U.S. Department of the Interior

U.S. Geological Survey

Occurrence and Status of Volatile Organic 

Compounds in Ground Water from Rural, 

Untreated, Self-Supplied Domestic Wells

in the United States, 1986-99

Water-Resources Investigations Report 02-4085

By Michael J. Moran, Wayne W. Lapham, Barbara L. Rowe, and John S. Zogorski
U.S. Department of the Interior

GALE A. NORTON, Secretary

U.S. Geological Survey

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and 

does not constitute endorsement by the U.S. Government.

Rapid City, South Dakota: 2002

For additional information write to:

District Chief

U.S. Geological Survey

1608 Mt. View Road

Rapid City, SD 57702

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286
FOREWORD

Foreword III

The U.S. Geological Survey (USGS) is 

committed to serve 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. 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 longterm availability of water that is clean and safe for 

drinking and recreation and that is suifor 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 WaterQuality Assessment (NAWQA) Program 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. NAWQA results can 

contribute to informed decisions that result in practical 

and effective water-resource 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. 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 waterquality 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 assessments on pesticides, nutrients, volatile organic compounds, trace 

metals, and aquatic ecology are developed at the 

national scale through comparative analysis of the 

Study-Unit findings. 

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 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 costeffective 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

Contents V

CONTENTS

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

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

Purpose and Scope................................................................................................................................................................. 3

Previous Investigations ................................................................................................................................................ 4

Maine................................................................................................................................................................. 5

Wisconsin .......................................................................................................................................................... 5

Study Approach ........................................................................................................................................................... 5

NAWQA Ground-Water Occurrence-Assessment Studies................................................................................ 6

Local, State, and Other Federal Water-Quality Monitoring Programs.............................................................. 8

Selection of VOC Data ...................................................................................................................................... 9

Statistical Methods in Data Analysis................................................................................................................. 11

Acknowledgments ....................................................................................................................................................... 12

Occurrence and Status........................................................................................................................................................... 12

Detections of at Least One VOC at an Assessment Level of 0.2 microgram per liter ................................................ 14

All Samples........................................................................................................................................................ 14

Samples that Contained VOCs .......................................................................................................................... 15

Areal Distribution of VOC Detections, Nationally and by State....................................................................... 15

Detections and Geographic Distribution of VOCs by Group at an Assessment Level of

0.2 microgram per liter .................................................................................................................................. 17

Detections of Individual VOCs at an Assessment Level of 0.2 microgram per liter................................................... 24

Status of VOC Concentrations..................................................................................................................................... 32

Detections of VOCs Using No Assessment Level....................................................................................................... 33

Detections of Mixtures of VOCs Using No Assessment Level................................................................................... 39

Potential Sources of VOCs and Factors Related to VOC Occurrence......................................................................... 41

Potential Sources of VOCs in Ground Water from Domestic Wells ................................................................. 41

Relations of VOCs to Hydrogeologic and Anthropogenic Variables ................................................................ 43

Fumigants ................................................................................................................................................ 43

Gasoline Hydrocarbons ........................................................................................................................... 46

Organic Synthesis .................................................................................................................................... 46

Gasoline Oxygenates ............................................................................................................................... 46

Refrigerants ............................................................................................................................................. 46

Solvents ................................................................................................................................................... 46

Trihalomethanes ...................................................................................................................................... 47

Summary and Conclusions .................................................................................................................................................... 47

References Cited.................................................................................................................................................................... 48

FIGURES

1. Map showing location of rural, untreated, self-supplied domestic wells that were sampled

for this study, and the location of wells where at least one volatile organic compound was

detected at an assessment level of 0.2 microgram per liter. ..................................................................................... 14

2. Graph showing detection frequency versus number of volatile organic compounds detected

at an assessment level of 0.2 microgram per liter..................................................................................................... 17

3. Graph showing detection frequency of volatile organic compound groups at an assessment

level of 0.2 microgram per liter................................................................................................................................ 19
VI Contents

FIGURES—Continued

4-10. Maps showing:

4. Locations where rural, untreated, self-supplied domestic well water was sampled

and where at least one trihalomethane was detected. ...................................................................................... 20

5. Locations where rural, untreated, self-supplied domestic well water was sampled

and where a least one solvent was detected..................................................................................................... 21

6. Locations where rural, untreated, self-supplied domestic well water was sampled

and where at least one refrigerant was detected .............................................................................................. 22

7. Locations where rural, untreated, self-supplied domestic well water was sampled

and where at least one gasoline hydrocarbon was detected............................................................................. 23

8. Locations (A) where rural, untreated, self-supplied domestic well water was sampled

and where at least one gasoline oxygenate was detected and (B) of Federal oxygenated

and reformulated gasoline program areas and locations ................................................................................. 25

9. Locations where rural, untreated, self-supplied domestic well water was sampled and

where at least one volatile organic compound used for organic synthesis was detected................................. 26

10. Locations where rural, untreated, self-supplied domestic well water was sampled and

where at least one fumigant was detected ....................................................................................................... 27

11. Bar chart showing detection frequencies of 54 volatile organic compounds at an assessment

level of 0.2 microgram per liter ............................................................................................................................... 31

12. Graph showing concentrations of volatile organic compounds detected at an assessment level

of 0.2 microgram per liter........................................................................................................................................ 34

13. Graph showing detection frequency of one or more volatile organic compounds versus

assessment level for samples from this study .......................................................................................................... 36

14. Graph showing detection frequency versus number of volatile organic compounds detected

using no assessment level ........................................................................................................................................ 39

TABLES

1. Ground-water withdrawals in the United States in 1995 by water-use category.................................................... 2

2. Domestic freshwater use in the United States from ground water in 1995 by water-supply category................... 2

3. General criteria for well-network design, well construction, and data selection used when

compiling retrospective data from local, State, and other Federal agencies........................................................... 8

4. Criteria used to select VOC data for analysis ......................................................................................................... 9

5. General characteristics of the NAWQA and retrospective data used for analysis in this report ............................ 11

6. Grouping of VOCs by predominant use ................................................................................................................. 13

7. Percent detections of at least one volatile organic compound in untreated water from 1,926 wells

at an assessment level of 0.2 microgram per liter................................................................................................... 15

8. Distribution of samples, by State, with a detection of at least one volatile organic compound

at an assessment level of 0.2 microgram per liter................................................................................................... 16

9. Summary of volatile organic compound detections, by group, at an assessment level

of 0.2 microgram per liter....................................................................................................................................... 18

10. Detections of individual volatile organic compounds at an assessment level of 0.2 microgram per liter.............. 28

11. Summary of detections of individual volatile organic compounds at an assessment level

of 0.2 microgram per liter....................................................................................................................................... 32

12. Frequencies of detection of individual volatile organic compounds at various assessment levels......................... 37

13. Top 10 most frequently occurring mixtures of volatile organic compounds using no assessment level................ 40

14. Consumer usage of 18 volatile organic compounds not detected and 7 volatile organic compounds

most frequently detected in rural, untreated, self-supplied domestic well water ................................................... 44
Contents VII

CONVERSION FACTORS, ABBREVIATIONS, AND ACRONYMS

Multiply By To obtain

acre 4,047 square meter

acre 0.4047 hectare

square mile (mi2

) 259.0 hectare

square mile (mi2

) 2.590 square kilometer 

gallons (gal) 3.785 liters

gallon per day (gal/d) 0.003785 cubic meter per day

million gallons per day (Mgal/d) 0.04381 cubic meter per second

ounce (oz) 28.35 grams

μg/L micrograms per liter

km2 square kilometers

mL milliliter

g/cm3 gram per cubic centimeter

AST Aboveground Storage Tank

BTEX Benzene, Toluene, Ethylbenzene, and Xylenes

CAA Clean Air Act

CWS Community Water System

DWA Drinking Water Advisory

GC/MS Gas Chromatography/Mass Spectrometry

GWSI U.S. Geological Survey Ground-Water Site Inventory

HA Health Advisory

MCL Maximum Contaminant Level

MCLG Maximum Contaminant Level Goal

NAAQS National Ambient Air Quality Standards

NAWQA U.S. Geological Survey National Water-Quality Assessment Program

NTNCWS Non-Transient Non-Community Water System

NWIS U.S. Geological Survey National Water Information System

NWQL U.S. Geological Survey National Water-Quality Laboratory

OXY Oxygenated Gasoline

QA Quality Assurance

QC Quality Control

PWS Public Water System

RFG Reformulated Gasoline

SDWA Safe Drinking Water Act

THMs Trihalomethanes (bromodichloromethane, chlorodibromomethane, 

tribromomethane, trichloromethane)

TRI U.S. Environmental Protection Agency Toxics Release Inventory

USGS U.S. Geological Survey

USEPA U.S. Environmental Protection Agency

UST Underground Storage Tank

VOCs Volatile Organic Compounds
VIII Contents

COMMON SYNONYMS AND ABBREVIATIONS OF SELECTED VOLATILE ORGANIC COMPOUNDS

IUPAC Name Common Synonym or Abbreviation

(1-Methylethyl)benzene Isopropylbenzene

1,1,1-Trichloroethane Methyl chloroform

1,1,2-Trichloro-1,2,2-trifluoroethane Freon 113, CFC 113

1,1-Dichloroethane Ethylidene dichloride

1,2-Dibromo-3-chloropropane DBCP

1,2-Dibromoethane EDB

1,2-Dichlorobenzene o-Dichlorobenzene

1,2-Dichloroethane Ethylene dichloride

1,2-Dimethylbenzene o-Xylene

1,3-Dichlorobenzene m-Dichlorobenzene

1,3-Dimethylbenzene m-Xylene

1,4-Dichlorobenzene p-Dichlorobenzene

1,4-Dimethylbenzene p-Xylene

2-Propenal Acrolein

2-Propenitrile Acrylonitrile

Bromoethene Vinyl bromide

Bromomethane Methyl bromide

Chloroethane Ethyl chloride

Chloroethene Vinyl chloride

Chloromethane Methyl chloride

Dichlorodifluoromethane Freon 12, CFC 12

Dichloromethane Methylene chloride

Diisopropyl ether DIPE

Ethenylbenzene Styrene

Ethyl tert-butyl ether ETBE

Methyl tert-butyl ether MTBE

Methylbenzene Toluene

tert-Amyl methyl ether TAME

Tetrachloroethene PCE

Tetrachloromethane Carbon tetrachloride

Tribromomethane Bromoform

Trichloroethene TCE

Trichlorofluoromethane Freon 11, CFC 11 

Trichloromethane Chloroform
Contents IX

DEFINITIONS OF SELECTED TERMS USED IN THIS REPORT 

Term Definition

Assessment level A level of censoring applied to water-quality data that have variable laboratory 

reporting levels either between subsets of VOC analytes or between individual 

VOC analytes. The assessment level is applied to data received from the 

laboratory and is applied subsequent to the laboratory reporting level. The 

primary purpose of the assessment level is for accurate comparison of detection 

frequencies between subsets of VOC analytes or between individual VOC 

analytes. 

Domestic well A self-supplied ground-water source for household water.

Domestic well water Untreated ground water collected from rural, self-supplied domestic wells at the 

wellhead.

Drinking-water criteria Drinking-water-quality measure that is not enforceable.

Drinking-water standard Drinking-water-quality measure that is enforceable.

Finished drinking water Water after treatment that might have been blended from multiple untreated water 

sources.

Laboratory reporting level A level of reporting concentrations of VOCs that is set by the laboratory to 

minimize the rate of false positives and false negatives. Concentration below 

the laboratory reporting level are denoted by a “<” preceding a concentration 

value.

Occurrence The presence or absence, frequencies of detection, concentrations, and ranges of 

concentrations of VOCs and the locations (areal patterns) of VOC detections in 

ground water.

Rural areas Areas that have a population density of less than 386 persons/km2

 (1,000 

persons/mi2

 or 1.56 persons/acre). 

Self-supplied water Water withdrawn from a source by a user rather than being obtained from a public 

supply.

Status Comparison of VOC concentrations relative to drinking-water standards, criteria, 

and taste and odor thresholds.

Target analyte One of 55 volatile organic compounds given emphasis in the U.S. Geological 

Survey’s National Water-Quality Assessment Program.

VOCs by group VOCs grouped by the primary use of each VOC.

Abstract 1

Occurrence and Status of Volatile Organic

Compounds in Ground Water from Rural,

Untreated, Self-Supplied Domestic Wells

in the United States, 1986-99

By Michael J. Moran, Wayne W. Lapham, Barbara L. Rowe, and John S. Zogorski

ABSTRACT

Samples of untreated ground water from 

1,926 rural, self-supplied domestic wells were 

analyzed for volatile organic compounds (VOCs) 

during 1986-99. This information was used to 

characterize the occurrence and status of VOCs in 

domestic well water. The samples were either 

collected as part of the U.S. Geological Survey’s 

National Water-Quality Assessment (NAWQA) 

Program occurrence-assessment studies or were 

compiled by NAWQA from existing ambient 

ground-water or source-water-quality monitoring 

programs conducted by local, State, and other 

Federal agencies. Water samples were collected at 

the wellhead prior to treatment or storage. In most 

samples, 55 target VOCs were analyzed, and 

occurrence and status information generally was 

computed at an assessment level of 0.2 μg/L 

(microgram per liter). 

