C

C

C O

C

C

H

H H

H

H

H H

H H

H

H

H

Br

Br

Cl

C

C C

H

H

H

H

H

C Cl

Cl

Cl

H

Cl Cl

Cl Cl

C C

C

C

C C

H

H H

H H

H

C

H

H

C

C

H

H

C C H Cl

Cl

Cl

Perchloroethene

Toluene

Methyl tert-butyl ether

Chloroform

Dibromochloropropane

1,1,1-Trichloroethane

U.S. Department of the Interior

U.S. Geological Survey

Circular 1292

The Quality of Our Nation’s Waters

National Water-Quality Assessment Program

Volatile Organic Compounds in the Nation’s 

Ground Water and Drinking-Water Supply Wells
“High quality water is more than the dream of the conservationists, 

more than a political slogan; high quality water, in the right quantity 

at the right place at the right time, is essential to health, recreation, 

and economic growth.”

Edmund S. Muskie, U.S. Senator

Cover illustration.  Three-dimensional molecular configuration

and composition of some of the compounds discussed in this

report.
The Quality of Our Nation’s Waters

Volatile Organic Compounds in

the Nation’s Ground Water and

Drinking-Water Supply Wells

By John S. Zogorski, Janet M. Carter, Tamara Ivahnenko, Wayne W. Lapham, 

Michael J. Moran, Barbara L. Rowe, Paul J. Squillace, and Patricia L. Toccalino

Circular 1292

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

Available from U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Additional information about this national assessment is available at http://water.usgs.gov/nawqa/vocs/national_

assessment

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Zogorski, J.S., Carter, J.M., Ivahnenko, Tamara, Lapham, W.W., Moran, M.J., Rowe, B.L., Squillace, P.J., and 

Toccalino, P.L., 2006, The quality of our Nation’s waters—Volatile organic compounds in the Nation’s ground water 

and drinking-water supply wells: U.S. Geological Survey Circular 1292, 101 p.

Library of Congress Cataloging-in-Publication Data

The Quality of our nation’s waters : volatile organic compounds in the nation’s ground water and 

drinking-water supply wells / by John S. Zogorski ... [et al.].

 p. cm. -- (Circular 1292)

 Includes bibliographical references and index.

 1. Organic water pollutants--United States. 2. Organic compounds. 3. Water quality management--United States. 4. Water chemistry--United States. I. Zogorski, John S. II. U.S. Geological 

Survey circular ; 1292. 

 TD427.O7Q83 2006

 363.738’420973--dc22

2005031595

ISBN 1-411-30836-0
Estimated use of ground water for drinking water (adapted from data source(1))

Ground water is among the Nation’s most important natural resources. 

Very large volumes of ground water are pumped each day for industrial, 

agricultural, and commercial use. Also, ground water is a drinking-water 

source for about one-half of the Nation’s population, including almost all 

residents in rural areas. Ground water is important as a drinking-water 

supply in every State.

Information on the quality and quantity of ground water is important 

because of the Nation’s increasing population and dependency on this 

resource. Although the population that used domestic wells for drinkingwater supplies decreased between 1950 and 2000, estimated withdrawal 

increased by about 70 percent during that time period. The population 

dependent on public water systems that used ground water for drinkingwater supplies increased between 1950 and 2000, and the estimated 

withdrawal increased about five-fold during that time period.

The quality and availability of ground water will continue to be an 

important environmental issue for the Nation’s citizens. Long-term 

conservation, prudent development, and management of this natural 

resource are critical for preserving and protecting this priceless national 

asset. Continued research by scientists, guidance and regulation by 

governmental agencies, and pollution abatement programs by industry 

are necessary to preserve the Nation’s ground-water quality and quantity 

for future generations.

Donna N. Myers

Chief, National Water-Quality Assessment (NAWQA) Program

U.S. Geological Survey

!LASKA

(AWAII

%80,!.!4)/.

%STIMATEDPERCENTOFPOPULATIONUSING

GROUNDWATERASDRINKINGWATER

TOPERCENT

TOPERCENT

TOPERCENT

TOPERCENT
Foreword

The U.S. Geological Survey (USGS) is committed to serving the Nation with accurate and timely 

scientific information that helps enhance and protect the overall quality of life, and facilitates 

effective management of water, biological, energy, and mineral resources (http://www.usgs.

gov/). Information on the quality of the Nation’s water resources is of critical interest to the 

USGS because it is so integrally linked to the long-term availability of water that is clean and 

safe for drinking and recreation and that is suitable for industry, irrigation, and habitat for fish 

and wildlife. Escalating population growth and increasing demands for the multiple water uses 

make water availability, now measured in terms of quantity and quality, even more critical to the 

long-term sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program (http://

water.usgs.gov/nawqa/) to support national, regional, and local information needs and decisions related to water-quality management and policy. Shaped by and coordinated with ongoing 

efforts of other Federal, State, and local agencies, the NAWQA Program is designed to answer: 

What is the condition of our Nation’s streams and ground water? How are the 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. 

NAWQA results can contribute to informed decisions that result in practical and effective waterresource management and strategies that protect and restore water quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more than 

50 of the Nation’s most important river basins and aquifers, referred to as Study Units (http://

water.usgs.gov/nawqa/nawqamap.html)

1

. Collectively, these Study Units account for more 

than 60 percent of the overall water use and population served by public water supply, and are 

representative of the Nation’s major hydrologic landscapes, priority ecological resources, and 

agricultural, urban, and natural sources of contamination. 

Each assessment is guided by a nationally consistent study design and methods of sampling 

and analysis. The assessments thereby build local knowledge about water-quality issues and 

trends in a particular stream or aquifer while providing an understanding of how and why water 

quality varies regionally and nationally. The consistent, multi-scale approach helps to determine 

if certain types of water-quality issues are isolated or pervasive, and allows direct comparisons 

of how human activities and natural processes affect water quality and ecological health in the 

Nation’s diverse geographic and environmental settings. Comprehensive national assessments 

on pesticides, nutrients, volatile organic compounds, trace elements, and aquatic ecology are 

developed through national data analysis and comparative analysis of the Study-Unit findings 

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

The USGS places high value on the communication and dissemination of credible, timely, and 

relevant science so that the most recent and available knowledge about water resources can be 

NAWQA National Water-Quality Assessment Program
applied in management and policy decisions. We hope this NAWQA publication will provide you 

the needed insights and information to meet your needs, and thereby foster increased awareness and involvement in the protection and restoration of our Nation’s waters. 

The NAWQA Program recognizes that a national assessment by a single program cannot 

address all water-resource issues of interest. External coordination at all levels is critical for a 

fully integrated understanding of watersheds and for cost-effective management, regulation, 

and conservation of our Nation’s water resources. The Program, therefore, depends extensively on the advice, cooperation, and information from other Federal, State, interstate, Tribal, 

and local agencies, non-government organizations, industry, academia, and other stakeholder 

groups. The assistance and suggestions of all are greatly appreciated.

 Robert M. Hirsch

Associate Director for Water

1

Summaries of water-quality studies for the 51 Study Units 

assessed in the first decade of the NAWQA Program, as well as 

Study Units scheduled for assessments in the Program’s second 

decade, are available at http://water.usgs.gov/nawqa/.

2

The name of each Study Unit and other areas are given in 

Appendix 1.

Study Units where the NAWQA Program has completed an occurrence study 

of volatile organic compounds in aquifers.2

Alaska

Hawaii

HPGW

YELL

GAFL

MISE

UMIS

RIOG

OZRK

MOBL

USNK

REDN

CAZB

SANJ

SCTX

NROK

CNBR

ALBE

SPLT

SACR LERI

ACAD

SANT

NECB

TRIN

UTEN

LIRB

EIWA

LTEN

ACFB

SOFL

WMIC ALMN

UCOL

CONN

GRSL

WILL

CCPT

HDSN

PUGT

UIRB

DELR

KANA

LSUS

MIAM NVBR

LINJ

DLMV

SANA

COOK

OAHU

OKLA

EXPLANATION

National Water-Quality Assessment Program

 Study Units

High Plains Regional Ground Water Study
This report is one of a series of publications, The Quality of Our Nation’s Waters, that describe 

major findings of the National Water-Quality Assessment (NAWQA) Program on water-quality 

issues of national and regional concern. This report is on volatile organic compounds (VOCs) in 

ground water and drinking-water supply wells. It is a synthesis of NAWQA and other investigations. Fifty-five VOCs are emphasized in NAWQA’s field studies, and these compounds are 

the focus of this report. During NAWQA’s first decade of Study-Unit investigations, samples 

from more than 2,500 wells were analyzed for VOCs. In addition, carefully selected VOC data 

from more than 1,700 well samples were compiled from other agencies or collected in other 

USGS studies. Collectively, these VOC analyses are the basis for this report’s assessment, 

which is (1) the first national assessment of a large number of VOCs in the Nation’s aquifers 

and (2) the most recent national characterization of VOCs in samples from domestic and public 

wells used for drinking water.

Subsequent reports in this series will cover other water-quality constituents of concern, such 

as pesticides, nutrients, trace elements, as well as physical and chemical effects on aquatic 

ecosystems. Each report will build toward a more comprehensive understanding of national 

and regional water resources as additional investigations are completed and as scientific 

models and tools that link water-quality conditions, dominant sources, and environmental 

characteristics are developed.

The information in this report is intended primarily for scientists and engineers interested or 

involved in resource management, conservation, regulation, and policy making at national, 

regional, and State levels. In addition, the information in this report is intended for public 

health agencies and water utilities who wish to know more about specific contaminant groups 

such as VOCs.

 P. Patrick Leahy, Acting Director

U.S. Geological Survey

Introduction to this report and the NAWQA series

The Quality of Our Nation’s Waters

Pesticides

Nutrients

Trace Elements

VOCs

Ecology

Photograph by Charles G. Crawford, U.S. Geological Survey

Photograph by Janet M. Carter, U.S. Geological Survey

Photograph by Stephen R. Moulton II, U.S. Geological Survey

Photograph courtesy of South Dakota Department of Environment and Natural Resources

Photograph by Janet M. Carter, U.S. Geological Survey
Contents

The first chapter provides an overview of major findings and conclusions for groundwater management, monitoring, and policies. The second chapter describes the 

assessment’s purpose, scope, and approach. More detailed findings for ground water 

are given in the third chapter, and findings for samples from drinking-water supply 

wells are presented in the fourth chapter. Additional information for some frequently 

and widely detected compounds is presented in the fifth chapter.

1 Major findings and conclusions................................................................................. 2

2 Introduction .................................................................................................................... 8

3 VOCs in ground water................................................................................................... 16

4 VOCs in samples from drinking-water supply wells .............................................. 28

5 Additional information for selected VOCs ................................................................ 42

– Chloroform and other trihalomethanes

– Chlorinated solvents—methylene chloride, perchlorethene, 1,1,1-trichloroethane, 

and trichloroethene

– Methyl tert-butyl ether and other gasoline oxygenates

– Gasoline hydrocarbons

References...................................................................................................................... 56

Glossary........................................................................................................................... 62

Appendixes..................................................................................................................... 66

A list of acronyms is included as Appendix 2. 

A glossary of common terms used in this report 

is included on p. 62–65. Beginning in Chapter 2, 

glossary terms are presented in boldface type 

when first used in the text.
   

Chapter 1—Major Findings and Conclusions

This national assessment of 55 volatile organic compounds (VOCs) in ground water gives emphasis to the 

occurrence of VOCs in aquifers that are used as an important supply of drinking water. In contrast to the monitoring 

of VOC contamination of ground water at point-source 

release sites, such as landfills and leaking underground 

storage tanks (LUSTs), our investigations of aquifers are 

designed as large-scale resource assessments that provide 

a general characterization of water-­quality conditions. 

Nearly all of the aquifers included in this assessment have 

been identified as regionally extensive aquifers or aquifer 

systems.(2) The assessment of ground water (Chapter 3) 

included analyses of about 3,500 water samples collected 

during 1985–2001 from various types of wells, representing almost 100 different aquifer studies. This is the first 

national assessment of the occurrence of a large number of 

VOCs with different uses, and the assessment addresses 

key questions about VOCs in aquifers. The assessment also 

provides a foundation for subsequent decadal assessments 

of the U.S. Geological Survey (USGS) National WaterQuality Assessment (NAWQA) Program to ascertain longterm trends of VOC occurrence in these aquifers.

The occurrence of VOCs in samples collected from 

drinking-water supply wells, specifically domestic and 

public wells, also is included (and discussed separately from 

aquifer studies) in this assessment (­Chapter 4), recognizing 

that various agencies, organizations, decision makers, and 

others have different interests and information needs. 

Occurrence findings are compared between domestic and 

public wells to distinguish the separate issues for these 

well types related to supply, environmental setting, and 

sources of VOCs. For this purpose, the occurrence of 55 

VOCs is based on analyses of samples collected at the well 

head, and before any treatment or blending, from about 

2,400 domestic wells and about 1,100 public wells. Findings 

from domestic well samples update earlier USGS studies 

and provide improved national coverage of sampled wells. 

As such, this assessment provides important information 

on VOC occurrence for domestic well samples that may be 

useful to public health agencies. Findings for public well 

samples constitute the most current understanding of the 

occurrence of a large number of VOCs in untreated ground 

water used by public water systems (PWSs) across the 

Nation. Our assessment of public well water complements 

compliance monitoring by water utilities that typically focus 

on drinking water delivered to the public.

Major findings that may be most relevant to the management and monitoring of the Nation’s ground water and 

drinking-water supply wells are emphasized in the following 

discussion. Additional information is included in subsequent 

chapters of this report and at a supporting Web site (http://

water.usgs.gov/nawqa/vocs/national_assessment).

Some household products contain VOCs or 

chemicals that form VOCs when added to 

water. (Photograph courtesy of Joel Beamer, 

professional photographer.)
    

Chapter 1

VOCs were detected in many aquifers across the Nation. Almost 20 percent of the water samples from aquifers contained one or more of the 55 

VOCs, at an assessment level of 0.2 microgram per liter (μg/L). This detection frequency increased to slightly more than 50 percent for the subset 

of samples analyzed with a low-level analytical method and for which an 

order-of-magnitude lower assessment level (0.02 μg/L) was applied. VOCs 

were detected in 90 of 98 aquifer studies completed across the Nation, with 

most of the largest detection frequencies in California, Nevada, Florida, 

and the New England and Mid-Atlantic States. Trihalomethanes (THMs), 

which may originate as chlorination by-products, and solvents were the most 

frequently detected VOC groups. Furthermore, detections of THMs and 

solvents and some individual compounds were geographically widespread; 

however, a few compounds, such as methyl tert-butyl ether (MTBE), ethylene dibromide (EDB), and dibromochloropropane (DBCP), had regional 

or local occurrence patterns. The widespread occurrence of VOCs indicates 

the ubiquitous nature of VOC sources and the vulnerability of many of the 

Nation’s aquifers to low-level VOC contamination. The findings for VOCs 

indicate that other compounds with widespread sources and similar behavior 

and fate properties also may be occurring. (See p. 16, 18, 20, and 21.)

CONCLUSIONS

Many of the Nation’s aquifers are vulnerable to low-level VOC contamination, indicating the need to include VOCs in groundwater monitoring programs to track the 

trend of the low-level VOC contamination 

identified in this assessment.

It is important to continue to control 

sources of VOCs, as well as to enhance 

information about the location, composi-

•

•

Many VOCs were detected, but typically at low concentrations. In water 

samples from aquifers, the concentrations of each VOC and the total concentration of all VOCs analyzed generally were low (defined in this report as 

concentrations less than 1 μg/L). For example, 90 percent of the total VOC 

concentrations in samples were less than 1 μg/L. Forty-two of the 55 VOCs 

were detected in one or more samples at an assessment level of 0.2 μg/L. 

Furthermore, VOCs in each of the seven VOC groups considered in this 

assessment were detected in the samples; these groups included fumigants, 

gasoline hydrocarbons, gasoline oxygenates, organic synthesis compounds, 

refrigerants, solvents, and THMs. The finding that most VOC concentrations 

in ground water are less than 1 μg/L is important because many previous 

monitoring programs did not use low-level analytical methods and therefore 

would not have detected such contamination. (See p. 16, 17, 23, and Appendixes 6 and 7.)

CONCLUSION

VOC contamination in aquifers may be 

more prevalent than previously reported in 

•

Photograph by Barbara L. Rowe, U.S. Geological Survey
   

Some VOCs were detected more frequently than others. Although 42 VOCs 

were detected in aquifer samples, only 15 occurred in about 1 percent or 

more of the samples. The most frequently detected VOCs include 7 solvents, 

4 THMs, 2 refrigerants, 1 gasoline oxygenate, and 1 gasoline hydrocarbon. 

The THM chloroform was the most 

frequently detected compound, and 

its source is attributed, in part, to the 

recycling of chlorinated waters to 

aquifers. The solvent perchloroethene (PCE) and the gasoline 

oxygenate MTBE were the second 

and third most frequently detected 

compounds, respectively. Overall, 

the 15 most frequently detected 

compounds comprise a large fraction of the low-level VOC contamination and provide a logical focus 

for future monitoring of aquifers 

and for follow-up studies to better 

understand their sources and pathways to aquifers. (See p. 22 and 

Appendix 6.)

CONCLUSIONS

Future studies to understand how VOC 

contamination of aquifers is occurring 

can focus on relatively few compounds.

Additional source control and/or 

remediation measures, if deemed warranted, also can focus on relatively few 

compounds, yet would address much of 

the low-level VOC contamination evident 

in this assessment.

•

•

Explaining VOC contamination in aquifers is complex—VOC occurrence is

determined not only by sources but also by natural and anthropogenic factors that affect the transport and fate of VOCs in aquifers. The complexity of 

explaining VOC contamination in aquifers was affirmed in this assessment 

through statistical models for 10 frequently detected compounds. Factors 

describing the source, transport, and fate of VOCs were all important in 

explaining the national occurrence of these VOCs. For example, the occurrence of PCE was statistically associated with the percentage of urban land 

use and density of septic systems near sampled wells (source factors), depth 

to top of well screen (transport factor), and presence of dissolved oxygen 

(fate factor). National-scale statistical analyses provide important insights 

about the factors that are strongly 

associated with the detection of 

specific VOCs, and this information may benefit many local aquifer 

investigations in selecting compound- and aquifer-specific information to be considered. Continued efforts to reduce or eliminate 

low-level VOC contamination 

will require enhanced knowledge 

of sources of contamination and 

aquifer characteristics. (See p. 24 

and 25.)

CONCLUSIONS

The natural and anthropogenic factors 

important to VOC occurrence in a particular aquifer need to be understood in 

order to effectively manage and protect 

aquifers that are susceptible to VOC 

contamination.

A careful review of the importance 

and feasibility of further reducing or 

eliminating VOC sources to aquifers also 

is needed to manage and protect these 

aquifers.

•

•

VOCs found in about 1 percent or more of aquifer

samples, at an assessment level of 0.2 μg/L (compounds listed by decreasing detection frequency)

Compound name VOC group

Chloroform trihalomethane

Perchloroethene solvent

Methyl tert-butyl ether gasoline oxygenate

Trichloroethene solvent

Toluene gasoline hydrocarbon

Dichlorodifluoromethane refrigerant

1,1,1-Trichloroethane solvent

Chloromethane solvent

Bromodichloromethane trihalomethane

Trichlorofluoromethane refrigerant

Bromoform trihalomethane

Dibromochloromethane trihalomethane

trans-1,2-Dichloroethene solvent

Methylene chloride solvent

1,1-Dichloroethane solvent

Factors most commonly associated with VOCs

in aquifers

• Septic systems

• Urban land

Resource Conservation and Recovery Act 

(RCRA) hazardous-waste facilities

•

• Gasoline storage and release sites

• Climatic conditions

• Hydric (anoxic) soils

• Dissolved oxygen in ground water

• Type of well

• Depth to top of well screen
    

Chapter 1

Despite the short period of its extensive use, MTBE was one of the most

frequently detected VOCs. As noted previously, MTBE was the third most 

frequently detected VOC in aquifers. MTBE production peaked in the 1990s 

with the majority of it used voluntarily by refineries for the Nation’s Reformulated Gasoline (RFG) Program. Concentrations of MTBE in aquifer 

samples were rarely of concern relative to the U.S. Environmental Protection 

Agency’s (USEPA) drinking-water advisory based on taste and odor; however, MTBE concentrations in ground water were detected more frequently 

in RFG Program areas than in other areas. The relatively frequent detection 

of MTBE in aquifers was not an anticipated outcome at the commencement 

of NAWQA’s assessment because of MTBE’s short and recent use. A period 

of only a decade or less was required for the detection of MTBE in some 

of the Nation’s aquifers. MTBE findings demonstrate how quickly some 

anthropogenic chemicals, especially those that are mobile and persistent like 

MTBE, may reach aquifers that are especially susceptible to land-surface or 

atmospheric contamination. (See p. 22, 50–53.)

CONCLUSIONS

Some VOCs that are mobile and persistent 

may reach especially susceptible aquifers 

within a decade or less of extensive use, 

and potentially adversely affect groundwater quality.

The environmental behavior and fate properties of anthropogenic compounds should 

be included in decision-making processes 

•

•

Some VOCs were not detected in aquifer samples. Thirteen of the VOCs 

included in this national assessment were not detected in any aquifer samples at a concentration of 0.2 μg/L or larger. The 13 compounds include 5 

VOCs predominantly used in organic synthesis, 4 solvents, 2 fumigants, 

1 gasoline hydrocarbon, and 1 gasoline oxygenate. The specific reason(s) 

why each of these compounds was not detected has not been ascertained; 

however, their lack of occurrence likely is attributed to 

one or more of the following factors: (1) limited use 

in industry, commerce, 

and household products; 

(2) small releases to water 

and land; (3) most use 

occurs in controlled industrial processes or in organic 

synthesis; (4) the compound 

degrades quickly to other 

compounds in the environment; and (5) insufficient 

time has elapsed to allow 

the compound to reach wells 

sampled in this assessment. 

(See Appendix 6.)

VOCs not detected in aquifer samples, at an assessment

level of 0.2 μg/L (compounds listed by VOC group)

Compound name VOC group

Acrolein organic synthesis compound

Acrylonitrile organic synthesis compound

Hexachlorobutadiene organic synthesis compound

1,2,3-Trichlorobenzene organic synthesis compound

Vinyl bromide organic synthesis compound

1,3-Dichlorobenzene solvent

Hexachloroethane solvent

1,2,4-Trichlorobenzene solvent

1,1,2-Trichloroethane solvent

cis-Dichloropropene fumigant

trans-Dichloropropene fumigant

Styrene gasoline hydrocarbon

Ethyl tert-butyl ether gasoline oxygenate

CONCLUSION

Some of these VOCs may not warrant continued inclusion in large-scale 

resource assessments, such as aquifer 

studies completed in the NAWQA 

Program, if it is confirmed that their use, 

release, and behavior and fate characteristics pose a small or negligible likelihood 

of ground-water contamination.

•

Photograph by Janet M. Carter, U.S. Geological Survey
   

Although VOCs were detected frequently in samples from domestic and

public wells, only a small percentage of samples had VOC concentrations

of potential human-health concern. One or more VOCs were detected in 

about 14 and 26 percent of domestic and public well samples, respectively, 

at an assessment level of 0.2 μg/L. However, only about 1 to 2 percent of 

domestic and public well samples had concentrations of potential humanhealth concern (defined in this report as concentrations greater than a 

USEPA Maximum Contaminant Level (MCL) or concentrations greater than 

a Health-Based Screening Level (HBSL) for compounds without an MCL). 

Eight compounds were detected at concentrations of potential concern, and 

three of these compounds occurred in both domestic and public well samples. Most of the concentrations of potential concern were attributed to the 

fumigant DBCP (in domestic well samples only) and the solvents PCE and 

trichloroethene (TCE) in 

samples from both well 

types. Because NAWQA’s 

assessment is based on 

samples collected at the 

wellhead, it is unknown if 

those domestic and public 

well samples with concentrations of potential 

concern actually result 

in concentrations greater 

than MCLs in drinking 

water. (See p. 30–35.)

VOCs found at concentration(s) of potential human-health concern

(compounds listed by decreasing number of concentrations of

potential concern).

Compound name VOC group

Domestic

wells

Public

wells

Trichloroethene solvent X X

Dibromochloropropane fumigant X

Perchloroethene solvent X X

1,1-Dichloroethene organic synthesis 

compound X X

1,2-Dichloropropane fumigant X

Ethylene dibromide fumigant X

Methylene chloride solvent X

Vinyl chloride organic synthesis 

compound X

CONCLUSIONS

Most samples from domestic and public 

wells had VOC concentrations less than 

MCLs and HBSLs, indicating that these 

concentrations are not anticipated to cause 

adverse human-health effects.

Some samples had VOC concentrations 

greater than MCLs, indicating possible 

adverse human-health effects if drinking 

water with these concentrations was 

consumed over a lifetime. However, there 

are uncertainties about actual drinkingwater exposure and health effects of water 

from these supply wells. Further study of 

these wells is warranted to understand 

contaminant sources and VOC concentrations in drinking water.

•

•

Additional VOCs may warrant inclusion in a low-concentration, trendsmonitoring program. Nine VOCs that did not occur at concentrations of 

potential concern in samples from domestic and/or public wells were 

detected at concentrations below but within a factor of 10 of an MCL. The 

9 compounds include 4 solvents, 4 THMs, and 1 gasoline hydrocarbon. 

These 9 VOCs, plus the 8 compounds with concentrations of potential concern, are important compounds to consider including in a low-concentration, 

trends-monitoring program, such as the NAWQA Program. Such programs 

seek to identify compounds in 

domestic and public well samples 

before concentrations reach levels 

of potential concern. Also noteworthy is the finding that the solvents PCE and TCE had, relative 

to other VOCs, a large number of 

concentrations in both domestic 

and public well samples below 

but within a factor of 10 of their 

MCLs. (See p. 32, 34, and Appendixes 9 and 11.)

CONCLUSIONS

Comparing concentrations to MCLs and 

HBSLs helps prioritize which compounds 

merit further study or monitoring. This 

assessment identified 17 VOCs that may 

warrant consideration for inclusion in a 

low-concentration, trends-monitoring 

program for domestic and public wells.

NAWQA’s occurrence information for these 

17 compounds is important information 

considered in the USEPA’s Contaminant 

Candidate List (CCL) Program.

Because of the relatively large number of 

concentrations near and greater than their 

MCLs, the solvents PCE and TCE appear to 

warrant special emphasis to understand 

their sources and their capture by both 

domestic and public wells.

•

•

•

Additional VOCs that may warrant inclusion in a

low-concentration, trends-monitoring program

(compounds listed by VOC group)

Compound name VOC group

Benzene gasoline hydrocarbon

Carbon tetrachloride solvent

1,2-Dichloroethane solvent

cis-1,2-Dichloroethene solvent

1,1,1-Trichloroethane solvent

Bromodichloromethane trihalomethane

Bromoform trihalomethane

Chloroform trihalomethane

Dibromochloromethane trihalomethane
    

Chapter 1

In general, public wells are more vulnerable to low-level VOC contamination than are domestic wells. The detection frequencies of nearly all of the 

most frequently detected compounds and mixtures of VOCs were larger 

in samples from public wells than from domestic wells, at an assessment 

level of 0.2 μg/L. Mixtures of 2 or more of the 55 VOCs were found in 

about 13 percent of the public well samples—more than three times more 

frequently than in domestic well samples—and the likelihood of detecting a 

mixture of VOCs in public well samples was about the same as detecting a 

single compound. Furthermore, 10 of the 15 most frequently detected VOCs 

in public well samples were either THMs or solvents, and all but one of the 

most common VOC mixtures included THMs. The larger detection frequencies in public well samples than in domestic well samples is attributed, in 

part, to the larger withdrawal rates of public wells and their proximity to 

developed areas. The larger pumping rates may increase the capture and 

movement of VOC contamination to public wells. The proximity of public 

wells to developed areas increases the likelihood of VOC sources. (See 

p. 36–41.)

CONCLUSIONS

The frequent detection of VOCs in public 

well samples reinforces the critical importance of effective well-head protection 

programs for public wells and the need to 

further identify and control sources of VOC 

contamination in these programs.

Toxicity testing of VOCs historically has 

focused on individual compounds, typically without consideration of compound 

mixtures. NAWQA studies contribute to 

toxicity studies for VOCs by identifying 

the most commonly occurring chemical 

mixtures in samples from drinking-water 

supply wells.

•

•

Water that has been chlorinated or exposed to household products containing chlorine is an important source of chloroform and possibly other

compounds in ground water supplying domestic and public wells. Chloroform was the most frequently detected VOC in domestic and public well 

samples. The chloroform detected in ground water may have potential 

sources associated with its use as a solvent and an extractant, and as an 

intermediate product in organic synthesis. Also, chloroform and other THMs 

are by-products of the chlorination of drinking waters and wastewaters, 

and the disinfection of domestic and public wells. These compounds also 

may be present in the effluent of septic systems from the use of household 

products containing chlorine, such as bleach. Furthermore, artificial recharge 

of chlorinated water containing THMs and potentially other compounds is 

becoming more common, especially in western States due to, in part, the 

limited supply of drinking water. The chlorination of water to control waterborne diseases has been a common practice in the United States for nearly 

a century. This long-term use has allowed ample time for the recharge of 

waters containing THMs to reach many of the sampled wells. Once introduced to ground water, chloroform and other THMs may persist and move 

long distances in some aquifers. The relative detection frequencies of the 

THMs in well samples, and the common occurrence of mixtures of THMs in 

public well samples, indicate that waters with a history of chlorination and 

that contain these compounds have reached some of the sampled wells. (See 

p. 42–45.)

CONCLUSIONS

The occurrence of THMs in samples from 

drinking-water supply wells, especially 

public wells, is attributed to anthropogenic sources, including most notably the 

capture of recycled water with a history of 

chlorination.

The practice of artificial recharge of 

chlorinated waters to aquifers may require 

additional evaluation to understand the 

concentrations and potential concerns of 

THMs and other chlorination by-products, 

especially for those aquifers used for 

drinking-water supply.

