1 Herbicide (URA) 0.008, 0.01 1,000 HBSL 2,116 425 2,541Terbacil μg/L 82665 5902-51-2 Herbicide (URC) 0.007, 0.017 90 HBSL 2,108 420 2,528Terbufos μg/L 82675 13071-79-9 Insecticide (OPH) 0.009, 0.013 0.4 HBSL 2,115 424 2,539Thiobencarb μg/L 82681 28249-77-6 Herbicide (CAB) 0.002 70 HBSL 2,115 424 2,5392,4,5-TP (Silvex) μg/L 39762 93-72-1 Herbicide (CPA) 0.021 50 MCL 920 362 1,282Triallate μg/L 82678 2303-17-5 Herbicide (CAB) 0.001 20 HBSL 2,115 424 2,539Triclopyr μg/L 49235 55335-06-3 Herbicide (OCL) 0.022–0.25 400 HBSL 1,372 397 1,769Trifluralin μg/L 82661 1582-09-8 Herbicide (DNA) 0.002, 0.005 20 HBSL 2,115 424 2,539
118 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 Table 4–1. Physical properties and contaminants analyzed in samples collected from domestic wells for the NAWQA Program, 2001–2004, and associated human-health benchmarks for drinking water.—Continued [USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Common method reporting levels (MRLs) and long-term method detection levels (LT-MDLs) are those used for at least about 10 percent of samples. Predominant use group is from Gilliom and others (2006) and Moran and others (2005) and includes compound class for pesticides (AMD, amide; CAB, carbamate; CBE, chlorobenzoic acid esters; CPA, chlorophenoxy acids; DNA, dinitroanalines; MSA, miscellaneous acids; MSC, miscellaneous; NPH, nitrophenols; OCL, organochlorines; OPH, organophosphates; PHN, phenols; PYR, pyrethroids; SFE, sulfite esters; TRZ, triazines, URC, uracils; URA, ureas). Pesticide degradation products are designated by use group “DP” with parent compound indicated in parentheses. MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public water supplies; AMCL, USEPA proposed Alternative Maximum Contaminant Level for public water supplies; HBSL, Health-Based Screening Level developed by USGS using USEPA toxicity information and methods (Toccalino and Norman, 2006); °C, degrees Celsius; μS/cm, microsiemens per centimeter; CaCO3, calcium carbonate; mg/L, milligrams per liter; ROE, residue on evaporation; μg/L, micrograms per liter; pCi/L, picocuries per liter; CFU/100 mL, colony-forming units per 100 milliliters; --, not available or not applicable] Property or contaminant Units USGS parameter code CAS number Predominant use group Common MRLs and LT-MDLs Human-health benchmark Number of analyses Value Type Aquifer studies Agri- cultural land-use studies Aquifer and agricultural land-use studies Volatile organic compounds Acetone μg/L 81552 67-64-1 Solvent 2–5 6,000 HBSL 1,536 37 1,573 Acrylonitrile μg/L 34215 107-13-1 Organic synthesis 0.6–2 20.06 HBSL 1,537 37 1,574 tert-Amyl methyl ether (Methyl tert-pentyl ether) μg/L 50005 994-05-8 Gasoline oxygenate 0.04–0.112 -- -- 1,537 37 1,574 Benzene μg/L 34030 71-43-2 Gasoline hydro- carbon 0.017–0.2 5 MCL 1,957 313 2,270 Bromobenzene μg/L 81555 108-86-1 Solvent 0.018–0.2 -- -- 1,956 313 2,269 Bromochloromethane μg/L 77297 74-97-5 Personal care and domestic use product 0.022–0.2 90 HBSL 1,956 312 2,268 Bromodichloromethane μg/L 32101 75-27-4 Trihalomethane 0.024–0.2 380 MCL 1,956 313 2,269 Bromoform (Tribromomethane) μg/L 32104 75-25-2 Trihalomethane 0.03–0.2 380 MCL 1,955 313 2,268 Bromomethane μg/L 34413 74-83-9 Fumigant 0.074–0.2 100 HBSL 1,956 313 2,269

n-Butylbenzene μg/L 77342 104-51-8 Gasoline hydrocarbon 0.05–0.2 -- -- 1,956 313 2,269sec-Butylbenzene μg/L 77350 135-98-8 Gasoline hydrocarbon 0.016–0.2 -- -- 1,956 313 2,269tert-Butylbenzene μg/L 77353 98-06-6 Gasoline hydrocarbon 0.03–0.2 -- -- 1,956 313 2,269Carbon disulfide μg/L 77041 75-15-0 Organic synthesis 0.013–0.18 700 HBSL 1,536 37 1,573Carbon tetrachloride 