At least one VOC was detected in 

12 percent of samples (232 samples) at an assessment level of 0.2 μg/L. This detection frequency is 

relatively low compared to the 26 percent detection frequency of at least one VOC in public supply wells sampled by NAWQA, and the difference 

may be due, in part, to the higher pumping rates, 

pumping stress factors, and larger contributing 

areas of public supply wells. Samples with detections of at least one VOC were collected from 

wells located in 31 of 39 States.

Solvents were the most frequently detected 

VOC group with detections in 4.6 percent of 

samples (89 samples) at an assessment level of 

0.2 μg/L. The geographic distribution of detections of some VOC groups, such as fumigants 

and oxygenates, relates to the use pattern of compounds in that group. With the exception of compounds used in organic synthesis, detection 

frequencies of VOCs by group are proportional to 

the average half-life of compounds in the group. 

When the organic synthesis group is excluded 

from the analysis, a good correlation exists 

between the detection frequency of VOCs by 

group and average half-life of compounds in the 

group. 

Individually, VOCs were not commonly 

detected at an assessment level of 0.2 μg/L, with 

the seven most frequently detected VOCs found in 

only 1 to 5 percent of samples. Mixtures (two or 

more compounds) were a common mode of occurrence for VOCs when no assessment level was 

applied, and mixtures occurred in one-half of all 

samples that contained at least one VOC. Only 

1.4 percent of samples (27 samples) had one or 

more VOC concentrations that exceeded a federally established drinking-water standard or health 

criterion. Only 0.1 percent of samples (2 samples) 

had one or more VOC concentrations that 

exceeded a taste/odor threshold.

Potential point sources of VOCs near 

domestic wells are numerous. Leaks from underground storage tanks and aboveground storage 

tanks that hold gasoline, diesel fuel, or heating oil 
2 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

have the potential to be major point sources of contaminants to domestic wells. Shock chlorination 

may be a source of trichloromethane and other 

trihalomethanes in some domestic wells. Septic 

systems are believed to be an important source of 

contaminants to domestic wells, but extensive 

research on this subject does not exist. VOCs 

frequently are ingredients in household products 

such as cleansers and insecticides, and some 

VOCs have been found in septic systems. 

INTRODUCTION

More than 76 billion gallons per day of ground 

water is withdrawn in the United States for various uses 

(table 1). Withdrawals for domestic (also called residential) uses that are self supplied account for about 

4.4 percent of total ground-water withdrawals. 

Domestic uses include water used for household purposes, such as drinking, food preparation, bathing, 

washing clothes and dishes, flushing toilets, and 

watering lawns and gardens (Solley and others, 1998). 

Self-supplied water is withdrawn from a source by the 

user rather than being obtained from a public supply. 

As of 2002, about 40 million people in the United 

States receive their household water from individual 

private wells, and the number increases every year 

(Job, 2002).

Table 1. Ground-water withdrawals in the United States in 

1995 by water-use category (from Solley and others, 1998)

[Mgal/d, million gallons per day]

Water-use category

Ground-water

withdrawal

(Mgal/d)

Percent of total 

ground-water

withdrawal

Irrigation 49,000 64.1

Public supply1 15,100 19.8

Industrial 4,090 5.4

Domestic (self supplied) 3,350 4.4

Livestock 2,260 3.0

Mining 1,070 1.4

Commercial 939 1.2

Thermoelectric 565 .7

Total 76,374 100

1Public-supply withdrawals are withdrawn by public and private 

water suppliers and delivered to users for a variety of uses, such as domestic, commercial, thermoelectric power, industrial, and public water supply.

Ground water is the source of drinking water for 

one-half of the Nation’s population and to virtually all 

people living in rural areas (U.S. Environmental 

Protection Agency, 1999a). Ground-water use for 

domestic purposes was approximately 26 billion 

gallons per day in 1995 (table 2). Of this use, about 

13 percent was self supplied and 87 percent was 

obtained from public suppliers. Approximately 42.4 

million people, or 16 percent of the U.S. population, 

relied on self-supplied water for domestic purposes in 

1995. Nearly all of this self-supplied water was 

obtained from ground water (Solley and others, 1998).

Although a large number of people in the United 

States rely on self-supplied well water for drinkingwater supply and other domestic water uses, the quality 

of water from these wells does not receive the same 

level of health scrutiny as water from public supplies. 

Public supplies are regulated by the U.S. Environmental Protection Agency (USEPA) and by State 

health or environmental departments. USEPA regulations require that public suppliers ensure that their 

water meets drinking-water standards, such as Maximum Contaminant Levels (MCLs), which are regulated under the Safe Drinking Water Act (SDWA) (U.S. 

Environmental Protection Agency, 1991, 2000a). This 

requirement is met by routinely testing the water for the 

presence of contaminants and, if needed, by treating the 

water to remove or reduce specific contaminants to 

levels that will not adversely affect human health (U.S. 

Environmental Protection Agency, 1999a). 

The MCLs established under the SDWA apply 

only to community water systems (CWSs) and nontransient non-community water systems (NTNCWSs) 

Table 2. Domestic freshwater use in the United States 

from ground water in 1995 by water-supply category (from 

Solley and others, 1998)

[Mgal/d, million gallons per day]

Water-supply

category

Use

(Mgal/d)

Use

(percent)

Population

served

(thousands)

Population

served

(percent)

Self supplied 3,350 13 42,400 16

Supplied from 

public 

suppliers

22,700 87 225,000 84

Total 26,050 100 267,400 100
Purpose and Scope 3

(U.S. Environmental Protection Agency, 1999b). A 

CWS is a public water system that serves at least 15 

service connections used by year-round residents or 

regularly serves at least 25 year-round residents. A 

NTNCWS is a public water system that serves at least 

25 of the same people over 6 months per year (U.S. 

Environmental Protection Agency, 1999b). This is in 

contrast to most self-supplied, domestic wells that 

generally serve only a single family (usually much less 

than 25 people). Because drinking-water quality from 

domestic wells is not regulated, owners of these wells 

are solely responsible for the quality of the water provided (U.S. Environmental Protection Agency, 1990a).

A primary objective of the U.S. Geological 

Survey’s (USGS) National Water-Quality Assessment 

(NAWQA) Program is to characterize the quality of 

ambient, untreated water in the United States. In its 

sampling of ambient ground water, the NAWQA 

Program has sampled ground water from many 

domestic wells. Data from NAWQA’s occurrenceassessment studies and from existing ambient groundwater or source-water-quality monitoring programs 

were analyzed for information on occurrence and status 

of VOCs in ground water from domestic wells.

Volatile organic compounds (VOCs) have chemical and physical properties that allow them to move 

freely between water and air (Bender and others, 1999). 

The USEPA has established MCLs in drinking water 

for many VOCs because of human health concerns such 

as short-term toxic effects and long-term chronic 

effects such as carcinogenicity (U.S. Environmental 

Protection Agency, 2000a). Because of the potential 

risk of some VOCs to human health, the occurrence of 

VOCs in water from self-supplied, domestic wells is of 

concern, and this concern is exacerbated by the widespread and extensive use of VOCs in households and 

the mobility and persistence of some VOCs in ground 

water.

VOCs are produced in large quantities for a multitude of uses and have been produced for a relatively 

long period of time. Products containing VOCs are used 

extensively in households (Bloeman and Burn, 1993; 

Bender and others, 1999) as discussed in detail later in 

this report. Some examples of household products that 

contain VOCs include cosmetics, room deodorizers, 

deodorants, fabric softeners, polishes, moth cakes, 

paint, adhesives, cleaning and polishing products, pesticides and fumigants, solvent degreasers, refrigerants, 

and fuels. VOCs probably have been used extensively 

in the past in domestic settings, and if these VOCs have 

sources to ground water near domestic wells the likelihood is high that VOCs will be found in ground 

water from domestic wells.

In addition to household use, considerable 

quantities of VOCs are produced and used in the 

United States in a variety of commercial and industrial 

applications, such as in fuels (gasoline, diesel fuel, 

etc.), dry cleaning, and the manufacture of plastics and 

pharmaceuticals. The production of synthetic organic 

chemicals, many of which are VOCs, increased by 

more than an order of magnitude between 1945 and 

1985 (Ashford and Miller, 1991). 

The production and use of VOCs in the United 

States results in considerable quantities of VOCs 

being released to the environment based on data 

reported in the USEPA Toxics Release Inventory 

(TRI) (U.S. Environmental Protection Agency, 1996, 

1998). The TRI provides information on the release of 

toxic chemicals from manufacturing facilities in the 

United States. During 1996 and 1998, for example, 10 

of the 20 chemicals with the largest total on-site and 

off-site releases to the environment were VOCs (U.S. 

Environmental Protection Agency, 1996, 1998). Total 

releases of these 10 chemicals exceeded 700 million 

pounds in both 1996 and 1998.

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 can be persistent, degrading little 

over a period of years, or decades. In addition, VOCs 

can have varying magnitudes of solubility and sorption, complicating their occurrence and transport in 

ground water. For example, methyl tert-butyl ether 

(MTBE), which has a high solubility in water and 

sorbs only weakly to soil and aquifer materials, also 

generally resists degradation in the anaerobic conditions usually found in ground water (Squillace and 

others, 1996). Thus, MTBE can persist in ground 

water and travel long distances through advection.

PURPOSE AND SCOPE

The purpose of this report is to provide information on the occurrence and status of VOCs at or above 

0.2 μg/L (microgram per liter) in samples of ground 

water from rural, self-supplied domestic wells 

collected by NAWQA during 1986-99, or existing 

data compiled by NAWQA. Occurrence information 
4 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

is given for: (1) all VOCs as a single group, (2) seven 

groups of VOCs, and (3) 55 individual VOCs. Information also is presented on occurrence and status of 

individual VOCs and mixtures of VOCs using no 

assessment level. Occurrence describes the presence or 

absence of VOCs, detection frequencies, ranges of concentrations, and areal patterns of occurrence. Status 

compares VOC concentrations to federally established 

drinking-water standards, health criteria, and taste/odor 

thresholds.

Previous Investigations

Several national surveys of VOCs in drinking 

water in the United States have been completed. However, no national surveys are known that specifically 

have investigated the occurrence and status of VOCs in 

self-supplied, domestic well water. 

The USEPA completed two national surveys 

between 1975 and 1981 to determine the occurrence of 

VOCs in finished drinking water from public supplies 

obtained from ground- and surface-water sources 

(Westrick and others, 1984; Westrick, 1990). These 

surveys did not address the quality of self-supplied, 

domestic well water. 

A national survey of the occurrence and status of 

VOCs in untreated, ambient ground water in the conterminous United States was completed by NAWQA in 

1999 using data collected during 1985-95 (Squillace 

and others, 1999). In that study, occurrence pertained to 

the incidence and location of VOCs in ground water, 

and status pertained to VOC concentrations relative to 

drinking-water standards and criteria. Ambient ground 

water was defined as ground water in areas where there 

were no known point sources of contamination prior 

to sampling. All VOC detection frequencies were 

reported at or above 0.2 μg/L, with the exception of 

1,2-dibromo-3-chloropropane, which was reported at 

or above 1.0 μg/L. A total of 2,948 wells were used for 

analysis, consisting of several different types: 192 

public-supply, 606 domestic, and 166 monitoring, with 

the remainder of the wells not having a type designation. The results of this analysis indicated that 

14 percent of ground water from wells in rural areas 

had a detection of at least one VOC.

In 1998, the Centers for Disease Control (CDC) 

completed a study of the quality of water from 

domestic wells in nine midwestern States (Centers for 

Disease Control, 1998). This study followed the Mississippi valley flood of 1993, and the CDC collected 

samples from 5,520 households with domestic wells. 

The samples were analyzed for bacteria, nitrate, and 

atrazine but no VOCs.

In 1999, the USEPA completed an analysis of the 

occurrence of contaminants in public water systems 

(PWSs) regulated under the SDWA (U.S. Environmental Protection Agency, 1999c). Finished drinking 

water was analyzed in this study. Compliancemonitoring data for the SDWA were the primary data 

source. Data from eight States, selected to represent the 

national range of hydrologic regimes and pollution 

potential, were used to develop a national cross section 

of contaminant occurrence. More than 25 percent of 

the U.S. population using public water supplies was 

represented by the eight States. The results of this study 

indicated that VOCs commonly were detected in public 

drinking-water systems but exceedances of MCLs were 

not common.

A multi-year national and ongoing survey of 

MTBE and other VOCs in sources of drinking water 

began in 1999 by the Metropolitan Water District of 

Southern California, the USGS, and the Oregon 

Graduate Institute (Ivahnenko and others, 2001). The 

objective of the survey was to provide information on 

the frequency of detection, concentrations, and distribution of MTBE, other ether oxygenates, ether oxygenate degradation by-products, and other VOCs in 

sources of drinking water in the United States. The 

approach included a literature review and sampling of 

source waters for CWSs. Both randomly selected and 

focused ground- and surface-water sources were 

selected for sampling. The random sampling selected 

CWSs from 52 geographical entities, including the 

50 States, Native American Lands, and Puerto Rico. 

Selection of CWSs was balanced to consider both 

ground- and surface-water sources. A total sample size 

of 954 CWSs provided information on VOC exposure 

from drinking water for an estimated 80 million people. 