•

•

Photograph by Michael R. Rosen, U.S. Geological Survey
   

1.  What are VOCs?

VOCs are a subset of organic compounds with 

inherent physical and chemical properties 

that allow these compounds to move between 

water and air. This behavior is the fundamental basis for the USGS’s laboratory analysis of 

VOCs in water samples, in which compounds 

that are sufficiently volatile are purged from a 

water sample by an inert gas and then identified and quantified by gas chromatography/

mass spectrometry (GC/MS). In general, VOCs 

have high vapor pressures, low-to-medium 

water solubilities, and low molecular weights. 

Some VOCs may occur naturally in the environment, other compounds occur only as a result 

of manmade activities, and some compounds 

have both origins.

Chapter 2—Introduction

Background and National Significance

The presence of elevated concentrations of VOCs in 

drinking water may be a concern to human health.

Volatile organic compounds (VOCs) are ground-water contaminants of concern because of very large environmental releases, human 

toxicity, and a tendency for some compounds to persist in and migrate with 

ground water to drinking-water supply wells (sidebar 1). Some VOCs, 

such as chlorinated solvents, have been used in commerce and industry for 

almost 100 years,(3) and chloroform and other trihalomethanes (THMs)

have undoubtedly been present in chlorinated drinking water since the first 

continuous municipal application of chlorination in 1908.(4) The production 

and use of manmade organic compounds, many of which are classified as 

VOCs, increased by an order of magnitude between 1945 and 1985.(5) Some 

VOCs have had, and continue to have, very large and ubiquitous usage. An 

example is the widespread use of gasoline, which contains many VOCs. 

Furthermore, VOCs have had numerous uses in industry, commerce, households, and military sites (sidebar 2).

The large-scale use of solutions of VOCs and products containing some 

VOCs has resulted in considerable quantities of VOCs released to the environment. Historically, many waste chemicals were disposed of indiscriminately. Because of this practice, VOCs often are the most frequently detected 

contaminants in soil and ground water at abandoned landfills and dumps, 

and at many industrial, commercial, and military sites across the Nation. 

Federal regulation of VOCs commenced in the 1970s with the passage of 

the Clean Air Act, Clean Water Act, Safe Drinking Water Act (SDWA), 

Resource Conservation and Recovery Act (RCRA), and other environmental 

acts. Collectively, much has been done in the past 30-plus years to mitigate 

pollution. Especially noteworthy examples for mitigating VOC ground-water 

contamination are (1) improved designs, operations, and disposal practices 

for the use of chlorinated solvents at industrial, commercial, and military 

sites; and (2) the cleanup of commercial gasoline release sites and the implementation of measures to minimize gasoline releases in the future. Despite 

these exemplary accomplishments, environmental releases of some VOCs 

from manufacturing facilities in the United States remain high. In 2001, 

for example, 4 of the 20 chemicals with the largest total on-site and off-site 

releases to the environment were VOCs, with a cumulative estimated release 

of more than 200 million pounds.(6)
    

Chapter 2

2.  How are VOCs Used?

VOCs have been used extensively in the 

United States since the 1940s. VOCs are 

common components or additives in many 

commercial and household products including 

gasoline, diesel fuel, other petroleum-based 

products, carpets, paints, varnishes, glues, 

spot removers, and cleaners. Example industrial applications include the manufacturing 

of automobiles, electronics, computers, wood 

products, adhesives, dyes, rubber products, 

and plastics, as well as in the synthesis of 

other organic compounds. VOCs also are used 

in the dry cleaning of clothing, in refrigeration 

units, and in the degreasing of equipment 

and home septic systems. VOCs are present 

in some personal care products such as 

perfumes, deodorants, insect repellents, skin 

lotions, and pharmaceuticals. Some VOCs also 

have been applied as fumigants in agriculture 

and in households to control insects, worms, 

and other pests.

The detection of VOCs in ground water is a concern to 

officials involved in the management of aquifers because 

such an occurrence implies aquifer vulnerability.

The detection of VOCs in aquifers is important because of the widespread, large, and increasing use of ground water for drinking water. In 

2000, about 50 percent of the Nation’s population obtained their supply of 

drinking water from ground water (p. 28 and 29).

The presence of elevated VOC concentrations in drinking water may 

be a concern to human health because of their potential carcinogenicity. In 

addition to cancer risk, VOCs may adversely affect the liver, kidney, spleen, 

stomach, and heart, as well as the nervous, circulatory, reproductive, and 

respiratory systems. Some VOCs may affect cognitive abilities, balance, 

or coordination, and some are eye, skin, and/or throat irritants. Because of 

known or suspected human-health concerns, the USEPA has established 

Maximum Contaminant Levels (MCLs) that apply to 29 VOCs in drinking 

water supplied by public water systems (PWSs). In addition, some States 

have set MCLs for additional VOCs and in some cases have established 

more stringent standards than the USEPA values. The human-health consequences of low-concentration exposure of VOCs in drinking water (that is, at 

concentrations less than MCLs) are uncertain.

In addition to human-health concerns, scientists and engineers involved 

in the management of aquifers and water-supply development are concerned 

about the detection of VOCs in ground water because such an occurrence

implies aquifer vulnerability. Identifying additional source-control strategies or enhancing existing measures may be warranted if anthropogenic 

compounds are detected frequently in ground water. The detection of a 

VOC in ground water also may be of concern because it denotes that a pathway exists by which other persistent and potentially toxic compounds may 

reach drinking-water supply wells.

Products containing VOCs have

many uses in commerce and

households. (Photographs by: 

left, Connie J. Ross; middle,

Janet M. Carter; right, Rika

Lashley, U.S. Geological Survey.) !UTOMOTIVE PAINTING

3MALL ENGINE FUEL

0AINT STRIPPING
10   

VOCs were selected for emphasis in the USGS’s NAWQA Program primarily because of the previously reported occurrence of some of 

these compounds in many of the Nation’s water supplies.(3, 7, 8, 9, 10) The overall intent of the Program’s VOC assessment is to provide an improved understanding of the occurrence and geographical distribution of selected VOCs 

in the Nation’s water resources, with emphasis on ground water. The assessment includes both new VOC data collected in the Program’s Study-Unit 

investigations and VOC data from previous studies with a similar design.

Previous findings from the Program’s assessment of VOCs were 

reported initially in 1999 with emphasis on (1) the occurrence of VOCs in 

samples from wells in urban and rural areas; and (2) the probability of 

detecting one or more VOCs in ground water on the basis of population 

density.(11) Subsequently, the Program’s scientists have reported nationalscale occurrence findings for (1) mixtures of VOCs, pesticides, and nitrate in 

samples from domestic and public wells;

(12) (2) VOCs in the water supply of 

selected community water systems (CWSs);

(13, 14) (3) MTBE and gasoline 

hydrocarbons in ground water;(15) and (4) VOCs in domestic well samples(16) and in shallow, urban ground water.(17)

This report presents additional salient findings of the national VOC 

assessment and gives emphasis to the occurrence of VOCs in the Nation’s 

ground water (sidebar 3) and in samples from drinking-water supply wells 

(sidebar 4). This includes information about the detection frequency, concentration, geographical distribution, and mixtures of VOCs. Also described 

are natural and anthropogenic factors that were found to be associated with 

the occurrence of some of the frequently detected VOCs. Additionally, this 

report presents information and more in-depth findings for selected VOCs 

including (1) chloroform and other THMs; (2) chlorinated solvents—methylene chloride, PCE, 1,1,1-trichloroethane (TCA), and TCE; (3) MTBE and 

other gasoline oxygenates; and (4) gasoline hydrocarbons.

Information on the occurrence of VOCs is presented separately in this 

report for ground water (Chapter 3) and drinking-water supply wells, specifically domestic and public wells (Chapter 4). It is recognized that various 

agencies, organizations, researchers, resource managers, decision makers, 

and the public have different interests and information needs regarding the 

use and management of ground-water resources and the protection and oversight of drinking-water supplies. NAWQA aquifer studies are large-scale 

resource assessments of ground water that provide a general characterization 

This Assessment’s Purpose and Scope

The overall intent of the NAWQA Program’s VOC 

assessment is to provide an improved understanding 

of the occurrence and distribution of selected VOCs 

in the Nation’s water resources.

3.  Assessing the Quality of Ground

Water

Ground water is an important supply of drinking water in the United States, and the study 

of aquifers is a large component of NAWQA’s 

ground-water assessments. Aquifer studies 

have been completed in nearly every NAWQA 

Study Unit and have provided a comprehensive picture of the chemical quality of water 

in locally and regionally important aquifers. 

More information on specific aquifer studies is 

available on the Circular’s Web site.

Many pesticides, VOCs, nutrients, and 

naturally occurring chemicals are monitored 

in aquifer studies. Typically the aquifer (or 

portion thereof) selected for study is locally 

one of the most intensively used aquifers for 

drinking water. Aquifer studies are designed 

to provide an overall picture of the aquifer’s 

water-quality condition and, as such, are considered resource assessments. To achieve this 

spatially large aquifer characterization, wells 

selected for sampling are randomly located but 

distributed approximately equally across the 

study area. A variety of well types with different water uses are included in the assessment 

of aquifer studies. None of the sampled wells 

were selected because of prior knowledge of 

nearby contamination.
    11

Chapter 2

of water-quality conditions in locally important aquifers or portions thereof. 

When completed in many locations, these studies collectively provide an 

important national perspective on the current extent of VOC contamination and regional patterns of VOC occurrence in ground water. In addition, 

aquifer studies characterize the vulnerability of ground-water systems to 

VOCs, as well as to other contaminants with similar sources and environmental properties. This information may be especially valuable for national 

and regional decisions about the need for future ground-water protection and 

associated policies and regulations. 

The occurrence of VOCs in samples from domestic and public wells 

is presented separately in order to distinguish the separate issues for these 

well types related to supply, environmental setting, and sources of VOCs. 

Samples from these wells provide information about VOC contamination 

that may reach tap water unless the supply is treated to remove any VOCs or 

is diluted with other water supplies. Occurrence information for individual 

VOCs provides important insights about the concentrations of potential 

human-health concern in drinking-water supply wells and the need for 

controlling their sources of contamination. This information often is sought 

by water utilities, public health agencies, the public, and rural citizens who 

rely on private wells for drinking water.

A total of 55 VOCs are included in this assessment, and a sample from 

each well was routinely analyzed for nearly all of these compounds. The 

selection procedure for the inclusion of these VOCs in NAWQA’s routine 

monitoring is described elsewhere(18) and included, for example, consideration of the feasibility of laboratory analysis, known or suspected humanhealth concerns, frequency of occurrence in water resources based on prior 

investigations, and potential for large-scale use.

4.  Assessing the Quality of Ground

Water Captured by Drinking-Water

Supply Wells

NAWQA’s studies of drinking-water supply 

wells focus on the quality of ground water 

captured by domestic and public wells, in 

contrast to the quality of tap water (that is, 

drinking water). USGS field personnel collect 

samples of ground water from domestic and 

public wells at the wellhead and before any 

treatment or blending. As such, NAWQA’s 

studies complement drinking-water-compliance-monitoring programs required by other 

agencies; these programs usually specify monitoring after treatment or blending. Comparisons of concentrations for domestic and public 

well samples to primary drinking-water

standards and Health-Based Screening

Levels (HBSLs) in this report are made only 

in the context of the quality of untreated and 

unblended ground water. Human exposure 

from tap water and other pathways is not 

quantified.

During NAWQA’s first decade of assessments, 

many domestic wells and some public wells 

were sampled. During its second decade, 

additional emphasis has been placed on understanding the quality of drinking-water supplies 

including the monitoring of river intakes and 

production wells of large CWSs, as well as the 

continued sampling of domestic wells. In addition, major factors that influence the transport 

of chemicals to public wells are being studied.

Studies of drinking-water supplies are important because these studies (1) identify the 

presence and concentrations of those chemicals that may reach domestic and public wells 

(or surface-water intakes); and (2) provide 

information on the need for enhanced source 

control. Through these studies, the USGS will 

continue to collaborate with other agencies, 

organizations, and water utilities involved with 

the supply of the Nation’s drinking water.

The primary purpose of this report is to present important findings of the assessment of VOCs in the Nation’s 

ground water and drinking-water supply wells.

Example Key Questions About VOCs That NAWQA’s Findings Address:

Which VOCs are detected most frequently in aquifers? In samples from domestic and 

public wells? At what concentrations?

Which of the aquifers studied are most vulnerable to VOC contamination? 

Which natural and anthropogenic factors are associated with VOC occurrence in 

aquifers and samples from domestic and public wells?

Are the frequently detected VOCs found everywhere in aquifers across the Nation or are 

local/regional occurrence patterns evident?

Are specific mixtures of VOCs common? Which mixtures occur most frequently?

Do domestic or public wells have more low-level VOC contamination? Why?

Which VOCs are detected at concentrations of potential human-health concern in 

samples from domestic and public wells?

Which VOC occurrence findings provide insights for future ground-water protection?

•

•

•

•

•

•

•

•
12   

This section describes some aspects of the assessment’s approach. Additional details are presented elsewhere(19) and in Appendix 3. Two 

primary objectives of this assessment included determination of (1) VOCs 

in ambient ground water from aquifer studies; and (2) VOCs in samples 

from actively used domestic and public wells. Samples from 3,498 wells 

with a variety of water uses were selected for analysis of VOCs in aquifer 

studies (table 1). VOC data from 2,401 domestic wells and 1,096 public 

wells were available from aquifer studies, shallow ground-water studies, 

and a national source-water survey (table 2) to characterize the occurrence 

of VOCs in these two well types. One VOC analysis per well was included 

in the assessment. Well selection criteria and maps showing the locations of 

wells are presented in Appendix 3.

VOC data for domestic well samples are a large subset of data for 

aquifer studies because existing wells, including many domestic wells, were 

selected for sampling. Domestic wells commonly were chosen for aquifer 

studies because their distribution in most areas best fit the study objective 

of assessing the quality of aquifers using randomly selected and spatially 

distributed sampling points for a large area.

All samples for NAWQA studies were collected and analyzed by USGS 

personnel using approved USGS methods. For nearly all of the ground-water 

samples analyzed by the USGS, compounds were identified and concentrations were quantified using GC/MS. For data not collected or analyzed by 

USGS, laboratory certification and use of GC/MS methods were required for 

inclusion of data in this assessment.

This Assessment’s Approach

Samples for VOC determination are collected and

analyzed by established methods that ensure highquality occurrence information. (Photographs by

Barbara L. Rowe, U.S. Geological Survey.)

Table 1. Number of wells with VOC data for aquifer studies by water use.

Use of water Aquifer studies

Number of wells Percent of wells

Domestic supply 2,138 61.1

Public supply 513 14.7

Monitoring 335 9.6

Other 461 13.2

Unknown 51 1.5

Total 3,498 100

Table 2. Number of domestic and public wells with VOC data by data source.

Data source

Domestic wells Public wells

Number

of wells

Percent

of wells

Number

of wells

Percent

of wells

Aquifer studies 12,138 89.0 1513 46.8

Shallow ground-water studies 263 11.0 8 .7

National source-water survey 0 0 575 52.5

Total 2,401 100 1,096 100

1

Same wells used in aquifer studies (table 1).
    13

Chapter 2

As noted previously, 55 VOCs were included in this assessment. 

These VOCs were assigned to the following groups on the basis of their 

primary usage (or origin): (1) fumigants, (2) gasoline hydrocarbons, (3) 

gasoline oxygenates, (4) organic synthesis compounds, (5) refrigerants, 

(6) solvents, and (7) THMs (chlorination by-products). Other uses and additional information for the 55 VOCs can be found in Appendix 4.

Most detection frequencies were computed by applying an assessment 

level of 0.2 μg/L (sidebar 5). The assessment level of 0.2 μg/L was chosen to 

represent the laboratory reporting value for USGS prior to April 1996 and to 

be compatible with other agencies. For this assessment level, data from all 

sampled wells were used in the computation of detection frequencies. The 

number of samples with laboratory analyses varied among the 55 VOCs.

For some computations, an assessment level of 0.02 μg/L also was 

applied. This assessment level was selected to represent the occurrence of 

VOCs using a new, low-level analytical method developed by the USGS 

for natural waters. When applying this assessment level for aquifer studies, 

the samples from a subset of 1,687 wells that were analyzed using the new 

method were used in the computation of detection frequencies. Data from 

a subset of 1,208 wells were available for computations for domestic well 

samples; however, insufficient data were available for computations for 

public well samples at an assessment level of 0.02 μg/L.

A variety of ancillary data and statistical models were used to relate the 

occurrence of VOCs to various hydrogeologic and anthropogenic variables. 

The hydrogeologic variables that were used in the relational analyses represented the transport and fate of VOCs in ground water. The anthropogenic 

variables used in the relational analyses represented some of the potential 

sources of VOCs to ground water. A listing of the ancillary data used in 

these analyses can be found elsewhere.(19)

For those compounds with Federal drinking-water standards, VOC 

concentrations in samples from domestic and public wells were compared to 

USEPA MCLs. Concentrations for 15 unregulated compounds were compared to HBSLs (p. 30), which were developed by the USGS in collaboration with the USEPA, New Jersey Department of Environmental Protection, 

and the Oregon Health & Science University. HBSLs are not enforceable 

regulatory standards but are concentrations of contaminants in water that 

warrant scrutiny because they may be of potential human-health concern.(20)

5.  What are Assessment Levels,

and Why are They Used?

The detection frequency of VOCs in ground 

water is an important indicator of water 

quality in occurrence assessments. In order to 

compare detection frequencies for individual 

VOCs, groups of VOCs, or VOC data from different agencies with different reporting levels, 

an “assessment level” must be established. 

An assessment level is a fixed concentration that is the basis for computing detection 

frequencies.

An assessment level is necessary because the 

detection frequency computed for a specific 

VOC depends on the laboratory reporting

level for that compound.(21) Laboratory reporting levels for VOCs may vary from compound 

to compound and from one laboratory to 

another due to differences in laboratory 

equipment, equipment sensitivity, experience 

and skill of equipment operators, or laboratory 

conditions. In addition, data sets collected for 

different monitoring objectives or analyzed by 

different laboratory methods also can have 

different reporting levels. Thus, different 

detection frequencies for VOC data sets with 

different reporting levels may not represent 

true differences in water quality, but rather 

they may only reflect the above noted factors.

Various quality-control criteria were used to select 

wells and VOC data for this national assessment.
14   

VOCs are used in numerous industrial, commercial, and domestic applications and can contaminate ground water through sources such 

as landfills and dumps, leaking storage tanks, septic systems, leaking water 

and sewer lines, stormwater runoff, and the atmosphere. These sources 

differ, however, in their potential to cause elevated concentrations of VOCs 

in ground water (sidebar 6). Many household products contain VOCs and 

can be discarded to septic systems or disposed of improperly. In commerce 

and industry, VOCs are used in numerous applications (sidebar 2), and these 

uses result in considerable quantities of VOCs being released to the environment.(22) Once in the environment, many VOCs move between the atmosphere, soil, ground water, and surface water. Although many VOCs have 

relatively short half-lives in certain media because of degradation, other 

VOCs such as DBCP, TCA, and MTBE can persist in ground water and 

degrade only slightly over a period of years or decades.

VOCs can be transported through the unsaturated zone in recharge, in 

soil vapor, or as a non-aqueous-phase liquid. Any hydrologic condition that 

shortens residence time within the unsaturated zone can result in increased 

amounts of VOCs to the water table; for example, manmade structures like 

recharge basins and shallow injection wells can accelerate transport through 

the unsaturated zone. Furthermore, a shallow water table and abundant 

recharge will favor more rapid transport through the unsaturated zone and 

increase the likelihood of VOCs reaching ground water. Some VOCs also 

can move slowly through the unsaturated zone with air and enter the top of 

the water table by partitioning between soil air and ground water; however, 

this type of transport also is enhanced by the movement of recharge.(23)

The movement of solutes by the bulk motion of flowing ground water is 

known as advection. The rate of advective transport varies by many orders 

of magnitude.(24) The tendency of solutes to spread out from the path that 

would be expected from advective flow is known as dispersion. VOCs in 

ground water can eventually be captured by pumping wells or discharged to 

surface waters if traveltimes are short enough to prevent the complete attenuation of VOCs. 

The transport of VOCs dissolved in ground water also may be slowed 

by sorption to organic carbon in the aquifer material. The effect of sorption 

on VOC transport is dependent on the solubility of the VOC, the amount of 

organic carbon in the aquifer, and aquifer density and porosity. Some very 

Sources, Transport, and Fate of VOCs in Ground Water—An Overview

6.  How Do Ground-Water

Concentrations from VOC Sources

Differ?

VOC contamination can originate from 

the release of liquids, such as petroleum 

hydrocarbons or solvents, at one location. The 

release of VOCs from a LUST is an example 

of such contamination and commonly results 

in concentrations of VOCs in ground water 

near the source at the milligram or gram per 

liter level. These large concentrations are one 

reason why this type of contamination can 

spread over a large area.

Contamination also can originate over large 

areas from sources such as leaking water and 

sewer lines, stormwater runoff, and atmospheric deposition. Typically, these sources 

result in small concentrations (microgram per 

liter or smaller) in water.

Manmade structures, such as recharge basins and 

shallow injection wells, can hasten the transport of 

VOCs to ground water.

A possible source of VOCs is illustrated by the

leaking barrels from a Superfund site. (Photograph

courtesy of U.S. Environmental Protection Agency.)
    15

Chapter 2

soluble VOCs like MTBE have a small sorption tendency and thus move as 

quickly as ground water, whereas other less soluble VOCs like carbon tetrachloride have a larger sorption tendency and may move slowly relative to the 

rate of ground-water flow.(25)

The fate of VOCs in ground water is largely dependent on their persistence under the conditions present in the aquifer. VOCs that are persistent 

in water are more likely to be detected in ground water because they can 

travel greater distances from their source before degradation and dilution 

occur. In ground water, VOCs may undergo selective abiotic (not involving 

microorganisms) and biotic (involving microorganisms such as bacteria and 

fungi) degradation. An example of abiotic degradation is the degradation 

of TCA to 1,1-dichloroethene (1,1-DCE) by reaction with water. For most 

VOCs, biotic degradation generally is more important than abiotic degradation. Some VOCs can be degraded biotically under a range of redox conditions,

(25) whereas others may persist in ground water until a particular redox 

condition occurs. An example of biotic degradation is the degradation of 

PCE to TCE.

Bacteria may be unable to use VOCs as a sole source of food when the 

compounds are present at nanogram per liter or low microgram per liter 

concentrations.(26) This may slow the degradation of VOCs in ground water. 

A decline in the degradation rate with decreasing concentration may account 

for the low VOC concentrations detected in this assessment for some VOCs 

that degrade quickly at larger concentrations.

VOCs can be transported with precipitation to

ground water and stormwater runoff. (Bottom

photograph by Charles G. Crawford, U.S. Geological

Survey.)

Some VOCs, such as DBCP, TCA, and MTBE, can 

persist in ground water with little degradation 

over years or decades.

Two other possible sources of VOCs are

demonstrated by contamination originating from

automobiles and this leaking underground storage

tank. (Bottom photograph courtesy of the Utah

Department of Environmental Quality.)
16   

Chapter 3—VOCs in Ground Water

Occurrence of One or More VOCs in Aquifers

7.  Occurrence Information Helps

in Managing Ground-Water

Resources

The occurrence of VOCs in aquifers provides 

important information to those responsible for 

managing ground-water resources. Contamination of aquifers by one or more VOCs also is 

a national issue of potential concern because 

of the widespread and long-term use of many 

of these compounds.

Detecting one or more VOCs in aquifer 

samples provides evidence that conditions 

favor VOCs reaching the sampled wells. Contaminant occurrence depends on aquifer properties, the associated sources of water to the 

aquifer, and stresses on the aquifer such as 

pumping. Contamination also depends on the 

locations and types of VOC sources, the relative locations of wells, and the transport and 

fate of VOCs.(27) Knowledge that VOC contamination is present in an aquifer provides the 

rationale for assessment of the human-health 

significance of the contamination, as well as 

the possible need for more in-depth studies 

to determine the source(s) of contamination 

and remedial action if concentrations are of 

potential concern. The occurrence of lowlevel contamination of one or more VOCs in 

an aquifer also can provide managers with an 

early indication of the presence of VOCs that 

eventually might adversely affect the quality 

of water from domestic and public wells.

Figure 1. Total VOC

concentrations were less

than 1 microgram per liter

(μg/L) in about 90 percent

of the 867 aquifer samples

with VOC detections

analyzed using the lowlevel method.

Detection of VOCs in aquifer samples 

demonstrates the vulnerability of many of the 

Nation’s aquifers to VOC contamination.

About 19 percent of the ground-water samples from 3,498 wells in aquifer studies (hereafter referred to as aquifer samples) contained 

one or more VOCs at an assessment level of 0.2 μg/L. A larger percent 

occurrence of 51 percent was evident for a subset of samples from 1,687 

wells that were analyzed using the low-level analytical method, for which an 

order-of-magnitude lower assessment level (0.02 μg/L) was applied.

Possible reasons why no VOCs were detected in some aquifer samples 

include (1) no VOC sources were present near the sampled wells, (2) the 

water sampled was recharged before VOCs were in use, (3) the water 

sampled was old enough that VOCs had time to undergo degradation, (4) the 

ground water sampled was a mix of water not containing VOCs with water 

containing VOCs, which resulted in any VOCs present being diluted to concentrations below detection levels, (5) VOCs were present in the aquifer but 

had not reached the wells yet, or (6) some combination of these and other 

reasons. VOC occurrence or non-occurrence could vary within different 

parts of an aquifer as well as among aquifers. At the local scale, additional 

studies are needed to help explain reasons for VOC occurrence or nonoccurrence. 

The finding that one or more VOCs were detected in about one-half of 

the samples analyzed using the low-level method demonstrates the vulnerability of many of the Nation’s aquifers to low-level VOC contamination 

,ESSTHAN±G,

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LESSTHAN±G,

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    17

Chapter 3

8.  Urban Land Use Contributes

More VOCs to Ground Water than

Do Other Land Uses

Detection frequencies of 1 or more of the 55 

VOCs differ in shallow ground water partly 

depending on the overlying land use—38 percent in residential/commercial urban settings 

and 11 percent in agricultural settings at an 

assessment level of 0.2 μg/L. The residential/

commercial findings may be attributable to 

one or more of several factors related to VOC 

sources in the urban environment compared 

to other settings. For example, the urban 

setting may have more sources and releases 

of VOCs than other settings. Also, recharge 

of VOCs to ground water may be enhanced in 

urban areas by structures such as recharge 

basins and shallow injection wells. In addition, 

differences in detection frequencies could be 

attributable to distance traveled by VOCs and 

to the transport and fate properties of the 

VOCs associated with the land-use setting.

The finding that urban settings contribute 

more VOCs to underlying ground water 

indicates that these waters generally are 

more vulnerable to VOC contamination than 

ground water underlying other settings. 

However, this is not always the case locally. 

In Oahu, Hawaii, for example, the largest 

VOC contamination occurs in the agricultural 

areas of central Oahu, where fumigants have 

been intensively applied but the aquifers 

are unconfined, as compared to the minimal 

contamination underlying urban Honolulu, 

where the aquifers are somewhat protected 

by a confining unit.(28)

Figure 2. VOC contamination occurs in aquifers across the Nation, albeit over a large

range of concentrations.

Although infrequent, total VOC concentrations 

of 10 μg/L or greater were found in many States 

throughout the Nation.

(sidebar 7). This finding also indicates that VOCs might be detected in other 

aquifers across the Nation if samples are analyzed using a low-level method.

Total concentrations of the 55 VOCs in samples provide an overall 

national perspective on the extent of VOC contamination in aquifers. About 

90 percent of samples analyzed using the low-level method had total VOC 

concentrations less than 1 μg/L (fig. 1). Conversely, total VOC concentrations of 10 μg/L or greater were found in slightly more than 1 percent of all 

samples with VOC detections.

Nearly three-quarters (42 out of 55) of the VOCs in NAWQA’s assessment were detected in one or more samples at a concentration of 0.2 μg/L or 

greater. The number of VOCs detected, however, did vary markedly among 

aquifer studies, ranging from 1 to 31 VOCs.

VOC contamination occurs in aquifers across the Nation, albeit over 

a large range of concentrations (fig. 2). Total concentrations of VOCs of 

10 μg/L or greater occur infrequently but in many States throughout the 

Nation. Many factors, such as land use, hydrogeology of the aquifer, geochemistry of the ground water, and the transport and fate properties of 

VOCs, affect the occurrence of VOCs in ground water (sidebars 7 and 8, and 

p. 14 and 15).

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18   

Occurrence of One or More VOCs by Aquifer Study, Principal Aquifer,

and Aquifer Lithology

9.  Hydrogeologic Conditions Can

Partly Control the Occurrence of

VOCs in an Aquifer

The Edwards aquifer is a sole-source 

carbonate aquifer used for drinking-water 

supply in south-central Texas. This aquifer 

demonstrates the control that hydrogeologic 

conditions can have on VOC occurrence in an 

aquifer.(29) VOC detection frequencies in the 

Edwards aquifer for the unconfined recharge 

area (61 percent) differed from the confined 

area (38 percent). The aquifer’s recharge 

area is a faulted and fractured limestone that 

allows unrestricted downward movement 

of water and contaminants into the aquifer. 

The confined part of the Edwards aquifer, 

however, is overlain by a unit composed of 

several hundred feet of low-permeability

rocks (the Navarro-Del Rio confining unit). 

This unit restricts the downward movement 

of water and contaminants to the underlying 

confined part of the Edwards aquifer, resulting 

in a smaller VOC occurrence in the confined 

area than in the unconfined recharge area.

Cross section of the Edwards aquifer near San Antonio,

Texas, showing the recharge zone and the confined

zone of the Edwards aquifer.(29)

Figure 3. VOCs were detected in many aquifer studies throughout the Nation.

VOCs were detected throughout the Nation, with the 

largest detection frequencies generally in the West 

and the New England and Mid-Atlantic States.