(Tetrachloromethane)

μg/L 32102 56-23-5 Solvent 0.03–0.2 5 MCL 1,956 313 2,269

Chlorobenzene 

(Monochlorobenzene)

μg/L 34301 108-90-7 Solvent 0.014–0.2 100 MCL 1,957 313 2,270

Chloroethane μg/L 34311 75-00-3 Solvent 0.06–0.2 -- -- 1,955 313 2,268
Appendix 4 119

Table 4–1. Physical properties and contaminants analyzed in samples collected from domestic wells for the NAWQA Program, 2001–2004, and associated human-health benchmarks for drinking water.—Continued[USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Common method reporting levels (MRLs) and long-term method detection levels (LT-MDLs) are those used for at least about 10 percent of samples. Predominant use group is from Gilliom and others (2006) and Moran and others (2005) and includes compound class for pesticides (AMD, amide; CAB, carbamate; CBE, chlorobenzoic acid esters; CPA, chlorophenoxy acids; DNA, dinitroanalines; MSA, miscellaneous acids; MSC, miscellaneous; NPH, nitrophenols; OCL, organochlorines; OPH, organophosphates; PHN, phenols; PYR, pyrethroids; SFE, sulfite esters; TRZ, triazines, URC, uracils; URA, ureas). Pesticide degradation products are designated by use group “DP” with parent compound indicated in parentheses. MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public water supplies; AMCL, USEPA proposed Alternative Maximum Contaminant Level for public water supplies; HBSL, Health-Based Screening Level developed by USGS using USEPA toxicity information and methods (Toccalino and Norman, 2006); °C, degrees Celsius; μS/cm, microsiemens per centimeter; CaCO3, calcium carbonate; mg/L, milligrams per liter; ROE, residue on evaporation; μg/L, micrograms per liter; pCi/L, picocuries per liter; CFU/100 mL, colony-forming units per 100 milliliters; --, not available or not applicable]

Property or contaminant Units

USGS 

parameter 

code

CAS number Predominant use 

group

Common 

MRLs and 

LT-MDLs

Human-health benchmark Number of analyses

Value Type Aquifer studies

Agricultural 

land-use 

studies

Aquifer and 

agricultural 

land-use 

studies

Volatile organic compounds—Continued

Chloroform 

(Trichloromethane)

μg/L 32106 67-66-3 Trihalomethane 0.012–0.2 380 MCL 1,956 313 2,269

Chloromethane μg/L 34418 74-87-3 Organic synthesis 0.09–0.25 30 HBSL 1,901 256 2,1573-Chloropropene μg/L 78109 107-05-1 Organic synthesis 0.03–0.196 -- -- 1,537 37 1,5742-Chlorotoluene (o-) μg/L 77275 95-49-8 Solvent 0.013–0.2 100 HBSL 1,956 313 2,2694-Chlorotoluene (p-) μg/L 77277 106-43-4 Solvent 0.03–0.2 100 HBSL 1,956 313 2,269Dibromochloromethane μg/L 32105 124-48-1 Trihalomethane 0.05–0.2 380 MCL 1,956 313 2,269Dibromochloropropane 

(DBCP)