Approximately 140 source waters were selected for a 

focused source-water sampling. Source waters selected 

for the focused survey were those known or suspected 

to be susceptible to gasoline contamination. Approximately 80 of these 140 source waters were groundwater sources. The results of this study indicated that 

VOCs were commonly detected in sources for CWSs 

but were more frequently detected in the focused 

sources compared to the randomly selected sources.

An assessment of the occurrence of MTBE and 

other VOCs in drinking water in the northeast and midAtlantic regions of the United States was completed in 

2000 (Grady and Casey, 1999, 2001; Moran and others, 
Purpose and Scope 5

2001). The assessment summarizes information on 

the quality of drinking water obtained from SDWA 

compliance-monitoring data for 2,110 randomly 

selected CWSs in the 12 States included in that study 

(Grady and Casey, 2001). Water-quality data included 

more than 21,000 chemical analyses of VOCs in 

drinking-water samples collected during 1993-98. The 

results of this study indicated that 39 percent of the 

2,110 CWSs reported a detection of one or more VOCs 

at or above 1.0 μg/L.

At least two States have conducted statewide 

studies and possibly other States have conducted 

studies of the occurrence and status of VOCs in selfsupplied, domestic well water. These statewide studies 

are summarized briefly below. 

Maine

In 1998, the State of Maine conducted a statewide, statistically based study of the occurrence of 

MTBE, benzene, toluene, ethylbenzene, and xylenes 

(BTEX) in drinking water (State of Maine, 1998). Sampled drinking-water supplies included 951 randomly 

selected household sources (919 wells, 23 springs, and 

9 lakes) collectively grouped together as wells in the 

report. Also sampled were 793 of the 830 regulated 

CWSs and NTNCWSs in the State. The laboratory 

reporting level for all compounds except total xylenes 

was 0.1 μg/L. The laboratory reporting level for total 

xylenes was 0.3 μg/L. MTBE was detected in 150 of 

the 951 household wells (15.8 percent). Concentrations 

of MTBE in 1.1 percent of the wells were greater than 

the State’s drinking-water standard of 35 parts per 

billion.

Extrapolated statewide, this percentage translates 

to an estimated 1,400 to 5,200 private wells in Maine 

with concentrations of MTBE that may exceed the 

drinking-water standard (State of Maine, 1998). MTBE 

concentrations were less than 1 part per billion in 

92.3 percent of the sampled household wells, and 

between 1 and 35 parts per billion in 6.6 percent of the 

wells. Compared to MTBE, other gasoline compounds 

were detected infrequently, and concentrations of 

BTEX compounds were well below drinking-water 

standards. Both the location of the water supply in areas 

of reformulated gasoline (RFG) use and high population density were associated with detectable concentrations of MTBE. Factors that were found not to be 

associated with MTBE detections included type of well 

or water supply and the proximity to gasoline storage 

tanks. 

Wisconsin

The State of Wisconsin, Department of Natural 

Resources, has been testing for VOCs in well water 

since 1982 and has completed sampling of all community wells, surface-water sources, and about 1,500 

non-community public and private wells (Wisconsin 

Department of Natural Resources, 1998). Of the 1,500 

non-community public and private wells tested, 355 

(or 24 percent) had at least one detection of a VOC. 

Many of the private wells were selected for sampling 

because they were considered to be especially vulnerable to contamination, were near other contaminated 

wells, or were delivering water with objectionable 

tastes or odors (Wisconsin Department of Natural 

Resources, 1998). Therefore, detections of VOCs 

might be higher than detections resulting from a 

random selection of private wells throughout 

Wisconsin.

Study Approach

An extensive and representative inventory of 

available water-quality data is required to determine 

the occurrence and status of VOCs in ground water 

from rural, self-supplied, domestic wells in the United 

States. The sources of data and the procedures and criteria used in selecting data for analysis are described 

in this section. The VOC data used for analysis in this 

report were compiled from two sources: (1) untreated 

water from rural, self-supplied domestic wells sampled by NAWQA personnel and water samples analyzed at the USGS National Water-Quality Laboratory 

(NWQL) between 1993 and 1999 as part of 

NAWQA’s occurrence-assessment studies; and 

(2) untreated water from rural, self-supplied, domestic 

wells collected and analyzed as part of ambient 

ground-water or source-water-quality monitoring programs conducted by local, State, and other Federal 

agencies between 1986 and 1995. In both data 

sources, water samples were collected at the wellhead 

prior to treatment or storage. Only wells in rural areas 

were selected in order to examine the quality of water 

in domestic wells. A rural area is defined as having a 

population density of less than 386 persons/km2

(1,000 persons/mi2

 or 1.56 persons/acre). Too few 

wells have been sampled to characterize the occurrence of VOCs in domestic wells in urban areas.
6 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

NAWQA Ground-Water Occurrence-Assessment 

Studies

In 1991, the NAWQA Program began full-scale 

implementation. The long-term goals of the NAWQA 

Program are to describe the status and trends in the 

quality of a large representative part of the surfacewater and ground-water resources of the United States 

and to provide an improved understanding of the primary natural and human factors that affect the quality 

of these resources. The NAWQA Program has two 

major operational components (Gilliom and others, 

1995): (1) hydrologic investigations of large river 

basins and aquifer systems, referred to as Study-Unit 

Investigations; and (2) national synthesis assessments 

that summarize results from NAWQA studies with 

information from other programs, agencies, and 

researchers to produce regional and national assessments for priority water-quality issues. National synthesis assessments of nutrients and pesticides began in 

1991, a synthesis of VOCs began in 1994, and syntheses of trace elements and ecology began in 1999. 

This report is a product of the VOC national synthesis 

assessment.

NAWQA ground-water occurrence-assessment 

studies, as part of the hydrologic investigations of 

Study Units, include a broad characterization of water 

quality in one or more aquifers underlying each Study 

Unit (Gilliom and others, 1995). Aquifers selected for 

this characterization generally are those with the largest 

withdrawals for water supply in each Study Unit. The 

characterization of water quality in each aquifer is 

achieved by sampling a network of spatially distributed, randomly selected existing wells throughout the 

aquifer. Each network consists of about 30 wells. 

Domestic wells are selected preferentially over other 

types of wells, if available, provided that the spatially 

distributed, randomly selected network design is maintained. In the NAWQA data used for analysis in this 

report, the majority of wells were sampled as a part of 

these types of assessments.

Study Units also conduct studies to determine the 

relation between shallow ground-water quality and 

overlying land use (Gilliom and others, 1995). Networks of wells within areas of specific types of land use 

(for example, agricultural or urban uses) and that are 

screened or open at or near the water table are sampled 

for these studies. In contrast to the studies of major 

aquifers, sites to be sampling for the land-use studies 

were required to have a minimum of 50 percent of the 

targeted land use within a 1⁄4-mile radius of the well. 

Most of the wells for these studies are monitoring wells 

installed by the NAWQA Study Units; however, some 

other types of wells, possibly including domestic wells, 

also might be sampled if the cost of installing monitoring wells is prohibitive. In the NAWQA data used 

for analysis in this report, some of the wells were 

sampled as part of the agricultural land-use studies.

Samples collected as part of the NAWQA Program consist of untreated water because the primary 

objective of NAWQA ground-water occurrence and 

status assessments is to characterize the quality of the 

ground-water resource, not the quality of treated or finished drinking water. To meet this objective, samples 

of finished water are not collected because the quality 

of finished drinking water may not be a valid measure 

of the quality of the ground-water resource. Finished 

water may be a blend of water from various sources, 

including both ground and surface water. Even if the 

source water is derived only from ground water, the 

quality of the finished water (after blending) might 

vary depending on which wells were being pumped and 

the relative rates of pumping from those wells 

(Squillace and others, 1999). In addition, VOC concentrations might increase or decrease during water treatment and distribution.

As of 2001, about 4,600 wells had been sampled 

by NAWQA Study Units as part of the studies 

described above. Details about network design, well 

selection, sampling protocols, and other data-collection 

requirements for NAWQA Study-Unit ground-water 

investigations are described in Scott (1990), Gilliom 

and others (1995), Koterba and others (1995), Lapham 

and others (1995), and Koterba (1998). A minimum 

amount of information about a well generally must be 

known and entered into the USGS National Water 

Information System (NWIS) Ground-Water Site Inventory (GWSI) database (Maddy and others, 1989) before 

a well is selected by NAWQA for water-quality sampling. In addition to the minimum requirements, other 

existing data for each well are compiled and entered 

into the GWSI database.

Sampling protocols for ground-water samples 

collected by NAWQA for VOC analysis consist of 

collecting an unfiltered sample in a 40-mL amber 

glass vial from each well after purging a minimum of 

three well volumes. Completion of well purging is 

determined from evaluation of stability of field 

measurements of water temperature, specific conductance, dissolved oxygen, pH, and alkalinity (Koterba 
Study Approach 7

and others, 1995). Samples are collected using 

polytetrafluoroethylene tubing connected to a spigot on 

each well as close as possible to the wellhead and prior 

to water treatment and pressure or holding tanks 

(Lapham and others, 1995). Samples are collected in a 

sample-collection chamber designed to isolate the 

sample from potential contamination by VOCs in the 

atmosphere and from other possible sources of VOCs, 

such as emissions from nearby motor vehicles and generators (Wilde and others, 1999). Each vial is allowed 

to overflow, preserved with one to five drops of 1:1 

hydrochloric acid to pH 2, and then capped leaving no 

headspace. If there was evidence of air bubbles in the 

vial after the sample was preserved and capped, the 

sample was discarded and a new sample was collected 

in a new vial.

All sampling equipment in contact with a water 

sample, including sampling tubes, is cleaned between 

sampled wells. Cleaning is accomplished by circulating 

a non-phosphate detergent in a tap water solution 

through the equipment, and then rinsing the equipment 

successively with tap water, methanol, and deionized 

water (Koterba and others, 1995). All equipment is 

wrapped in either aluminum foil or clear plastic for 

storage.

The method of analysis for VOCs in NAWQAcollected samples is purge and trap gas chromatography/mass spectrometry (GC/MS) (Rose and 

Schroeder, 1995; Connor and others, 1998). The 

analytical schedule used from 1993 to April 1996 by the 

NAWQA Program included 60 VOCs. The NWQL 

used a custom analytical method from April 1996 to 

October 1996, and the current analytical schedule was 

used after October 1996. The methods used since April 

1996 include analyses for 86 VOCs plus a search for 

nontarget analytes. The laboratory reporting level for 

each VOC using schedules prior to April 1996 generally was 0.2 μg/L. The laboratory reporting levels for 

newer methods generally are about one-half an order of 

magnitude lower than those for older methods because 

of new, long-term approaches for determining method 

detection levels (Oblinger Childress and others, 1999). 

The laboratory reporting levels for analytes in the 

newer schedules also are variable between different 

VOCs and can vary throughout time for individual 

compounds (Oblinger Childress and others, 1999). 

As of 1999, 55 VOCs were selected by the 

NAWQA Program for emphasis and are considered 

national target analytes (Bender and others, 1999). 

These include 21 halogenated alkanes, 10 halogenated 

alkenes, 3 aromatic hydrocarbons, 9 alkyl benzenes, 

6 halogenated aromatics, 4 ethers, 1 aldehyde, and 1 

nitrile (Bender and others, 1999). Only these 55 VOCs 

are examined in this report because of the emphasis 

given to them in the NAWQA Program. A list of target 

analytes, by compound use group, is presented later in 

this report. Concentrations of 1,3-dimethylbenzene 

(m-xylene) and 1,4-dimethylbenzene (p-xylene) are 

reported as the sum of these two isomers by NWQL 

because they cannot be separated by current GC/MS 

methods (Bender and others, 1999). Also, 2-propenal 

was deleted from the NWQL schedules in May 1998 

because of erratic and poor instrument response after 

a new concentrator was installed. Nevertheless, results 

for 2-propenal are included in this report because the 

NAWQA Program collected acceptable data during 

1996-98 (Bender and others, 1999). Of the 55 VOCs, 

29 have federally established MCLs, 28 are classified 

as known, probable, or possible human carcinogens, 

35 have potential non-carcinogenic human health 

effects but currently do not have national regulations 

(Drinking Water Equivalent Level (DWEL) or Lifetime Health Advisory (HA)), and 33 have documented 

taste/odor thresholds (Bender and others, 1999).

Prescribed, consistent quality-assurance (QA) 

procedures are followed in the field and in the laboratory for NAWQA studies. QA procedures in the field 

include standard, written protocols for purging wells, 

collecting field measurements, collecting VOC samples, cleaning equipment (decontamination) after each 

visit to a well, and collecting quality-control (QC) 

samples (Koterba and others, 1995).