The occurrence of 1 or more of the 55 VOCs in aquifers was reported collectively to provide an overall national perspective (p. 16 and 17) 

on the extent of VOC contamination. Additional insights about the variability in occurrence of at least one or more VOCs across the Nation, at aquifer 

or regional scales, and by aquifer characteristics, such as lithology and 

hydrogeologic conditions (sidebar 9), are presented here and are relevant to 

most regional and local ground-water managers.

Detection frequencies of one or more VOCs for the 98 aquifer studies 

conducted as part of the Study-Unit investigations ranged from 0 to about 

77 percent at an assessment level of 0.2 μg/L (fig. 3; Appendix 5). VOCs 

were detected in many studies throughout the Nation, with most of the 

largest detection frequencies in California, Nevada, Florida, and the New 

England and Mid-Atlantic States. No VOCs were detected in eight aquifer 

studies that were widely distributed across the Nation.

When the sampling was grouped by 33 principal aquifers and 3 other 

aquifers (sidebar 10), detection frequencies of one or more VOCs at an 

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    19

Chapter 3

Detection of VOCs differed markedly between 

and within principal aquifers.

Figure 4. VOC detection frequencies in principal and other aquifers varied widely

among lithologic categories.

10.  Analysis and Reporting at the

Principal-Aquifer Scale Help Link

National and Local-Scale Findings

 Analysis and reporting of NAWQA’s first 

decade of sampling have focused on national 

and Study-Unit (local-scale) assessments. 

Future NAWQA efforts will expand this 

focus to include analysis and reporting at the 

principal-aquifer scale. National assessments 

provide summaries of the national occurrence 

and distribution of water-quality conditions. 

However, the large variability in hydrogeologic 

and other conditions across the Nation often 

confound the scientist’s ability to sort out 

factors that affect water quality. Study-Unit 

assessments describe water-quality conditions locally, and often the scientist is able 

to determine the factors that affect water 

quality. Extrapolating those findings to beyond 

the study area often is problematic. Analysis 

and reporting at an intermediate regional 

scale, such as by principal aquifer, is intended 

to help link the findings between the national 

and local scales.

The principal aquifers used as the framework 

for this intermediate scale of analysis and 

reporting are located throughout the United 

States. Sixty-two principal aquifers have been 

identified as regionally extensive aquifers or 

aquifer systems that could potentially be used 

as a source of potable water.(2) NAWQA sampled parts of 33 of these 62 principal aquifers 

during its first decade of assessments. The 

principal aquifers vary widely in size, thickness, hydrogeologic properties, yield, and use 

as drinking-water supplies. Basic descriptions 

of these principal aquifers and many of their 

characteristics are available at http://www.

nationalatlas.gov.

assessment level of 0.2 μg/L varied from 0 to 51 percent (Appendix 5). This 

variability between principal aquifers is of the same order of magnitude as 

the variability within principal aquifers. For example, detection frequencies 

in the glacial deposit aquifers ranged from 0 to 43 percent.

The two clusters of relatively large detection frequencies (in the New 

England and Mid-Atlantic States and in California and Nevada) (fig. 3) 

include multiple principal aquifers. Large detection frequencies occurred 

in one or more aquifer studies within four principal or other aquifers in 

the Northeast—the New England part of the New York and New England 

crystalline rock aquifer, the glacial deposit aquifers, the Northern Atlantic Coastal Plain aquifer system, and the Early Mesozoic basin aquifers. 

Large detection frequencies occurred in one or more aquifer studies in two 

principal aquifers in California—the Central Valley aquifer system and 

the California Coastal basin aquifers in and near Los Angeles—and in the 

Basin and Range basin-fill aquifers in Nevada. The relatively large detection frequencies of VOCs in these principal aquifers likely are the result of 

a combination of factors such as long-term use of VOCs, high population 

densities, high rainfall (in the Northeast), artificial recharge (in California), 

and use of VOCs that are relatively persistent in ground water (such as 

DBCP in the Central Valley of California).

VOCs were detected in principal and other aquifer studies for all 

lithologic categories, and with the exception of the sandstone and carbonate aquifers, a wide range of detection frequencies were evident for each 

category (fig. 4). Noteworthy also is that VOCs were detected in nearly all 

studies. In general, lithology alone is not a good indicator of aquifer vulnerability nor of how frequently VOCs will be detected in a specific aquifer. !15)&%2,)4(/,/'9#!4%'/29

      

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20   

Figure 5. Detection frequencies in aquifers vary between VOC groups.

THMs and solvents were the most frequently 

detected groups of VOCs in aquifers.

11.  Production Rates Do Not Fully

Explain VOC Group Detection

Frequencies

If estimated production rates of VOCs (shown 

below) alone were the primary governing factor 

explaining detection frequencies, the gasoline 

hydrocarbons would be detected in aquifers 

much more frequently than the other six VOC 

groups. Likewise, the fumigants, refrigerants, 

and THMs would have smaller detection 

frequencies than the other four VOC groups. 

Comparison of production rates with detection

frequencies of VOCs by group (fig. 5) shows 

that this generally is not the case.

There are many possible reasons for this lack 

of correspondence between VOC production 

rates and detection frequency in aquifers. For 

example, production data (see Circular’s Web 

site) are not available for all VOCs in each group, 

so actual production could be considerably more 

than the estimates shown above. In addition, 

even if the production data were complete, production is not necessarily an exact measure of a 

VOC source that is contributing a VOC to ground 

water. Although preferable in this analysis, 

national data sets of releases for all VOC groups 

are not available. Additionally, factors such as 

the hydrogeologic setting, geochemistry of the 

ground water, and transport and fate properties 

of VOCs can control the occurrence of VOCs in 

aquifers (p. 14 and 15).

Occurrence of VOC Groups in Aquifers

The most frequently detected VOC groups in aquifers were THMs and solvents (fig. 5). Both groups were detected in about 8 percent of 

aquifer samples at an assessment level of 0.2 μg/L. One or more compounds 

in each of the remaining five VOC groups also were detected, but at frequencies less than 4 percent. At an assessment level of 0.02 μg/L, a detection of 

one or more THMs, solvents, and gasoline hydrocarbons occurred in about 

1 out of every 5 wells. Production of the VOC groups alone does not fully 

explain VOC group occurrence (sidebar 11).

Most total concentrations for each VOC group were less than 1 μg/L, 

and more than one-half of the samples with detections had concentrations 

less than 0.2 μg/L for all groups. THMs, solvents, and gasoline hydrocarbons 

had the largest numbers of detections at concentrations less than 0.2 μg/L.

Solvents (fig. 6), THMs, gasoline hydrocarbons, and, less frequently, 

refrigerants had a widespread distribution throughout the Nation. Fumigants, 

gasoline oxygenates, and organic synthesis compounds were not detected in 

many aquifers. Presumably, the spatial patterns of detections/non-detections 

may reflect, in part, the more spatially focused historical or continued use 

of particular VOC groups. For example, the association between fumigant 

use and occurrence in aquifers in Oahu, Hawaii, and the Central Valley of 

California illustrates effects from historical use and provides an example 

of how local and national detection frequencies of VOC groups can differ 

(sidebar 12; fig. 7). The gasoline oxygenates, specifically MTBE, also show 

spatial patterns of occurrence that are related to use (p. 52 and 53). Additional maps of the national occurrence patterns of VOC groups are available 

(see Circular’s Web site).

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    21

Chapter 3

12.  Local and National Detection

Frequencies of VOC Groups Can

Differ

Fumigant detections in aquifers on the Island 

of Oahu, Hawaii, and in the Central Valley of 

California provide examples of a VOC group 

with much higher local detection frequencies 

than the national detection frequency of about 

2 percent (assessment level of 0.2 μg/L). The 

fumigant detections in Oahu are the result of 

fumigant application to pineapple fields. In 

1970, for example, about 1.8 million pounds of 

fumigants were applied to combat rootworms.(30) Fumigant formulations containing 

1,2-dichloropropane, 1,2,3-trichloropropane, 

and EDB were banned in the late 1970s-early 

1980s after about 20 to 30 years of use.(31, 32) In 

spite of the discontinuation of their use more 

than 20 years ago, fumigants were detected 

in more than 30 percent of wells sampled in 

NAWQA’s aquifer study. Fumigants occur in 

ground water in Oahu because of a combination of factors, including extensive use in 

recharge areas of the unconfined aquifer 

in central Oahu, high rainfall that promotes 

infiltration from the surface, and slow rates of 

biodegradation.

(28)

Fumigant detections in the Central Valley of 

California also are associated with the historical application of a fumigant—in this case, 

DBCP—on vineyards and almond orchards. 

DBCP was detected in shallow ground water 

beneath the vineyards and orchards as well as 

in the regional aquifer. Detection frequencies 

of DBCP were as large as 60 percent in shallow ground water beneath the vineyards and 

orchards and about 10 percent in the regional 

aquifer.(33)

Figure 6. Solvents were detected in aquifers throughout the Nation.

Figure 7. Fumigant detections in aquifers generally are related to areas of known

fumigant use.

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22   

13.  Specific VOC Mixtures

Occurred Infrequently in Aquifer

Samples

Specific mixtures of VOCs in the 3,498 aquifer 

samples occurred relatively infrequently 

at an assessment level of 0.2 μg/L. Of the 

10 most common mixtures, the two most 

frequently detected VOC mixtures, PCE–TCE 

and PCE–chloroform, occurred in 1.5 percent 

of samples (table below). Only one other 

mixture, TCE–chloroform, occurred in more 

than 1 percent of the samples. Of the 55 VOCs 

measured, only 7 compounds (5 solvents and 

2 THMs) were found in the 10 most frequently 

occurring mixtures. Although specific VOC 

mixtures are a relatively infrequent occurrence 

at an assessment level of 0.2 μg/L, mixtures 

do occur more frequently when lower VOC 

concentrations are considered.(12)

[PCE, perchloroethene; TCE, trichloroethene; TCA, 

1,1,1-trichloroethane; DCE, dichloroethene; DCA, 

dichloroethane]

Rank VOC mixture

Detection

frequency,

in percent

1 PCE–TCE 1.5

1 PCE–chloroform 1.5

3 TCE–chloroform 1.3

4 PCE–TCE–chloroform .94

5 TCA–chloroform .89

5 PCE–TCA .89

7 TCE–TCA .83

8 TCE–trans-1,2-DCE .74

9 TCE–1,1-DCA .71

10 Chloroform–chlorodibromomethane

.69

Occurrence of Individual VOCs in Aquifers

Forty-two of the 55 VOCs were detected in aquifers 

at an assessment level of 0.2 μg/L; chloroform 

was the most frequently detected compound.

Figure 8. The 15 most frequently detected VOCs in aquifers are from 5 of the 7 VOC

groups.

Forty-two of the 55 VOCs were detected in aquifers at an assessment level of 0.2 μg/L (Appendix 6). Of those 42 VOCs, 12 were detected in 

more than 1 percent of the samples, and 3 other VOCs had detection frequencies slightly less than 1 percent (fig. 8). Specific VOC mixtures also 

occur, but infrequently (sidebar 13). Some of the VOCs mixtures in aquifer 

samples may be the result of degradation of parent compounds (sidebar 14).

The 15 most frequently detected VOCs represent most of the use 

groups (fig. 8) and include 7 solvents, 4 THMs, 2 refrigerants, 1 gasoline 

oxygenate, and 1 gasoline hydrocarbon. Fumigants and organic synthesis 

compounds were not among the 15 most frequently detected VOCs.

In general, VOC detection frequencies were larger at an assessment 

level of 0.02 μg/L than at an assessment level of 0.2 μg/L (fig. 8). However, 

the same general pattern of occurrence among the 15 VOCs was observed. 

For example, chloroform, PCE, MTBE, and toluene were among the top five 

most frequently detected VOCs at both assessment levels.

Chloroform was the most frequently detected VOC in aquifers regardless of the assessment level. This finding has not been previously documented for ambient ground water nationally (p. 42–45). Like chloroform, 

0 2 4 6 8 10 12 14 16 18 20 22

trans-1,2-Dichloroethene

(trans-1,2-DCE)

Methylene chloride

Dibromochloromethane

Bromoform

1,1-Dichloroethane

 (1,1-DCA)

Trichlorofluoromethane

 (CFC-11)

Bromodichloromethane

Chloromethane

1,1,1-Trichloroethane (TCA)

Dichlorodifluoromethane

(CFC-12)

Toluene

Trichloroethene (TCE)

Methyl tert-butyl ether

(MTBE)

Perchloroethene (PCE)

Chloroform

DETECTION FREQUENCY, IN PERCENT

Trihalomethane (THM)

Gasoline hydrocarbon

Gasoline oxygenate

Refrigerant

Solvent

Assessment level of 0.02 microgram per liter

Assessment level of 0.2 microgram per liter
    23

Chapter 3

Most of the concentrations of the 15 most frequently 

detected VOCs were less than about 1 μg/L.

Figure 9. Concentrations varied widely for each of the 15 most frequently detected

VOCs in aquifers.

14.  Some VOC Detections Could be

the Result of the Degradation of a

Parent Compound

Some VOCs can degrade through abiotic or 

biotic processes to another VOC or other compound under oxic and/or anoxic conditions. 

Several possible degradation by-products are 

among the 15 most frequently detected VOCs 

in aquifers. Four of these are (1) methylene 

chloride from chloroform; (2) chloromethane 

from methylene chloride; (3) TCE from PCE; 

and (4) 1,1-dichloroethane (1,1-DCA) from TCA. 

There is a high degree of co-occurrence of 

these four by-product/parent VOCs in aquifer 

samples where the parent compound was 

detected.

Some VOCs can originate both as degradation 

by-products and from industrial production 

for use in industrial, commercial, or domestic 

applications. For VOCs with these dual origins, 

information on the sources of both the parent 

and potential by-product to ground water 

would be helpful to determine whether a 

detected VOC was a degradation by-product or 

a result of anthropogenic use.

most of the other frequently detected VOCs are halogenated aliphatic 

organic compounds (exceptions are toluene and MTBE).

Toluene was the only VOC of the gasoline hydrocarbon group that was 

among the 15 most frequently detected VOCs (fig. 8). Many of the gasoline 

hydrocarbons might be expected to be among the most frequently detected 

VOCs given the very high production and the large and long-term use of the 

gasoline hydrocarbons compared to other VOC groups. Additional discussion of gasoline hydrocarbons is included in Chapter 5 (p. 54 and 55).

Concentrations reported by the laboratory for the 15 most frequently 

detected VOCs in aquifers ranged from about 0.002 to about 350 μg/L 

(fig. 9; Appendix 7). Most of the VOC concentrations, however, were less 

than about 1 μg/L, and all 15 VOCs display this same general concentration 

pattern. However, the number of samples with concentrations in various concentration ranges differ among compounds. For example, concentrations less 

than 0.2 μg/L accounted for relatively large percentages of all of the concentrations for chloroform, toluene, and TCA. Conversely, concentrations 

less than 0.2 μg/L accounted for a relatively small percentage of all of the 

concentrations for some VOCs such as bromoform. 

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24   

15.  How were Associations

Developed for the Occurrence of

VOCs in Aquifers?

Many natural and anthropogenic factors 

were tested in individual logistic regression 

models, hereafter termed “statistical models,” 

for 10 frequently detected VOCs. Numerous 

models were tested for each VOC, but one was 

selected as the final model on the basis of 

several statistical measures. Details of these 

measures and the general procedures for the 

modeling are described elsewhere.(19) The 

detections of individual VOCs were significantly associated with particular natural or 

anthropogenic factors (table 4). In these models, numerous factors are considered together 

in a single model allowing one to account 

for differences in one factor (for example, 

sources) while testing for significance of other 

factors (for example, dissolved oxygen).

Ten frequently detected VOCs were associated 

with factors that would affect their source, 

transport, and fate in ground water.

Natural and Anthropogenic Factors Associated with Selected VOCs in Aquifers

Managing the land area

around a well helps to prevent

ground-water contamination.

(Photograph by Larry D. Putnam,

U.S. Geological Survey.)

Ten frequently detected VOCs were associated with natural or a mix of natural and anthropogenic factors that would affect their source, 

transport, and fate in ground water (table 3). Dissolved oxygen, which controls the fate of many compounds in ground water, was the most common 

explanatory factor for the occurrence of these 10 VOCs (sidebar 15; table 4). 

Other important factors included source factors of urban land use, RCRA

hazardous-waste facilities, gasoline storage sites, and septic systems; transport factors of depth to top of well screen, climate, and soil characteristics; 

and the indeterminate factor of type of well.

Important similarities and differences are evident in factors that were 

associated with the occurrence of VOCs within three groups—gasoline 

hydrocarbons, solvents, and THMs—that are represented by nine of the 

VOCs considered in the statistical models. The number of LUST sites or 

underground storage tank (UST) sites was an important source factor associated with the gasoline hydrocarbons (1,2,4-trimethylbenzene and toluene) 

and also with the gasoline oxygenate MTBE. Subsurface leakage or surface runoff from these sites may be the source of these three VOCs. Cool 

climates, which tend to reduce volatilization of VOCs from land surfaces to 

the atmosphere, were associated with the occurrence of 1,2,4-trimethylbenzene, toluene, and MTBE in ground water. Toluene and MTBE were weakly 

associated with oxic conditions.

Many factors that were associated with the occurrence of solvents 

(chloromethane, methylene chloride, TCA, TCE, and PCE) were similar. 

Septic system density, percentage of urban land use, and number of RCRA 

hazardous-waste facilities all were identified as sources associated with the 

occurrence of solvents. High silt, sparse sand, low organic content of soils, 

and shallow wells or screens are transport factors associated with the occurrence of solvents. High silt and sparse sand content of soils (indicating low 

permeability) were associated with the occurrence of chloromethane and 

methylene chloride. Under these conditions, the slower transport of recharge 
    25

Chapter 3

Table 4. Positive associations, in order of

decreasing importance, for 10 frequently detected

VOCs in aquifers.

[TCA, 1,1,1-trichloroethane; TCE, trichloroethene; PCE, 

perchloroethene; MTBE, methyl tert-butyl ether; RCRA, 

Resource Conservation and Recovery Act; LUST, leaking 

underground storage tank; UST, underground storage tank; 

F, fate; S, source; T, transport; I, indeterminate].

Compound Occurrence

associated with:

Type

of

variable

Gasoline hydrocarbons

1,2,4-

Trimethylbenzene

few septic systems S

cool climates T

dry climates T

gasoline UST sites S

Toluene cool climates T

old construction S

gasoline LUST sites S

domestic wells I

oxic water F

hydric soils T

Gasoline oxygenate

MTBE wet climates T

MTBE use areas S

shallow depth to top of 

well screen T

public wells I

cool climates T

oxic water F

gasoline LUST sites S

Solvents

Chloromethane

anoxic water F

high silt in soil T

undeveloped land S

Methylene 

chloride

domestic wells I

shallow well depth T

septic systems S

sparse sand in soil T

TCA oxic water F

shallow depth to top of

well screen T

low soil organic content T

cool climates T

septic systems S

urban land S

RCRA facilities S

old construction S

TCE urban land S

oxic water F

wet climates T

public wells I

sparse hydric soils T

septic systems S

RCRA facilities S

PCE shallow depth to top of 

well screen T

oxic water F

public wells I

urban land S

septic systems S

Trihalomethanes (THMs)

Bromodichloromethane

oxic water F

sewer systems S

low ground-water recharge T

public wells I

Chloroform urban land S

oxic water F

wet climates T

public wells I

sparse hydric soils T

septic systems S

RCRA facilities S

The concentration of dissolved oxygen was the 

most common explanatory factor associated 

with the occurrence of many VOCs.

through the unsaturated zone may enhance the degradation of chloroform 

to methylene chloride and chloromethane (p. 26 and 27). The occurrence 

of methylene chloride, TCA, and PCE was associated with either shallow 

well depth or shallow well screen depth. The occurrence of TCA, TCE, and 

PCE was strongly related to oxic water conditions, whereas chloromethane 

occurrence was strongly related to anoxic conditions. These relations are not 

surprising given that TCA, TCE, and PCE are more stable under oxic conditions, and chloromethane is more stable under anoxic conditions. 

The occurrence of THMs (bromodichloromethane and chloroform) was 

associated with oxic conditions and public wells. Bromodichloromethane 

was detected more frequently in areas with low ground-water recharge and 

in areas with sewer systems. In contrast, chloroform was detected more frequently in areas with wet climates (generally indicating high ground-water 

recharge) and in areas with several possible sources of contamination including urban land use, septic systems, and RCRA hazardous-waste facilities.

The detection frequencies of many of the compounds were associated 

with a particular well type (domestic well or public well), but the reason for 

this association is not fully known. Noteworthy is the association of bromodichloromethane, chloroform, PCE, TCE, and MTBE with public wells. 

Plausible reasons for this association are the large pumping rates of public 

wells and their proximity to developed areas, where multiple sources or uses 

of those compounds may be present (p. 40 and 41).

Table 3. Factors most commonly associated with VOCs in aquifers.

Source Factors

• Septic systems

• Urban land

• Resource Conservation and Recovery Act (RCRA) hazardous-waste facilities

• Gasoline underground storage tank and leaking underground storage tank sites

Transport Factors

• Climatic conditions

• Depth to top of well screen

• Hydric (anoxic) soils

Fate Factor

Oxic ground water (dissolved-oxygen concentration greater than or equal to 

0.5 milligram per liter)

•

Indeterminate

• Type of well
26   

Dissolved Oxygen and VOC Occurrence in Aquifers

The detection frequencies of most VOCs were 

dependent on the dissolved-oxygen conditions 

of ground water and the type of VOC.

16.  Dissolved Oxygen Varies Along

a Ground-Water Flowpath

Dissolved-oxygen concentrations in ground 

water can vary by location in an aquifer and 

with the age of the ground water. Young

ground water usually has a larger dissolvedoxygen concentration compared to old

ground water. This is because dissolved 

oxygen can become depleted along a flowpath 

through various abiotic and biotic processes.

Samples collected by NAWQA with age-date 

information indicate that water recharged 

after 1955 (referred to here as young 

ground water) had higher dissolved-oxygen 

concentrations compared to ground water 

recharged prior to 1955 (referred to here as 

old ground water). Because ground water in 

recharge areas of aquifers is younger than 

ground water farther along a flowpath, a 

comparison of detection frequencies of VOCs 

between young, oxic ground water and old, 

anoxic ground water should be similar to a 

comparison of ground water at points along a 

hypothetical flowpath (fig. 11).

Dissolved oxygen in ground water was the factor most commonly associated with the occurrence of VOCs (p. 24 and 25). Oxygen is the 

electron acceptor preferred by many microorganisms in their respiration

of organic compounds.(34) Although the biodegradation of many VOCs can 

occur in either oxic or anoxic ground-water conditions, the rates of biodegradation usually are not equal.(35) Because the rates of biodegradation of VOCs 

in oxic and anoxic conditions differ, the detection frequencies of VOCs also 

can be expected to vary with differences in the dissolved-oxygen condition 

of ground water.

The type of VOC (major chemical class) also is important in determining the rate of biodegradation in various dissolved-oxygen conditions. 

This is evident from the observation that, with the exception of MTBE and 

toluene, all of the other frequently detected VOCs in aquifers are halogenated aliphatic compounds (fig. 8). In general, halogenated aliphatic VOCs 

biodegrade more rapidly in anoxic conditions than in oxic conditions (see 

Circular’s Web site). Because about three-quarters of the samples from 

aquifer studies were oxic, compounds that biodegrade more slowly in oxic 

ground water, like halogenated aliphatic VOCs, should be more persistent 

and more frequently detected than compounds that degrade quickly in oxic 

ground water, like many petroleum hydrocarbons.

The ratios of the detection frequencies of 10 frequently occurring VOCs 

in oxic ground water compared to their detection frequencies in anoxic 

ground water differ markedly (fig. 10). Some VOCs, such as TCA, chloroform, and PCE, were detected more frequently in oxic ground water than in 

anoxic ground water. Other VOCs, such as methylene chloride and chloromethane, were detected more frequently in anoxic ground water. The differences in detection frequencies for some of these VOCs are consistent with 

published rates of biodegradation for these VOCs under different dissolvedoxygen conditions.(35) For example, TCA has an aerobic half-life that is 

nearly twice as long as its anaerobic half-life (see Circular’s Web site). This 

indicates that TCA should be more persistent in oxic ground water than in 

anoxic ground water, which was confirmed by the relatively large detection 

frequency ratio of TCA.

A conceptual model illustrates how chloroform may undergo biodegradation along a hypothetical flowpath in an aquifer, along which dissolved 

oxygen becomes depleted (sidebar 16; fig. 11). A subset of samples from 
    27

Chapter 3

Figure 10. The

occurrence of most

VOCs was dependent

on dissolved-oxygen

conditions in ground

water at an assessment level of

0.02 microgram per

liter.

Figure 11. Chloroform in young,

oxic ground water biodegrades

along a hypothetical groundwater flowpath to form methylene

chloride and chloromethane in old,

anoxic ground water.

aquifers was used to represent this hypothetical flowpath and to characterize 

changes in the detection frequency of chloroform and two potential byproducts. The detection frequency of chloroform was lower in old, anoxic 

ground water compared to young, oxic ground water. In contrast, the 

detection frequencies of both chloromethane and methylene chloride, both 

potential by-products of chloroform degradation, were larger in old, anoxic 

ground water than in young, oxic ground water (fig. 11). These data support 

the conceptual model in which chloroform biodegrades along a groundwater flowpath.

Dissolved-oxygen concentration data from aquifer samples could be 

used to ascertain if VOCs of local interest would tend to persist in ground 

water or be degraded to a more or less toxic compound. Aquifer conditions 

that favor the persistence of the parent compound or formation of a toxic byproduct would warrant scrutiny.

   


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28   

Chapter 4—VOCs in Samples from Drinking-Water Supply Wells

Ground Water as a Drinking-Water Supply

Ground water provides a drinking-water supply for about one-half the Nation’s population, including almost all of the people who reside 

in rural areas.(1) Ground water supplies domestic wells and public wells 

in every State (fig. 12). Domestic wells are privately owned, self-supplied 

sources for domestic water use.(36) Public wells are privately or publicly 

owned and supply ground water for PWSs. In this report, the discussion 

of public wells refers to the quality of water captured by wells that supply 

drinking water to PWSs. As defined by the USEPA,(37) PWSs supply drinking water to at least 15 service connections or regularly serve at least 25 

individuals daily at least 60 days a year.

Ground water is used as a drinking-water supply for 

about one-half the Nation’s population, including 

almost all people residing in rural areas.

About 150 million people in the United States received their drinking 

water from domestic and public wells in 2000.(38, 39) Estimated withdrawals 

from domestic and public wells increased by about 60 and 100 percent, 

respectively, from 1965 to 2000 (fig. 13). In 2000, average daily withdrawal 

rates from domestic and public wells for drinking-water supply were 3.5 and 

16 billion gallons per day (Bgal/d), respectively.(39)

Figure 12.  In most States, a greater percentage of the population is dependent on public wells than on domestic wells as a

drinking-water supply.

Photograph courtesy of Mike Wolforth, Light Images

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    29

Chapter 4

For this NAWQA assessment, detection frequencies and concentrations 

of individual VOCs and VOC mixtures were examined to characterize the 

quality of ground water captured by drinking-water supply wells (sidebar 4). 

VOC detection frequencies for domestic well samples were determined 

using a two-tiered assessment level approach of 0.2 μg/L for 2,401 wells 

and 0.02 μg/L for a subset of 1,208 wells. Through a collaborative effort 

with researchers and water utilities, VOC detection frequencies also were 

determined for 1,096 public well samples at an assessment level of 0.2 μg/L. 

All samples from domestic and public wells were collected at the well head 

before any treatment or blending of the water.

In this chapter, a three-step approach was used to assess the relevance 

of VOC concentrations in domestic and public well water to human health 

and to assess monitoring needs for VOCs. First, VOC concentrations were 

compared to USEPA’s MCLs for regulated compounds (sidebar 17), and 

to HBSLs for unregulated compounds (sidebar 18). Comparison of VOC 

concentrations to benchmarks aids in identification of VOC concentrations 

that may be of potential human-health concern (hereafter referred to as concentrations of potential concern). The spatial distribution of VOC concentrations of potential concern also are examined in this step. Second, the relative 

proportions of concentrations of potential concern for individual VOCs were 

determined for both well types. Third, VOCs detected at concentrations less 

than but within a factor of 10 of MCLs and HBSLs were identified. These 

VOCs, along with the compounds determined in the first step, may warrant 

inclusion in a low-concentration, trends-monitoring strategy, such as the 

approach used by the NAWQA Program. This monitoring may provide 

an early indication of VOC concentrations approaching levels of potential 

concern.

In addition, sources of contamination to domestic and public wells are 

discussed, and anthropogenic and hydrogeologic factors associated with the 

detection of VOCs are described for each well type. Lastly, VOC occurrence 

findings for domestic wells and public wells are compared. 

17. MCLs Serve as DrinkingWater-Quality Benchmarks for

PWSs

Under the authority of the SDWA, the USEPA 

establishes drinking-water standards, such 

as MCLs, to limit the level of contaminants 

in the Nation’s drinking water. An MCL is a 

legally enforceable standard that sets the 

maximum permissible level of a contaminant 

in water that is delivered to any user of a 

PWS.(40) When setting an MCL, the USEPA 

also establishes a non-enforceable health 

goal or Maximum Contaminant Level Goal

(MCLG). The MCLG is the maximum level of 

a contaminant in drinking water at which no 

known or anticipated adverse effect on human 

health would occur, and which allows an 

adequate margin of safety.(40) The MCL is set 

as close to the MCLG as feasible, taking into 

account the best available technology, treatment techniques, and cost considerations, as 

well as expert judgment and public comments. 

The USEPA reviews drinking-water standards 

every 6 years to determine if revisions are 

needed.

Established MCLs apply to 29 VOCs included in 

this NAWQA assessment. However, because 

MCLs apply to drinking water supplied to the 

public by PWSs, comparisons of VOC concentrations for samples collected at the well head 

in this assessment to MCLs are used only to 

indicate concentrations of potential humanhealth concern. Actual human exposure from 

drinking water is not described (sidebar 4). 