μg/L 82625 96-12-8 Fumigant 0.107–1.0 0.2 MCL 1,956 313 2,269

Dibromomethane μg/L 30217 74-95-3 Solvent 0.025–0.2 -- -- 1,956 313 2,2691,2-Dichlorobenzene (o-) μg/L 34536 95-50-1 Solvent 0.024–0.2 600 MCL 1,957 313 2,2701,3-Dichlorobenzene (m-) μg/L 34566 541-73-1 Solvent 0.015–0.2 600 HBSL 1,957 313 2,2701,4-Dichlorobenzene (p-) μg/L 34571 106-46-7 Fumigant 0.025–0.2 75 MCL 1,957 313 2,270trans-1,4-Dichloro-2-butene μg/L 73547 110-57-6 Organic synthesis 0.346–5 -- -- 1,537 37 1,574Dichlorodifluoromethane μg/L 34668 75-71-8 Refrigerant 0.07–0.2 1,000 HBSL 1,957 313 2,2701,1-Dichloroethane μg/L 34496 75-34-3 Solvent 0.033–0.2 -- -- 1,956 313 2,2691,2-Dichloroethane μg/L 32103 107-06-2 Solvent 0.05–0.2 5 MCL 1,945 312 2,2571,1-Dichloroethene μg/L 34501 75-35-4 Solvent 0.02–0.2 7 MCL 1,956 313 2,269cis-1,2-Dichloroethene μg/L 77093 156-59-2 Solvent 0.019–0.2 70 MCL 1,956 313 2,269trans-1,2-Dichloroethene μg/L 34546 156-60-5 Solvent 0.016–0.2 100 MCL 1,956 313 2,2691,2-Dichloropropane μg/L 34541 78-87-5 Fumigant 0.014–0.2 5 MCL 1,956 313 2,2691,3-Dichloropropane μg/L 77173 142-28-9 Fumigant 0.013–0.2 -- -- 1,956 313 2,2692,2-Dichloropropane μg/L 77170 594-20-7 Fumigant 0.02–0.2 -- -- 1,955 313 2,2681,1-Dichloropropene μg/L 77168 563-58-6 Organic synthesis 0.018–0.2 -- -- 1,955 313 2,268cis-1,3-Dichloropropene μg/L 34704 10061-01-5 Fumigant 0.046–0.2 2,40.3 HBSL 1,956 313 2,269
120 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 Table 4–1. Physical properties and contaminants analyzed in samples collected from domestic wells for the NAWQA Program, 2001–2004, and associated human-health benchmarks for drinking water.—Continued [USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Common method reporting levels (MRLs) and long-term method detection levels (LT-MDLs) are those used for at least about 10 percent of samples. Predominant use group is from Gilliom and others (2006) and Moran and others (2005) and includes compound class for pesticides (AMD, amide; CAB, carbamate; CBE, chlorobenzoic acid esters; CPA, chlorophenoxy acids; DNA, dinitroanalines; MSA, miscellaneous acids; MSC, miscellaneous; NPH, nitrophenols; OCL, organochlorines; OPH, organophosphates; PHN, phenols; PYR, pyrethroids; SFE, sulfite esters; TRZ, triazines, URC, uracils; URA, ureas). Pesticide degradation products are designated by use group “DP” with parent compound indicated in parentheses. MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public water supplies; AMCL, USEPA proposed Alternative Maximum Contaminant Level for public water supplies; HBSL, Health-Based Screening Level developed by USGS using USEPA toxicity information and methods (Toccalino and Norman, 2006); °C, degrees Celsius; μS/cm, microsiemens per centimeter; CaCO3, calcium carbonate; mg/L, milligrams per liter; ROE, residue on evaporation; μg/L, micrograms per liter; pCi/L, picocuries per liter; CFU/100 mL, colony-forming units per 100 milliliters; --, not available or not applicable] Property or contaminant Units USGS parameter code CAS number Predominant use group Common MRLs and LT-MDLs Human-health benchmark Number of analyses Value Type Aquifer studies Agri- cultural land-use studies Aquifer and agricultural land-use studies Volatile organic compounds—Continued trans-1,3-Dichloropropene μg/L 34699 10061-02-6 Fumigant 0.04–0.2 2,40.3 HBSL 1,955 313 2,268 Diethyl ether μg/L 81576 60-29-7 Solvent 0.085–0.17 1000 HBSL 1,537 37 1,574 Diisopropyl ether μg/L 81577 108-20-3 Gasoline oxygenate 0.05–0.1 -- -- 1,427 37 1,464 Ethyl methacrylate μg/L 73570 97-63-2 Organic synthesis 0.09–1 -- -- 1,536 37 1,573 Ethyl tert-butyl ether 

(tert-Butyl ethyl ether)

μg/L 50004 637-92-3 Gasoline oxygenate 0.027–0.1 -- -- 1,537 37 1,574

Ethylbenzene μg/L 34371 100-41-4 Gasoline hydrocarbon 0.015–0.2 700 MCL 1,957 313 2,270Ethylene dibromide 

(1,2-Dibromoethane)

μg/L 77651 106-93-4 Fumigant 0.018–0.2 0.05 MCL 1,955 313 2,268

2-Ethyltoluene μg/L 77220 611-14-3 Gasoline hydrocarbon 0.03–0.1 -- -- 1,537 37 1,574Hexachlorobutadiene μg/L 39702 87-68-3 Organic synthesis 0.07–0.2 20.9 HBSL 1,956 313 2,269Hexachloroethane μg/L 34396 67-72-1 Solvent 0.05–0.362 0.7 HBSL 1,537 37 1,574Iodomethane μg/L 77424 74-88-4 Organic synthesis 0.05–0.350 -- -- 1,537 37 1,574Isopropylbenzene μg/L 77223 98-82-8 Gasoline hydrocarbon 0.016–0.2 700 HBSL 1,956 313 2,269n-Isopropyltoluene 