QC samples for VOCs include a variety of types 

and generally comprise about 10 to 15 percent of the 

samples collected. The review of QC data collected by 

NAWQA indicated that a small number of samples 

were subject to random contamination resulting from 

collection, handling, shipping, processing, and/or 

analyses. However, the majority of samples showed 

no systematic contamination on a national basis 

(G.C. Delzer, U.S. Geological Survey, written 

commun., 2001). If the QC data indicated evidence of 

systematic contamination, the associated environmental data were coded to indicate the possibility of 

contamination and these data were not used in subsequent analyses (G.C. Delzer, U.S. Geological Survey, 

written commun., 2000).
8 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

Local, State, and Other Federal Water-Quality 

Monitoring Programs

NAWQA sampling does not cover all areas of 

the United States. However, some local, State, and 

other Federal agencies sample domestic and other 

types of wells for VOCs as part of ambient groundwater or source-water-quality monitoring programs. To 

augment the NAWQA data, VOC data from some of 

these monitoring programs have been compiled 

(Lapham and others, 1997). These data are referred to 

as retrospective data. The locations where retrospective 

VOC data were compiled included areas in NAWQA 

Study Units that had not been sampled at the time of the 

data compilation and areas outside of the NAWQA 

Study Units. VOC data from approximately 15,500 

wells from 43 ambient monitoring programs or well 

networks in 27 States had been compiled as part of this 

retrospective effort as of August 1997 (Lapham and 

others, 1997). As of summer 2001, data from an additional 1,283 wells had been compiled. VOC data collected by local, State, and other Federal agencies were 

selected only if they met specific criteria in terms of 

monitoring objectives, network design, well construction, and methods of sample collection and laboratory 

analysis (table 3). These criteria were followed to help 

ensure consistency between the NAWQA and retrospective data.

Table 3. General criteria for well-network design, well construction, and data selection used when compiling retrospective 

data from local, State, and other Federal agencies (modified from Lapham and Tadayon, 1996)

[VOC, volatile organic compound; μg/L, micrograms per liter]

Category of criteria Description of criteria

Well-network design Wells are suitably located in relation to the desired spatial and depth design.

All wells or a subset of wells from one or more well networks collectively result in a generally unbiased, 

random, equal-area distribution of sampling sites throughout the aquifer or part of the aquifer of interest.

Well construction Well-construction information is sufficient to ensure that the water level measured in the well represents the

water level in the aquifer and that the sample water collected is from that aquifer.

Ideally, the well construction and pumping equipment in the well do not affect concentrations of VOCs in the 

water sample.

Data selection National methods for collection and analysis of VOCs are used. The analytical method and method detection 

limits or the reporting levels for each VOC analyte are known and are less than about 5 μg/L.

Analysis is done by a laboratory certified by the U.S. Environmental Protection Agency.

The location of the well from which the sample is collected is known by latitude and longitude.

The sample is collected from untreated (source) water.

The water sample is collected at or near the wellhead before being held in a pressure or holding tank.

The analyte name is identified by parameter code (P-code), and the analyte concentration is known.

The date (at a minimum, the year) of sample collection is known. (In general, the latest measured concentration 

of the analyte from a well after January 1, 1985, is used; however, in some cases this criteria might not apply, 

as discussed in Lapham and Tadayon (1996)).

Quality-control data are used to evaluate and, if necessary, to censor and/or codify the environmental data prior 

to its compilation at a national level to the extent that quality-control data are available.
Study Approach 9

Field and laboratory methods and QA procedures followed for the collection and laboratory analysis of the retrospective data were documented in a 

general way when the data were compiled. This 

approach ensured that, to the extent possible, all data 

met a minimum standard of quality. Accordingly, only 

data that met the specific selection criteria listed in 

table 3 were selected. All retrospective data should 

meet the following critical QA requirements: national 

methods for collection and analysis of VOCs were 

used; the analytical method and method detection 

limits or the reporting levels for each VOC analyte 

were known and are less than about 5 μg/L; analyses 

were done by a laboratory certified by the USEPA; and 

QC data were used, to the extent they were available, to 

evaluate and censor and/or codify, if necessary, the 

environmental data prior to its compilation at a national 

level.

Selection of VOC Data

NAWQA and retrospective VOC data had to 

meet the requirements listed in table 4 to be included in 

the analyses for this report. Many types of wells 

including domestic, public supply, monitoring, and 

others were sampled by both NAWQA and other agencies conducting water-quality monitoring; however, 

only domestic wells were included for analyses in this 

report. For some samples, particularly in the retrospective data set, information such as type of well was not 

documented, and these samples were omitted from the 

data set. Other missing information about a well 

resulted in the elimination of additional data. The general limitations imposed by lack of supporting information about each well diminishes the usefulness of some 

of the retrospective data in this analysis, and this is discussed in more detail in Lapham and others (1997) and 

Lapham and others (2000).

Table 4. Criteria used to select VOC data for analysis

[VOC, volatile organic compound; NAWQA, National Water-Quality Assessment Program; SU, study unit; km, kilometer; >, greater than; ≤, less than or 

equal to; μg/L, micrograms per liter; NWQL, U.S. Geological Survey National Water-Quality Laboratory]

Study objective National occurrence and status assessment of one or more VOCs, groups of VOCs, and individual VOCs.

Data sources 1991 and 1994 NAWQA SU data and retrospective data.

Sampling period 1986 through 1999 (generally, the most recent sample from the well).

Well type Wells used for domestic water supply in rural areas.1

Sample collection point At the wellhead.

Sample type Untreated water (collection prior to water treatment and pressure or holding tanks).

Target analytes As many of 55 VOCs as possible (the number of VOCs in each analysis might differ, but a minimum of 24 

VOCs were required).2,3

Other criteria Well spacing >1 km.4

Only one sample from each well.

The laboratory reporting levels for each VOC in the sample generally was ≤ 0.2 μg/L.

1Rural areas are areas with a population density less than 386 people per square kilometer. 255 VOCs were selected for emphasis in the NAWQA Program. However, concentrations of 1,3-dimethylbenzene (m-xylene) and 1,4-dimethylbenzene (p-xylene) are reported together by the NWQL because these isomers cannot be separated by the gas chromatography/mass spectrometry method 

(Bender and others, 1999). Therefore, concentrations for only 54 analytes are reported. 2-propenal was deleted from the NWQL schedules in May 1998 

because of erratic and poor instrument response after a new concentrator was installed. However, this VOC is included as a VOC target analyte because 

21⁄2 years of acceptable data were collected by the NAWQA Program in 1996-98 (Bender and others, 1999, p. 9). 3The list of VOCs is provided later in this report in table 6.

4Well spacing is greater than 1 km to avoid spatial autocorrelation between samples.
10 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

The NAWQA and retrospective data sets combined contain VOC analyses from 2,479 samples of 

ground water from domestic wells. Only one primary 

environmental sample from each domestic well was 

analyzed in this report. Because each well had only one 

environmental sample analyzed, the words “well” and 

“sample” will be used interchangeably from this point 

on. If the type of well was not identified, the well was 

not selected. Only a subset of 1,926 samples met five 

additional selection criteria required for inclusion in 

data analysis. The five selection criteria used include: 

(1) limiting the location of the well to a rural area; 

(2) limiting each well to only one water sample; 

(3) specifying the number of VOCs included in the laboratory analysis of the water sample; (4) specifying 

minimum laboratory reporting levels for the majority 

of analytes; and (5) specifying a minimum separation 

distance between wells (table 4).

Only domestic wells located in rural areas were 

selected to avoid any potential bias that might be associated with population density. Higher population density in the vicinity of a well has been shown to be 

strongly associated with a higher probability of 

detecting VOCs (Squillace and others, 1999). Because 

only 4 percent of wells were located in urban areas, 

only a small amount of data was eliminated. The population density associated with each well was interpolated between grid cells that were entirely or partly 

within the 1-km radius of each well. Urban wells 

were identified and removed if the population density 

within a 1-km radius of the well was greater than 386 

people/km2

; a population density of 386 people/km2 or 

more is considered urban (Hitt, 1994). Populationdensity data were available as a geographic information 

system grid from the Consortium for International 

Earth Science Information Network (1996). This step 

removed 91 samples from the data set.

Only one sample from each well was selected 

(table 4). Removal of duplicate samples was necessary 

in order to accurately describe occurrence of VOCs in 

the ground-water resource. Computing occurrence with 

a varying number of samples per well does not provide 

an accurate picture of water quality by well. Removal 

of duplicate samples eliminated 129 samples from the 

data set. Generally, the sample used for analysis from 

each well was the one with the most recent sampling 

date. In the case of NAWQA data, the sample selected 

for analysis was the one classified as the primary environmental sample for the well. If there was a choice of 

samples collected by NAWQA and a local, State, or 

other Federal monitoring program, samples collected 

by NAWQA were selected. Data from NAWQA samples were preferred to data from other sources because 

the number of VOCs measured by NAWQA (55 

VOCs) usually is larger than that measured by other 

monitoring programs. In addition, documented and 

consistent field sampling methods and laboratory analyses are used in NAWQA data collection that minimizes potential data bias and variability.

The third step in the selection process was to 

limit the data set to only those samples that included 

analyses for at least 24 VOCs. Samples collected by 

NAWQA had analyses for at least 55 VOCs, and results 

for 55 VOCs are included in this report. However, the 

number of VOCs analyzed in each well or network in 

the retrospective data set varied considerably. Therefore, the purpose of this third step was to ensure, to the 

extent possible, that all wells had analyses for a relatively large number of VOCs. A minimum number of 

24 VOCs was selected as a threshold value after the 

distribution of the number of VOCs analyzed in each 

sample was examined. Then comparisons of frequencies of detections among VOCs would be based on 

approximately the same number of analyses for each 

VOC. This step removed 137 samples from the data set.

The fourth step was to select only those samples 

for which the laboratory reporting levels for each VOC 

in the sample generally were less than or equal to 

0.2 μg/L. The level of 0.2 μg/L was chosen because 

this concentration is the most frequently occurring 

reporting level in the data set. This step was performed 

because large numbers of analytes with high reporting 

levels can potentially bias detection frequencies low. In 

order to avoid this, only samples that had at least 

75 percent of all reporting levels at or below 0.2 μg/L 

were retained. This step removed 138 samples from the 

data set. 

Finally, a fifth step in data selection was completed to avoid potential bias in computing detection 

frequency and potential problems with spatial autocorrelation. Spatial autocorrelation is caused when wells 

are too closely spaced and thus do not provide sample 

independence (Barringer and others, 1990). This can 

cause inaccuracy and inefficiency in tests relating 

occurrence of water-quality indicators to variables such 

as land use (Barringer and others, 1990). Barringer and 

others (1990) have shown that wells separated by a distance of less than 1 km show distinct autocorrelation 

with respect to cropland use. Spatial autocorrelation 
Study Approach 11

also could potentially cause bias in the computation of 

detection frequencies for water-quality constituents. 

In order to prevent spatial autocorrelation and 

potential bias in detection frequency, each well in the 

data set was separated from other wells by at least a 

1-km distance. An algorithm preferentially eliminated 

wells that were within the separation distance. Wells 

sampled by NAWQA were retained if there was a 

choice to select NAWQA-sampled wells versus retrospective wells. This step removed an additional 58 

samples from the data set.

After these five selection steps, the final data set 

consisted of 1,926 samples. Of these samples, 1,325 

were samples collected by NAWQA and 601 were 

samples collected by local, State, and other Federal 

agencies in other ambient ground-water or sourcewater-quality monitoring programs (table 5). Other 

general characteristics of the data set used in this report 

are described in table 5.

Statistical Methods in Data Analysis

To better understand the significance of some of 

the analytical results and the hydrologic processes that 

may have caused these results, statistical tests were performed to evaluate and compare groups of data. Two 

statistical tests were performed: (1) the Spearman correlation test for determining correlation between two 

continuous variables, and (2) Pearson’s chi-square test 

of independence for determining dependence between 

two categorical variables where both variables are 

nominal. 

The Spearman correlation test was performed to 

determine both the correlation between detection frequency of one or more VOCs by State and the number 

of wells sampled by State (Occurrence and Status - All 

Samples section) and correlation between detection 

frequency of VOCs by group and average half-life 

(Occurrence and Status - Detections of VOCs by Group 

section). The null hypothesis for this test states that 

there is no correlation between the two variables 

(Helsel and Hirsch, 1992). If the null hypothesis is 

rejected, then an association exists between the two 

variables at the specified statistical significance level. 

The computed statistic for this test is referred to as rho. 

The p-value is the probability that rejection of the null 

hypothesis is wrong or the probability of committing a 

Type I error (Sheskin, 1997).

The Pearson’s chi-square test of independence 

was performed to determine if detection frequency of 

one or more VOCs is dependent on well type for 

domestic versus public supply wells (Occurrence and 

Status - Geographic Distribution section). The null 

hypothesis for this test states that there is no dependence of one variable (detection frequency) upon the 

other (well type) (Helsel and Hirsch, 1992). If the null 

hypothesis is rejected, then the value of one variable is 

dependent upon the other. Again, the p-value is the 

probability that rejection of the null hypothesis is 

wrong or the probability of committing a Type I error 

(Sheskin, 1997).