Figure 13.  From 1965 to 2000, domestic well

withdrawals increased by about 60 percent, whereas

public well withdrawals increased nearly 100 percent.



















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30   

VOCs in Domestic Well Samples

18. HBSLs Can be Applied to VOCs

with no MCLs

HBSLs are estimates of benchmark concentrations of contaminants in water that may be 

of potential human-health concern. HBSLs 

are based on health effects alone and have 

been calculated for unregulated contaminants (those with no MCLs) analyzed by the 

NAWQA Program. HBSLs were developed by 

the USGS in collaboration with others (p. 13) 

using (1) standard USEPA Office of Water 

methodologies; and (2) the most current, 

USEPA peer-reviewed, publically available 

human-health toxicity information. HBSLs are 

regularly reviewed and, as needed, revised to 

incorporate the most recent toxicity information and research findings.

HBSLs are not regulatory standards and 

are not legally enforceable. HBSLs were 

calculated for 15 of the 26 unregulated VOCs 

in this assessment, but were not calculated 

for the remaining 11 VOCs due to a lack of 

toxicity information. Measured contaminant 

concentrations may be compared to HBSLs 

to evaluate water-quality data in a humanhealth context. Such comparisons can provide 

an early indication of when contaminant 

concentrations in water resources may merit 

additional study or monitoring.

Since 1998, the USGS, in collaboration with 

others, has made substantial progress in 

providing additional information about the 

potential human-health implications of its 

water-quality findings. USGS will continue its 

research to develop and refine approaches 

to expand its ability to evaluate contaminant 

concentrations in a human-health context at 

the State and national scales.

Additional information about HBSLs and ongoing research is available in other publications(20, 41, 42) and at http://water.usgs.

gov/nawqa/HBSL/.

Domestic well water may be vulnerable to low-level 

VOC contamination from many compounds.

One or more VOCs were detected in 14 percent of the 2,401 domestic well samples at an assessment level of 0.2 μg/L. VOCs in these 

samples were not limited to a few compounds—more than two-thirds of 

the monitored VOCs were detected. In contrast, nearly one-half of 1,208 

samples from a subset of these domestic wells had VOC detections using the 

low-level analytical method, for which an order-of-magnitude lower assessment level (0.02 μg/L) was applied. Furthermore, about 90 percent of the 

total VOC concentrations in samples with VOC detections were less than 

1 μg/L.

Figure 14.  Detection frequencies in domestic well samples differed for

the 15 most frequently occurring VOCs at assessment levels of 0.2 and

0.02 microgram per liter.

Six VOCs had detection frequencies of 1 percent or larger at an assessment level of 0.2 μg/L (fig. 14, Appendix 8). Chloroform had the largest 

detection frequency, almost double that of MTBE, the second most frequently detected VOC. The 15 most frequently detected VOCs in domestic 

well samples represent six groups (fig. 14), indicating multiple contaminant 

sources.

The gasoline oxygenate, refrigerant, solvent, and THM groups each 

were detected in more than 2 percent of the domestic well samples. VOCs 

with multiple uses and/or widespread sources, for example VOCs within 

the solvent group, were detected throughout the Nation. Gasoline oxygenates were detected most frequently in domestic well samples in the New 

England and Mid-Atlantic States. Few samples contained fumigants, and 

          

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    31

Chapter 4

most of these occurred in the Central Valley of California and in New Jersey, 

Arizona, and Washington. VOCs used in organic synthesis seldom were 

detected. Additional information about spatial occurrence of VOC groups 

and selected compounds is available from the Circular’s Web site.

Six VOCs had concentrations greater than MCLs—DBCP, 1,2-dichloropropane, EDB, 1,1-DCE, PCE, and TCE (fig. 15A). VOC concentrations of 

potential concern occurred in about 1 percent of the domestic well samples. 

Fumigants accounted for about two-thirds of the 32 VOC concentrations of 

potential concern, and DBCP comprised about one-half of these.

Samples with concentrations of potential concern were localized 

(fig. 15B) and may be associated with a specific VOC use, such as the 

historical application of DBCP on crops in the Central Valley of California 

from the late 1950s until the compound’s ban in the late 1970s.(43) DBCP 

About 1 percent of domestic well samples had VOC 

concentrations of potential human-health concern.

Figure 15.  VOC concentrations of potential human-health concern in domestic well samples were predominantly

for fumigants and solvents: (A) the fumigant DBCP accounted for nearly one-half of all these concentrations; and (B)

concentrations of fumigants were most common in California.

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may have a half-life of about 6 years in ground water on the basis of an 

investigation in the eastern San Joaquin Valley, California.(43) This persistence, when coupled with the intrinsic susceptibility of the sand and gravel 

aquifers in the Central Valley, has resulted in ground-water contamination in 

an area where about one-tenth of the population relies on domestic wells for 

drinking-water supplies.

All six of the previously mentioned VOCs that were detected at concentrations of potential concern warrant inclusion in low-concentration, trendsmonitoring programs. In addition, benzene, bromoform, carbon tetrachloride, chloroform, dibromochloromethane, 1,2-dichloroethane (1,2-DCA), 

methylene chloride, TCA, and vinyl chloride were detected at concentrations 

less than but within a factor of 10 of MCLs (Appendix 9). These VOCs also 

may warrant inclusion in such a monitoring program.

VOCs in Domestic Well Samples—Continued

Septic systems and USTs are important potential 

sources of VOC contamination to domestic wells.

Leaking gasoline, heating oil, and diesel

fuel from storage tanks can result in lowlevel VOC contamination in ground water

that serves as a domestic drinking-water

supply. (Photograph by Connie J. Ross,

U.S. Geological Survey.)

The finding that some VOC concentrations in domestic well samples 

were greater than or within a factor of 10 of an MCL is particularly noteworthy because testing of water from domestic wells is not federally mandated nor uniformly monitored (sidebar 19). Although HBSLs exist for 15 

unregulated VOCs (sidebar 18), none of the compounds had concentrations 

greater than or within a factor of 10 of these benchmarks.

VOCs detected in domestic well samples could be from contaminant 

sources near the home, including septic systems, underground and aboveground storage tanks, fumigant applications, spills, pipelines, and sewer 

lines. Household septic systems are important potential sources of contamination to domestic wells (p. 45). USTs used to store fuels also are recognized as potential contaminant sources to domestic wells.(44)

19. Most Government Agencies Do

Not Require Routine Monitoring of

Water Quality for Domestic Wells

Although regulations vary by State, and also 

within States, the quality of water from privately owned domestic wells generally is the 

homeowner’s responsibility. Routine monitoring is not required; however, most States 

and some local agencies provide guidance to 

domestic well owners through Web sites and 

printed materials.(45)

Raising awareness about the importance of 

regularly testing private wells is an important 

step towards ensuring a safe drinking-water 

supply for the population relying on domestic 

wells. As such, private well owners are advised 

by State and local agencies to test water 

annually to identify possible contaminants 

such as coliform bacteria, nitrate and nitrite, 

pesticides, radionuclides, heavy metals, and 

VOCs, and to compare test results to USEPA 

and State standards. No States currently 

require homeowners to take action to improve 

water quality if contaminants are detected in 

domestic well water. However, some States 

have introduced measures to assess water 

quality to aid in protection of human health. 

For example, in 2002 New Jersey passed a law 

that required “raw” or untreated water to be 

tested in wells included in real estate transactions.(46) Additionally, landlords must test water 

every 5 years and provide the test results to 

new tenants. VOCs, including benzene and 

TCE, were among the required compounds to 

be tested in the well samples.

Because water from domestic wells usually is 

not treated prior to use, the VOC occurrence 

data provided by this NAWQA assessment 

may reflect the quality of tap water used by 

many rural households. Prior to NAWQA’s 

assessment, no major national studies had 

been conducted for a large number of VOCs in 

domestic well samples. 
    33

Chapter 4

One or more VOCs were detected in about one-fourth of the 1,096 public well samples at an assessment level of 0.2 μg/L. Total VOC 

concentrations in about one-half of the samples with detections were less 

than 1 μg/L. Furthermore, about three-fourths of the 55 monitored VOCs 

were detected (Appendix 10), indicating that many VOCs are potential 

contaminants in public well water. Because the public well samples were 

collected before any treatment or blending of the water, these findings do 

not necessarily reflect the quality of drinking water ultimately supplied by 

the large number of PWSs that use ground water as a drinking-water supply 

(sidebar 20).

Fifteen VOCs had detection frequencies of 1 percent or larger (fig. 16). 

Chloroform was the most frequently detected VOC in public well samples 

with a frequency of about 11 percent. Additionally three other THMs—bromoform, dibromochloromethane, and bromodichloromethane—each were 

detected in about 4 percent of public well samples. MTBE was the second 

most frequently detected VOC. The solvents PCE, TCE, TCA, 1,1-dichloroethane (1,1-DCA), and cis- and trans-1,2-dichloroethene (cis- and trans1,2-DCE) were detected in 1 to 5 percent of the public well samples. In a 

national survey conducted by the USEPA (1981–1982), these solvents were 

among the most frequently detected VOCs in treated water from PWSs.(8)

VOCs in Public Well Samples

Figure 16.  Trihalomethanes, solvents, and the gasoline oxygenate MTBE were among the

15 most frequently detected VOCs in public well samples.

20. Ground Water is Used by

Many PWSs

Nearly 145,000 PWSs provide ground water 

for human consumption to about 112 million 

people in the United States.(38) PWS categories established by the USEPA include CWSs 

and non-community water systems (NCWSs). 

CWSs serve a residential population such as 

a municipality, mobile home park, or nursing 

home. NCWSs are divided into non-transient, 

non-community water systems (NTNCWSs), 

such as schools, hospitals, and factories; 

and transient non-community water systems 

(TNCWSs), such as campgrounds, motels, and 

gasoline stations. Nearly 60 percent of PWSs 

are TNCWSs, but more than 80 percent of the 

U.S. population is served by CWSs.(38)

Ownership and size of population served by 

PWSs may vary from very small, privately 

owned systems whose primary business is 

something other than water supply (such as 

mobile home parks) to large, publicly owned 

water utilities that serve millions of people.(47)

About one-fourth of the public well samples contained 

VOCs; however, total VOC concentrations generally 

were less than 1 μg/L.





















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THMs and solvents had the largest detection frequencies among VOC 

groups in public well samples (15 and 10 percent, respectively). In addition, gasoline oxygenates, predominantly MTBE, occurred in about 5 percent of the samples. All other groups were detected in about 3 percent or 

less of the samples. Spatial patterns of occurrence differed for VOC groups 

(see Circular’s Web site). Detections of solvents, THMs, and gasoline 

hydrocarbons were distributed throughout the Nation. Gasoline oxygenates 

were detected primarily in the New England and Mid-Atlantic States, and 

in Florida and California. Detections of fumigants were predominantly in 

Hawaii and in the eastern coastal area of the United States.

Drinking water from PWSs is monitored for regulated contaminants and 

also for selected unregulated contaminants that may be considered for new 

drinking-water standards (sidebar 21). Five of the 29 regulated VOCs had 

one or more concentrations greater than MCLs (fig. 17A). These VOCs with 

concentrations of potential concern are 1,1-DCE, methylene chloride, PCE, 

TCE, and vinyl chloride and generally occurred in highly populated areas of 

the Nation (fig. 17B). None of the 15 unregulated VOCs with HBSLs had 

concentrations greater than these benchmarks. The solvents PCE and TCE 

comprised about three-quarters of the concentrations of potential concern.

The regulated compounds benzene, bromodichloromethane, bromoform, carbon tetrachloride, chloroform, dibromochloromethane, 1,2-DCA, 

1,2-dichloropropane, and cis-1,2-DCE were detected at concentrations less 

than but within a factor of 10 of MCLs (Appendix 11). In contrast, none of 

the 15 unregulated VOCs with HBSLs had concentrations within a factor 

of 10 of these benchmarks. The 5 VOCs with concentrations greater than 

their MCLs and the 9 VOCs within a factor of 10 of their benchmarks may 

warrant inclusion in a low-concentration, trends-monitoring program.

Potential sources of VOC contamination to public wells include leaking aboveground and underground storage tanks, sewer lines, effluent from 

septic systems, landfills, industrial sites, accidental spills, and areas where 

chemicals have been disposed of improperly.(48) Additionally, businesses 

associated with populated areas, such as dry cleaners, gasoline stations, 

auto repair and re-painting shops, and industrial fabricators that use VOCs 

About 2 percent of public well samples had VOC 

concentrations of potential human-health concern.

21. Unregulated Contaminants,

Including VOCs, are Monitored in

PWSs

The 1996 SDWA Amendments require 

the USEPA to identify and publish a list of 

unregulated contaminants (referred to as 

the CCL) that are known or anticipated to 

occur in PWSs and that may require regulation with a national primary drinking-water 

standard.(49) In making regulatory determinations for compounds on the CCL, USEPA must 

determine whether (1) the contaminant may 

have an adverse effect on human health; 

(2) the contaminant is known to occur or there 

is substantial likelihood that the contaminant 

will occur in PWSs with a frequency and at 

levels of public health concern; and (3) in the 

sole judgment of the Administrator, regulation 

of such contaminant presents a meaningful 

opportunity for health risk reduction for people 

served by PWSs.(50) SDWA requires that the 

USEPA publish the CCL every 5 years and 

make regulatory determinations for at least 

five contaminants (also every 5 years).

The Unregulated Contaminant Monitoring 

(UCM) Program is the mechanism used to 

collect data for unregulated contaminants suspected to occur in drinking water.(51) The UCM 

list is revised every 5 years by the USEPA and 

is based primarily on the CCL.

Five VOCs monitored by NAWQA are currently 

listed on the second CCL published by the 

USEPA(52) and are prioritized for research and 

data collection efforts by the USEPA. These 

VOCs include bromomethane (methyl bromide), 1,1-DCA, 1,3-dichloropropene, MTBE, 

and 1,2,4-trimethylbenzene.(52) In public well 

samples collected for this national assessment, the isomers cis- and trans-1,3-dichloropropene were not detected, and bromomethane and 1,2,4-trimethylbenzene were detected 

in less than 1 percent of the samples. MTBE 

and 1,1-DCA had the largest detection frequencies of these five VOCs, 5.4 percent and 

2.0 percent, respectively (Appendix 10).

VOCs in Public Well Samples—Continued
    35

Chapter 4

to clean and degrease material and equipment or complete other processes, 

may be potential contaminant sources. Other possible VOC sources include 

industrial and motor vehicle emissions to the atmosphere,(53) highway runoff, 

and urban stormwater runoff.(54)

The detection of VOCs in public well samples has been shown to be 

associated with the CWS size (sidebar 22). VOCs were detected more 

frequently and at greater concentrations in samples from very large systems 

than from smaller systems.(13) A study by the USEPA Office of Drinking 

Water (1981–1982) also found that samples from large PWSs had more frequent VOC detections and greater VOC concentrations than small systems.(8)

Figure 17.  VOC concentrations of potential human-health concern in public well samples included solvents and compounds

used in organic synthesis: (A) solvents accounted for about 85 percent of these concentrations; and (B) these concentrations

occurred predominantly in the highly populated areas of southern California and the New England and Mid-Atlantic States.

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Comparison of VOCs in Samples from Domestic and Public Wells

Figure 18.  Although the top 3 of the 10 most frequently detected VOCs

were the same for samples from domestic and public wells, the overall

signature of VOC detections associated with domestic and public wells

differed.

VOCs were detected more frequently and had more 

concentrations of potential human-health concern in 

public well samples than in domestic well samples.

22. VOCs Occur More Frequently

in the Water Supply of Very Large

CWSs than Other System Sizes

The USGS assisted in a nationwide survey 

(1999–2002) to characterize the occurrence 

of VOCs in ground water that served as 

a drinking-water supply for CWSs.(13) The 

survey used a statistically stratified design 

for sampling 575 public wells from randomly 

selected CWSs representative of the five size 

categories as follows:

CWS Size Population Served

Very small less than 500

Small 501 to 3,300

Medium 3,301 to 10,000

Large 10,001 to 50,000

Very large more than 50,000

In general, VOCs were detected most 

frequently in the very large CWSs. As of 

1998, very large CWSs using ground water 

collectively provided drinking water to about 

26 million people. VOC detections were 

significantly related to urban land use and 

population density associated with large systems.(13) In particular, detections of gasoline 

hydrocarbons, solvents, and refrigerants were 

detected more frequently in ground-water supplies from very large CWSs than from smaller 

sized systems.

VOC occurrence in samples from domestic and public wells is compared in this section. Additionally, VOC data for public well samples 

are compared to the findings of previous investigations (sidebar 23). Domestic well samples had fewer compounds and mixtures, lower detection frequencies, and smaller VOC concentrations than public well samples.

The 10 most frequently detected VOCs for each well type are shown 

in figure 18. The three most frequently detected VOCs were the same for 

samples from domestic and public wells—chloroform, MTBE, and PCE. 

Some of the 55 VOCs were not detected at the 0.2 μg/L assessment level 

(table 5) in any domestic or public well samples (18 and 14 VOCs, respectively). VOCs within the organic synthesis group had the greatest proportion of compounds with no detections for both well types. In contrast, all 

of the VOCs in the THM and refrigerant groups were detected in samples 

from both well types. All of the VOCs in the gasoline hydrocarbon group 

were detected in public well samples, whereas four of these VOCs were not 

detected in domestic well samples (table 5).





















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    37

Chapter 4

The 15 most frequently detected VOCs in domestic well samples represented all VOC groups with the exception of organic synthesis compounds 

(fig. 14), whereas VOCs in public well samples were predominantly THMs, 

solvents, and the gasoline oxygenate MTBE (fig. 16). Total concentrations for each VOC group were summed for each sample with detections. 

For domestic well samples, organic synthesis compounds had the greatest median total concentration (1.1 μg/L), and refrigerants had the lowest 

(0.3 μg/L). For public well samples, solvents had the greatest median total 

concentration (0.7 μg/L), and gasoline hydrocarbons had the lowest 

(0.4 μg/L). 

Table 5.  VOCs with no detections in samples from domestic or

public wells at an assessment level of 0.2 microgram per liter.

[ND, VOC not detected; --, VOC detected]

Compound

Well type

Domestic Public

Fumigants

Bromomethane ND1 --

Dibromochloropropane (DBCP) -- ND

cis-1,3-Dichloropropene ND1 ND

trans-1,3-Dichloropropene ND1 ND

Ethylene dibromide (EDB) -- ND

Gasoline hydrocarbons

n-Butylbenzene ND --

Styrene ND --

o-Xylene ND --

m- and p-Xylenes2 ND --

Gasoline oxygenate

Ethyl tert-butyl ether (ETBE) ND1 --

Organic synthesis compounds

Acrolein ND1 ND

Acrylonitrile ND1 ND

Hexachlorobutadiene ND1 ND

1,2,3-Trichlorobenzene ND1 ND

Vinyl bromide ND1 ND

Solvents

1,3-Dichlorobenzene ND ND

Hexachloroethane ND1 ND

n-Propylbenzene -- ND

1,2,4-Trichlorobenzene ND1 ND

1,1,2-Trichloroethane ND ND

1

VOC also was not detected in a subset of domestic well samples analyzed using a low-level method and with no assessment level applied.

2

Considered as 2 of the 55 compounds included in this assessment.

23. NAWQA’s VOC Assessment Can

Be Placed in the Context of Other

Studies for Public Wells

This NAWQA study provides one of the few 

existing national assessments of VOCs in 

public well samples. The few previous studies 

of VOC occurrence generally were based on 

samples of drinking water. One such study was 

completed by the USEPA during 1981–1982 

and focused on 29 VOCs in treated water 

from CWSs.(8) In this study, VOC concentrations greater than 5 μg/L were found in 

2.9 percent of samples from CWSs serving 

less than 10,000 people and in 6.5 percent of 

the samples from CWSs serving more than 

10,000 people. Although untreated water 

was sampled for this NAWQA assessment, 

VOC occurrence in these size categories was 

similar, 2.4 and 7.8 percent, respectively.

A recent study completed in 2002 by the 

USEPA,(55) based on compliance monitoring 

data, also provided information on the quality 

of drinking water from PWSs. In the USEPA 

study, nine VOCs—benzene, carbon tetrachloride, 1,2-DCA, 1,1-DCE, methylene chloride, 

1,2-dichloropropane, PCE, 1,1,2-trichloroethane, 

and TCE—each had concentrations greater 

than their MCL in less than 0.7 percent of 

the PWSs sampled. In public wells sampled 

by the NAWQA Program, only four of these 

VOCs—1,1-DCE, methylene chloride, PCE, 

and TCE—each had concentrations greater 

than their MCL in less than 0.9 percent of the 

samples.
38   

Samples from domestic and public wells had about the same number of 

VOCs with concentrations of potential concern (6 and 5 VOCs, respectively) 

(table 6). VOCs with concentrations of potential concern in both well types 

included 1,1-DCE, PCE, and TCE. Domestic wells had three additional 

VOCs with concentrations of potential concern—DBCP, 1,2-dichloropropane, and EDB; and public wells had two additional VOCs with concentrations of potential concern—methylene chloride and vinyl chloride (Appendixes 9 and 11). Overall, domestic well samples had a smaller percentage 

of samples with VOC concentrations of potential concern than public well 

samples (1.2 and 1.5 percent, respectively).

VOCs with the largest proportion of concentrations of potential concern 

differed between well types (figs. 15A and 17A). For domestic well samples, 

fumigants and solvents had the most concentrations of potential concern, 

with DBCP having the greatest proportion. For public well samples, solvents 

and compounds used for organic synthesis had the most concentrations of 

potential concern, with PCE and TCE each having the greatest and equal 

proportions. VOC concentrations of potential concern for domestic wells 

were located predominantly in the Central Valley and other areas of California, whereas concentrations of potential concern for public wells generally 

occurred in southern California and in the New England and Mid-Atlantic 

States.

Occurrence of mixtures may result from persistence of VOCs in the 

environment, co-occurrence of parent compounds with their degradation byproducts, multiple compounds sharing the same source, and wide distributions of VOCs that may have overlapping sources.(12) VOC mixtures occurred 

less frequently in domestic well samples (3.9 percent) than in public well 

samples (13.4 percent) at an assessment level of 0.2 μg/L. Furthermore, a 

single VOC occurred more frequently than multiple VOCs in domestic well 

samples, whereas detections of single VOCs and multiple VOCs were about 

equal in public well samples (table 6). Also, concentrations of individual 

VOCs in mixtures generally were greater than the compound’s concentration 

when detected alone in a sample.

Solvents, THMs, and the gasoline oxygenate MTBE comprised the 10 

most frequently detected VOC mixtures for domestic well samples (sidebar 24). In contrast, all but 1 of the 10 most frequently occurring VOC mixtures for public well samples were mixtures of THMs; the exception was the 

24. Specific VOC Mixtures

Occurred Infrequently, But Were

More Common in Public Well

Samples

All specific mixtures of VOCs occurred in less 

than 1 percent of domestic well samples, and 

in less than 4 percent of public well samples 

at an assessment level of 0.2 μg/L. The table 

below ranks the 10 most common VOC mixtures by their detection frequency in domestic 

well samples and in public well samples, 

and shows the more common occurrence of 

mixtures in public well samples.

[MTBE, methyl tert-butyl ether; PCE, perchloroethene; 

TCA, 1,1,1-trichloroethane; TCE, trichloroethene]

Rank VOC mixture

Detection

frequency,

in percent

Domestic well samples

1 PCE–TCA 0.62

2 Chloroform–PCE .50

2 Chloroform–MTBE .50

2 PCE–TCE .50

5 Dibromochloromethane–

chloroform

.42

6 Chloroform–TCA .37

6 TCA–TCE .37

8 PCE–MTBE .33

9 Bromoform–

dibromochloromethane

.29

9 Bromoform–chloroform .29

Public well samples

1 Bromodichloromethane–

dibromochloromethane

3.4

2 Bromodichloromethane–

chloroform

3.2

3 Bromoform–

dibromochloromethane

2.9

4 Dibromochloromethane–

chloroform

2.5

5 Bromodichloromethane–

dibromochloromethane–

chloroform

2.4

6 PCE–TCE 2.3

7 Bromodichloromethane–

bromoform

2.0

7 Bromodichloromethane–

bromoform–dibromochloromethane

2.0

9 Bromoform–chloroform 1.7

10 Bromoform–

dibromochloromethane–

chloroform

1.6

VOC mixtures were detected more frequently in public 

well samples than in domestic well samples.

Comparison of VOCs in Domestic and Public Well Samples—Continued
    39

Chapter 4

solvent mixture PCE–TCE. Solvents, however, were common components 

of VOC mixtures in public well samples that occurred at detection frequencies lower than those for the mixtures listed in sidebar 24.

As previously described for ground water in general (p. 24 and 25), 

many factors can be associated with the source, transport, and fate of 

VOCs. Results of statistical models used in this assessment indicated that 

for samples from both domestic and public wells, hydrogeologic factors 

Table 6.  Statistics for VOC occurrence in samples from domestic and public wells at an assessment level of 0.2 microgram

per liter.

[MCL, Maximum Contaminant Level; HBSL, Health-Based Screening Level; μg/L, micrograms per liter]

Occurrence information

Well type

Domestic Public

Detections

Number of VOCs detected 37 41

Frequency of one or more VOCs, in percent 14.0 26.2

Frequency of VOC groups, in percent trihalomethanes (THMs), 5.3 

solvents, 4.9 

gasoline oxygenates, 2.9

refrigerants, 2.1

gasoline hydrocarbons, 1.5

fumigants, 1.2 

organic synthesis compounds, 0.2 

trihalomethanes (THMs), 14.7

solvents, 9.9

gasoline oxygenates, 5.3

refrigerants, 2.6

gasoline hydrocarbons, 2.2

fumigants, 1.3

organic synthesis compounds, 1.4

Most frequently detected VOCs, in percent chloroform, 5.2

methyl tert-butyl ether (MTBE), 2.9

perchloroethene (PCE), 2.0

chloroform, 11.4

methyl tert-butyl ether (MTBE), 5.4

perchloroethene (PCE), 5.3

Concentrations

Total VOC concentrations less than 1 μg/L, in percent 62.2 49.8

Total VOC concentrations greater than 10 μg/L, in percent 6.6 8.4

Median concentration of samples with detections for the three most 

frequently detected VOCs, in μg/L

chloroform, 0.5

methyl tert-butyl ether (MTBE), 0.6 

perchloroethene (PCE), 0.4

chloroform, 0.6

methyl tert-butyl ether (MTBE), 0.6 

perchloroethene (PCE), 0.7

Detections and concentrations of potential human-health concern

Number of VOCs with concentrations greater than their MCL 6 5

Frequency of concentrations greater than their MCL, in percent 1.2 1.5

Number of VOCs with concentrations greater than their HBSL 0 0

Frequency of concentrations greater than their HBSL, in percent 0 0

Total number of VOCs with concentrations of potential human-health 

concern 6 5

Multiple VOCs

Total number of VOCs that occurred in mixtures 38 40

Number of VOCs occurring only in mixtures 16 18

Frequency of well samples with a single VOC detection, in percent 10.1 12.8

Frequency of mixtures in all wells, in percent 3.9 13.4

Frequency of mixtures in well samples with VOC detections, in percent 27.7 51.2

Generally, public wells are more vulnerable to lowlevel VOC contamination than domestic wells.
40   

Comparison of VOCs in Domestic and Public Well Samples—Continued

associated with an increased probability of detecting VOCs included high 

aquifer recharge and high soil permeability. Furthermore, dissolved-oxygen 

concentration in samples from domestic and public wells was an important 

factor associated with the occurrence of individual VOCs.

Anthropogenic factors strongly associated with the increased probability of VOC detections in both well types included population density 

and percentage of urban land use near the sampled wells. Additionally, the 

probability of detecting VOCs was associated with the number of nearby 

RCRA hazardous-waste facilities. For public well samples, increased MTBE 

occurrence was associated with local use of this VOC in gasoline.

Although hydrogeologic and anthropogenic factors associated with 

VOC occurrence were similar for both well types, findings from this assessment indicate that public well water has the potential for more frequent 

detections of individual VOCs and mixtures and for greater concentrations 

than domestic well water. These findings are apparent despite the much 

deeper median depth of sampled public wells (303 feet) than domestic wells 

(104 feet). In general, deep public wells are presumed to be less vulnerable 

than shallow domestic wells to anthropogenic contaminants that originate on 

or near the land surface. However, several factors including large withdrawal 

rates from public wells and proximity to developed areas explain, at least 

in part, why public wells have a larger vulnerability to VOC contamination 

despite their typically greater depths.

Two recently completed NAWQA Study-Unit investigations explain, 

and figure 19 illustrates, reasons for the differences in VOC occurrence 

between domestic and public wells. A study of paired domestic and public 

wells within the High Plains aquifer system in the central part of the United 

States indicated that water containing surface-derived anthropogenic compounds from near the water table was drawn more quickly to the higher 

Photograph courtesy of Mike Wolforth, Light Images
    41

Chapter 4

Figure 19.  Differences in hydrogeologic and anthropogenic factors can affect the quality of ground water that supplies

domestic and public wells.

volume pumping systems.(56) Production rates of public wells create a larger 

capture zone, greater drawdown, and faster movement of water from the top 

of the water table to the well screen than the comparatively low production 

rates of domestic wells. 

Relatively young water in recently recharged ground water can be intercepted by both domestic and public wells (fig. 19). The frequent occurrence 

of MTBE in samples from public wells in the Kirkwood-Cohansey aquifer 

system in southern New Jersey was attributed to the capture of young ground 

water through the interception of flowpaths with short traveltimes.(57) Furthermore, for the same aquifer system, public wells intercepted VOCs from 

multiple land uses and point sources within a large contributing area.(48, 58)

Additionally, deep public wells also can intercept ground water flowing 

along extensive paths associated with long residence times. This water may 

contain degradation by-products from parent compounds. VOCs that had 

substantial historical use, but whose use has been reduced or phased out, 

may be potential contaminants in ground water with relatively long flowpaths.
42   

Chapter 5—Additional Information for Selected VOCs

Chloroform and Other THMs

Chloroform was the most frequently detected VOC 

in the Nation’s aquifers; however, concentrations 

of chloroform generally were less than 1 μg/L.