(4-Isopropyltoluene)

μg/L 77356 99-87-6 Gasoline hydrocarbon 0.03–0.2 -- -- 1,956 313 2,269

Methyl acrylate μg/L 49991 96-33-3 Organic synthesis 0.612–2 -- -- 1,537 37 1,574Methyl acrylonitrile 

(Methacrylonitrile)

μg/L 81593 126-98-7 Organic synthesis 0.3–2 0.7 HBSL 1,537 37 1,574

Methyl butyl ketone 

(n-Butyl methyl ketone)

μg/L 77103 591-78-6 Solvent 0.378–5 -- -- 1,537 37 1,574

Methyl ethyl ketone 

(Ethyl methyl ketone)

μg/L 81595 78-93-3 Solvent 0.825–5 4,000 HBSL 1,537 37 1,574
Appendix 4 121

Table 4–1. Physical properties and contaminants analyzed in samples collected from domestic wells for the NAWQA Program, 2001–2004, and associated human-health benchmarks for drinking water.—Continued[USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Common method reporting levels (MRLs) and long-term method detection levels (LT-MDLs) are those used for at least about 10 percent of samples. Predominant use group is from Gilliom and others (2006) and Moran and others (2005) and includes compound class for pesticides (AMD, amide; CAB, carbamate; CBE, chlorobenzoic acid esters; CPA, chlorophenoxy acids; DNA, dinitroanalines; MSA, miscellaneous acids; MSC, miscellaneous; NPH, nitrophenols; OCL, organochlorines; OPH, organophosphates; PHN, phenols; PYR, pyrethroids; SFE, sulfite esters; TRZ, triazines, URC, uracils; URA, ureas). Pesticide degradation products are designated by use group “DP” with parent compound indicated in parentheses. MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public water supplies; AMCL, USEPA proposed Alternative Maximum Contaminant Level for public water supplies; HBSL, Health-Based Screening Level developed by USGS using USEPA toxicity information and methods (Toccalino and Norman, 2006); °C, degrees Celsius; μS/cm, microsiemens per centimeter; CaCO3, calcium carbonate; mg/L, milligrams per liter; ROE, residue on evaporation; μg/L, micrograms per liter; pCi/L, picocuries per liter; CFU/100 mL, colony-forming units per 100 milliliters; --, not available or not applicable]

Property or contaminant Units

USGS 

parameter 

code

CAS number Predominant use 

group

Common 

MRLs and 

LT-MDLs

Human-health benchmark Number of analyses

Value Type Aquifer studies

Agricultural 

land-use 

studies

Aquifer and 

agricultural 

land-use 

studies

Volatile organic compounds—Continued

Methyl isobutyl ketone 

(Isobutyl methyl ketone)

μg/L 78133 108-10-1 Solvent 0.180–5 -- -- 1,537 37 1,574

Methyl methacrylate μg/L 81597 80-62-6 Organic synthesis 0.175–1 10,000 HBSL 1,537 37 1,574Methyl tert-butyl ether μg/L 78032 1634-04-4 Gasoline oxygenate 0.08–0.2 -- -- 1,956 312 2,268Methylene chloride 

(Dichloromethane)

μg/L 34423 75-09-2 Solvent 0.08–0.38 5 MCL 1,956 313 2,269

Naphthalene μg/L 34696 91-20-3 Gasoline hydrocarbon 0.125–0.5 100 HBSL 1,956 313 2,269Perchloroethene 

(Tetrachloroethene)