Table 5. General characteristics of the NAWQA and 

retrospective data used for analysis in this report

[NAWQA, National Water-Quality Assessment Program; VOC, volatile 

organic compound]

General Information

Number of domestic wells 1,926

Number of domestic wells sampled by 

NAWQA

1,325

Number of domestic wells sampled in other 

monitoring programs (retrospective data)

601

Number of States covered 39

Sampling period 1986-99

VOC Analyses

Range in the number of VOCs analyzed per 

sample

24 to 55

Range in the reporting level, in micrograms 

per liter

0.012 to 1,000

Well Characteristics

Range in depth of wells, in feet below land 

surface

6 to 1,500

Median depth of wells, in feet below land 

surface

140

Lithologic Information

Percentage of wells screened in 

unconsolidated aquifers

34

Percentage of wells screened in or open to 

consolidated aquifers

47

Percentage of wells for which lithologic 

information was not recorded

19
12 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

The statistical significance level is the accepted 

level of probability of committing a Type I error 

(Sheskin, 1997). In both statistical tests for this study, 

the statistical significance level (α) used was 0.05. If 

the p-value associated with a statistical test is less than 

the statistical significance level, the null hypothesis is 

rejected. If the p-value associated with a statistical test 

is greater than or equal to the statistical significance 

level, the null hypothesis is not rejected.

Acknowledgments

The authors acknowledge and thank the many 

people that assisted in this study. In particular, the 

authors appreciate the help of project personnel from 

the 1991, 1994, and 1997 USGS NAWQA Study Units 

who collected ground-water samples in their Study 

Units or compiled VOC and related data from local or 

State ground-water monitoring programs in and near 

their Study Units. The authors also would like to thank 

the domestic well owners that allowed USGS personnel 

access to their property to sample their wells.

The authors also thank the colleagues who provided peer reviews of this report: Michael Focazio, 

Lori Apodaca, and Jennifer Shelton. The authors also 

thank Connie Ross and Ella Decker who prepared this 

report for publishing.

OCCURRENCE AND STATUS

Six major topics related to the occurrence and 

status of VOCs in ground water from rural, selfsupplied, domestic wells are presented. These six 

topics are: (1) detections of at least one VOC at an 

assessment level of 0.2 μg/L; (2) detections of VOCs 

by predominant use group at an assessment level of 

0.2 μg/L; (3) detections of individual VOCs at an 

assessment level of 0.2 μg/L; (4) status of VOC concentrations; (5) detections of VOCs using no assessment level; and (6) detections of mixtures of VOCs 

using no assessment level. The VOC analytes targeted 

in this study, by predominant use group, are presented 

in table 6. The total sample size varies between individual VOCs because not every sample from the retrospective data was analyzed for all 55 target VOCs. The 

number of VOCs analyzed in a sample ranged from 24 

to 55 (table 5). Analytical results for 1,3-dimethylbenzene and 1,4-dimethylbenzene are reported as the sum 

of concentrations for the two compounds by the 

NWQL; therefore, the number of target analytes with 

reported concentrations used for occurrence and status 

in this report is 54.

A uniform assessment level of 0.2 μg/L was 

chosen for analyses in some of the following sections, 

and the basis for this strategy will be explained in more 

detail. The assessment level should be distinguished 

from the laboratory reporting level. The laboratory 

reporting level is a level of censoring, or lack of censoring, that is applied at the analytical laboratory prior 

to the data being released. A detailed discussion of laboratory reporting levels is beyond the scope of this 

report but can be found in Oblinger Childress and 

others (1999). An assessment level is a level of censoring applied to water-quality data that have variable 

laboratory reporting levels either between subsets of 

VOC analytes or between individual VOC analytes. 

The assessment level is applied to data received from 

the laboratory and is subsequent to the laboratory 

reporting level. The primary purpose of the assessment 

level is for accurate comparison of detection frequencies between subsets of VOC analytes or between individual VOC analytes.

Because detection frequency is strongly dependent on the assessment level, comparisons of detection 

frequencies between groups of VOCs or between individual VOCs may not reflect true differences in water 

quality if the laboratory reporting level varies between 

VOCs. Most VOCs analyzed by NAWQA since April 

1996 have variable laboratory reporting levels. The 

different laboratory reporting levels are based on the 

differing responses of compounds to the analytical 

instrumentation, and many compounds have laboratory 

reporting levels that vary as method changes are implemented or new instrumentation is used (Oblinger 

Childress and others, 1999). For retrospective data, the 

laboratory reporting levels also are quite variable 

between compounds, and this variability is most likely 

due to differing laboratory censoring criteria. 

The procedure used to establish a uniform 

assessment level applied to every analysis in every 

sample and only applied to quantified concentrations. 

For example, if a VOC in a sample had a detectable 

concentration of 0.1 μg/L and an analysis was done at 

an assessment level of 0.2 μg/L, then the reported concentration for that VOC in that sample was censored to 

less than 0.2 μg/L (a non-detection). Detectable concentrations equal to or greater than 0.2 μg/L were 

unaltered. 
Occurrence and Status 13

Table 6. Grouping of VOCs by predominant use

Fumigants Gasoline hydrocarbons Organic synthesis Oxygenates Refrigerants Solvents Trihalomethanes

Bromomethane1,2-Dibromo-3-

chloropropane 

1,2-Dibromoethane1,4-Dichlorobenzenecis-1,3-Dichloropropenetrans-1,3-Dichloropropene

Benzenen-Butylbenzene1,2-Dimethylbenzene1,3-Dimethylbenzene3

1,4-Dimethylbenzene3EthylbenzeneMethylbenzeneNaphthalene1,2,4-Trimethylbenzene

BromoetheneChloroethene 1,1-DichloroetheneEthenylbenzene Hexachlorobutadiene(1-Methylethyl)benzene2-Propenal2-Propenenitrile1,2,3-Trichlorobenzene

tert-Amyl methyl etherDiisopropyl etherEthyl tert-butyl ether

Methyl tert-butyl ether

ChloromethaneDichlorodifluoromethaneTrichlorofluoromethane1,1,2-Trichloro-1,2,2-

trifluoroethane

ChlorobenzeneChloroethane1,2-Dichlorobenzene1,3-Dichlorobenzene1,1-Dichloroethane1,2-Dichloroethanecis-1,2-Dichloroethenetrans-DichloroetheneDichloromethane1,2-DichloropropaneHexachloroethanen-PropylbenzeneTetrachloroetheneTetrachloromethane 1,2,4-Trichlorobenzene1,1,1-Trichloroethane1,1,2-TrichloroethaneTrichloroethene1,2,3-Trichloropropane

Bromodichloromethane1

Dibromochloromethane1Tribromomethane2Trichloromethane2

1Although classified as a trihalomethane, is used predominantly in organic synthesis.2Although classified as a trihalomethane, is used predominantly as a solvent.3Analytical results reported as the sum of concentrations for these two compounds by U.S. Geological Survey National Water-Quality Laboratory.
14 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

Detections of at Least One VOC at an 

Assessment Level of 0.2 microgram per liter

Samples of untreated water from 1,926 rural, 

self-supplied domestic wells (fig. 1) were analyzed to 

determine the national occurrence and the distribution 

of at least one VOC. Detection frequencies are reported 

at an assessment level of 0.2 μg/L. Three sets of results 

are presented: (1) detection frequencies of at least one 

VOC from all 1,926 samples; (2) detection frequencies 

for the subset of samples that contained VOCs; and 

(3) the areal distribution of VOC detections, nationally 

and by State.

All Samples

Detection frequencies for at least one VOC were 

computed at an assessment level of 0.2 μg/L. Most 

samples contained no VOCs at or above a concentration of 0.2 μg/L. No VOCs were detected at or above 

0.2 μg/L in 1,694 of the 1,926 samples, or about 

88 percent. Conversely, one or more VOCs were 

detected in 232 of the 1,926 samples, or about 

12 percent (table 7). This detection frequency is relatively low compared to the 26-percent detection frequency of one or more VOCs in public supply wells 

sampled by the NAWQA program from 1993-2000 

at an assessment level of 0.2 μg/L (2002 analysis of 

Figure 1. Location of rural, untreated, self-supplied domestic wells that were sampled for this study, 

and the location of wells where at least one volatile organic compound was detected at an assessment 

level of 0.2 microgram per liter.

Oahu, Hawaii

EXPLANATION

Conterminous United States and Hawaii

 (no wells sampled in Alaska)

Detection of at least one volatile organic

 compound with a concentration at or

 above 0.2 microgram per liter

SAMPLED WELLS (1,926)

Sampled wells (no detections)

0 500 MILES

0 500 KILOMETERS

Base from U.S. Geological Survey,

digital data, 1:2,000,000, 1994

Albers Equal-Area Conic projection

Standard Parallels: 29o30',45o30'

Central Meridan: 96oW
Occurrence and Status 15

current NAWQA data). These results are statistically 

different at a significance level of α = 0.05 (p = 0; 

chi-square independence test). The higher detection 

frequency in public supply wells compared to domestic 

wells may be due, in part, to the higher pumping rates, 

pumping stress factors, and larger contributing areas of 

public supply wells.

Samples that Contained VOCs

For the samples in which one or more VOCs 

were detected, the detection frequency of VOCs in 

samples decreases as the number of VOCs detected 

increases (fig. 2). Seventy-two percent of the 232 samples (8.7 percent of all 1,926 samples) in which one or 

more VOCs were detected contained only one VOC. 

Twenty-eight percent of the samples that had a detection of at least one VOC had two or more VOCs 

detected (table 7). Three VOCs were detected in about 

6 percent of 232 samples. Only about 5 percent of 232 

samples contained 4 or more VOCs.

Areal Distribution of VOC Detections, Nationally

and by State

The 232 samples with detections of at least one 

VOC in rural, self-supplied, domestic wells occur 

throughout the conterminous United States and Hawaii 

(fig. 1). Explaining the observed areal distribution of 

VOCs in domestic well water is problematic for several 

reasons. One reason is that the compounds that are 

detected differ from well to well. Another reason is that 

the most important sources of VOCs, the compounds 

being released, and the amounts entering ground water 

likely differ in different areas of the United States. The 

distribution of these sources across the United States is 

not well documented. For example, fumigants might be 

the most important VOC source in agricultural areas.

Most of the States where domestic wells were 

sampled had at least one sample in which one or more 

VOCs were detected (fig. 1; table 8). Overall, samples 

with detections of at least one VOC were collected 

from wells located in 31 of the 39 States. The most frequent detections of at least one VOC occurred in New 

Jersey where one or more VOCs were detected in 44 of 

the 97 wells sampled. California had detections of at 

least one VOC in 39 of the 132 wells sampled in that 

State (table 8). Detection frequencies by State are presented in table 8 only for those States in which there 

were at least 30 wells sampled. A sample size of 30 was 

considered to be a minimum sample size for calculation 

of detection frequency for this report. Bias in the detection frequency by State might be expected because of 

the variability in the numbers of domestic wells sampled by State; however, a strong correlation does not 

exist between the number of wells sampled per State 

and detection frequency of one or more VOCs 

(Spearman’s rho = -0.127, p = 0.616).

Table 7. Percent detections of at least one volatile organic compound in untreated water from 1,926 wells at an assessment 

level of 0.2 microgram per liter

[VOC, volatile organic compound; >, greater than; N, number of samples]

VOC occurrence Number of samples with

indicated occurrence

Percent detection from samples

that contained at least one VOC

[N = 232]

Percent detection

from all samples

[N = 1,926]

>5 VOCs detected 3 1 0.2

5 VOCs detected 5 2 0.3

4 VOCs detected 4 2 0.2

3 VOCs detected 13 6 0.7

2 VOCs detected 40 17 2.1

1 VOC detected 167 72 8.7

At least one VOC detected 232 100 12

No VOCs detected 1,694 Not applicable 88
16 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

Table 8. Distribution of samples, by State, with a detection of at least one volatile

organic compound at an assessment level of 0.2 microgram per liter

[NC, not computed; VOC, volatile organic compound]

State

Number of

samples

analyzed

Number of samples

with one or more

VOCs detected

Frequency of

detection

(percent)

States with Samples from Greater Than or Equal to 30 Wells

Arizona 71 2 3

California 132 39 30

Colorado 54 8 15

Iowa 57 7 12

Idaho 287 7 2

Illinois 52 8 15

Minnesota 173 8 5

Missouri 30 7 23

New Jersey 97 44 45

Nevada 73 8 11

New York 53 2 4

Oklahoma 94 8 9

Oregon 56 4 7

Pennsylvania 164 20 12

South Carolina 48 3 6

Texas 96 8 8

Washington 88 13 15

West Virginia 39 4 10

States with Samples from Less Than 30 Wells

Arkansas 12 4 NC

Connecticut 12 6 NC

Delaware 8 2 NC

Georgia 22 5 NC

Kansas 1 0 NC

Louisiana 2 0 NC

Massachusetts 6 2 NC

Maryland 11 2 NC

Michigan 29 3 NC

Mississippi 2 0 NC

North Carolina 21 1 NC

Nebraska 9 2 NC

New Hampshire 3 0 NC

New Mexico 22 0 NC

Ohio 6 0 NC

Rhode Island 1 0 NC

Tennessee 26 1 NC

Virginia 29 1 NC

Vermont 3 1 NC

Wisconsin 28 2 NC

Wyoming 9 0 NC

Total 1,926 232 NC
Detections and Geographic Distribution of VOCs by Group 17

The eight States in which no VOCs were detected 

were Kansas, Louisiana, Mississippi, New Hampshire, 

New Mexico, Ohio, Rhode Island, and Wyoming 

(fig. 1; table 8). Very few wells were sampled in most 

of these States (table 8), so the absence of detections 

should be interpreted with caution. Given the widespread number and uses of VOCs in and near households throughout the United States, one or more VOCs 

might be detected in some domestic wells in these eight 

States if more sampling was done. 