25.  Chloroform has a Long History

of Use, Research, and Regulation The THM chloroform was the most frequently detected VOC in aquifers 

(fig. 8), in 7 percent of all samples at an assessment level of 0.2 μg/L. 

The other three THMs—bromodichloromethane, dibromochloromethane, 

and bromoform—were each detected in about 1 percent of aquifer samples. 

Concentrations of chloroform and other THMs generally were less than 

1 μg/L. Of samples with detections, the median concentration was 0.08 μg/L 

for chloroform, and the median total trihalomethane (TTHM) concentration was 0.09 μg/L (Appendix 6).

Chloroform has a long history of use, research, and regulation (sidebar 25). In 1999, 98 percent of chloroform produced by industry (fig. 20) 

was used in the production of the refrigerant hydrochlorofluorocarbon-22 

(HCFC-22) to replace dichlorodifluoromethane (CFC-12) for home air 

conditioners and commercial freezers. The remaining 2 percent of 

chloroform was used for other purposes, including reagents and extraction 

solvents,(59, 60) fumigants, insecticides, and a precursor for dyes and pesticides.(61, 62) In 1998, the most recent year with production data, about 46 million gallons of chloroform were produced by industry. 

In contrast to chloroform, the production of the other three THMs has 

been relatively small; for example, only about 46,000 gallons of bromoform 

were produced in 1977, the most recent year for which production data are 

available.(63) Furthermore, there is no information on the U.S. production of 

bromodichloromethane and chlorodibromomethane, presumedly because 

of the limited or non-existent use of these compounds. Examples of limited 

current or prior use(s) of these three THMs are as solvents for fats, waxes 

and resins, heavy liquid for geological assays, fire-extinguisher agents, and 

as an intermediate in chemical synthesis.

Chloroform and other THMs are commonly produced during the chlorination of water and wastewater (sidebar 26). Chlorination of water and 

wastewater produces an estimated 0.37 million gallons of chloroform in the 

United States annually,(64) which includes the treatment of drinking water, 

wastewater from municipalities and the bleaching process of pulp and paper, 

and rinse water from domestic and commercial cleaning and laundry operations. In addition, some THMs occur naturally in the environment (sidebar 27).

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Chapter 5

26.  THMs Result from Water

Chlorination

THMs are a group of VOCs classified as 

disinfection by-products. THMs in drinking 

water were first identified by Rook(77) and are 

formed as a result of the haloform reaction 

when dissolved chlorine combines with dissolved organic matter, such as humic and fulvic 

acids. The disinfection of drinking water in the 

United States by chlorination commonly uses 

chlorine gas, sodium hypochlorite, and calcium 

hypochlorite. The chemistry associated with 

the chlorination of water and the formation 

of chloroform and the brominated THMs are 

described elsewhere.(71) The primary purpose 

of chlorination of drinking water is to prevent 

the spread of waterborne diseases, which 

can include such fatal diseases as cholera 

and typhoid. Another advantage of chlorination is that a chlorine residual is retained, 

which provides ongoing disinfection within 

the distribution system.(78) Although the 

chlorination of drinking water provides many 

advantages, THMs remain a human-health 

concern (sidebar 28). Because of this concern, 

the USEPA has established an MCL of 80 μg/L 

for the combined concentrations of four 

THMs (chloroform, bromodichloromethane, 

dibromochloromethane, and bromoform), also 

known as TTHMs.

27.  Some THMs Occur Naturally

Chloroform was originally considered solely of 

anthropogenic origins; however, several natural sources of chloroform recently have been 

identified. These include volcanic gases,(79)

soil fungi,(64) and marine algae.(80, 81) Although 

natural sources contribute approximately 

90 percent of the total global chloroform 

flux,(64) evidence in the NAWQA-collected 

data is inconclusive about whether natural 

sources contribute to chloroform in ground 

water. Marine algae have been identified as 

a natural source for bromodichloromethane, 

dibromochloromethane, and bromoform.(80)

In studies of the formation of multiple THMs during water chlorination, it has been shown that the relative concentrations (and their relative 

detection frequencies) usually decrease with increasing bromine content (chloroform > bromodichloromethane > dibromochloromethane > 

bromoform).(65,66) This occurrence pattern also was noted in the NAWQAcollected data,(11, 13, 36, 67, 68, 69, 70) and indicates that chlorinated water is 

an important source of chloroform and other THMs to ground water.(71)

Sources of chlorinated water to ground water may include irrigation of 

lawns, gardens, golf courses, and parks; leaking drinking-water distribution and sewer pipes; artificial recharge of wastewater; regulated discharges 

of cooling process blowdown water from electric power-generating plants; 

combined sewer overflows;(62, 72) and unintended backflow of chlorinated 

water to supply wells.

Chloroform has been detected in urban air(73) and in rainwater.(74) In 

studies of chloroform occurrence in shallow ground water in Indiana(75) and 

New Jersey,(76) however, the concentrations of chloroform in the atmosphere 

and rainwater were not high enough to account for all the mass of chloroform measured in shallow ground water. The sources of the chloroform 

frequently detected in ground water in the New Jersey study were attributed 

to use of chlorinated drinking water and the associated recharge from septic 

tanks or from the irrigation of crops and lawns.(76)

Chloroform has been detected in ground water beneath a variety of 

land-use settings.(11, 68) In NAWQA’s assessments, THMs were detected more 

frequently beneath urban residential areas than beneath agricultural areas 

(fig. 21). Furthermore, the percentage of urban land, the number of RCRA 

hazardous-waste facilities, and septic system density were shown to be 

Figure 20.

Industrial

production of

chloroform

continues to

increase in the

United States.

Trihalomethanes (THMs) are produced during 

water and wastewater treatment when chlorine 

is added for disinfection.

















      

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44   

statistically associated with chloroform in aquifers, whereas sewer systems 

were associated with the occurrence of bromodichloromethane (table 4).

Chloroform was the most frequently detected VOC in samples from 

domestic and public wells. Detection frequencies of chloroform were about 

5 percent in domestic well samples and about 11 percent in public well 

samples at an assessment level of 0.2 μg/L (fig. 22). As previously mentioned, this difference may be a consequence of the higher pumping rates 

of public wells and their proximity to developed areas, compared to domestic wells (p. 40 and 41). A pattern of detection frequencies (chloroform > 

bromodichloromethane > dibromochloromethane > bromoform) is apparent 

for domestic well samples (fig. 22). This pattern indicates that chlorinated 

water is an important source of THMs in domestic well water. The frequent 

occurrence of brominated THMs and THM mixtures in public well samples 

supports the importance of chlorinated water as a source of THMs to these 

wells.

Although chloroform was detected frequently, most TTHM concentrations were two orders of magnitude lower than the USEPA MCL of 80 μg/L. 

No TTHM concentrations in samples from domestic or public wells were 

Chloroform and Other THMs—Continued

28.  THMs are Associated with

Human-Health Problems

THMs are associated with both acute and 

chronic human-health problems, including 

nausea, vomiting, dry mouth, dizziness, headaches, and damage to the liver and kidneys 

with prolonged exposure.(62) USEPA Office of 

Water indicates that three THMs—chloroform, bromodichloromethane, and bromoform—are likely to be carcinogenic to humans. 

Chloroform, however, is likely to be carcinogenic only at high doses. Chloroform is the 

only potentially carcinogenic THM for which 

the more stringent, but not regulatory, MCLG 

has been raised. Historically, the USEPA has 

established an MCLG of zero for all carcinogens, including chloroform, based on the 

presumption that any exposure to carcinogens 

represents a non-zero health risk. Because 

chloroform is presumed to be non-carcinogenic to humans at concentrations less than 

those that cause cell regeneration, the USEPA 

removed the zero MCLG in May 2000(82) and 

more recently revised the MCLG to 70 μg/L.(83) 

Only one sample from a domestic well and no 

samples from public wells in this assessment 

had concentrations greater than the MCLG for 

chloroform, and no samples had concentrations greater than the MCL for TTHMs.

Recent studies have shown a weak association between the ingestion of THMs and 

adverse birth outcomes (such as low birth 

weight, stillbirths, spontaneous abortions, and 

neural tube defects); however, many of these 

studies were considered inconclusive. More 

systematic epidemiological data on reproductive anomalies, including health effects of 

mixtures of THMs, and better exposure characterization would improve such studies(84) and 

aid regulatory agencies in making informed 

decisions to further protect human health.

Figure 22. Detection

frequencies of

trihalomethanes

(THMs), especially the

brominated species,

were much greater in

public well samples

than in domestic well

samples.

Figure 21. Detection

frequencies of

chloroform in shallow

ground water were

larger beneath urban

areas than beneath

agricultural areas.

Chlorinated water is an important source 

of chloroform to ground water.

        

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Chapter 5

greater than the MCL (Appendixes 9 and 11). However, each of the four 

THMs had concentrations less than but within a factor of 10 of the TTHM 

MCL.

THMs have been detected in septic tank effluent,(85, 86) and in improperly designed, maintained, or operated septic systems, resulting in shallow 

ground-water contamination in the vicinity of the septic system.(82) Water 

from a domestic well may be contaminated if the well is drawing contaminated water from the vicinity of an improperly operating septic system. 

Other potential sources of chloroform and other THMs to domestic wells are 

laundry wastewater containing bleach and the well disinfection practice of 

shock chlorination. In both cases, chloroform and other THMs are produced 

through the haloform reaction of the chlorine in bleach with organic matter.

Chloroform and other THMs are persistent under oxic conditions and 

have relatively low tendencies to sorb to soil and aquifer organic carbon. 

Because of these properties, THMs are expected to persist and may migrate 

substantial distances through the subsurface, especially in aquifers with 

detectable concentrations of dissolved oxygen and low amounts of organic 

carbon.(71, 87) These properties are important considerations in the western 

United States where drinking-water sources are sparse, and the injection of 

chlorinated wastewater into aquifers for future supply and to reduce land 

subsidence may become a more common practice (sidebar 29).

Monitoring chloroform and other THMs in samples from domestic and 

public wells is important because these compounds likely are indicators of 

sources of contamination. It also is important to evaluate trends in the occurrence of these compounds and to better understand the origin and humanhealth relevance of the low concentrations in water supplies.

29.  THMs Persist in an Aquifer in

the Western United States

Chlorinated surface water was injected into an 

oxic, unconsolidated sand and gravel aquifer 

in Antelope Valley, California, as part of a 

program to assess the long-term feasibility 

of using injection, storage, and recovery as a 

water-supply method and to reduce waterlevel declines and land subsidence in Antelope 

Valley.(88) In the sand and gravel aquifer, THM 

formation continued after the treated water 

was injected. Once all the residual chlorine 

reacted, the concentrations of THMs were 

controlled primarily by mixing and dilution with 

native ground water. Potential for natural THM 

attenuation in the aquifer by biodegradation 

and sorption was low because the aquifer 

has oxic conditions and low organic matter 

content. 

A model used to forecast the effects of 

repeated cycles of injection, storage, and 

recovery indicated that the cycles increased 

concentrations of THMs in the aquifer. These 

repeated cycles could yield aquifer THM 

concentrations approaching 100 percent of the 

injection-water THM concentration within 10 

annual cycles, provided mixing within the aquifer does not lower concentrations markedly. 

Detection frequencies of THMs were larger in public 

well samples than in domestic well samples; 

however, no concentrations were greater than 

the USEPA drinking-water standard.

Trihalomethanes (THMs) in chlorinated irrigation water

may reach ground water, especially if recharge occurs.

(Photograph courtesy of Rain Bird Corporation.)
46   

Chlorinated Solvents—Methylene Chloride, PCE, TCA, and TCE

Perchloroethene (PCE) is the most commonly used

solvent at commercial dry cleaners. (Photograph by

Connie J. Ross, U.S. Geological Survey.)

Figure 23. Production of most solvents in the United States began

declining as early as the 1970s.(3, 89, 90)

Seven of the 15 most frequently detected 

VOCs in aquifers were solvents.

30.  Production and Use of

Chlorinated Solvents have Declined

Chlorinated solvents are used in the aerospace and electronics industries, dry cleaning, 

manufacture of flexible urethane foam, paint 

removal/stripping, manufacture of pharmaceuticals, metal cleaning and degreasing, and 

wood manufacturing.(91) Solvents also are present in a variety of household consumer products including drain and pipe cleaners, oven 

cleaners, shoe polish, household degreasers, 

deodorizers, leather dyes, photographic supplies, tar remover, waxes, and pesticides.(92)

Some solvents, such as carbon tetrachloride, 

have been used as fumigants in grain storage 

bins.

Production of solvents began in the United 

States in the early 20th century, and usage 

of solvents increased markedly after World 

War II. Production has since declined. Nonetheless, relatively large quantities of solvents 

continue to be used in industrial, commercial, 

and domestic applications. For example, 

methylene chloride is an active ingredient in 

many formulations of paint removers, and PCE 

is used by more than 80 percent of commercial 

dry cleaners.(91)

The solvent group is one of the two most frequently detected VOC groups in the Nation’s aquifers (the other is the THM group; fig. 5). 

Seven solvents were among the 15 most frequently detected VOCs (fig. 8). 

PCE was the second most frequently detected VOC, and TCE, TCA, and 

methylene chloride were the 4th, 7th, and 14th most frequently detected 

VOCs, respectively.

Chlorinated solvents are organic compounds that are used in a variety 

of industrial, commercial, and domestic applications (sidebar 30). Four of 

the most commonly used solvents are given focus here—methylene chloride, 

PCE, TCA, and TCE. Although relatively large quantities of these solvents 

are still used today, their production began declining as early as the 1970s 

(fig. 23) in response to human-health and environmental concerns.(3) Specifically, all four solvents have known human-health concerns, and methylene 

chloride and TCE are classified as probable human carcinogens (sidebar 31).

One or more of the four commonly used chlorinated solvents given 

focus here were detected in about 6 percent of aquifer samples at an assessment level of 0.2 μg/L. Among the four solvents, detection frequencies were 

about 4 percent for PCE, 3 percent for TCE, 2 percent for TCA, and 1 percent for methylene chloride. The detection frequencies of these solvents, 























        

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    47

Chapter 5

Figure 24. The

detection frequencies

of most solvents

were higher in public

well samples than

in domestic well

samples.

Figure 25. Concentrations of solvents in aquifers typically were less than

1 microgram per liter.

31.  Some Solvents are Associated

with Human-Health Problems

Chlorinated solvents have been associated 

with both cancer and non-cancer humanhealth problems. USEPA Office of Water 

indicates that both methylene chloride and TCE 

are probable human carcinogens, although the 

cancer classification of TCE is under review. 

TCA is not classifiable for carcinogenicity by 

the USEPA because of the lack of reported 

human data and the inadequacy of the available animal studies. The USEPA has not issued 

any qualitative judgment on the carcinogenicity of PCE.(83) The USEPA currently is reassessing the health effects of all four solvents. The 

final drafts of the reassessments are expected 

during 2006–2008.(93)

MCLs for drinking water (Appendix 9) have 

been set for all the solvents considered 

here.(82) The potential of solvents to affect 

drinking water is large, in part, because the 

water solubilities of the solvents are much 

greater than their MCLs. This means that even 

small spills can result in ground-water concentrations of potential human-health concern.

In samples from domestic and public wells, solvents 

were among the VOCs that most frequently had 

concentrations greater than USEPA MCLs.

A variety of household products contain

chlorinated solvents. (Photograph courtesy of

Mike Wolforth, Light Images.)

except for methylene chloride, were higher in public well samples than in 

domestic well samples (fig. 24). Solvents also were detected frequently in 

drinking water supplied by CWSs (sidebar 32).

The median concentrations of detections of all four solvents in aquifer 

samples were less than 0.2 μg/L (fig. 25). TCE had the highest median 

concentration of detections, followed by PCE, TCA, and methylene chloride. If all laboratory analyses (including non-detections) are considered, the 

ranking of solvents by concentration is the same as the ranking by detection 

frequency.

In samples from domestic and public wells, the concentrations of 

8 VOCs were greater than their MCL, and 3 of these were the solvents PCE, 

TCE, and methylene chloride. In domestic well samples, TCE and PCE were 

ranked second and third among VOCs with concentrations that were greater 

than their MCLs. In public well samples, PCE and TCE were both ranked 

first, and methylene chloride was ranked third among VOCs with concentrations that were greater than their MCLs. In addition, PCE and TCE had, 

      

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48   

PCE degrades to TCE in aquifers with 

anoxic conditions.

32.  Solvents Occur Frequently in

Drinking Water Supplied by CWSs

Drinking water supplied by CWSs sampled 

during 1993–1998 in 12 New England and 

Mid-Atlantic States for a previous study had 

a larger occurrence of solvents compared 

to samples from domestic wells in the same 

12-State area(70) . One or more solvents were 

detected in about 10 percent of samples from 

CWSs that supply drinking water from ground 

water and in about 6 percent of domestic 

well samples. Individual solvents also were 

detected more frequently in CWS samples 

than in domestic well samples with the exception of methylene chloride.

Individual solvents were detected much more 

frequently in large CWSs than in small CWSs. 

The differences in detection frequencies 

by CWS size probably are related to larger 

pumping rates and to more urban land use and 

higher population density in areas surrounding 

the supply wells of large CWSs compared to 

small ones.

In drinking water from CWSs, concentrations 

of three solvents were greater than MCLs 

more often than other VOCs. Concentrations of 

PCE, TCE, and methylene chloride were greater 

than their MCLs in 1.5 percent, 1.2 percent, 

and 0.2 percent, respectively, of CWS samples.

Chlorinated Solvents—Continued

relative to other VOCs, a large number of concentrations less than but within 

a factor of 10 of their MCLs in samples from public and domestic wells 

(Appendixes 9 and 11).

The four solvents occurred together in any mixture in 5 percent of aquifer samples at an assessment level of 0.2 μg/L. The most frequently occurring two-solvent mixture, PCE–TCE, probably represents the degradation of 

PCE to TCE, which occurs through a process called “reductive dechlorination.”(94) In reductive dechlorination, a chlorine atom in PCE is replaced 

by a hydrogen atom producing TCE and is mediated by microbes (fig. 26). 

Several lines of evidence indicate that at least some PCE is being degraded 

in aquifers with anoxic conditions (sidebar 33).

A statistical model of solvents in aquifers and shallow ground water 

indicated that the occurrence of one or more of the four solvents increased 

with dissolved-oxygen concentrations (table 7). The probability of occurrence of the four solvents also increased with increasing percentage of urban 

land use, population density, number of RCRA hazardous-waste facilities, 

and septic system density. Conversely, the probability of occurrence of the 

four solvents decreased with increasing sand content of soil and depths to 

the top of the screen/open interval in the well.

The sources of solvents to ground water are numerous and include 

releases associated with the production, transport, or use of these chemicals. Urban land-use practices, regulated hazardous-waste facilities, and 

dry-cleaning businesses are likely sources of solvents. Conditions such as 

recharge, vertical soil permeability, and dissolved oxygen control the transport and fate of the solvents. Although solvents occur in air in urban areas, 

concentrations are not large enough for atmospheric deposition to account 

for all the mass of solvents detected in ground water.(76)

Figure 26. Perchloroethene (PCE) degrades to trichloroethene (TCE) under anoxic

conditions through reductive dechlorination.

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    49

Chapter 5

Figure 27. The concentration of a chlorinated solvent may increase with depth in an aquifer.

In some cases, concentrations of PCE and 

TCE increase with depth in an aquifer.

Table 7. Explanatory factors associated with the occurrence in ground water of

one or more of the four solvents—methylene chloride, perchloroethene (PCE), 1,1,1-

trichloroethane (TCA), and trichloroethene (TCE).

[RCRA, Resource Conservation and Recovery Act]

Explanatory factor Rank Positive (+) or

negative (-) association

Sand content of soil 1 -

Percentage of urban land use 2 +

Dissolved-oxygen concentration 3 +

Population density 4 +

Depth to screen/open interval 5 -

Number of RCRA facilities within 1 kilometer 6 +

Septic system density within 1 kilometer 7 +

Because of their physical and chemical properties, different solvents 

move through and react in ground water in unique ways (sidebar 33). 

In some cases, the concentrations of PCE and TCE have been shown to 

increase with increasing depth of the water in the aquifer.(95) Because pure 

solvents are denser than water, they can penetrate the water table and collect in pools on top of less permeable layers in an aquifer. In this way, they 

become a source of contamination at the bottom of the aquifer. Through 

dissolution, diffusion, and dispersion, the solvent mass can then move 

upward in an aquifer and produce a concentration gradient that increases 

with depth (fig. 27).

33.  What are the Properties of

Solvents?

In general, chlorinated solvents, as pure liquids, have relatively high densities, vapor pressures, and solubilities. In ground water, they 

also have relatively long half-lives. All solvents 

considered here have densities greater than 1. 

This means that they are denser than water 

and that releases of pure solvents can penetrate the water table (fig. 27). The relatively 

high vapor pressures of the four solvents 

means that these compounds can volatilize 

when spilled onto a surface or exposed to 

the atmosphere. The aqueous solubilities of 

the four solvents also generally are high.(96)

Consequently, some mass of solvents exposed 

to land surfaces can move in solution to the 

water table. Finally, the biotic half-lives of solvents in ground water are longer than those of 

other commonly used VOCs, like the gasoline 

hydrocarbons.(97) This means that solvents 

biodegrade slowly and therefore can persist 

for long periods of time in certain aquifers.

For more highly chlorinated molecules, the 

biodegradation of one chlorinated solvent can 

result in a by-product that also is a chlorinated 

solvent. For example, PCE can degrade to TCE 

(fig. 26), especially under anoxic conditions. 

If this occurs, the detection frequencies and 

concentrations of PCE should be larger in 

oxic ground water compared to anoxic ground 

water. PCE was detected in about 13 percent 

of samples from oxic ground water, but in 

only about 6 percent of samples from anoxic 

ground water. In addition, the concentration 

ratios of PCE to TCE were three times larger in 

oxic ground water compared to anoxic ground 

water. These lines of evidence indicate that 

some PCE may be degraded to TCE in aquifers 

with anoxic conditions.
50   

MTBE and Other Gasoline Oxygenates

Figure 28. Compared to methyl tert-butyl ether, other gasoline oxygenates were

detected infrequently in samples from domestic and public wells.

MTBE was the second most frequently detected VOC in 

samples from domestic and public wells despite its 

relatively short production and use history.

34.  What is

MTBE and Why Does it Persist in

Ground Water?

MTBE was first introduced in gasoline in 

1979 as an octane enhancer resulting from 

the phase-out of leaded gasoline.(99) Since the 

mid-1990s, MTBE also has been used in large 

quantities as an oxygenate in reformulated 

gasoline.

MTBE is an organic compound with ether 

bonds. The ether bonds and tertiary carbon 

atom of MTBE make it slow to biodegrade in 

ground water.

The physical properties of MTBE include 

high water solubility compared to gasoline 

hydrocarbons, low sorption to organic matter 

in soil and aquifer material, and a tendency to 

partition from air into water. MTBE also can 

undergo significant vapor phase transport in 

the unsaturated zone.(100, 101, 102) Collectively, 

these properties can allow MTBE to reach 

ground water and to travel faster and farther 

than other common gasoline components.

In ground water, MTBE is slow to biodegrade. (102, 103) MTBE is much less biodegradable than the hydrocarbons benzene, toluene, 

ethylbenzene, and xylenes (BTEX) in ground 

water; thus, dissolved MTBE can persist 

longer in aquifer systems relative to BTEX 

hydrocarbons.(104)

MTBE, a gasoline oxygenate, was the second most frequently detected VOC in samples from domestic and public wells at an 

assessment level of 0.2 μg/L. The detection frequency of MTBE was about 

3 percent in domestic well samples and about 5 percent in public well 

samples (fig. 28). Public wells generally have larger pumping rates than 

domestic wells, draw water from larger areas, and thus are more likely to 

be affected by releases of MTBE. Because of the large overlap in samples 

between studies of aquifers and domestic wells, the detection frequency of 

MTBE in aquifers is the same as for domestic well samples.

The NAWQA Program also analyzed samples for three other ether gasoline oxygenates—tert-amyl methyl ether (TAME), diisopropyl ether (DIPE), 

and ethyl tert-butyl ether (ETBE). These VOCs were detected infrequently 

in samples from domestic and public wells (fig. 28).

Gasoline oxygenates are compounds that contain oxygen and are added 

to gasoline to improve combustion and reduce harmful motor vehicle emissions. Since the 1990s, oxygenates have been used in gasoline in areas where 

certain air-quality standards have not been attained. In 2004, MTBE was the 

most commonly used gasoline oxygenate (sidebar 34); however, MTBE has 

been completely or partially banned in some States (sidebar 35). The Energy 

Policy Act of 2005 eliminated the oxygen requirement in gasoline.(98)

Potential point sources of MTBE include leaking storage tanks and 

associated piping, tank overflow spills, leaks from transport pipelines or bulk 

      

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Chapter 5

35.  MTBE has been Completely or

Partially Banned in Some States

As of June 2004, 19 States had enacted 

legislation to completely or partially ban 

MTBE use in gasoline.(111) Also, several Federal 

bills and resolutions have been introduced in 

Congress that would ban or limit the use of 

MTBE nationwide. As of 2004, however, none 

of the Federal legislation to restrict MTBE use 

had been enacted.

After 1995, most of the gasoline in California 

contained 2 percent oxygen by weight 

(11 percent by volume) in order to comply 

with Federal regulations.(112) Because of its 

favorable blending and transfer characteristics in gasoline, MTBE was used as the 

oxygenate in gasoline in California to comply 

with air-quality standards. However, because 

of concerns about the occurrence of MTBE in 

water supplies, an Executive Order was issued 

by the Governor of California in 1999 to ban 

the use of MTBE in gasoline by the end of 

2002. In 2002, implementation of the ban was 

extended to the end of 2003.(112)

In 2005, Congress passed the Energy Policy 

Act that eliminated the oxygen requirement in 

gasoline as established by the Clean Air Act 

Amendments (CAAA) of 1990.(98) Application 

of the elimination of the oxygen requirement 

in gasoline can begin as of the date of the 

Act (Aug. 8, 2005) for States that received a 

waiver under the CAAA. Application of the 

elimination of the oxygen requirement for any 

other State can begin 270 days after the date 

of the Act.(98) The elimination of an oxygen 

requirement is expected to result in less use 

of oxygenates, such as MTBE, in reformulated 

gasoline.

Figure 29. The

annual production

rate of methyl

tert-butyl ether

(MTBE) in the United

States generally

has increased until

1999, but has since

decreased.(89, 108, 109, 110)

gasoline stations, and automobile and truck accidents. Potential nonpoint 

sources of MTBE include evaporative losses from tanks or pipelines, incomplete combustion in automobile engines, urban stormwater runoff, exhaust 

from motorized watercraft, and leaks from watercraft tanks. MTBE has been 

detected in used motor oil, which also could be a source of MTBE in ground 

water.(105) In some situations, the atmosphere can be a source of MTBE to 

ground water. This source of MTBE to shallow ground water was studied in 

a network of sites in southern New Jersey. At some sites, concentrations of 

MTBE in ambient air and in shallow ground water indicated that the atmosphere was a possible source of MTBE in ground water.(76) At other sites, the 

concentrations of MTBE in ground water indicated that other sources, such 

as leaking tanks or road runoff, were the dominant source of MTBE.(106) In 

California, artificial recharge in some areas may be a source of MTBE to 

aquifers (sidebar 36).

Production of MTBE has increased markedly since 1985, with large 

volumes of MTBE produced in the United States since about 1990 (fig. 29). 

The locations of ground-water samples analyzed for MTBE, as collected or 

compiled by the NAWQA Program, and samples where MTBE was detected 

are shown in figure 30. The locations include samples from studies of aquifers, domestic wells, and public wells. As is evident in this figure, many of 

the detections of MTBE were in the New England and Mid-Atlantic States.

MTBE has been detected in drinking water of some CWSs (sidebar 37). 

The current concern with MTBE in drinking water is primarily taste and 

odor because the human-health effects of MTBE have not yet been clearly 

established. Because of its chemical properties, MTBE can be smelled and 

tasted at low concentrations by some people. In 1997, the USEPA issued a 

Drinking-Water Advisory for MTBE.(107) The advisory indicates that keeping 

MTBE concentrations in drinking water below the range of 20 to 40 μg/L 

will likely avert unpleasant taste and odor effects and also be protective of 

human health. The USEPA currently (2005) is conducting a risk assessment 

on the health effects of MTBE.











     

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52   

Smog attributed to motor vehicle emissions was

common in many metropolitan areas of the United

States prior to the implementation of Federal and

State gasoline programs.

MTBE and Other Gasoline Oxygenates—Continued

36.  Artificial Recharge Areas are a

Source of MTBE to Some California

Aquifers

In a study of public wells in the Los Angeles 

Basin (aquifer) and Orange County of southern 

California, factors such as population density 

and the density of LUSTs were not associated 

with the occurrence of VOCs like MTBE.(114)

This information indicates that localized 

sources like LUSTs and vertical migration of 

contaminants are not the dominant factors 

affecting the occurrence of MTBE in this 

area. Instead, movement of ground water 

along flowpaths from artificial recharge 

areas appeared to be the dominant factor 

in the occurrence of MTBE in active public 

wells.(114) In general, ground water near the 

focused recharge areas contained more VOCs 

like MTBE compared to water farther out in 

the basins. The occurrence of MTBE close 

to the areas of recharge is consistent with 

the relatively recent use of MTBE as a fuel 

additive and the lateral downgradient flow of 

recharge water in this area. Further investigation of ground water from public wells in three 

aquifers in southern California confirmed 

that MTBE was being introduced into areas 

of artificial recharge in the aquifers and that 

concentrations of MTBE decreased along 

flowpaths in the aquifers.(115)

Only one sample from a domestic well and no samples 

from public wells had MTBE concentrations greater 

than the USEPA Drinking-Water Advisory.