μg/L 34475 127-18-4 Solvent 0.013–0.2 5 MCL 1,927 313 2,240

n-Propylbenzene μg/L 77224 103-65-1 Solvent 0.021–0.2 -- -- 1,956 313 2,269Styrene μg/L 77128 100-42-5 Gasoline hydrocarbon 0.021–0.2 100 MCL 1,951 313 2,2641,1,1,2-Tetrachloroethane μg/L 77562 630-20-6 Solvent 0.015–0.2 70 HBSL 1,955 313 2,2681,1,2,2-Tetrachloroethane μg/L 34516 79-34-5 Solvent 0.04–0.2 0.3 HBSL 1,956 313 2,269Tetrahydrofuran μg/L 81607 109-99-9 Solvent 1.1–5 -- -- 1,536 37 1,5731,2,3,4-Tetramethylbenzene μg/L 49999 488-23-3 Gasoline hydrocarbon 0.05–0.23 -- -- 1,537 37 1,5741,2,3,5-Tetramethylbenzene μg/L 50000 527-53-7 Gasoline hydrocarbon 0.05–0.24 -- -- 1,537 37 1,574Toluene μg/L 34010 108-88-3 Gasoline hydrocarbon 0.03–0.2 1,000 MCL 1,891 313 2,2041,2,3-Trichlorobenzene μg/L 77613 87-61-6 Organic synthesis 0.133–0.266 -- -- 1,956 313 2,2691,2,4-Trichlorobenzene μg/L 34551 120-82-1 Solvent 0.09–0.2 70 MCL 1,956 313 2,2691,1,1-Trichloroethane μg/L 34506 71-55-6 Solvent 0.016–0.2 200 MCL 1,957 313 2,2701,1,2-Trichloroethane μg/L 34511 79-00-5 Solvent 0.03–0.2 5 MCL 1,956 313 2,269Trichloroethene μg/L 39180 79-01-6 Solvent 0.019–0.2 5 MCL 1,956 313 2,269
122 Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 Table 4–1. Physical properties and contaminants analyzed in samples collected from domestic wells for the NAWQA Program, 2001–2004, and associated human-health benchmarks for drinking water.—Continued [USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Common method reporting levels (MRLs) and long-term method detection levels (LT-MDLs) are those used for at least about 10 percent of samples. Predominant use group is from Gilliom and others (2006) and Moran and others (2005) and includes compound class for pesticides (AMD, amide; CAB, carbamate; CBE, chlorobenzoic acid esters; CPA, chlorophenoxy acids; DNA, dinitroanalines; MSA, miscellaneous acids; MSC, miscellaneous; NPH, nitrophenols; OCL, organochlorines; OPH, organophosphates; PHN, phenols; PYR, pyrethroids; SFE, sulfite esters; TRZ, triazines, URC, uracils; URA, ureas). Pesticide degradation products are designated by use group “DP” with parent compound indicated in parentheses. MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public water supplies; AMCL, USEPA proposed Alternative Maximum Contaminant Level for public water supplies; HBSL, Health-Based Screening Level developed by USGS using USEPA toxicity information and methods (Toccalino and Norman, 2006); °C, degrees Celsius; μS/cm, microsiemens per centimeter; CaCO3, calcium carbonate; mg/L, milligrams per liter; ROE, residue on evaporation; μg/L, micrograms per liter; pCi/L, picocuries per liter; CFU/100 mL, colony-forming units per 100 milliliters; --, not available or not applicable] Property or contaminant Units USGS parameter code CAS number Predominant use group Common MRLs and LT-MDLs Human-health benchmark Number of analyses Value Type Aquifer studies Agri- cultural land-use studies Aquifer and agricultural land-use studies Volatile organic compounds—Continued Trichlorofluoromethane μg/L 34488 75-69-4 Refrigerant 0.046–0.2 2,000 HBSL 1,957 313 2,270 1,2,3-Trichloropropane μg/L 77443 96-18-4 Organic synthesis 0.08–0.2 40 HBSL 1,956 313 2,269 Trichlorotrifluoroethane (1,1,2-Trichloro-1,2,2- trifluoroethane) μg/L 77652 76-13-1 Refrigerant 0.016–0.2 200,000 HBSL 1,955 313 2,268 1,2,3-Trimethylbenzene μg/L 77221 526-73-8 Gasoline hydro- carbon 0.05–0.124 -- -- 1,537 37 1,574 1,2,4-Trimethylbenzene μg/L 77222 95-63-6 Gasoline hydro- carbon 0.028–0.2 -- -- 1,913 306 2,219 1,3,5-Trimethylbenzene μg/L 77226 108-67-8 Gasoline hydro- carbon 0.022–0.2 -- -- 1,956 313 2,269 Vinyl bromide (Bromoethene) μg/L 50002 593-60-2 Organic synthesis 0.050–0.11 -- -- 1,537 37 1,574 Vinyl chloride μg/L 39175 75-01-4 Organic synthesis 0.056–0.2 2 MCL 1,957 313 2,270

o-Xylene μg/L 77135 95-47-6 Gasoline hydrocarbon 0.019–0.064 510,000 MCL 1,536 37 1,573m- and 

p-Xylenes μg/L 85795 108-38-3 and 

106-42-3

Gasoline hydrocarbon 0.0300–0.11 510,000 MCL 1,530 37 1,567

Fecal indicator bacteria

Total coliform, all methods CFU/100 mL Various -- -- -- See text -- 397 86 483Escherichia coli, all methods CFU/100 mL Various -- -- -- See text -- 378 55 433