Detections and Geographic Distribution of 

VOCs by Group at an Assessment Level

of 0.2 microgram per liter

Detection frequencies of VOCs by group were 

computed at an assessment level of 0.2 μg/L. Although 

VOCs frequently have multiple uses, the 55 analytes 

were organized into seven groups based on their

primary use for purposes of comparison in this report. 

These groups are based on a classification developed 

by Bender and others (1999). Assigning an individual 

VOC to a use group does not imply that the designated 

usage is the only use for that compound. Table 6 lists 

the seven groups and the VOCs in each group. Most 

VOCs have numerous uses in industry, commerce, 

and households. Six of the groups include: (1) fumigants, (2) gasoline hydrocarbons, (3) compounds used 

for organic synthesis, (4) gasoline oxygenates, 

(5) refrigerants, and (6) solvents (table 9). A seventh 

group used in this report is the trihalomethanes 

(THMs). Although the title THMs does not reflect a 

predominant use of VOCs, THMs have been included 

here because they are frequently detected in ground 

water. For the seven groups of VOCs discussed, detection frequency is defined as the number of wells with 

a detection of any VOC in the group divided by the 

number of wells with an analysis of any VOC in the 

group, multiplied by 100. 

Figure 2. Detection frequency versus number of volatile organic compounds detected at 

an assessment level of 0.2 microgram per liter.

0

1

2

3

4

5

6

7

8

9

10

DETECTION FREQUENCY, IN PERCENT

12 4 3 5 greater than 5

NUMBER OF VOLATILE ORGANIC COMPOUNDS DETECTED
18 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

Table 9. Summary of volatile organic compound detections, by group, at an assessment level of 0.2 microgram per liter

[VOCs, volatile organic compounds; >, greater than; ≤, less than or equal to]

Occurrence

category

Number of

VOCs

Percent of

54 VOCs

Number of VOCs in the group detected in ground water

Fumigants

Gasoline 

hydrocarbons

Organic 

synthesis

Gasoline

oxygenates

RefrigerantsSolventsTrihalomethanes

Compounds not detected 18 33 3 3 6 1 0 5 0

Compounds with a 

detection frequency

>0 and <1 percent

29 54 2 3 4 2 3 12 3

Compounds with a 

detection frequency

>1 and <5 percent

7 13 1 1 0 1 1 2 1

Total 54 100 6 7 10 4 4 19 4

By group, the 54 VOCs examined in this report 

include 6 fumigants, 7 gasoline hydrocarbons, 10 compounds used for organic synthesis, 4 gasoline oxygenates, 4 refrigerants, 19 solvents, and 4 THMs. All 

VOCs within the refrigerant and THM groups were 

detected; however, most of the VOCs in all of the 

groups (87 percent) were either not detected or detected 

at a frequency of equal to or less than 1 percent 

(table 9). Only seven VOCs (13 percent of the total 

number of analytes with reported concentrations) had 

percent detection frequencies between 1 and 5 percent. 

Fumigants, gasoline hydrocarbons, gasoline oxygenates, refrigerants, and THMs all had one compound 

with a detection frequency between 1 and 5 percent. 

Solvents had two VOCs with detection frequencies 

between 1 and 5 percent. Compounds used for organic 

synthesis had no VOCs with a detection frequency 

greater than 1 percent. The 18 VOCs with no detections 

(33 percent of the total number of VOCs) include 3 of 

the 6 fumigants; 3 of the 7 gasoline hydrocarbons; 6 of 

the 10 compounds used for organic synthesis; 1 of the 4 

gasoline oxygenates; and 5 of the 19 solvents.

Detection frequencies of VOCs by groups ranged 

from 0.5 percent for VOCs used in organic synthesis to 

greater than 4 percent for both solvents and THMs 

(fig. 3). The detection frequencies of the other five use 

groups are less than one-half the detection frequencies 

of solvents and THMs. Detection frequencies of these 

groups, listed in decreasing order of detection, are 

as follows: 2.2 percent for gasoline oxygenates; 

1.6 percent for refrigerants; 1.3 percent for gasoline 

hydrocarbons; 1.0 percent for fumigants; and 

0.5 percent for compounds used for organic synthesis. 

Of the 1,926 samples with analyses for solvents, 89 

samples (4.6 percent) contained one or more solvents. 

The high detection frequency for solvents could result 

from the large number of compounds (19) in that group 

and/or the common use of VOCs within the group. 

Sixty-three of the samples had only one solvent 

detected. These 63 detections included 10 different 

VOCs, which indicates that no single compound predominated the detection frequency of solvents. 

Many of the detections within each of the groups 

consist of the detection of a single VOC in that group. 

For example, trichloromethane detected by itself comprised 77 of the 84 detections of one or more THMs.

MTBE, one of the four VOCs in the gasoline oxygenate 

group, comprised 20 of the 30 detections (about 

67 percent) of one or more gasoline oxygenates.

In addition to the detection frequency, the 

average half-life of each VOC group is shown in 

figure 3. Environmental degradation half-lives in 

figure 3 were obtained from Howard and others (1991) 

and represent the average half-life in ground water for 

all of VOCs in each group. With the exception of compounds used in organic synthesis, VOC groups have 

detection frequencies that generally are proportional to 

average half-lives. One reason for the discrepancy 

between average half-life and the detection frequency 

for compounds used for organic synthesis could be 
Detections and Geographic Distribution of VOCs by Group 19

minimal use of these chemicals in households. When 

the organic synthesis group is excluded from the analysis, there is a good correlation between the detection 

frequency of VOCs by group and the average half-life 

in days (Spearman’s rho = 0.841, p = 0.038). 

Detections of some VOC groups were widely 

distributed throughout the conterminous United States, 

whereas detections of other VOC groups were geographically concentrated in specific areas. The geographic distribution of detections of some VOC groups 

might relate to the use pattern of that group, whereas the 

distribution of detections of other VOC groups has no 

obvious explanation. In addition to use, factors such as 

climate, hydrogeology, recharge, soil and aquifer permeability, organic content of soils, ground-water redox 

conditions, and land use likely affect the geographic 

distribution of VOC groups. A more detailed review of 

some hydrogeologic and anthropogenic factors that 

could relate to VOC detections by group follows this 

section.

THMs had the widest geographic distribution of 

any of the VOC groups (fig. 4). THMs were detected in 

84 domestic wells distributed throughout 27 States. 

Detections of solvents also had wide geographic distribution with 89 wells having detectable concentrations of solvents distributed in 20 States (fig. 5). 

Detections of refrigerants also had widespread distribution throughout the conterminous United States. 

The 31 domestic wells with detections of refrigerants 

were distributed throughout 18 States (fig. 6).

The 26 domestic wells with detections of gasoline hydrocarbons were fairly widely distributed 

throughout 15 States (fig. 7). Because of the ubiquitous use of gasoline products and widespread geographic distribution of releases to the environment, 

detections in ground water are expected, particularly 

in highly populated areas. Most of the 26 detections of 

gasoline hydrocarbons occurred in higher population 

density areas of the eastern United States and California. However, less populated States such as 

Arkansas, Idaho, Iowa, Minnesota, Missouri, and 

Texas also had detections supporting the expectation 

of widespread releases to the environment. 

Figure 3. Detection frequency of volatile organic compound groups at an assessment level

of 0.2 microgram per liter. Left side of y-axis represents frequency of detection of groups (in

percent), and right side of y-axis represents the average half-life of groups (in days) (Howard

and others, 1991).

Gasoline Oxygenates Refrigerants Solvents THMs

hydrocarbons

Fumigants Organic

synthesis

DETECTION FREQUENCY, IN PERCENT

AVERAGE HALF-LIFE OF GROUPS, IN DAYS

Detection frequency

Average half-life

5.0 600

500

400

300

200

100

0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0
20 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

Figure 4. Locations where rural, untreated, self-supplied domestic well water was sampled and 

where at least one trihalomethane was detected.

Oahu, Hawaii EXPLANATION

SAMPLED WELLS (1,926)

Sampled wells (no detections)

Detection of at least one trihalomethane

 compound with a concentration at or

 above 0.2 microgram per liter

Conterminous United States and Hawaii

 (no wells sampled in Alaska)

0 500 MILES

0 500 KILOMETERS

Base from U.S. Geological Survey,

digital data, 1:2,000,000, 1994

Albers Equal-Area Conic projection

Standard Parallels: 29o30',45o30'

Central Meridan: 96oW
Detections and Geographic Distribution of VOCs by Group 21

Figure 5. Locations where rural, untreated, self-supplied domestic well water was sampled and

where a least one solvent was detected.

Oahu, Hawaii EXPLANATION

SAMPLED WELLS (1,926)

Sampled wells (no detections)

Detection of at least one solvent

 compound with a concentration at

 or above 0.2 microgram per liter

Conterminous United States and Hawaii

 (no wells sampled in Alaska)

0 500 MILES

0 500 KILOMETERS

Base from U.S. Geological Survey,

digital data, 1:2,000,000, 1994

Albers Equal-Area Conic projection

Standard Parallels: 29o30',45o30'

Central Meridan: 96oW
22 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

Figure 6. Locations where rural, untreated, self-supplied domestic well water was sampled and 

where at least one refrigerant was detected.

Oahu, Hawaii EXPLANATION

SAMPLED WELLS (1,926)

Sampled wells (no detections)

Detection of at least one refrigerant

 compound with a concentration at

 or above 0.2 microgram per liter

Conterminous United States and Hawaii

 (no wells sampled in Alaska)

0 500 MILES

0 500 KILOMETERS

Base from U.S. Geological Survey,

digital data, 1:2,000,000, 1994

Albers Equal-Area Conic projection

Standard Parallels: 29o30',45o30'

Central Meridan: 96oW
Detections and Geographic Distribution of VOCs by Group 23

Figure 7. Locations where rural, untreated, self-supplied domestic well water was sampled and

where at least one gasoline hydrocarbon was detected.

Oahu, Hawaii EXPLANATION

SAMPLED WELLS (1,926)

Sampled wells (no detections)

Detection of at least one gasoline hydro-

 carbon compound with a concentration

 at or above 0.2 microgram per liter

Conterminous United States and Hawaii

 (no wells sampled in Alaska)

0 500 MILES

0 500 KILOMETERS

Base from U.S. Geological Survey,

digital data, 1:2,000,000, 1994

Albers Equal-Area Conic projection

Standard Parallels: 29o30',45o30'

Central Meridan: 96oW
24 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

The 30 domestic wells with detections of gasoline oxygenates were distributed throughout 8 States, 

but detections were concentrated predominantly in the 

eastern portion of the United States (fig. 8A). As a 

result of the enactment of the Clean Air Act (CAA) 

Amendments of 1990 (U.S. Environmental Protection 

Agency, 1990b), oxygen-containing compounds must 

be added to gasoline in areas that do not meet National 

Ambient Air Quality Standards (NAAQS) for carbon 

monoxide and ozone. The two gasoline programs of the 

CAA Amendments that require oxygenate use are: 

(1) the oxygenated gasoline (OXY) program in which 

gasoline must contain 2.7 percent oxygen by weight 

during the cold season in areas that fail to meet 

NAAQS for carbon monoxide, and (2) the reformulated gasoline (RFG) program in which gasoline must 

contain 2 percent oxygen by weight year-round in areas 

having the highest levels of tropospheric ozone 

(Moran, Clawges, and Zogorski, 1999). Consequently, 

the use of gasoline oxygenates is concentrated in the 

OXY and RFG program areas. Locations of both 

former and current OXY and RFG areas are shown in 

figure 8B. 

The four compounds in the gasoline oxygenate 

group are tert-amyl methyl ether, diisopropyl ether,

ethyl tert-butyl ether, and MTBE. Any of these can be 

used in gasoline as an oxygenate, but MTBE is used 

most frequently. The first cold season oxygenated gasoline program in the United States was voluntarily 

implemented in Denver, Colorado, in 1988 (Office of 

Science and Technology, 1997). One oxygenate was 

detected in a domestic well within this area (fig. 8B). 