Figure 30. Most detections of methyl tert-butyl ether (MTBE) are in the highly

populated New England and Mid-Atlantic States.(15, 113)

Concentrations of MTBE generally were less than the Drinking-Water 

Advisory, with a median for samples with detections of 0.6 μg/L for samples 

from both domestic and public wells using an assessment level of 0.2 μg/L 

(fig. 31). Only one sample from a domestic well had an MTBE concentration that was greater than the lower limit of the Drinking-Water Advisory of 

20 μg/L. No samples from public wells had MTBE concentrations greater 

than 20 μg/L.

In the NAWQA-collected data, the single concentration of MTBE 

greater than 20 μg/L likely originated from some type of point source.(113)

Concentrations lower than this level could originate from either point or 

nonpoint sources. In general, MTBE did not occur frequently with other 

gasoline components in ground water except when detected at high concentrations.(113) The detection of MTBE without other common gasoline hydrocarbons likely is the result of MTBE’s higher solubility, lower sorption, and 

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Chapter 5

Figure 31. Most concentrations of methyl tert-butyl ether (MTBE) in samples

from domestic and public wells are much less than the U.S. Environmental

Protection Agency’s Drinking-Water Advisory level.

MTBE in drinking water has been a public

concern because of its low taste and odor

threshold and uncertain human-health effects.

(Photograph by Paul J. Squillace, U.S. Geological

Survey.)

Public well water and shallow ground water 

underlying urban land-use areas had the highest 

probability of MTBE contamination.

37.  MTBE has been Detected in

Drinking Water of Some CWSs

MTBE was the fifth most frequently detected 

VOC in CWS samples collected during 1993–

1998 for a previous study in 10 New England 

and Mid-Atlantic States.(70) It was detected 

in about 9 percent of 985 CWS samples. 

However, only 0.9 percent of CWSs reported 

MTBE concentrations greater than 20 μg/L, 

the lower limit of the USEPA’s Drinking-Water 

Advisory. Most MTBE concentrations in drinking water were less than 5.0 μg/L.

The USEPA required monitoring of MTBE 

in drinking water provided by public water 

systems under the UCM, which was designed 

to support drinking-water regulations. As of 

January 2005, 1,859 public water systems 

with ground-water sources have been 

sampled for MTBE under the UCM. Only 

four systems reported MTBE concentrations 

greater than 20 μg/L.(116)

greater persistence in ground water relative to common gasoline hydrocarbons.

In statistical analyses reported previously,(113) the probability of detecting MTBE in ground water was greater in areas with high population density, in areas where MTBE is used as an oxygenate in gasoline, and in areas 

with high ground-water recharge, compared to other areas of the Nation. 

Public well water and shallow ground water underlying urban land-use 

areas had a greater probability of containing detectable levels of MTBE than 

domestic well water and ground water underlying rural land-use areas.(113)

When MTBE use in gasoline was applied as a surrogate for MTBE 

release to the environment, the detection frequency of MTBE was significantly larger in areas of high use than in areas of low use.(113) These results 

indicate that the use of MTBE in large amounts in gasoline in some areas 

of the Nation has resulted in increased detection frequencies of MTBE in 

ground water.(113) MTBE was detected in some low MTBE-use areas presumedly as a result of its small volume in gasoline for octane enhancement.

The quick arrival of MTBE in ground water relative to its production 

history (fig. 29) indicates that MTBE is an important concern with respect to 

ground-water management. Aquifers that may be vulnerable to MTBE contamination warrant evaluation with consideration given to the development 

of strategies to reduce the likelihood of contamination. Continued groundwater monitoring is warranted to evaluate trends in the occurrence of MTBE 

and to better understand the sources and transport of MTBE to ground water 

and to domestic and public wells.

   

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54   

Gasoline Hydrocarbons

38.  Why are Gasoline Hydrocarbons

Detected Infrequently in Aquifers?

A variety of physical and chemical properties 

can limit the movement of gasoline hydrocarbons to and transport by ground water. In 

ground water, a compound’s solubility is one of 

the most important chemical properties. The 

median solubility of gasoline hydrocarbons is 

the lowest of any VOC group.(118) Thus, gasoline hydrocarbons have less chance to reach 

ground water if released because less mass 

can be dissolved in recharge water. 

At a gasoline release site in Beaufort, South 

Carolina, aerobic biodegradation and 

volatilization were found to be important in 

limiting the transport and occurrence of gasoline hydrocarbons in the unsaturated zone.(119)

This could help to explain, in part, the lower 

detection frequency of gasoline hydrocarbons 

relative to other VOC groups.

Another important property of organic chemicals is their ability to adsorb to organic carbon 

in soil and aquifer material. A good measure 

of the sorption potential is the organic carbon 

partitioning coefficient. Gasoline hydrocarbons have the highest median organic carbon 

partitioning coefficient of any other VOC 

group.(118) Because of this, once in the subsurface the movement of gasoline hydrocarbons 

will be retarded with respect to the velocity of 

ground water. Also, less mass is left in ground 

water as sorption occurs.

Finally, and perhaps most importantly, biodegradation can reduce the mass of gasoline 

hydrocarbons in the unsaturated zone and 

in ground water. For aerobic biodegradation, 

gasoline hydrocarbons have some of the 

shortest half-lives of VOCs (see Circular’s Web 

site). This means that gasoline hydrocarbons 

biodegrade quickly in oxic ground water relative to other VOCs.

Gasoline hydrocarbons were detected infrequently 

in aquifers at an assessment level of 0.2 μg/L.

The gasoline hydrocarbon group is defined here as aromatic hydrocarbons or alkyl benzenes whose predominant use is in motor vehicle 

gasoline. Eleven of the 55 VOCs analyzed by NAWQA are in this group: 

benzene, n-butylbenzene, ethylbenzene, isopropylbenzene, naphthalene, 

styrene, toluene, 1,2,4-trimethylbenzene, o-xylene, m-xylene, and p-xylene.

Gasoline hydrocarbons are among the most intensively and widely 

used VOCs. Compared to other VOC groups, production of gasoline hydrocarbons has been extremely large. From 1980 to 2000, the median annual 

production of these compounds for all uses (including motor gasoline) was 

about 20 trillion gallons.(117) This production number was at least four orders 

of magnitude larger than other VOC groups (sidebar 11) and comprised 

99.9 percent of the total production of all VOC groups for the time period.

Although the production of gasoline hydrocarbons from 1980 to 2000 

was substantially larger than that of any other VOC group, it was not the 

most frequently detected VOC group in aquifers. Both THMs and solvents 

were detected more frequently than gasoline hydrocarbons at an assessment 

level of 0.2 μg/L (fig. 5). The smaller detection frequency of gasoline hydrocarbons relative to these other VOC groups probably is attributed to their 

transport and fate properties (sidebar 38).

Gasoline hydrocarbons were detected most frequently at concentrations less than 0.2 μg/L and comprised a substantial fraction (24 percent) of 

all low-level VOC detections. The frequent occurrence of low concentrations explains why the median concentration of samples with detections of 

gasoline hydrocarbons was among the lowest of any VOC group (fig. 32). 

In drinking water from CWSs, few concentrations of gasoline hydrocarbons 

were greater than MCLs (sidebar 39).

Individual gasoline hydrocarbons were detected infrequently in aquifers. Only one gasoline hydrocarbon, toluene, was detected in more than 

1 percent of aquifer samples (fig. 8). No individual gasoline hydrocarbons 

were in the top 10 mixtures of VOCs in aquifers (sidebar 13). Furthermore, gasoline hydrocarbons occurred more frequently alone than as mixtures. Two or more gasoline hydrocarbons occurred together in only about 

15 percent of samples in which gasoline hydrocarbons were detected.

The sources of most gasoline hydrocarbons in aquifers probably are 

releases of gasoline or other finished fuel products. However, in addition to 

their use in gasoline, many of the compounds identified as gasoline hydrocarbons have other uses (Appendix 4) and thus may have other sources to 

ground water. Gasoline hydrocarbons have been detected in used motor oil, 

which could be a source for some of these VOCs in ground water.(105)
    55

Chapter 5

Figure 32. The median concentration of gasoline hydrocarbons in aquifer samples

with detections was the second lowest of all seven VOC groups.

39.  Gasoline Hydrocarbons Occur

More Frequently in Drinking Water

from CWSs than in Domestic Wells

Drinking water from CWSs sampled during 

1993–1998 for a previous study in 12 New 

England and Mid-Atlantic States(70) had a more 

frequent occurrence of one or more gasoline 

hydrocarbons compared to domestic well 

samples in the same 12-State area. One or 

more gasoline hydrocarbons were detected 

in about 8 percent of CWSs deriving drinking 

water from ground water compared to about 

2 percent of domestic well samples. Individual 

gasoline hydrocarbons also were detected 

more frequently in CWSs than in domestic 

well samples, with the exception of isopropylbenzene. The larger detection frequency 

of gasoline hydrocarbons in CWSs than in 

domestic well samples is likely due to several 

factors as discussed previously (p. 40 and 41).

Few concentrations of gasoline hydrocarbons 

in drinking water from CWSs or in domestic 

well samples were greater than MCLs. One 

CWS had a concentration of one gasoline 

hydrocarbon—benzene—that was greater 

than its MCL. In domestic well samples from 

the same 12-State area, no concentrations 

of gasoline hydrocarbons were greater than 

MCLs.

More than 1.5 million storage tanks have been

removed from service since 1984 to protect soil

and ground water from gasoline contamination.

(Photograph courtesy of South Dakota Petroleum

Release Compensation Fund.)

The median concentration of gasoline hydrocarbons 

was among the lowest of any VOC group.

Because of the infrequent detection of individual gasoline hydrocarbons 

in aquifers, few associations with anthropogenic or hydrogeologic factors 

that might control their occurrence could be made in this assessment. The 

likelihood of detecting either toluene or 1,2,4-trimethylbenzene, however, 

was determined to be larger in areas with higher density of gasoline storage 

tanks (table 4).

In a study of the occurrence of MTBE and gasoline hydrocarbons 

in ground water of the United States, the detection frequencies of individual gasoline hydrocarbons were smaller than the detection frequency of 

MTBE.(113) Although both are components of gasoline, the smaller detection 

frequencies of gasoline hydrocarbons were believed to be the result of differences in transport and fate properties of these compounds compared to those 

of MTBE.

In the Reformulated Gasoline (RFG) Program, the content of benzene was limited to 1 percent by volume. Normally, gasoline contains 

between 1 and 1.5 percent benzene by volume.(120) RFG usually contains the 

oxygenate MTBE. The limitation of benzene content in RFG should have 

resulted in smaller detection frequencies of benzene in areas of high MTBE 

use compared to areas of low MTBE use. An examination of ground-water 

data from across the United States revealed that this was the case.(113) The 

smaller detection frequency of benzene, a known human carcinogen, in high 

MTBE-use areas could be a result of the lower benzene content in RFG and, 

if so, would be an important change for improving ground-water quality, 

especially for ground water that is used as a supply for drinking water.

     

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56   

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129. Sittig, Marshall, 1991, Handbook of toxic and hazardous chemicals and carcinogens (3d ed.), Vol. 1 and 2: Park 

Ridge, N.J., Noyes Publications, 1,685 p.
62   

Abiotic  Pertaining to the nonliving parts of a system.

Abiotic degradation  The transformation of a compound

without involvement of living organisms.

Advection  Transport of contaminants due only to the flow of

water.

Aerobic biodegradation  The breakdown of organic contaminants by microorganisms when oxygen is present. Aerobic

biodegradation also is known as aerobic respiration.

Ambient ground water  Untreated ground water that is characteristic of the aquifer resource. Studies of ambient ground

water by the NAWQA Program typically exclude contaminated ground water at regulated point-source release sites.

Anoxic  Ground water that has no dissolved oxygen or a

very low concentration of dissolved oxy­gen (that is, less than

0.5 milligrams per liter).

Anthropogenic  Derived from, or caused by human activity.

Anthropogenic compound  A compound that occurs in the

environment primarily as a result of human activity.

Aquifer  A geologic formation, group of formations, or part

of a formation that contains sufficient saturated permeable

material (soil, sand, gravel and/or rock) to yield significant

quantities of water to wells and springs.

Aquifer sample  As used in this report, a water sample collected as part of an aquifer study.

Aquifer study  A study to assess the general water quality

of an aquifer, a part of an aquifer, or an aquifer system by

sampling primarily existing wells. Typically 20 to 30 wells are

sampled once. Wells are randomly selected using a stratified,

areally distributed design. Wells are distributed over a large

area, and the study constitutes a resource assessment.

Assessment level  A concentration selected by a hydrologist

and applied to water-quality data that have variable laboratory

reporting levels either for a specific compound or between

individual compounds. The assessment level is applied to

data received from the laboratory and is applied subsequent

to the laboratory reporting level. Concentrations reported by

the laboratory but below the assessment level are considered

as “non-detections” in the calculation of occurrence statistics.

The primary purpose of the assessment level is for accurate

comparison of detection frequencies and median concentrations between individual VOCs, groups of VOCs, and to

previous studies.

Atmospheric deposition  The process by which chemical

constituents are deposited from the atmosphere to the earth’s

surface by rain, sleet, and snow.

Biodegradation  See definition for biotic degradation.

Biotic Pertaining to the living parts of a system.

Biotic degradation  As used in this report, the conversion of

a parent VOC to a by-product by microorganisms. Also known

as biodegradation.

By-product  A compound that results from the degradation of

another (that is, parent) compound.

Chlorinated solvent  An organic compound that contains

chlorine and is used in a variety of indus­trial, commercial, and

domestic applications. In general, chlorinated solvents have

relatively high densities, relatively high vapor pressures and

solubilities, and relatively long half-lives in ground water.

Community water system (CWS)  A public water system that

supplies water to the same population year-round. A CWS

serves a residential population, such as a municipality, mobile

home park, or nursing home.

Concentration  The amount or mass of a substance present

in a given volume or mass of sample. Concentrations in this

report generally are expressed in micrograms per liter, but are

also expressed in milligrams per liter and nanograms per liter.

Concentration of potential human-health concern  As used in

this report: (1) for a regulated compound with a U.S. Environmental Protection Agency drinking-water standard, a concentration greater than the Maximum Contaminant Level; and

(2) for an unregulated compound, a concentration greater than

the Health-Based Screening Level.

Degradation  The breakdown of substances like VOCs

through abiotic or biotic processes.

Detection frequency  The frequency of detection of an individual VOC that was computed as the number of samples with

a detection of an individual VOC divided by the number of

samples in which the VOC was analyzed, times 100. In most

cases, the detection frequency reported in this assessment was

computed at a prescribed assessment level.

Detection frequency of VOCs by group  The frequency of

detection of one or more VOCs from a particular use group

that was computed as the number of samples with a detection

of one or more VOCs from a particular use group divided by

the number of samples analyzed for the particular use group,

times 100. In all cases, the detection frequency of VOCs by

group was computed at a prescribed assessment level.

Dispersion  The process whereby solutes are mixed and

spread during advective transport due to velocity variations.

Domestic well A privately owned well that typically serves

one home and supplies water for human consumption and

other homeowner uses.

Domestic well water  Self-supplied water that is withdrawn

from a private well and used for human consumption and other

Glossary
    63

Glossary

homeowner uses. Water supplied for domestic wells often is

untreated and is not subject to federally enforceable drinkingwater standards.

Drinking water  Water for human consumption that meets all

applicable Federal, State, and local requirements.

Fifteen most frequently detected VOCs  The 15 compounds

with the largest detection frequency in samples from aquifers,

domestic wells, or public wells, based on samples from all

wells and an assessment level of 0.2 microgram per liter.

Fumigant  A compound or mixture of compounds that produces a gas, vapor, fumes, or smoke intended to destroy, repel,

or control organisms such as insects, bacteria, or rodents. Bromomethane is an example of a fumigant used for large-scale

strawberry farming.

Gasoline hydrocarbon  A straight, branched, and (or) cyclic

structured organic compound containing only carbon and

hydrogen atoms that is a common ingredient in gasoline and

other petroleum product for­mulations. Benzene, toluene, ethylbenzene, and xylenes, commonly referred to as BTEX, are a

subset of the gasoline hydrocarbons.

Gasoline oxygenate  A compound that contains oxygen and

was added to gasoline in order to meet the requirements of

the 1990 Clean Air Act Amendments. As used in this report,

gasoline oxy­genates include the four ethers MTBE, TAME,

DIPE, and ETBE.

Ground water  Water beneath the land surface in the saturated

zone.

Half-life  The time required for the concentration of a

compound in a given environmental medium to be reduced to

one-half of its original value by one or more processes, such as

degradation or transport into another environmental medium.

Halogenated aliphatic organic compound  A compound

belonging to a group of compounds that consist of carbon

and hydrogen atoms, and any of the five nonmetalic elements

including bromine, chlorine, fluorine, iodine, or astatine.

Atoms are linked in an open chain.

Health-Based Screening Level (HBSL)  An estimate of concentration (for a noncarcinogen) or concentration range (for a

carcinogen) in water that (1) may be of potential human-health

concern; (2) can be used as a threshold value against which

measured concentrations of contaminants in ambient groundwater samples can be compared; and (3) is consistent with

USEPA Office of Water methodologies.

Hydric soil  Soil that formed under conditions of saturation,

flooding, or ponding long enough during the growing season to develop anoxic conditions in the upper part of the soil

profile.

Intrinsic susceptibility  A measure of the ease with which

a contaminant in water enters and moves through an aquifer.

It is a characteristic of the aquifer and overlying material and

hydrologic conditions, and is independent of the chemical

characteristics of the contaminant and its sources.

Laboratory reporting level  A level of reporting concentrations of VOCs in a water sample that is set by the laboratory

to minimize the rate of false positives and false negatives.

Concentrations below the laboratory reporting level are

denoted by a less than designation, “<,” preceding a concentration value.

Lithology  The physical character of a rock based on such

characteristics as color, structure, mineralogical composition,

and grain size.

Low-level (analytical) method  A new GC/MS method for the

analysis of VOCs in ambient water samples, USGS method

0–4127–96, which was implemented in 1996.

Low-level contamination  As used in this report, concentrations of individual VOCs or VOC groups, and total VOC

concentrations less than 1 microgram per liter. Although

arbitrary, this concentration is approximately the laboratory

reporting level for VOCs by many agencies. The USGS lowlevel method identifies VOCs at concentrations 2–3 orders of

magnitude below this benchmark.

Maximum Contaminant Level (MCL)  As used in this report,

a USEPA drinking-water standard that is legally enforceable,

and that sets the maximum permissible level of a contaminant

in water that is delivered to any user of a public water system.

Maximum Contaminant Level Goal (MCLG)  As established by

the USEPA, a non-enforceable health goal that is set at a level

at which no known or anticipated adverse effect on the health

of persons occurs and which allows an adequate margin of

safety.(121)

Median  A compound’s concentration for which 50 percent

of the laboratory analyses, when arranged in order of magnitude, lie on each side. In this report, this median is based on

samples from all wells.

Median (concentrations) of detections  A compound’s

concentration for which 50 percent of the detections, when

arranged in order of magnitude, lie on each side. For the

0.02-microgram per liter assessment level, this median for this

report is based on the subset of wells for which samples were

analyzed by the low-level (analytical) method.

Non-aqueous-phase liquid  An organic liquid that can exist

in a separate phase in equilibrium with water after the dissolved concentration in water has reached the saturation limit

for water.

Occurrence  The presence, frequencies of detection, concentrations, and ranges of concentrations of VOCs and the locations (areal patterns) of VOC detections in ground water.

Old ground water  As used in this report, ground water

recharged prior to 1955.
64   

Organic synthesis compound  A compound that is used in

the formation of other organic compounds. Chloroethene is an

example of an organic synthesis compound used in the production of polyvinyl chloride plastics.

Oxic  Ground water that has a concentration of dissolved

oxygen greater than or equal to 0.5 milligrams per liter.

Permeability  A measure of the relative ease with which a

porous medium can transmit a fluid.

Primary drinking-water standard  A regulation that (1)

applies to public water systems; (2) specifies contaminants

that may have any adverse effect on the health of persons;

(3) specifies for each contaminant a maximum contaminant

level or treatment technique; and (4) contains criteria and

procedures to ensure compliance.

Principal aquifer  A regionally extensive aquifer or aquifer

system that has the potential to be used as a source of potable

water.

Probable human carcinogen  Under the USEPA’s cancer

classification, an agent that is likely to be carcinogenic to

humans.(122) Probable human carcinogens also have been

expressed as a USEPA cancer “group” and included agents for

which the weight of evidence of human carcinogenicity based

on epidemiologic studies was limited (“Group B1”), and those

for which the weight of evidence of carcinogenicity based on

animal studies was “sufficient” (“Group B2”).(121)

Public water system (PWS)  Any publicly or privately owned

water system that provides water for human consumption if

such system has 15 connections or regularly serves at least

25 people for at least 60 days out of the year. PWSs include

(1) community water systems, such as municipalities, mobile

home parks, or nursing homes; (2) transient non-community

water systems, such as campgrounds, motels, and gasoline

stations; and (3) non-transient, non-community systems, such

as schools, factories, and hospitals.

Public well  As used in this report, a privately or publicly

owned well that provides water to a public water system

(PWS).

Public well water  As used in this report, water provided by

a public water system that is for human consumption and other

homeowner uses. Water may be treated or blended prior to

distribution.

RCRA Hazardous-Waste Facilities  As used in this report,

includes treatment, storage, and disposal facilities regulated

under the Resource Conservation and Recovery Act.

Recharge  The process involved in the addition of water to

the saturated zone. Also, the amount of water added.

Redox condition  As used in this report, redox condition

refers to the position that a system is in for the redox scale

between very oxidizing and very reducing.

Reductive dechlorination  A reductive process, usually

mediated by bacteria, in which chlorine atoms on a chlorinated aliphatic hydrocarbon are replaced sequentially with

hydrogen. Also known as dehalogenation, hydrogenolysis, and

hydrogenolytic dehalogenation.

Reformulated Gasoline (RFG) Program  A program applied to

an area established under the Clean Air Act Amendments in

which gasoline contained 2 percent oxygen by weight yearround to control levels of tropospheric ozone.

Refrigerant  A compound used for producing refrigeration,

either as a working substance in a refrigerator or by direct

absorption of heat. The chlorofluorocarbons (CFCs) are classified as refrigerants for the purposes of this report.

Regulated compound  As used in this report, a compound for

which a Federal drink­ing-water standard has been established.

Respiration  The metabolic processes whereby certain organisms obtain energy from organic molecules.

Rural area  An area that has a population density of less than

386 persons/km2 (1,000 persons/mi2 or 1.56 persons/acre).

Saturated zone  The region in the subsurface in which all

the interstices or voids are filled with water under a pressure

exceeding that of the atmosphere.

Shallow ground-water study  An investigation of the concentrations and distribution of water-quality constituents in

recently recharged ground water (generally less than 10 years

old) associated with a particular land use. For each study,

usually about 20-30 shallow monitoring wells are sampled.

Solvent  A compound that is used to dissolve other substances. Two of the more common solvents are trichloroethene

(TCE) and perchloroethene (PCE).

Sorption  The interaction, through binding or association, of

a solute ion or molecule with a solid.

Source of contamination  Includes any natural or anthropogenic chemical or physical property of the ground-water

resource that is not desirable from a health or other perspective

such as interference with water-treatment practices.

Study Unit  A major hydrologic system of the United States

in which NAWQA studies are focused, geographically defined

by surface- or ground-water features and usually encompassing more than 10,000 km2 of land area. The NAWQA studies

during the first decade of assessments included 51 of these

systems, collectively covering a large part of the Nation,

encompassing the majority of population and water use, and

including diverse hydrologic settings that differ widely in the

natural and human factors that affect water quality.

Study-Unit investigation  The systematic study of water

quality in a NAWQA Study Unit. Study Units are organized

into three groups that are studied on a rotational schedule, with

3-year intensive study periods repeated about every decade.

Susceptibility  See definition for intrinsic susceptibility.
    65

Glossary

Total trihalomethane concentration  The sum of all quanti-

fied concentrations for bromodichloromethane, bromoform,

chloroform, and dibromochloromethane ina water sample.

Total VOC concentration  The sum of all quantified concen-

trations for all VOCs analyzed ina sample.

Total xylene concentration  The sum of all quantified con-

centrations form-xylene,o-xylene, andp-xylene ina water

sample.

Trihalomethane (THM)  As used in this report,a compound

belonging toa group of VOCs that includes bromodichloro-

methane, bromoform, chloroform, and dibromochloromethane.

These compounds are known by-products of water chlorina-

tion.

Unregulated compound  As used in this report,a compound

for which no Federal drinking-water standard has been estab-

lished. Note thata compound that is unregulated under the

Safe Drinking Water Act may be regulated in other contexts

and under other statutes.

Unsaturated zone  The subsurface region above the water

table in which the pore spaces may containa combination of

air and water.

Urban area  An area that hasa population density of 386 per-

sons/km

2 (1,000 persons/mi2 or 1.56 persons/acre) or greater.

VOC mixture  The co-occurrence of two or more VOCs ina

water sample.

Volatile organic compound (VOC)  An organic chemical that

hasa high vapor pressure relative to its water solubility. VOCs

include components of gasoline, fuel oils, and lubricants, as

well as organic solvents, fumigants, some inert ingredients

in pesticides, refrigerants, some compounds used in organic

synthesis, and some by-products of water chlorination.

Vulnerability  The tendency or likelihood for contaminants

to reacha specified position in the ground-water system after

introduction at some location above the uppermost aquifer.

The vulnerability ofa ground-water resource to contamination

depends on its intrinsic susceptibility as well as the locations

and types of sources of naturally occurring and anthropogenic

contamination, relative location of wells, and the fate and

transport of the contaminant(s). As used in this report, an aqui-

fer is considered vulnerable if at least one VOC was detected

in aquifer samples.

Well sample  As used in this report, water collected (prior to

treatment and blending) froma domestic or public well built

into or drilled into the zone of saturation.

Young ground water  As used in this report, ground water

recharged after 1955.
66   

ACAD	 Acadian–Pontchartrain Drainages

ACFB	 Apalachicola–Chattahoochee–Flint River

Basins

ALBE	 Albemarle–Pamlico Drainage Basin

ALMN	 Allegheny and Monongahela River Basin

CAZB	 Central Arizona Basins

CCPT	 Central Columbia Plateau

CNBR	 Central Nebraska Basins

CONN	 Connecticut, Housatonic, and Thames River

Basins

COOK	 Cook Inlet Basin

DELR	 Delaware River Basin

DLMV	 Delmarva Peninsula

EIWA	 Eastern Iowa Basins

GAFL	 Georgia–Florida Coastal Plain

GRSL	 Great Salt Lake Basins

HDSN	 Hudson River Basin

HPGW	 High Plains Regional Ground Water Study

KANA	 Kanawha–New River Basins

LERI	 Lake Erie–Lake Saint Clair Drainages

LINJ	 Long Island–New Jersey Coastal Drainages

LIRB	 Lower Illinois River Basin

LSUS	 Lower Susquehanna River Basin

LTEN	 Lower Tennessee River Basin

MIAM	 Great and Little Miami River Basins

MISE	 Mississippi Embayment

MOBL	 Mobile River Basin

NECB	 New England Coastal Basins

NROK	 Northern Rockies Intermontane Basins

NVBR	 Nevada Basin and Range

OAHU	 Oahu

OKLA	 Oklahoma Ground-Water Pilot Study

OZRK	 Ozark Plateaus

PUGT	 Puget Sound Basin

REDN	 Red River of the North Basin

RIOG	 Rio Grande Valley

SACR	 Sacramento River Basin

SANA	 Santa Ana Basin

SANJ	 San Joaquin–Tulare Basins

SANT	 Santee River Basin and Coastal Drainages

SCTX	 South–Central Texas

SOFL	 Southern Florida

SPLT	 South Platte River Basin

TRIN	 Trinity River Basin

UCOL	 Upper Colorado River Basin

UIRB	 Upper Illinois River Basin

UMIS	 Upper Mississippi River Basin

USNK	 Upper Snake River Basin

UTEN	 Upper Tennessee River Basin

WILL	 Willamette Basin

WMIC	 Western Lake Michigan Drainages

YELL	 Yellowstone River Basin

Appendixes

Appendix 1.  Key to NAWQA Study Units that Completed Aquifer Studies for

VOCs
    67

Appendixes

Appendix 2.  Abbreviations and Acronyms

Bgal/d	 billion gallons per day

km	 kilometer

μg/L	 micrograms per liter

BTEX	 Benzene, Toluene, Ethylbenzene, and Xylenes

CAAA	 Clean Air Act Amendments

CAS	 Chemical Abstract Services

CCL	 Drinking-Water Candidate Contaminant List

CFC-11	 Trichlorofluoromethane

CFC-12	 Dichlorodifluoromethane

CFC-113	 Trichlorotrifluoroethane

CWS	 Community Water System

DBCP	 Dibromochloropropane

DCA	 Dichloroethane

DCE	 Dichloroethene

DIPE	 Diisopropyl Ether

EDB	 Ethylene Dibromide

ETBE	 Ethyl tert-Butyl Ether

GC/MS	 Gas Chromatography/Mass Spectrometry

HBSL	 Health-Based Screening Level

HCFC-22	 Hydrochlorofluorocarbon-22

IUPAC	 International Union of Pure and Applied

Chemistry

LUST	 Leaking Underground Storage Tank

MCL	 Maximum Contaminant Level

MCLG	 Maximum Contaminant Level Goal

MTBE	 Methyl tert-Butyl Ether

NAWQA	 U.S. Geological Survey National WaterQuality Assessment Program

NTNCWS	 Non-Transient, Non-Community Water

System

NCWS	 Non-Community Water System

PCE	 Perchloroethene

PWS	 Public Water System

RCRA	 Resource Conservation and Recovery Act

RFG	 Reformulated Gasoline

SDWA	 Safe Drinking Water Act

TAME	 tert-Amyl Methyl Ether

TCA	 1,1,1-Trichloroethane

TCE	 Trichloroethene

THM	 Trihalomethane (includes bromodichloromethane, bromoform, chloroform, and dibromochloromethane)

TNCWS	 Transient, Non-Community Water System

TTHM	 Total Trihalomethane

UCM	 Unregulated Contaminant Monitoring

USEPA	 U.S. Environmental Protection Agency

USGS	 U.S. Geological Survey

UST	 Underground Storage Tank

VOC	 Volatile Organic Compound
68   

Appendix 3.  Additional Information on the Approach of this Assessment

Figure 33.  Locations of wells sampled in aquifer studies for the national assessment of VOCs.