Summary

Total number of analytes -- -- -- -- -- -- -- 219 219 219Total number of analyses -- -- -- -- -- -- -- 373,578 63,189 436,767

1CAS number is for nitrate plus nitrite; see text for additional information.2Value is low end of range, associated with 10-6 cancer risk (Toccalino and others, 2006). 3MCL is for sum of four trihalomethanes.4HBSL is for sum of cis-1,3-dichloropropene and trans-1,3-dichloropropene. 5MCL is for sum of xylenes.
Appendix 5 123

Appendix 5. Summary statistics for physical 

properties and concentrations of major ions, 

trace elements, nutrients, radionuclides, and 

fecal indicator bacteria in samples collected 

from domestic wells for the NAWQA Program, 

1991–2004

CD-ROM

[In Pocket]

Tables

5–1. Abbreviations for principal aquifer names

5–2. Summary statistics for physical properties of and concentrations of major ions, 

trace elements, nutrients, radionuclides, and fecal indicator bacteria in samples 

collected from domestic wells for the NAWQA Program in aquifer studies, 1991–2004

5–3. Summary statistics for physical properties of and concentrations of major ions, 

trace elements, nutrients, radionuclides, and fecal indicator bacteria in samples 

collected from domestic wells for the NAWQA Program in agricultural land-use 

studies, 1991–2004

5–4. Summary statistics for physical properties of and concentrations of major ions, 

trace elements, nutrients, radionuclides, and fecal indicator bacteria in samples 

collected from domestic wells for the NAWQA Program in aquifer and agricultural 

land-use studies, 1991–2004
THIS PAGE INTENTIONALLY LEFT BLANK
Appendix 6 125

Appendix 6. Percentages of wells in principal 

aquifers with values of physical properties or 

concentrations of major ions, trace elements, 

nutrients, radionuclides, or organic compounds 

greater than human-health benchmarks or 

guidelines for drinking water in samples 

collected from domestic wells for the NAWQA 

Program, 1991–2004

CD-ROM

[In Pocket]

Tables

6–1. Abbreviations for principal aquifer names

6–2. Percentage of wells in principal aquifers with values of physical properties or 

concentrations of major ions, trace elements, nutrients, radionuclides, or organic 

compounds greater than human-health benchmarks or non-health guidelines for 

drinking water in samples collected from domestic wells for the NAWQA Program in 

aquifer studies, 1991–2004

6–3. Percentage of wells in principal aquifers with values of physical properties or 

concentrations of major ions, trace elements, nutrients, radionuclides, or organic 

compounds greater than human-health benchmarks or non-health guidelines for 

drinking water in samples collected from domestic wells for the NAWQA Program in 

agricultural land-use studies, 1991–2004

6–4. Percentage of wells in principal aquifers with values of physical properties or 

concentrations of major ions, trace elements, nutrients, radionuclides, or organic 

compounds greater than human-health benchmarks or non-health guidelines for 

drinking water in samples collected from domestic wells for the NAWQA Program in 

aquifer and agricultural land-use studies, 1991–2004
THIS PAGE INTENTIONALLY LEFT BLANK
Appendix 7 127

Appendix 7. Detection frequencies of organic 

compounds at any concentration and at several 

common reporting levels in samples collected 

from domestic wells for the NAWQA Program, 

1991–2004

CD-ROM

[In Pocket]

Tables

7–1. Detection frequencies of organic compounds at any concentration and at three 

common reporting levels in samples collected from domestic wells for the 

NAWQA Program in aquifer studies, 1991–2004

7–2. Detection frequencies of organic compounds at any concentration and at three 

common reporting levels in samples collected from domestic wells for the 

NAWQA Program in agricultural land-use studies, 1991–2004

7–3. Detection frequencies of organic compounds at any concentration and at three 

common reporting levels in samples collected from domestic wells for the 

NAWQA Program in aquifer and agricultural land-use studies, 1991–2004
Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

Director

U.S. Geological Survey

Massachusetts-Rhode Island Water Science Center

10 Bearfoot Road

Northborough, MA 01532

dc_ma@usgs.gov

or visit our Web site at:

http://ma.water.usgs.gov
DeSimone—Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004—Scientific Investigations Report 2008–5227

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