Detections of oxygenates occurred in 5 of the 18 States 

with large areas that have participated, or presently are 

participating in the OXY and RFG programs. These 

include 12 detections in New Jersey, 6 detections in 

Pennsylvania, 3 detections in Connecticut, 2 detections 

in Massachusetts, and 1 detection in Illinois (fig. 8B). 

Although MTBE is used throughout California in the 

State’s reformulated gasoline program, no oxygenates 

were detected in domestic wells sampled there. One 

reason for the lack of oxygenate detections in California may be that all of the domestic wells sampled are 

located in the Central Valley where land use is predominantly agricultural. Ground water under areas of agricultural land use would be expected to have a lower 

overall usage of gasoline compared to other areas and 

thus have a smaller number of potential sources.

Compounds used in organic synthesis were 

geographically restricted with detections in only nine 

domestic wells in six States (fig. 9). Minimal detections of these VOCs in rural domestic wells would be 

expected because they are predominantly used as intermediates for production of other compounds and, as 

such, would most likely be associated with industrial 

settings and would not be expected to have many 

sources near domestic wells. 

Of all the VOC groups, fumigants had the most 

restricted geographic distribution with detections in 20 

domestic wells in only three States (fig. 10). Sixteen of 

these detections (80 percent) occurred in the Central 

Valley of California. Because fumigants are used for 

insect and nematode control in vineyards and orchards, 

detections in agricultural areas with emphasis on these 

types of crops, such as the Central Valley of California, 

would be expected. Washington and New Jersey had 

two wells each with detections of VOCs in the fumigant group.

Detections of Individual VOCs at an 

Assessment Level of 0.2 microgram per liter

Detection frequencies for individual VOCs were 

computed at an assessment level of 0.2 μg/L. Fifty-five 

VOCs are targeted by NAWQA (table 4), but the concentrations of 1,3-dimethylbenzene and 1,4-dimethylbenzene cannot be separated by the NWQL and are 

reported together. Therefore, results for only 54 individual analytes are presented here. Occurrence information presented for each VOC includes the number of 

detections of the VOC, the number of samples in which 

the VOC was analyzed, the detection frequency, VOC 

concentrations, and ranges in concentration.

VOC detections are organized into three groups 

in table 10: (1) VOCs not detected in any samples; 

(2) VOCs detected at a frequency of less than or equal 

to 1 percent; and (3) VOCs detected at a frequency of 

greater than 1 percent and less than 5 percent. No individual VOC had a detection frequency greater than 

5 percent. Of the 54 VOCs, 18 (33 percent) were not 

detected, 29 (54 percent) had detection frequencies of 

less than or equal to 1 percent, and 7 (13 percent) had 

detection frequencies of greater than 1 percent and less 

than 5 percent. The most frequently detected individual 

VOC was trichloromethane with a detection frequency 

of 4.3 percent (fig. 11). The detections of individual 

VOCs are summarized in table 11. Figure 11 illustrates 

the detection frequency of individual VOCs.
Detections of Individual VOCs at an Assessment Level of 0.2 microgram per liter 25

Figure 8. Locations (A) where rural, untreated, self-supplied domestic well water was sampled and where at 

least one gasoline oxygenate was detected and (B) of Federal oxygenated and reformulated gasoline program 

areas and locations.

Oahu, Hawaii

Oahu, Hawaii

EXPLANATION

SAMPLED WELLS (1,335)

Sampled wells (no detections)

Detection of at least one gasoline

 oxygenate compound with a

 concentration at or above 0.2

 microgram per liter

EXPLANATION

PROGRAM AREA DESIGNATIONS

Oxygenated and reformulated

 gasoline

Oxygenated gasoline

Reformulated gasoline

Former oxygenated gasoline

Former reformulated gasoline

Conterminous United States and

 Hawaii (no wells sampled in

 Alaska)

B

A

Conterminous United States and

 Hawaii (no wells sampled in

 Alaska)

0 500 MILES

0 500 KILOMETERS

0 500 MILES

0 500 KILOMETERS

Base from U.S. Geological Survey,

digital data, 1:2,000,000, 1994

Albers Equal-Area Conic projection

Standard Parallels: 29o30',45o30'

Central Meridan: 96oW

Base from U.S. Geological Survey,

digital data, 1:2,000,000, 1994

Albers Equal-Area Conic projection

Standard Parallels: 29o30',45o30'

Central Meridan: 96oW
26 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

Figure 9. Locations where rural, untreated, self-supplied domestic well water was sampled 

and where at least one volatile organic compound used for organic synthesis was detected.

Oahu, Hawaii EXPLANATION

SAMPLED WELLS (1,926)

Sampled wells (no detections)

Detection of at least one organic

 synthesis compound with a

 concentration at or above 0.2

 microgram per liter

Conterminous United States and Hawaii

 (no wells sampled in Alaska)

0 500 MILES

0 500 KILOMETERS

Base from U.S. Geological Survey,

digital data, 1:2,000,000, 1994

Albers Equal-Area Conic projection

Standard Parallels: 29o30',45o30'

Central Meridan: 96oW
Detections of Individual VOCs at an Assessment Level of 0.2 microgram per liter 27

Figure 10. Locations where rural, untreated, self-supplied domestic well water was sampled 

and where at least one fumigant was detected.

Oahu, Hawaii EXPLANATION

SAMPLED WELLS (1,926)

Sampled wells (no detections)

Detection of at least one fumigant

 compound with a concentration at

 or above 0.2 microgram per liter

Conterminous United States and Hawaii

 (no wells sampled in Alaska)

0 500 MILES

0 500 KILOMETERS

Base from U.S. Geological Survey,

digital data, 1:2,000,000, 1994

Albers Equal-Area Conic projection

Standard Parallels: 29o30',45o30'

Central Meridan: 96oW
28 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells Table 10. Detections of individual volatile organic compounds at an assessment level of 0.2 microgram per liter [VOC, volatile organic compound; Min., minimum; Max., maximum; MCL, Maximum Contaminant Level; HA, Health-Advisory Level, lifetime 70-kilogram adult consuming 2 liters of water per day; 10-4 CR, risk of one additional 70-kilogram adult in ten thousand (1 x 10-4) contracting cancer over a lifetime of exposure consuming 2 liters of water per day; μg/L, micrograms per liter; --, not applicable] Volatile organic compound Predominant use Number of detections/ samples Detected concentrations (in μg/L) Type of drinking-water standard, health criterion, or taste/odor threshold Min Median Max MCL, HA, 10-4 CR (μg/L) Number of concen- trations exceeding standard or criterion Taste/odor threshold1 (μg/L) Number of concen- trations exceeding taste/odor threshold VOCs Not Detected Bromomethane Fumigant 0/1,677 -- -- -- 10 (HA) 0 -- 0 cis-1,3-Dichloropropene Fumigant 0/1,685 -- -- -- 40 (10-4 CR) 0 -- 0 trans-1,3-Dichloropropene Fumigant 0/1,592 -- -- -- 40 (10-4 CR) 0 -- 0

n-Butylbenzene Gasoline hydrocarbon 0/1,454 -- -- -- -- 0 -- 01,2-Dimethylbenzene Gasoline hydrocarbon 0/805 -- -- -- 10,000 (MCL) 0 -- 01,3/1,4-Dimethylbenzene Gasoline hydrocarbon 0/799 -- -- -- 10,000 (MCL) 0 217 0

Bromoethene Organic synthesis 0/688 -- -- -- -- 0 -- 0Hexachlorobutadiene Organic synthesis 0/1,347 -- -- -- 1 (HA) 0 6 0

2-Propenal Organic synthesis 0/472 -- -- -- -- 0 110 02-Propenenitrile Organic synthesis 30/693 -- -- -- 6 (10-4 CR) 0 9,100 0

Ethenylbenzene Organic synthesis 0/1,915 -- -- -- 100 (MCL and 

HA)

0 11 0

1,2,3-Trichlorobenzene Organic synthesis 0/1,455 -- -- -- -- 0 10 0Ethyl tert-butyl ether Oxygenate 0/688 -- -- -- -- 0 -- 01,2-Dichlorobenzene Solvent 0/1,911 -- -- -- 600 (MCL) 0 24 01,3-Dichlorobenzene Solvent 0/1,340 -- -- -- 600 (HA) 0 77 0Hexachloroethane Solvent 0/698 -- -- -- 1 (HA) 0 10 01,2,4-Trichlorobenzene Solvent 0/1,464 -- -- -- 70 (MCL) 0 5 01,1,2-Trichloroethane Solvent 0/1,686 -- -- -- 5 (MCL) 0 50,000 0
Detections of Individual VOCs at an Assessment Level of 0.2 microgram per liter 29

Detection Frequency Greater Than 0 and Less Than or Equal to 1 Percent

1,2-Dibromoethane Fumigant 33/1,614 0.3 0.6 1.1 0.05 (MCL) 3 -- --

1,4-Dichlorobenzene Fumigant 1/1,925 1.2 1.2 1.2 75 (MCL) 0 4.5 0Benzene Gasoline hydrocarbon 6/1,892 0.2 0.5 4.4 5 (MCL) 0 170 0Ethylbenzene Gasoline hydrocarbon 2/1,926 0.2 2.8 5.4 700 (MCL) 0 29 0Naphthalene Gasoline hydrocarbon 2/1,464 1.8 2.1 2.5 20 (HA) 0 2.5 01,2,4-Trimethylbenzene Gasoline hydrocarbon 6/1,415 0.2 0.3 12 -- - 500 0Chloroethene Organic synthesis 1/1,917 0.4 0.4 0.4 2 (MCL) 0 3,400 01,1-Dichloroethene Organic synthesis 2/1,926 0.5 19.8 39 7 (MCL) 1 1,500 0(1-Methylethyl) benzene Organic synthesis 2/1,371 0.4 0.6 0.8 233 (10-4 CR) 0 0.8 0

tert-Amyl methyl ether Oxygenate 1/688 0.8 0.8 0.8 -- -- -- --

Diisopropyl ether Oxygenate 1/581 22 22 22 -- -- -- --Chloromethane Refrigerant 2/1,565 1.1 1.8 2.5 3 (HA) 0 -- --Trichlorofluoromethane Refrigerant 7/1,925 0.2 0.3 1.9 2,000 (HA) 0 28 0

1,1,2-Trichloro-1,2,2-

trifluoroethane

Refrigerant 1/1,515 0.3 0.3 0.3 2,100,000

(10-4 CR)

0 -- --

Chlorobenzene Solvent 1/1,926 3.5 3.5 3.5 100 (MCL) 0 50 0Chloroethane Solvent 2/1,677 0.3 0.5 0.7 -- -- 19 01,1-Dichloroethane Solvent 5/1,926 0.2 0.7 1.7 -- -- -- --1,2-Dichloroethane Solvent 4/1,910 0.2 1.5 2.9 5 (MCL) 0 7,000 0cis-1,2-Dichloroethene Solvent 2/1,705 3 3.7 4.4 70 (MCL) 0 -- --trans-1,2-Dichloroethene Solvent 1/1,767 0.4 0.4 0.4 100 (MCL) 0 4.3 0

Table 10. Detections of individual volatile organic compounds at an assessment level of 0.2 microgram per liter–Continued

[VOC, volatile organic compound; Min., minimum; Max., maximum; MCL, Maximum Contaminant Level; HA, Health-Advisory Level, lifetime 70-kilogram adult consuming 2 liters of water per day;10-4 CR, risk of one additional 70-kilogram adult in ten thousand (1 x 10-4) contracting cancer over a lifetime of exposure consuming 2 liters of water per day; μg/L, micrograms per liter; --, not applicable]

Volatile organic

compound Predominant use

Number of 

detections/

samples

Detected concentrations

(in μg/L) Type of drinking-water standard, health criterion, or taste/odor threshold

Min Median Max

MCL, HA,

10-4 CR

(μg/L)

Number of concentrations exceeding 

standard or criterion

Taste/odor 

threshold1

(μg/L)

Number of concentrations exceeding 

taste/odor threshold
30 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells Detection Frequency Greater Than 0 and Less Than or Equal to 1 Percent—Continued Dichloromethane Solvent 12/1,923 0.2 0.6 4 5 (MCL) 0 910 0 1,2-Dichloropropane Solvent 15/1,926 0.2 0.5 19.4 5 (MCL) 2 10 1

n-Propylbenzene Solvent 1/1,454 0.6 0.6 0.6 -- -- -- --Tetrachloromethane Solvent 5/1,925 0.2 0.4 0.6 5 (MCL) 0 520 0Trichloroethene Solvent 16/1,926 0.2 0.7 25 5 (MCL) 3 310 01,2,3-Trichloropropane Solvent 10/1,615 0.2 0.4 2.1 40 (HA) 0 -- --Bromodichloromethane4 Trihalomethane 7/1,926 0.2 1.5 7 80 (MCL) 0 -- --

Chlorodibromomethane4 Trihalomethane 7/1,926 0.3 0.6 11 80 (MCL) 0 -- --Tribromomethane4 Trihalomethane 4/1,925 0.3 0.7 8.2 80 (MCL) 0 300 0

Detection Frequency Greater Than 1 and Less Than or Equal to 5 Percent

1,2-Dibromo-3-chloropropane Fumigant 516/1,459 0.2 1.3 3.2 0.2 (MCL) 16 -- --

Methylbenzene Gasoline hydrocarbon 21/1,882 0.2 0.3 12 1,000 (MCL) 0 42 0Methyl tert-butyl ether Oxygenate 30/1,335 0.2 0.7 30.2 -- -- 620 1

Dichlorodifluoromethane Refrigerant 23/1,916 0.2 0.3 2 1,000 (HA) 0 -- --Tetrachloroethene Solvent 32/1,897 0.2 0.3 29 5 (MCL) 3 190 01,1,1-Trichloroethane Solvent 22/1,926 0.2 0.4 120 200 (MCL) 0 970 0Trichloromethane4 Trihalomethane 83/1,926 0.2 0.5 74 80 (MCL) 0 2,400 0

1Taste/odor thresholds from Bender and others, 1999. 21,3-Dimethylbenzene.3No additional detections exist below 0.2 μg/L. 4Bromodichloromethane, chlorodibromomethane, tribromomethane, and trichloromethane can also be classified as solvents or VOCs used in organic synthesis. 5When uncensored, the number of detections equals 19. 6Lower limit of U.S. Environmental Protection Agency Drinking Water Advisory.