The following maps show the locations of wells 

sampled in aquifer studies (fig. 33) and of domestic 

wells (fig. 34) and public wells (fig. 35) sampled for 

this national assessment of VOCs. The VOC occurrence 

information from these wells came from three sources: 

(1) the NAWQA Program that sampled aquifers throughout the United States from 1993 to 2002; (2) local, State, 

and Federal agencies that sampled aquifers in various 

parts of the United States from 1985 to 1997 (referred to 

as retrospective data); and (3) a collaborative study by 

several entities including the USGS, that sampled ground 

waters used by community water systems throughout the 

United States from 1999 to 2000.

Various criteria were used to select wells for inclusion in this assessment including: study type, number 

of wells in the study, intended purpose of the sample, 

minimum number of compounds analyzed in each well 

sample, analytical method, laboratory reporting levels, 

and minimum distance between wells. Additional information on the sources of the data and the procedures 

used to select wells can be found elsewhere.(19)

(AWAII

/AHU

!LASKA
    69

Appendixes

Figure 34.  Locations of domestic wells sampled for the national assessment of VOCs.

Figure 35.  Locations of public wells sampled for the national assessment of VOCs.

(AWAII

/AHU

!LASKA

(AWAII

/AHU

!LASKA
70   

Appendix 4.  Compound names used in this report, International Union of Pure and Applied Chemistry (IUPAC) names, Chemical

Abstract Service (CAS) numbers, predominant use group, and other use information in alphabetical order.

[--, not applicable or no information available]

Compound name

IUPAC name

(If different from

this report’s

name)

CAS number Predominant use

group (or source)

Other uses as parent compound

or additive and remarks1

Acrolein 2-Propenal 107–02–8 Organic synthesis Herbicide; algicide; biocide; leather tanning; 

protein supplements in poultry feed

Acrylonitrile 2-Propenenitrile 107–13–1 Organic synthesis Former fumigant for milling and bakery 

equipment and for stored tobacco

tert-Amyl methyl ether (TAME) 2-Methoxy-2-

methylbutane

994–05–8 Gasoline oxygenate --

Benzene -- 71–43–2 Gasoline hydrocarbon Solvent; organic synthesis 

Bromodichloromethane -- 75–27–4 Trihalomethane (chlorination by-product)Organic synthesis

Bromoform Tribromomethane75–25–2 Trihalomethane (chlorination by-product)Solvent; pharmaceutical manufacturing; 

ingredient of fire resistant chemicals and 

gauge fluid

Bromomethane -- 74–83–9 Fumigant Chemical intermediate; solvent

n-Butylbenzene -- 104–51–8 Gasoline hydrocarbon --

Carbon tetrachloride Tetrachloromethane56–23–5 Solvent Manufacture of chlorofluorocarbons; dry 

cleaning; fire extinguishers; organic synthesis; agriculture

Chlorobenzene -- 108–90–7 Solvent Organic synthesis; insecticides

Chloroethane -- 75–00–3 Solvent Used in the production of tetraethyl lead 

for gasoline, cellulose, chemicals, and 

pharmaceuticals

Chloroform Trichloromethane67–66–3 Trihalomethane (chlorination by-product)Industrial solvent; extractant; chemical 

intermediate

Chloromethane -- 74–87–3 Solvent Chemical intermediate for production of 

silicones, agricultural chemicals, and 

cellulose

Dibromochloromethane -- 124–48–1 Trihalomethane (chlorination by-product)Chemical intermediate for manufacture of 

aerosol propellants, refrigerants, pesticides, fire extinguishing agents
    71

Appendixes

Appendix 4.  Compound names used in this report, International Union of Pure and Applied Chemistry (IUPAC) names, Chemical

Abstract Service (CAS) numbers, predominant use group, and other use information in alphabetical order.—Continued

[--, not applicable or no information available]

Compound name

IUPAC name

(If different from

this report’s

name)

CAS number Predominant use

group (or source)

Other uses as parent compound

or additive and remarks1

Dibromochloropropane (DBCP) 1,2-Dibromo-3-

chloropropane

96–12–8 Fumigant Nematocide; fumigant used until 1979 except on pineapples in Hawaii until 1985

1,2-Dichlorobenzene -- 95–50–1 Solvent Intermediate for making herbicides and 

insecticides

1,3-Dichlorobenzene -- 541–73–1 Solvent --

1,4-Dichlorobenzene -- 106–46–7 Fumigant Insecticide and fungicide; organic synthesis; 

garbage and restroom deodorant

Dichlorodifluoromethane

(CFC-12)

-- 75–71–8 Refrigerant --

1,1-Dichloroethane (1,1-DCA) -- 75–34–3 Solvent Manufacture of plastic wrap, adhesives, and 

synthetic fiber 

1,2-Dichloroethane (1,2-DCA) -- 107–06–2 Solvent Chemical intermediate for manufacture of 

vinyl chloride, solvents, and fluorocarbons; fumigant; ingredient in paints and 

leaded gasoline

1,1-Dichloroethene (1,1-DCE) -- 75–35–4 Organic synthesis PVC (plastics); adhesives; refrigerants

cis-1,2-Dichloroethene 

(cis-1,2-DCE)

-- 156–59–2 Solvent Refrigerant; pharmaceutical manufacture; 

artificial pearl manufacture; extraction of 

fats from fish and meat; organic synthesis

trans-1,2-Dichloroethene 

(trans-1,2-DCE)

-- 156–60–5 Solvent Refrigerant; pharmaceutical manufacture; 

artificial pearl manufacture; extraction of 

fats from fish and meat; organic synthesis

1,2-Dichloropropane -- 78–87–5 Fumigant Organic synthesis; former soil fumigant; 

stain remover; ingredient in gasoline

cis-1,3-Dichloropropene -- 10061–01–5 Fumigant Solvent; chemical intermediate

trans-1,3-Dichloropropene -- 10061–02–6 Fumigant Solvent; chemical intermediate

Diisopropyl ether (DIPE) 2,2-Oxybis[propane]108–20–3 Gasoline oxygenate Solvent; chemical intermediate in organic 

synthesis
72   

Compound name

IUPAC name

(If different from

this report’s

name)

CAS number Predominant use

group (or source)

Other uses as parent compound

or additive and remarks1

Ethyl tert-butyl ether (ETBE) 2-Ethoxy-2-

methylpropane

637–92–3 Gasoline oxygenate --

Ethylbenzene -- 100–41–4 Gasoline hydrocarbon Solvent; production of styrene

Ethylene dibromide (EDB) 1,2-Dibromoethane106–93–4 Fumigant Former pesticide; solvent for resins, gums, 

and waxes; waterproofing preparations; 

dyes and pharmaceuticals; additive in 

leaded gasoline and aviation fuel

Hexachlorobutadiene 1,1,2,3,4,4-Hexachloro-1,3-

butadiene

87–68–3 Organic synthesis Heat transfer fluid; wash oil in chemical 

industry; solvent

Hexachloroethane 1,1,1,2,2,2-Hexachloroethane67–72–1 Solvent Military smoke and pyrotechnic devices; organic synthesis; fire extinguishing fluids; 

lubricants

Isopropylbenzene (1-Methylethyl)

benzene

98–82–8 Gasoline hydrocarbon --

Methyl tert-butyl ether (MTBE) 2-Methoxy-2-

methylpropane

1634–04–4 Gasoline oxygenate Formerly used to dissolve gallstones; laboratory reagent for semi-volatile organic 

compound extraction from liquids and 

solids

Methylene chloride Dichloromethane 75–09–2 Solvent Aerosol propellant; paint remover; solvent 

for spice oleoresins; caffeine extractant; 

fumigant for strawberries and grains; 

pharmaceuticals

Naphthalene -- 91–20–3 Gasoline hydrocarbon Mothballs; insecticide; organic synthesis; 

solvent

Perchloroethene2 (PCE) Tetrachloroethene 127–18–4 Solvent Chemical intermediate; dry cleaning; textile 

processing

n-Propylbenzene -- 103–65–1 Solvent --

Styrene Ethenylbenzene 100–42–5 Gasoline hydrocarbon Coatings; paint; rubber; adhesives

Toluene Methylbenzene 108–88–3 Gasoline hydrocarbon Solvents; production of urethane

1,2,3-Trichlorobenzene -- 87–61–6 Organic synthesis --

Appendix 4.  Compound names used in this report, International Union of Pure and Applied Chemistry (IUPAC) names, Chemical

Abstract Service (CAS) numbers, predominant use group, and other use information in alphabetical order.—Continued

[--, not applicable or no information available]
    73

Appendixes

Compound name

IUPAC name

(If different from

this report’s

name)

CAS number Predominant use

group (or source)

Other uses as parent compound

or additive and remarks1

1,2,4-Trichlorobenzene -- 120–82–1 Solvent Dye carrier; herbicides; wood preservatives; 

former soil treatment for termite control

1,1,1-Trichloroethane (TCA) -- 71–55–6 Solvent Dyeing; textile processing; ingredient in 

cosmetics; aerosol products 

1,1,2-Trichloroethane -- 79–00–5 Solvent Manufacture of vinylidene chloride, an 

intermediate for making synthetic fibers 

and plastic wraps

Trichloroethene3 (TCE) 1,1,2-Trichloroethene79–01–6 Solvent Caffeine extractant; dry-cleaning; paint 

and ink formulation; rubber processing 

industries

Trichlorofluoromethane

(CFC-11)

-- 75–69–4 Refrigerant Foaming agent; aerosol propellant

1,2,3-Trichloropropane -- 96–18–4 Fumigant Solvent; degreaser

Trichlorotrifluoroethane

(CFC-113)

1,1,2-Trichloro1,2,2-trifluoroethane76–13–1 Refrigerant Foaming agent; aerosol propellant

1,2,4-Trimethylbenzene -- 95–63–6 Gasoline hydrocarbon Solvents; dye and perfume manufacture; 

organic synthesis

Vinyl bromide Bromoethene 593–60–2 Organic synthesis Flame retardant

Vinyl chloride Chloroethene 75–01–4 Organic synthesis Rubber, paper, glass industries; PVC intermediate; small quantities in food wrappings & containers

m- and p-Xylene4 1,3- and 1,4-

Dimethylbenzene

108–38–3

106–42–3 

Gasoline hydrocarbon Chemical intermediate for the manufacture 

of polyesters, vitamins, and pharmaceuticals; ingredient in paint, thinner, and for 

p-xylene, insecticides

o-Xylene 1,2-Dimethylbenzene

95–47–6 Gasoline hydrocarbon Chemical intermediate for the manufacture 

of plasticizers, polyesters, vitamins, and 

pharmaceuticals; ingredient in paint, thinner, and insecticides

1

Sources of information on other uses.(123, 124, 125, 126, 127, 128, 129)

2

Also known as tetrachloroethylene, perchloroethylene, and perc.

3

Also known as trichloroethylene.

4

Considered as 2 of the 55 compounds included in this assessment.

Appendix 4.  Compound names used in this report, International Union of Pure and Applied Chemistry (IUPAC) names, Chemical

Abstract Service (CAS) numbers, predominant use group, and other use information in alphabetical order.—Continued

[--, not applicable or no information available]
74   

Principal or

other aquifer

Predominant

lithology

Map

number1

Aquifer study

in principal or

other aquifer2

Assessment level of 0.2 μg/L3 Assessment level of 0.02 μg/L4

Number

of wells

sampled

Number

of

detections

Percent of

samples with

a detection

of at least

one VOC

Number

of wells

sampled

Number

of

detections

Percent of

samples with

a detection

of at least

one VOC

Central Valley aquifer 

system

US&G -- All samples 200 101 50.5 30 9 30.0

1 sacr04 142 89 62.7 -- -- --

2 sacrsus1 30 3 10.0 30 9 30.0

3 sanjsus1 28 9 32.1 -- -- --

Northern Atlantic 

Coastal Plain 

aquifer system

S Sand -- All samples 220 106 48.2 30 23 76.7

4 delmarva 34 8 23.5 -- -- --

5 linj01 130 78 60.0 -- -- --

6 albesus1 11 1 9.1 -- -- --

7 albesus2 15 2 13.3 -- -- --

8 linjsus2 30 17 56.7 30 23 76.7

Biscayne aquifer Carb 9 soflsus1 29 12 41.4 29 20 69.0

California Coastal 

Basin aquifers

US&G -- All samples 69 26 37.7 69 48 69.6

10 sanasus1 27 12 44.4 27 22 81.5

11 sanasus2 20 6 30.0 20 13 65.0

12 sanasus3 22 8 36.4 22 13 59.1

Hawaiian volcanicrock aquifers - 

locally overlain 

by sedimentary 

deposits

B&V -- All samples 43 16 37.2 28 16 57.1

13 oahu02 15 3 20.0 -- -- --

14 oahusus1 28 13 46.4 28 16 57.1

New York and New 

England crystallinerock aquifers5

Cryst -- All samples 118 43 36.4 88 50 56.8

15 connsus1 30 11 36.7 -- -- --

16 linjsus1 30 19 63.3 30 23 76.7

17 necbsus1 28 8 28.6 28 15 53.6

18 necbsus2 30 5 16.7 30 12 40.0

Early Mesozoic basin 

aquifer

SS -- All samples 50 18 36.0 50 36 72.0

19 delrsus1 30 11 36.7 30 22 73.3

20 linjsus3 20 7 35.0 20 14 70.0

Silurian–Devonian 

aquifers

Carb -- All samples 49 15 30.6 33 29 87.9

21 eiwa03 16 7 43.8 -- -- --

22 eiwasus1 33 8 24.2 33 29 87.9

Rocky Mountain Front 

Range crystallinerock aquifers5

Cryst 23 spltsus1 26 7 26.9 -- -- --

Surficial aquifer 

system

US&G 24 gaflsus1 36 9 25.0 -- -- --

Appendix 5.  Detection frequency of at least one volatile organic compound (VOC) by principal or other aquifer and by aquifer study at

two assessment levels (in order of overall decreasing detection frequency).

[μg/L, micrograms per liter; --, not applicable; US&G, unconsolidated sand and gravel; S Sand, sand and (or) semiconsolidated sand; Carb, carbonate rocks; 

B&V, basaltic and (or) volcanic rocks; SS, sandstone; Cryst, crystalline rocks; SS&Carb, sandstone and carbonate rocks; ND, no detections]
    75

Appendixes

Principal or

other aquifer

Predominant

lithology

Map

number1

Aquifer study

in principal or

other aquifer2

Assessment level of 0.2 μg/L3 Assessment level of 0.02 μg/L4

Number

of wells

sampled

Number

of

detections

Percent of

samples with

a detection

of at least

one VOC

Number

of wells

sampled

Number

of

detections

Percent of

samples with

a detection

of at least

one VOC

Ozark Plateaus 

aquifer system

Carb -- All samples 49 12 24.5 -- -- --

25 ozrksus2a 33 8 24.2 -- -- --

26 ozrksus3a 16 4 25.0 -- -- --

Mississippian aquifers SS&Carb 27 ltensus16 32 6 18.8 32 23 71.9

Coastal Lowlands 

aquifer system

S Sand -- All samples 81 15 18.5 57 29 50.9

28 acadsus1 29 3 10.3 29 10 34.5

29 acadsus2 28 10 35.7 28 19 67.9

30 trinsus3 24 2 8.3 -- -- --

Unconsolidated deposit 

aquifers (Alaska) 

(Cook Inlet)

US&G 31 cooksus1a 28 5 17.9 28 20 71.4

Other sand and gravel 

aquifers5

US&G -- All samples 142 25 17.6 53 28 52.8

32 mise07 30 2 6.7 -- -- --

33 sofl03 13 10 76.9 -- -- --

34 yell01 19 2 10.5 -- -- --

35 cnbrsus2 27 2 7.4 -- -- --

36 ucolsus1 29 2 6.9 29 11 37.9

37 yellsus1 24 7 29.2 24 17 70.8

Puget Sound 

aquifer system

US&G 38 pugtsus1 30 5 16.7 30 23 76.7

Edwards–Trinity 

aquifer system

SS&Carb -- All samples 164 27 16.5 88 33 37.5

39 sctx01 52 18 34.6 -- -- --

40 sctxsus1 28 3 10.7 28 17 60.7

41 sctxsus2 31 1 3.2 31 5 16.1

42 sctxsus3 29 4 13.8 29 11 37.9

43 trinsus1 24 1 4.2 -- -- --

Glacial deposit 

aquifers7

US&G -- All samples 367 56 15.3 253 146 57.7

44 nneb05 69 15 21.7 -- -- --

45 almnsus2 30 3 10.0 30 12 40.0

46 delrsus3 16 2 12.5 16 6 37.5

47 eiwasus2 32 1 3.1 32 16 50.0

48 hdsnsus1 35 2 5.7 -- -- --

49 lerisus1 28 2 7.1 28 3 10.7

50 lirbsus1 30 ND ND 30 16 53.3

51 lirbsus2 30 8 26.7 30 26 86.7

Appendix 5.  Detection frequency of at least one volatile organic compound (VOC) by principal or other aquifer and by aquifer study at

two assessment levels (in order of overall decreasing detection frequency).—Continued

[μg/L, micrograms per liter; --, not applicable; US&G, unconsolidated sand and gravel; S Sand, sand and (or) semiconsolidated sand; Carb, carbonate rocks; 

B&V, basaltic and (or) volcanic rocks; SS, sandstone; Cryst, crystalline rocks; SS&Carb, sandstone and carbonate rocks; ND, no detections]
76   

Principal or

other aquifer

Predominant

lithology

Map

number1

Aquifer study

in principal or

other aquifer2

Assessment level of 0.2 μg/L3 Assessment level of 0.02 μg/L4

Number

of wells

sampled

Number

of

detections

Percent of

samples with

a detection

of at least

one VOC

Number

of wells

sampled

Number

of

detections

Percent of

samples with

a detection

of at least

one VOC

Glacial deposit 

aquifers7—

Continued

52 miamsus1 30 6 20.0 30 19 63.3

53 necbsus3 30 13 43.3 30 28 93.3

54 rednsus2 10 4 40.0 -- -- --

55 uirbsus1 27 ND ND 27 20 74.1

Basin and Range 

basin-fill aquifers

US&G -- All samples 328 46 14.0 127 50 39.4

56 carson 148 18 12.2 -- -- --

57 cazbsus1a 30 2 6.7 30 13 43.3

58 cazbsus2 27 5 18.5 27 7 25.9

59 cazbsus3 18 ND ND 18 13 72.2

60 grslsus1 52 5 9.6 52 17 32.7

61 nvbrsus1 21 11 52.4 -- -- --

62 nvbrsus2 16 5 31.2 -- -- --

63 nvbrsus3 16 ND ND -- -- --

Pennsylvanian aquifers SS -- All samples 60 8 13.3 60 32 53.3

64 almnsus1 30 4 13.3 30 22 73.3

65 kanasus1 30 4 13.3 30 10 33.3

Mississippi River 

Valley alluvial 

aquifer

US&G -- All samples 54 7 13.0 54 28 51.9

66 misesus1 29 7 24.1 29 17 58.6

67 misesus3 25 ND ND 25 11 44.0

Ordovician aquifers Carb 68 ltensus2 31 4 12.9 31 15 48.4

Columbia Plateau 

basaltic-rock 

aquifers

B&V 69 ccptsus1 32 4 12.5 -- -- --

Northern Rocky 

Mountains Intermontane Basins 

aquifer system

US&G -- All samples 61 7 11.5 61 13 21.3

70 nroksus1 31 4 12.9 31 7 22.6

71 nroksus2 30 3 10.0 30 6 20.0

Valley and Ridge 

aquifers

SS&Carb -- All samples 101 11 10.9 60 28 46.7

72 uten02 12 3 25.0 -- -- --

73 delrsus2 30 5 16.7 30 11 36.7

74 lsussus1 29 ND ND -- -- --

75 utensus1 30 3 10.0 30 17 56.7

Willamette Lowland 

basin-fill aquifers

US&G -- All samples 65 7 10.8 -- -- --

76 willlusag1 15 1 6.7 -- -- --

77 willlusag2 25 2 8.0 -- -- --

78 willsus1 25 4 16.0 -- -- --

Appendix 5.  Detection frequency of at least one volatile organic compound (VOC) by principal or other aquifer and by aquifer study at

two assessment levels (in order of overall decreasing detection frequency).—Continued

[μg/L, micrograms per liter; --, not applicable; US&G, unconsolidated sand and gravel; S Sand, sand and (or) semiconsolidated sand; Carb, carbonate rocks; 

B&V, basaltic and (or) volcanic rocks; SS, sandstone; Cryst, crystalline rocks; SS&Carb, sandstone and carbonate rocks; ND, no detections]
    77

Appendixes

Principal or

other aquifer

Predominant

lithology

Map

number1

Aquifer study

in principal or

other aquifer2

Assessment level of 0.2 μg/L3 Assessment level of 0.02 μg/L4

Number

of wells

sampled

Number

of

detections

Percent of

samples with

a detection

of at least

one VOC

Number

of wells

sampled

Number

of

detections

Percent of

samples with

a detection

of at least

one VOC

Floridan aquifer 

system

Carb -- All samples 56 6 10.7 30 4 13.3

79 acfbsus1 26 4 15.4 -- -- --

80 santsus2 30 2 6.7 30 4 13.3

Central Oklahoma 

aquifer

SS 81 oklahoma 120 10 8.3 -- -- --

Cambrian–Ordovician 

aquifer system

SS -- All samples 76 6 7.9 50 28 56.0

82 umissus3 25 3 12.0 25 18 72.0

83 umissus4 25 1 4.0 25 10 40.0

84 wmicsus1 26 2 7.7 -- -- --

Mississippi 

Embayment–Texas 

Coastal Uplands 

aquifer system

S Sand -- All samples 52 4 7.7 30 14 46.7

85 misesus2 30 3 10.0 30 14 46.7

86 trinsus2 22 1 4.5 -- -- --

Piedmont and Blue 

Ridge crystallinerock aquifers

Cryst -- All samples 70 5 7.1 60 23 38.3

87 kanasus2 30 3 10.0 30 12 40.0

88 lsussus2 10 1 10.0 -- -- --

89 santsus3 30 1 3.3 30 11 36.7

Snake River Plain 

basin-fill aquifers

US&G -- All samples 405 17 4.2 -- -- --

90 id01 385 16 4.2 -- -- --

91 usnksus3 20 1 5.0 -- -- --

Lower Tertiary 

aquifers

SS 92 yellsus2 28 1 3.6 28 20 71.4

Southeastern Coastal 

Plain aquifer system

S Sand -- All samples 57 2 3.5 57 24 42.1

93 moblsus1 30 ND ND 30 7 23.3

94 santsus1 27 2 7.4 27 17 63.0

High Plains aquifer US&G -- All samples 141 4 2.8 141 55 39.0

95 hpgwsus1a 74 1 1.4 74 39 52.7

96 hpgwsus1b 47 2 4.3 47 9 19.1

97 hpgwsus2 20 1 5.0 20 7 35.0

Rio Grande aquifer 

system

US&G 98 riogsus1 28 ND ND -- -- --

1

The map number refers to a corresponding map on the Circular’s Web site.

2

The name of each aquifer study is unique and can be used to find more detailed information about the aquifer study on the Circular’s Web site.

3

Detection frequencies are for all samples included in this assessment, regardless of the analytical method.

4

Detection frequencies are for the subset of samples that were analyzed with the U.S. Geological Survey’s low-level method 0–4127–96. At this assessment 

level, detection frequencies are estimates.(19)

5

Other aquifer.

6

Regolith overlying bedrock.

7

Sand and gravel aquifers north of the limit of Quaternary continental glaciation and east of the Rocky Mountains.

Appendix 5.  Detection frequency of at least one volatile organic compound (VOC) by principal or other aquifer and by aquifer study at

two assessment levels (in order of overall decreasing detection frequency).—Continued

[μg/L, micrograms per liter; --, not applicable; US&G, unconsolidated sand and gravel; S Sand, sand and (or) semiconsolidated sand; Carb, carbonate rocks; 

B&V, basaltic and (or) volcanic rocks; SS, sandstone; Cryst, crystalline rocks; SS&Carb, sandstone and carbonate rocks; ND, no detections]
78   

Compound name

Number

of

samples

Detection frequency at

selected assessment

levels1 (percent) Number

of

samples

Detection frequency at selected

assessment levels2 (percent)

Median

concentration3 (μg/L)

No

assessment

level

0.02 μg/L 0.2 μg/L 1 μg/L 5 μg/L 10 μg/L All

samples

Samples

with

detections

Fumigants

Bromomethane 1,687 ND ND 3,119 0.032 ND ND ND < 0.20 0.50

Dibromochloropropane 1,687 0.18 0.18 2,518 .71 0.56 ND ND < .50 1.4

1,4-Dichlorobenzene 1,687 2.5 .77 3,446 .15 .12 0.058 0.058 < .20 .011

1,2-Dichloropropane 1,686 1.3 1.0 3,497 .71 .31 .086 ND < .20 .23

cis-1,3-Dichloropropene 1,687 ND ND 3,118 ND ND ND ND < .10 --

trans-1,3-Dichloropropene 1,686 ND ND 3,117 ND ND ND ND < .13 --

Ethylene dibromide 1,686 ND ND 2,851 .070 ND ND ND < .10 .72

1,2,3-Trichloropropane 1,687 .59 .59 2,765 .61 .14 ND ND < .20 .50

Gasoline hydrocarbons

Benzene 1,687 4.1 1.7 3,497 0.63 0.23 0.057 0.057 < 0.20 0.020

n-Butylbenzene 1,687 .18 .18 2,461 .041 ND ND ND < .19 .040

Ethylbenzene 1,687 .83 .47 3,497 .26 .17 .057 .029 < .20 .035

Isopropylbenzene 1,687 1.0 .36 2,461 .081 .041 ND ND < .032 .014

Naphthalene 1,687 .30 .30 2,509 .16 .12 ND ND < .25 1.9

Styrene 1,665 2.3 .60 3,463 ND ND ND ND < .20 .015

Toluene 1,676 16.4 9.9 3,457 1.9 .46 .17 .087 < .20 .032

1,2,4-Trimethylbenzene 1,662 16.0 9.4 2,376 .63 .042 ND ND < .056 .020

o-Xylene 1,682 1.0 .54 1,739 .058 .058 .058 ND < .050 .010

m- and p-Xylenes4 1,683 2.5 1.2 1,717 .12 .058 .058 .058 < .060 .020

Total xylenes5 1,683 2.7 1.3 3,436 .38 .087 .058 .058 < .064 .022

Gasoline oxygenates

tert-Amyl methyl ether 1,683 0.65 0.65 1,710 0.18 ND ND ND < 0.11 0.11

Diisopropyl ether 1,547 .58 .58 1,574 .13 0.064 0.064 0.064 < .10 .10

Ethyl tert-butyl ether 1,683 .18 .059 1,710 ND ND ND ND < .054 .010

Methyl tert-butyl ether 1,687 7.1 7.0 2,422 2.8 .99 .12 .041 < .17 .20

Organic synthesis compounds

Acrolein 618 ND ND 670 ND ND ND ND < 2.0 --

Acrylonitrile 1,683 ND ND 1,762 ND ND ND ND < 1.2 --

1,1-Dichloroethene 1,686 2.2 1.5 3,497 0.66 0.34 0.14 0.086 < .20 0.068

Hexachlorobutadiene 1,687 .059 .059 2,509 ND ND ND ND < .20 .030

1,2,3-Trichlorobenzene 1,687 .059 .059 2,461 ND ND ND ND < .27 .020

Vinyl bromide 1,683 ND ND 1,710 ND ND ND ND < .10 --

Vinyl chloride 1,687 .12 .12 3,498 .26 .14 .029 ND < .20 1.1

Appendix 6. Detection frequencies and median concentrations for selected volatile organic compounds in samples from 

aquifer studies.

[μg/L, micrograms per liter; ND, compound not detected; <, less than; --, not applicable]
    79

Appendixes

Compound name

Number

of

samples

Detection frequency at

selected assessment

levels1 (percent) Number

of

samples

Detection frequency at selected

assessment levels2 (percent)

Median

concentration3 (μg/L)

No

assessment

level

0.02 μg/L 0.2 μg/L 1 μg/L 5 μg/L 10 μg/L All

samples

Samples

with

detections

Refrigerants

Dichlorodifluoromethane 1,687 4.1 4.1 3,496 1.9 0.34 0.029 ND < 0.20 0.28

Trichlorofluoromethane 1,686 2.4 2.1 3,495 1.1 .37 .057 .029 < .20 .20

Trichlorotrifluoroethane 1,686 1.0 .77 2,666 .26 .038 ND ND < .060 .092

Solvents

Carbon tetrachloride 1,686 1.3 1.1 3,497 0.31 0.086 ND ND < 0.20 0.077

Chlorobenzene 1,687 1.3 .41 3,498 .17 .11 0.029 ND < .20 .007

Chloroethane 1,686 .30 .30 3,113 .29 .064 ND ND < .12 .20

Chloromethane 1,676 7.9 6.0 2,988 1.1 .13 .033 0.033 < .20 .035

1,2-Dichlorobenzene 1,687 .53 .41 3,464 .12 .087 .029 .029 < .20 .041

1,3-Dichlorobenzene 1,687 .41 .12 2,347 ND ND ND ND < .054 .008

1,1-Dichloroethane 1,686 2.8 2.4 3,496 .86 .17 .029 ND < .20 .085

1,2-Dichloroethane 1,687 .18 .18 3,438 .47 .15 ND ND < .20 .30

cis-1,2-Dichloroethene 1,686 2.3 1.7 2,847 .42 .070 ND ND < .050 .047

trans-1,2-Dichloroethene 1,686 .30 .24 3,200 .91 .38 .12 .062 < .050 .60

Hexachloroethane 1,683 ND ND 1,759 ND ND ND ND < .19 --

Methylene chloride 1,685 5.0 3.6 3,487 .89 .37 .057 .057 < .20 .040

Perchloroethene 1,656 13.2 8.3 3,449 3.7 1.5 .70 .32 < .20 .090

n-Propylbenzene 1,687 .30 .24 2,461 .041 .041 ND ND < .042 .048

1,2,4-Trichlorobenzene 1,687 .059 .059 2,509 ND ND ND ND < .20 .020

1,1,1-Trichloroethane 1,687 8.2 4.4 3,498 1.7 .57 .17 .14 < .20 .043

1,1,2-Trichloroethane 1,687 .18 .12 3,119 ND ND ND ND < .10 .028

Trichloroethene 1,686 5.2 3.8 3,497 2.6 1.1 .46 .26 < .20 .20

Trihalomethanes

Bromodichloromethane 1,686 4.9 3.7 3,497 1.1 0.46 0.20 0.11 < 0.20 0.080

Bromoform 1,685 2.2 1.4 3,496 1.0 .31 .029 ND < .20 .30

Chloroform 1,686 29.8 21.2 3,495 7.4 2.3 .69 .31 < .20 .079

Dibromochloromethane 1,686 2.0 1.7 3,497 .94 .34 .14 .11 < .20 .20

Total trihalomethanes5 1,686 30.5 21.6 3,497 8.1 2.5 .71 .34 < .20 .090

1

These detection frequencies are for the subset of samples that were analyzed with the U.S. Geological Survey’s low-level method 0–4127–96. At this 

assessment level, detection frequencies are estimates.(19)

2

These detection frequencies are for all samples included in this assessment, regardless of the analytical method.

3

The analytical methods used for this assessment have varied sensitivity among compounds and comparison of the median concentrations between compounds is not appropriate. No assessment level was applied to determine the median.

4

Considered as 2 of the 55 compounds included in this assessment.

5

Not considered as 1 of the 55 compounds included in this assessment.

Appendix 6. Detection frequencies and median concentrations for selected volatile organic compounds in samples from 

aquifer studies.—Continued

[μg/L, micrograms per liter; ND, compound not detected; <, less than; --, not applicable]
80   

Appendix 7.  Concentrations of selected volatile organic compounds (VOCs) in samples of untreated ground water.

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COMPARISONOFTHEMEDIANCONCENTRATIONSSHOWNINTHECHARTS	BETWEENSTUDYTYPEWELLTYPEAND6/#SIS

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    81

Appendixes

Appendix 7.  Concentrations of selected volatile organic compounds (VOCs) in samples of untreated ground water.—Continued


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82   

Appendix 7.  Concentrations of selected volatile organic compounds (VOCs) in samples of untreated ground water.—Continued

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    83

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Appendix 7.  Concentrations of selected volatile organic compounds (VOCs) in samples of untreated ground water.—Continued

      





































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84   

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    85

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86   

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    87

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    91

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94   

Compound name

Number

of

samples

Detection

frequency at selected

assessment levels1

(percent) Number

of

samples

Detection frequency at selected

assessment levels2 (percent)

Median

concentration3

(μg/L)

No

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level

0.02 μg/L 0.2 μg/L 1 μg/L 5 μg/L 10 μg/L All

samples

Samples

with

detections

Fumigants

Bromomethane 1,208 ND ND 2,156 ND ND ND ND <0.20 --

Dibromochloropropane 1,208 0.25 0.25 1,962 0.71 0.51 ND ND <.50 1.4

1,4-Dichlorobenzene 1,208 1.9 .41 2,399 .083 .083 ND ND <.20 .011

1,2-Dichloropropane 1,207 .75 .58 2,400 .58 .29 0.12 0.042 <.20 .30

cis-1,3-Dichloropropene 1,208 ND ND 2,156 ND ND ND ND <.10 --

trans-1,3-Dichloropropene 1,207 ND ND 2,155 ND ND ND ND <.13 --

Ethylene dibromide 1,207 ND ND 2,085 .14 .048 ND ND <.04 .55

1,2,3-Trichloropropane 1,208 .17 .17 2,092 .43 ND ND ND <.20 .38

Gasoline hydrocarbons

Benzene 1,208 3.1 1.2 2,401 0.21 0.042 ND ND <0.20 0.015

n-Butylbenzene 1,208 .17 .17 1,932 ND ND ND ND <.19 .038

Ethylbenzene 1,208 .58 .33 2,401 .12 .083 0.042 ND <.12 .041

Isopropylbenzene 1,208 .75 .33 1,932 .10 ND ND ND <.050 .019

Naphthalene 1,208 .25 .25 1,939 .15 .10 ND ND <.20 1.1

Styrene 1,202 2.2 .25 2,395 ND ND ND ND <.17 .014

Toluene 1,203 17.9 10.7 2,386 1.0 .21 .042 0.042 <.20 .026

1,2,4-Trimethylbenzene 1,190 15.2 8.5 1,876 .32 .053 .053 .053 <.056 .020

o-Xylene 1,205 .66 .41 1,214 ND ND ND ND <.050 .038

m- and p-Xylenes4 1,206 2.3 1.0 1,208 ND ND ND ND <.060 .018

Total xylenes5 1,206 2.5 1.1 2,388 .21 .042 .042 .042 <.060 .020

Gasoline oxygenates

tert-Amyl methyl ether 1,206 0.50 0.50 1,215 0.082 ND ND ND <0.11 0.10

Diisopropyl ether 1,096 .36 .36 1,105 .090 0.090 0.090 0.090 <.10 .14

Ethyl tert-butyl ether 1,206 ND ND 1,215 ND ND ND ND <.054 --

Methyl tert-butyl ether 1,208 5.5 5.5 1,931 2.9 .98 .21 .052 <.17 .30

Organic synthesis compounds

Acrolein 445 ND ND 450 ND ND ND ND <2.0 --

Acrylonitrile 1,206 ND ND 1,220 ND ND ND ND <1.2 --

1,1-Dichloroethene 1,207 1.6 1.1 2,400 0.21 0.12 0.083 0.042 <.18 0.026

Hexachlorobutadiene 1,208 ND ND 1,939 ND ND ND ND <.20 --

1,2,3-Trichlorobenzene 1,208 ND ND 1,932 ND ND ND ND <.20 --

Vinyl bromide 1,206 ND ND 1,215 ND ND ND ND <.10 --

Vinyl chloride 1,208 .083 .083 2,401 .083 .042 ND ND <.20 .74

Appendix 8. Detection frequencies and median concentrations for selected volatile organic compounds in samples from domestic 

wells.

[μg/L, micrograms per liter; ND, compound not detected; <, less than; --, not applicable]
    95

Appendixes

Compound name

Number

of

samples

Detection

frequency at selected

assessment levels1

(percent) Number

of

samples

Detection frequency at selected

assessment levels2 (percent)

Median

concentration3

(μg/L)

No

assessment

level

0.02 μg/L 0.2 μg/L 1 μg/L 5 μg/L 10 μg/L All

samples

Samples

with

detections

Refrigerants

Dichlorodifluoromethane 1,208 3.6 3.6 2,401 1.4 0.17 ND ND <0.20 0.27

Trichlorofluoromethane 1,208 1.9 1.6 2,401 .62 .17 ND ND <.20 .16

Trichlorotrifluoroethane 1,207 .50 .33 2,083 .19 .048 ND ND <.060 .17

Solvents

Carbon tetrachloride 1,207 1.1 0.75 2,400 0.21 0.042 ND ND <0.20 0.043

Chlorobenzene 1,208 1.3 .17 2,401 .042 .042 ND ND <.11 .0040

Chloroethane 1,207 .33 .33 2,155 .093 ND ND ND <.12 .060

Chloromethane 1,207 9.7 7.3 2,059 .97 .097 ND ND <.20 .030

1,2-Dichlorobenzene 1,208 .33 .25 2,391 .042 .042 ND ND <.19 .092

1,3-Dichlorobenzene 1,208 .50 .17 1,894 ND ND ND ND <.054 .0096

1,1-Dichloroethane 1,207 2.2 2.0 2,400 .29 .042 ND ND <.20 .073

1,2-Dichloroethane 1,208 .17 .17 2,383 .21 .13 ND ND <.20 1.3

cis-1,2-Dichloroethene 1,207 .91 .83 2,177 .18 .092 ND ND <.050 .087

trans-1,2-Dichloroethene 1,207 ND ND 2,241 .045 ND ND ND <.050 .20

Hexachloroethane 1,206 ND ND 1,223 ND ND ND ND <.19 --

Methylene chloride 1,207 6.1 4.6 2,398 .67 .21 ND ND <.20 .029

Perchloroethene 1,179 11.0 6.5 2,371 2.0 .63 0.21 0.17 <.20 .058

n-Propylbenzene 1,208 .25 .25 1,932 .052 ND ND ND <.050 .061

1,2,4-Trichlorobenzene 1,208 ND ND 1,939 ND ND ND ND <.20 --

1,1,1-Trichloroethane 1,208 8.5 4.4 2,401 1.4 .21 .042 .042 <.020 .029

1,1,2-Trichloroethane 1,208 .083 .083 2,156 ND ND ND ND <.10 .028

Trichloroethene 1,207 3.4 2.6 2,400 .92 .46 .25 .21 <.20 .14

Trihalomethanes

Bromodichloromethane 1,207 2.8 2.3 2,400 0.58 0.25 0.083 ND <0.19 0.071

Bromoform 1,206 1.8 .66 2,399 .33 .13 .042 ND <.20 .010

Chloroform 1,207 25.6 18.0 2,400 5.2 1.7 .37 0.17 <.052 .059

Dibromochloromethane 1,207 1.1 1.1 2,400 .50 .17 .042 .042 <.20 .30

Total trihalomethanes5 1,207 26.5 18.0 2,400 5.3 1.8 .42 .21 <.20 .062

1

These detection frequencies are for the subset of samples that were analyzed with the U.S. Geological Survey’s low-level method 0–4127–96. At this 

assessment level, detection frequencies are estimates.(19)

2

These detection frequencies are for all samples included in this assessment, regardless of the analytical method.

3

The analytical methods used for this assessment have varied sensitivity among compounds, and comparison of the median concentrations between compounds is not appropriate. No assessment level was applied to determine the median.

4

Considered as 2 of the 55 compounds included in this assessment.

5

Not considered as 1 of the 55 compounds included in this assessment.

Appendix 8. Detection frequencies and median concentrations for selected volatile organic compounds in samples from domestic 

wells.—Continued

[μg/L, micrograms per liter; ND, compound not detected; <, less than; --, not applicable]
96   

Appendix 9.  Summary of concentrations of selected volatile organic compounds in domestic well samples in comparison to

U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) for regulated compounds and to HealthBased Screening Levels (HBSLs) for unregulated compounds, and concentrations within one order of magnitude of MCLs and

HBSLs.

[μg/L, micrograms per liter; --, no MCL established or HBSL reported]

Compound name

Number

of

samples

Maximum Contaminant Level Health-Based Screening Level

MCLs

(μg/L)

Number of

concentrations

greater than

the MCL

Number of

concentrations within

one order of

magnitude1

HBSLs

(μg/L)

Number of

concentrations

greater than

the HBSL

Number of

concentrations within

one order of

magnitude2

Fumigants

Bromomethane 2,156 -- -- -- 3, 4100 0 0

Dibromochloropropane 1,962 0.2 14 1 -- -- --

1,4-Dichlorobenzene 2,399 75 0 0 -- -- --

1,2-Dichloropropane 2,400 5 3 6 -- -- --

cis-1,3-Dichloropropene 1,208 -- -- -- 4, 5, 6.3 0 0

trans-1,3-Dichloropropene 1,207 -- -- -- 4, 5, 6.3 0 0

Total 1,3-dichloropropenes7 1,208 -- -- -- 4, 5.3 0 0

Ethylene dibromide 2,085 .05 3 0 -- -- --

1,2,3-Trichloropropane 2,092 -- -- -- 340 0 0

Gasoline hydrocarbons

Benzene 2,401 5 0 2 -- -- --

Ethylbenzene 2,401 700 0 0 -- -- --

Isopropylbenzene 1,932 -- -- -- 3700 0 0

Naphthalene 1,939 -- -- -- 3100 0 0

Styrene 2,395 100 0 0 -- -- --

Toluene 2,386 1,000 0 0 -- -- --

o-Xylene 1,214 8

10,000 0 0 -- -- --

m- and p-Xylene9 1,208 8

10,000 0 0 -- -- --

Total xylenes7 2,388 10,000 0 0 -- -- --

Organic synthesis compounds

Acrolein 445 -- -- -- 34 0 0

Acrylonitrile 1,220 -- -- -- 5.06 0 0

1,1-Dichloroethene 2,400 7 1 3 -- -- --

Hexachlorobutadiene 1,939 -- -- -- 31 0 0

Vinyl chloride 2,401 2 0 2 -- -- --

Refrigerants

Dichlorodifluoromethane 2,401 -- -- -- 31,000 0 0

Trichlorofluoromethane 2,401 -- -- -- 32,000 0 0

Trichlorotrifluoroethane 2,083 -- -- -- 3200,000 0 0
    97

Appendixes

Appendix 9.  Summary of concentrations of selected volatile organic compounds in domestic well samples in comparison to

U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) for regulated compounds and to HealthBased Screening Levels (HBSLs) for unregulated compounds, and concentrations within one order of magnitude of MCLs and

HBSLs.—Continued

[μg/L, micrograms per liter; --, no MCL established or HBSL reported]

Compound name

Number

of

samples

Maximum Contaminant Level Health-Based Screening Level

MCLs

(μg/L)

Number of

concentrations

greater than

the MCL

Number of

concentrations within

one order of

magnitude1

HBSLs

(μg/L)

Number of

concentrations

greater than

the HBSL

Number of

concentrations within

one order of

magnitude2

Solvents

Carbon tetrachloride 2,400 5 0 3 -- -- --

Chlorobenzene 2,401 100 0 0 -- -- --

Chloromethane 2,059 -- -- -- 330 0 0

1,2-Dichlorobenzene 2,391 600 0 0 -- -- --

1,3-Dichlorobenzene 1,894 -- -- -- 3600 0 0

1,2-Dichloroethane 2,383 5 0 3 -- -- --

cis-1,2-Dichloroethene 2,177 70 0 0 -- -- --

trans-1,2-Dichloroethene 2,241 100 0 0 -- -- --

Hexachloroethane 1,223 -- -- -- 10.7 0 0

Methylene chloride 2,398 5 0 9 -- -- --

Perchloroethene 2,371 5 5 17 -- -- --

1,2,4-Trichlorobenzene 1,939 70 0 0 -- -- --

1,1,1-Trichloroethane 2,401 200 0 1 -- -- --

1,1,2-Trichloroethane 2,156 5 0 0 -- -- --

Trichloroethene 2,400 5 6 9 -- -- --

Trihalomethanes

Bromodichloromethane 2,400 1180 0 0 -- -- --

Bromoform 2,399 1180 0 1 -- -- --

Chloroform 2,400 1180 0 4 -- -- --

Dibromochloromethane 2,400 1180 0 1 -- -- --

Total trihalomethanes7 2,400 80 0 5 -- -- --

1

Includes the number of concentrations equal to and less than the MCL that are within one order of magnitude of the MCL.

2

Includes the number of concentrations equal to and less than the HBSL that are within one order of magnitude of the HBSL.

3

Value calculated using USEPA’s formula for Lifetime Health Advisory.

4

Value is provisional and is under discussion with USEPA.

5

The HBSL is a range that is based on USEPA’s risk-specific dose at 10-6 to 10-4 cancer risk level. The lowest value of this range was used in this assessment.

6

The HBSL is for total 1,3-dichloropropene mixed isomers (cis- and trans-1,3-dichloropropene).

7

Not considered as 1 of the 55 compounds included in this assessment.

8

The MCL is for total xylenes (o-, m-, and p-xylene).

9

Considered as 2 of the 55 compounds included in this assessment.

10The HBSL was calculated using USEPA’s Lifetime Health Advisory for Class C carcinogens.

11The MCL is for total trihalomethanes.
98   

Compound name

Number

of

samples

Detection frequency at selected

assessment levels1 (percent)

Median concentration2

(μg/L)

0.2 μg/L 1 μg/L 5 μg/L 10 μg/L All samples

Samples

with

detections

Fumigants

Bromomethane 1,078 0.093 0.093 0.093 ND <0.17 6.4

Dibromochloropropane 378 ND ND ND ND <.5 --

1,4-Dichlorobenzene 1,067 .094 ND ND ND <.066 .025

1,2-Dichloropropane 1,078 .74 .19 ND ND <.056 .17

cis-1,3-Dichloropropene 1,078 ND ND ND ND <.048 --

trans-1,3-Dichloropropene 1,078 ND ND ND ND <.052 --

Ethylene dibromide 462 ND ND ND ND <.10 --

1,2,3-Trichloropropane 997 .80 .30 ND ND <.054 .70

Gasoline hydrocarbons

Benzene 1,095 0.46 0.27 ND ND < 0.058 0.13

n-Butylbenzene 950 .11 ND ND ND <.094 .21

Ethylbenzene 1,083 .46 .18 ND ND <.066 .32

Isopropylbenzene 944 .11 ND ND ND <.080 .19

Naphthalene 962 .10 ND ND ND <.11 .22

Styrene 1,074 .19 ND ND ND <.052 .13

Toluene 1,077 1.0 .46 .19 ND <.050 .040

1,2,4-Trimethylbenzene 938 .32 ND ND ND <.044 .018

o-Xylene 830 .48 ND ND ND <.056 .31

m- and p-Xylenes3 828 .60 .12 ND ND <.13 .38

Total xylenes4 1,069 .56 .19 ND ND <.060 .44

Gasoline oxygenates

tert-Amyl methyl ether 818 0.49 ND ND ND < 0.050 0.23

Diisopropyl ether 807 .37 0.12 ND ND <.15 .16

Ethyl tert-butyl ether 818 .12 ND ND ND <.068 .14

Methyl tert-butyl ether 913 5.4 1.6 0.11 ND <.078 .26

Organic synthesis compounds

Acrolein 126 ND ND ND ND < 2.0 --

Acrylonitrile 837 ND ND ND ND <.20 --

1,1-Dichloroethene 1,096 1.3 0.46 0.18 0.18 <.16 0.20

Hexachlorobutadiene 962 ND ND ND ND <.17 .030

1,2,3-Trichlorobenzene 950 ND ND ND ND <.084 .020

Vinyl bromide 818 ND ND ND ND <.17 --

Vinyl chloride 1,096 .18 .18 ND ND <.16 3.2

Appendix 10. Detection frequencies and median concentrations for selected volatile organic compounds in samples from public

wells.

[μg/L, micrograms per liter; ND, compound not detected; <, less than; --, not applicable]
    99

Appendixes

Compound name

Number

of

samples

Detection frequency at selected

assessment levels1 (percent)

Median concentration2

(μg/L)

0.2 μg/L 1 μg/L 5 μg/L 10 μg/L All samples

Samples

with

detections

Refrigerants

Dichlorodifluoromethane 1,096 1.7 0.64 0.18 0.091 < 0.24 0.22

Trichlorofluoromethane 1,096 1.1 .27 .091 .091 <.19 .25

Trichlorotrifluoroethane 931 .32 ND ND ND <.20 .094

Solvents

Carbon tetrachloride 1,096 0.73 0.27 ND ND < 0.098 0.38

Chlorobenzene 1,096 .18 .091 ND ND <.064 .31

Chloroethane 1,078 .28 .28 ND ND <.19 2.2

Chloromethane 1,045 .38 ND ND ND <.21 .070

1,2-Dichlorobenzene 1,087 .18 ND ND ND <.074 .15

1,3-Dichlorobenzene 913 ND ND ND ND <.058 .008

1,1-Dichloroethane 1,096 2.0 .46 0.18 0.091 <.072 .22

1,2-Dichloroethane 1,073 .56 .093 ND ND <.058 .39

cis-1,2-Dichloroethene 969 1.5 .62 .10 .10 <.048 .18

trans-1,2-Dichloroethene 1,050 1.0 .57 .095 ND <.080 .60

Hexachloroethane 829 ND ND ND ND <.17 --

Methylene chloride 1,094 .46 .37 .18 .18 <.042 1.6

Perchloroethene 1,093 5.3 2.3 .82 .46 <.098 .20

n-Propylbenzene 950 ND ND ND ND <.086 --

1,2,4-Trichlorobenzene 962 ND ND ND ND <.084 .020

1,1,1-Trichloroethane 1,095 2.2 1.1 .46 .18 <.090 .17

1,1,2-Trichloroethane 1,078 ND ND ND ND <.028 .018

Trichloroethene 1,093 4.3 2.2 .82 .37 <.068 .52

Trihalomethanes

Bromodichloromethane 1,095 4.2 1.3 0.46 0.091 <0.036 0.23

Bromoform 1,096 4.5 1.6 .27 .091 <.044 .50

Chloroform 1,092 11.4 3.1 .82 .46 <.050 .20

Dibromochloromethane 1,095 4.4 1.5 .18 .091 <.040 .43

Total trihalomethanes4 1,096 14.8 5.3 1.7 .82 <.048 .30

1

These detection frequencies are for all samples included in this assessment, regardless of the analytical method.

2

The analytical methods used for this assessment have varied sensitivity among compounds and, as such, comparison of the median concentrations 

between compounds is not appropriate. No assessment level was applied to determine the median.

3

Considered as 2 of the 55 compounds included in this assessment.

4

Not considered as 1 of the 55 compounds included in this assessment.

Appendix 10. Detection frequencies and median concentrations for selected volatile organic compounds in samples from public

wells.—Continued

[μg/L, micrograms per liter; ND, compound not detected; <, less than; --, not applicable]
100   

Compound name

Number

of

samples

Maximum Contaminant Level Health-Based Screening Level

MCLs

(μg/L)

Number of

concentrations

greater than

the MCL

Number of

concentrations within

one order of

magnitude1

HBSLs

(μg/L)

Number

of concentrations greater

than the HBSL

Number of

concentrations within

one order of

magnitude2

Fumigants

Bromomethane 1,078 -- -- -- 3, 4100 0 0

Dibromochloropropane 378 0.2 0 0 -- -- --

1,4-Dichlorobenzene 1,067 75 0 0 -- -- --

1,2-Dichloropropane 1,078 5 0 2 -- -- --

cis-1,3-Dichloropropene 1,078 -- -- -- 4, 5, 6.3 0 0

trans-1,3-Dichloropropene 1,078 -- -- -- 4, 5, 6.3 0 0

Total 1,3-dichloropropenes7 1,078 -- -- -- 4, 5.3 0 0

Ethylene dibromide 462 .05 0 0 -- -- --

1,2,3-Trichloropropane 997 -- -- -- 340 0 0

Gasoline hydrocarbons

Benzene 1,095 5 0 5 -- -- --

Ethylbenzene 1,083 700 0 0 -- -- --

Isopropylbenzene 944 -- -- -- 3700 0 0

Naphthalene 962 -- -- -- 3100 0 0

Styrene 1,074 100 0 0 -- -- --

Toluene 1,077 1,000 0 0 -- -- --

o-Xylene 830 8

10,000 0 0 -- -- --

m- and p-Xylene9 828 8

10,000 0 0 -- -- --

Total xylenes7 1,069 10,000 0 0 -- -- --

Organic synthesis compounds

Acrolein 126 -- -- -- 34 0 0

Acrylonitrile 837 -- -- -- 5.06 0 0

1,1-Dichloroethene 1,096 7 2 6 -- -- --

Hexachlorobutadiene 962 -- -- -- 31 0 0

Vinyl chloride 1,096 2 2 0 -- -- --

Refrigerants

Dichlorodifluoromethane 1,096 -- -- -- 31,000 0 0

Trichlorofluoromethane 1,096 -- -- -- 32,000 0 0

Trichlorotrifluoroethane 931 -- -- -- 3200,000 0 0

Appendix 11. Summary of concentrations of selected volatile organic compounds in public wells in comparison to U.S. Environmental 

Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) for regulated compounds and to Health-Based Screening Levels 

(HBSLs) for unregulated compounds, and concentrations within one order of magnitude of MCLs and HBSLs.

[μg/L, microgram per liter; --, no MCL established or HBSL reported]
    101

Appendixes

Compound name

Number

of

samples

Maximum Contaminant Level Health-Based Screening Level

MCLs

(μg/L)

Number of

concentrations

greater than

the MCL

Number of

concentrations within

one order of

magnitude1

HBSLs

(μg/L)

Number

of concentrations greater

than the HBSL

Number of

concentrations within

one order of

magnitude2

Solvents

Carbon tetrachloride 1,096 5 0 5 -- -- --

Chlorobenzene 1,096 100 0 0 -- -- --

Chloromethane 1,045 -- -- -- 330 0 0

1,2-Dichlorobenzene 1,087 600 0 0 -- -- --

1,3-Dichlorobenzene 913 -- -- -- 3600 0 0

1,2-Dichloroethane 1,073 5 0 2 -- -- --

cis-1,2-Dichloroethene 969 70 0 1 -- -- --

trans-1,2-Dichloroethene 1,050 100 0 0 -- -- --

Hexachloroethane 829 -- -- -- 10.7 0 0

Methylene chloride 1,094 5 2 2 -- -- --

Perchloroethene 1,093 5 9 26 -- -- --

1,2,4-Trichlorobenzene 962 70 0 0 -- -- --

1,1,1-Trichloroethane 1,095 200 0 0 -- -- --

1,1,2-Trichloroethane 1,078 5 0 0 -- -- --

Trichloroethene 1,093 5 9 27 -- -- --

Trihalomethanes

Bromodichloromethane 1,095 1180 0 1 -- -- --

Bromoform 1,096 1180 0 1 -- -- --

Chloroform 1,092 1180 0 6 -- -- --

Dibromochloromethane 1,095 1180 0 2 -- -- --

Total trihalomethanes7 1,096 80 0 10 -- -- --

1

Includes the number of concentrations equal to and less than the MCL that are within one order of magnitude of the MCL.

2

Includes the number of concentrations equal to and less than the HBSL that are within one order of magnitude of the HBSL.

3

Value calculated using USEPA’s formula for Lifetime Health Advisory.

4

Value is provisional and is under discussion with USEPA.

5

The HBSL is a range that is based on USEPA’s risk-specific dose at 10-6 to 10-4 cancer risk level. The lowest value of this range was used in this assessment.

6

The HBSL is for total 1,3-dichloropropene mixed isomers (cis- and trans-1,3-dichloropropene).

7

Not considered as 1 of the 55 compounds included in this assessment.

8

The MCL is for total xylenes (o-, m-, and p-xylene).

9

Considered as 2 of the 55 compounds included in this assessment.

10The HBSL was calculated using USEPA’s Lifetime Health Advisory for Class C carcinogens.

11The MCL is for total trihalomethanes.

Appendix 11. Summary of concentrations of selected volatile organic compounds in public wells in comparison to U.S. Environmental 

Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) for regulated compounds and to Health-Based Screening Levels 

(HBSLs) for unregulated compounds, and concentrations within one order of magnitude of MCLs and HBSLs.—Continued

[μg/L, microgram per liter; --, no MCL established or HBSL reported]

Acknowledgments

PROJECT MANAGER

John S. Zogorski

WRITERS

Janet M. Carter

Tamara Ivahnenko

Wayne W. Lapham

Michael J. Moran

Barbara L. Rowe

Paul J. Squillace

Patricia L. Toccalino

John S. Zogorski

GRAPHIC DESIGN

Ella M. Decker

Connie J. Ross

Barbara L. Rowe

REPORT PREPARATION

Janet M. Carter, Reports Specialist

Sandra C. Cooper, Approving Official

Ella M. Decker, Publications Assistant

John Evans, Visual Information Specialist

Connie J. Ross, Illustrator

Chester Zenone, Pre-approval Reviewer

RESEARCH ASSISTANCE

David A. Bender

Rika Lashley

Hopa Yellow Horse

Jon Scott

Joshua F. Valder

GEOGRAPHICAL INFORMATION 

SPECIALIST

Curtis V. Price

TECHNICAL PEER REVIEWERS

Neil M. Dubrovsky

Stephen J. Grady

James F. Pankow

Anthony J. Tesoriero

USEPA READERS

Veronica Blette

Thomas R. Carpenter

Ann M. Codrington

Joyce M. Donohue

Irene S. Dooley

Elizabeth A. Doyle

Marilyn H. Ginsberg

Wynne F. Miller

Amy Mills

Edward V. Ohanian

Susan Richardson

OTHER EXTERNAL READERS

A. Scott Andres,

Delaware Geological Survey

Judy Campbell Bird,

Consultant

Denise D. Fort,

University of New Mexico

Adam Harris,

California State Water Resources Control 

Board

Robert H. Johnson,

Center for Disease Control and 

Prevention

Julia E. Norman,

Oregon Health & Science University

Timothy Parker,

California Department of Water 

Resources

USGS READERS

William M. Alley

Leslie A. DeSimone

Michael J. Focazio

Robert J. Gilliom

Pixie A. Hamilton

Donna N. Myers

Gary L. Rowe, Jr.

William G. Wilber

SUPPORTING AGENCIES

The cooperation of water utilities, State 

agencies, and American Water Works Association Research Foundation to develop 

VOC data sets is greatly appreciated.

FOR ADDITIONAL INFORMATION 

ON THE NAWQA PROGRAM:

Chief, NAWQA Program

U.S. Geological Survey

413 National Center

Reston, VA 20192

Phone: (703) 648-5012

Fax: (703) 648-6693

Email: nawqa_whq@usgs.gov 

World Wide Web: http://water.usgs.gov/

nawqa

FOR ADDITIONAL INFORMATION 

ABOUT THIS NATIONAL 

ASSESSMENT:

Project Chief, VOC National Synthesis

U.S. Geological Survey

1608 Mt. View Road

Rapid City, SD 57702

Phone: (605) 394-3214

Email: jszogors@usgs.gov

World Wide Web: http://water.usgs.gov/

nawqa/vocs/national_assessment
Zogorski and others—Volatile Organic Compounds in the Nation’s Ground Water and Drinking-Water Supply Wells—Circular 1292

0RINTEDONRECYCLEDPAPER