Table 10. Detections of individual volatile organic compounds at an assessment level of 0.2 microgram per liter–Continued[VOC, volatile organic compound; Min., minimum; Max., maximum; MCL, Maximum Contaminant Level; HA, Health-Advisory Level, lifetime 70-kilogram adult consuming 2 liters of water per day;10-4 CR, risk of one additional 70-kilogram adult in ten thousand (1 x 10-4) contracting cancer over a lifetime of exposure consuming 2 liters of water per day; μg/L, micrograms per liter; --, not applicable]

Volatile organic

compound Predominant use

Number of 

detections/

samples

Detected concentrations

(in μg/L) Type of drinking-water standard, health criterion, or taste/odor threshold

Min Median Max

MCL, HA,

10-4 CR

(μg/L)

Number of concentrations exceeding 

standard or criterion

Taste/odor 

threshold1

(μg/L)

Number of concentrations exceeding 

taste/odor threshold
Detections of Individual VOCs at an Assessment Level of 0.2 microgram per liter 31

Figure 11. Detection frequencies of 54 volatile organic compounds at an assessment level of 0.2 microgram per 

liter.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Trichloromethane

Methyl tert-butyl ether

Tetrachloroethene

Dichlorodifluoromethane

1,1,1-Trichloroethane

Methylbenzene

1,2-Dibromo-3-chloropropane

Trichloroethene

1,2-Dichloropropane

Dichloromethane

1,2,3-Trichloropropane

1,2,4-Trimethylbenzene

Trichlorofluoromethane

Bromodichloromethane

Chlorodibromomethane

Benzene

Tetrachloromethane

1,1-Dichloroethane

1,2-Dichloroethane

Tribromomethane

1,2-Dibromoethane

Diisopropyl ether

(1-Methylethyl)benzene

tert-Amyl methyl ether

Naphthalene

Chloromethane

Chloroethane

cis-1,2-Dichloroethene

1,1-Dichloroethene

Ethylbenzene

n-Propylbenzene

1,1,2-Trichloro-1,2,2-trifluoroethane

trans-1,2-Dichloroethene

Chloroethene

1,4-Dichlorobenzene

Chlorobenzene

1,1,2-Trichloroethane

1,2,3-Trichlorobenzene

1,2,4-Trichlorobenzene

1,2-Dichlorobenzene

1,2-Dimethylbenzene

1,3-Dichlorobenzene

1,3/1,4-Dimethylbenzene

2-Propenal

2-Propenenitrile

Bromoethene

Bromomethane

Ethyl tert-butyl ether

Hexachlorobutadiene

Hexachloroethane

Ethenylbenzene

cis-1,3-Dichloropropene

n-Butylbenzene

trans-1,3-Dichloropropene

DETECTION FREQUENCY, IN PERCENT
32 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

Table 11. Summary of detections of individual volatile organic compounds at an assessment level of 

0.2 microgram per liter

[VOC, volatile organic compound; >, greater than; ≤, less than or equal to]

Occurrence category Number

of VOCs

Percent of

54 VOCs

Occurrences

exceeding a

drinking-water

standard, health

criterion, or taste/odor 

threshold

VOCs not detected 18 33 0

VOCs with a detection frequency >0 and <1 percent 29 54 10

VOCs with a detection frequency >1 and <5 percent 7 13 20

VOCs with a detection frequency >5 percent 0 0 0

Total 54 100 30

The VOCs listed in table 10 have a variety of 

uses. The 18 VOCs that were not detected and the 29 

VOCs with detection frequencies less than 1 percent 

represent six predominant uses. The seven VOCs that 

had the highest detection frequencies included: trichloromethane, MTBE, tetrachloroethene, dichlorodifluoromethane, 1,1,1-trichloroethane, methylbenzene, and 

1,2-dibromo-3-chloropropane. These VOCs represent 

all uses except organic synthesis. Detection or detection 

frequency of a VOC does not appear to be related to its 

use as indicated by the lack of clustering of VOCs in 

certain use groups by detection frequency. 

Status of VOC Concentrations

Table 10 gives ranges of concentrations for every 

VOC that was detected. Status information includes 

exceedances of VOCs relative to various drinkingwater standards (MCLs), health criteria (HA), and 

taste/odor thresholds. All concentration data for individual VOCs are considered at or above a concentration 

of 0.2 μg/L. 

Under the SDWA, the USEPA has established 

legally enforceable water-quality standards, or MCLs, 

for CWSs and NTNCWSs based on overall health risks 

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

MCL is the highest level of a contaminant that is 

allowed in drinking water and is set as close to the Maximum Contaminant Level Goals (MCLG) as is feasible 

based upon treatment technologies, costs, and other 

factors (U.S. Environmental Protection Agency, 

2000a). The USEPA also has established health criteria 

that are not legally enforceable such as Lifetime Health 

Advisories (HA) and excess lifetime cancer risks that 

are believed to be protective of carcinogenic and noncarcinogenic human health effects. In this report, health 

criteria refer to either HAs or excess lifetime cancer 

risks. Drinking-Water Advisories (DWA) and 

taste/odor thresholds, which are not legally enforceable, also have been recommended by the USEPA to 

address acceptable drinking-water concentrations for a 

chemical substance based on cosmetic effects such as 

tooth or skin discoloration or aesthetic effects such as 

taste, odor, and color (U.S. Environmental Protection 

Agency, 2000a). Values for MCLs, health criteria, and 

taste/odor thresholds were taken from Bender and 

others (1999).

Forty-two of the 54 VOCs have drinking-water 

standards or health criteria (table 10). Thirty-seven of 

the target VOCs have either MCLs, HAs, or both 

(1,3-dimethylbenzene and 1,4-dimethylbenzene are 

reported together), and 14 have only HAs or excess lifetime cancer risks. Thirty-three VOCs have either a 

DWA or taste/odor threshold.

The concentrations of VOCs relative to drinkingwater standards, health criteria, or taste/odor thresholds 

were examined in two ways: (1) the number of samples 

with one or more VOCs exceeding a drinking-water 

standard, health criterion, or taste/odor threshold, and 

(2) the total number of times that an individual VOC 

exceeded a drinking-water standard, health criterion, or 

taste/odor threshold. If a VOC did not have an MCL but 

had both an HA and an excess lifetime cancer risk, the 
Detections of VOCs Using No Assessment Level 33

lowest value of either was taken for comparison to VOC 

concentrations. The excess lifetime cancer risk was 

examined at 10-4 level.

Figure 12 illustrates the detected concentrations 

of all VOCs. Most concentrations were relatively low. 

Maximum concentrations of most VOCs detected were 

less than 10 μg/L, but 11 of the 54 VOCs had at least 

one detection with a concentration greater than 10 μg/L 

(table 10; fig. 12). Seven of these 11 VOCs had at least 

one detection with a concentration that exceeded 

20 μg/L. Trichloromethane and 1,1,1-trichloroethane 

had at least one detection with a concentration that 

exceeded 50 μg/L.

Twenty-seven samples (1.4 percent) had at least 

one VOC that exceeded either an MCL, a health criterion, or both. Two samples (0.1 percent) had at least one 

VOC that exceeded a DWA or taste/odor threshold. 

Only six VOCs exceeded either an MCL or 

health criterion in a total of 28 occurrences. The six 

VOCs that exceeded an MCL at least once were 1,2-

dibromoethane, 1,1-dichloroethene, 1,2-dichloropropane, trichloroethene, 1,2-dibromo-3-chloropropane, 

and tetrachloroethene. Sixteen of the 28 exceedances 

of an MCL (57 percent) were by one compound, 1,2-

dibromo-3-chloropropane (DBCP). The MCL for 

DBCP is relatively low (0.2 μg/L) compared to MCLs 

for other VOCs (table 10). Of the 16 exceedances of 

DBCP, 15 occurred in the Central Valley of California 

and one occurred in New Jersey. The exceedances in 

California are logical because DBCP was used extensively in vineyards and orchards as a soil fumigant for 

insects and nematodes prior to its ban in 1985, and these 

types of agricultural areas are common in the Central 

Valley of California (U.S. Environmental Protection 

Agency, 2001). Trichloroethene, tetrachloroethene, and 

1,2-dibromoethane exceeded their MCLs in three 

occurrences each. Trichloroethene and tetrachloroethene are both commonly used solvents and 1,2-dibromoethane is another commonly used fumigant. Only 

two VOCs exceeded a DWA or taste/odor threshold, 

each only once. The two VOCs that exceeded a DWA or 

taste/threshold were 1,2-dichloropropane and MTBE 

(table 10).

Detections of VOCs Using No Assessment 

Level

Previous occurrence results have been reported at 

an assessment level of 0.2 μg/L. Most VOCs analyzed 

by NAWQA since April 1996 have variable laboratory 

reporting levels that can differ between individual 

VOCs by as much as an order of magnitude and also 

can vary over time for an individual VOC (Oblinger 

Childress and others, 1999).

To fully understand the effect that differing laboratory reporting levels have on water-quality information, the detection frequency of one or more VOC 

versus assessment level should be examined. 

Figure 13 illustrates how detection frequency of one 

or more VOCs varies with assessment level for the 

data set used in this study. In this instance, the assessment level is used to illustrate the effect that varying 

laboratory reporting levels can have on detection 

frequency.

As assessment level is increased, the detection 

frequency decreases (fig. 13). Individual VOCs, or 

subsets of VOCs, that have different laboratory 

reporting levels, like the differing assessment levels 

shown in figure 13, can have different detection 

frequencies. Comparisons of detection frequencies 

between groups of VOCs or between individual VOCs 

that have variable laboratory reporting levels, without 

applying an assessment level, may not reflect true differences in water quality. Rather than demonstrating 

differences in water quality, differences in detection 

frequencies between analytes or VOC subsets with 

different reporting levels may simply show differences 

in resolving power of the analytical instrument with 

respect to various compounds or differences in laboratory reporting levels between various laboratories if 

the data are from various sources.

Applying an assessment level can improve comparisons between analytes or VOC subsets with different reporting levels by subjecting both to an equal 

level of assessment. However, applying an assessment 

level can cause a loss of important information. For 

those VOCs with many analyses at a laboratory 

reporting level lower than the chosen assessment level, 

a great deal of information on occurrence of that VOC 

may be lost. The frequency of detection of a particular 

VOC might be underestimated (biased low) when the 

assessment level is significantly higher than the laboratory reporting level. Therefore, it is important to 

understand the effect of assessment level on the 

frequency of detection of individual VOCs.
34 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

Figure 12. Concentrations of volatile organic compounds detected at an assessment level of 0.2 microgram per

liter.

Bromodichloromethane

Benzene

Chlorobenzene

Chlorodibromomethane

Chloroethane

Chloroethene

Chloromethane

1,2-Dibromo-3-chloropropane

1,4-Dichlorobenzene

1,1-Dichloroethane

1,2-Dichloroethane

tert-Amyl methyl ether 1,1-Dichloroethene

cis-1,2-Dichloroethene

trans-1,2-Dichloroethene

Dichlorodifluoromethane

Dichloromethane

1,2-Dichloropropane

CONCENTRATION, IN MICROGRAMS PER LITER

Ethylbenzene

Diisopropyl ether

(1-Methyethyl)benzene

Methylbenzene

Naphthalene

Tetrachloroethene

n-Propylbenzene

0.1 1,000 1 10 100

CONCENTRATION, IN MICROGRAMS PER LITER

0.1 1,000 1 10 100

Methyl tert-butyl ether
Detections of VOCs Using No Assessment Level 35

Figure 12. Concentrations of volatile organic compounds detected at an assessment level of 0.2 micrograms per 

liter.—Continued

CONCENTRATION, IN MICROGRAMS PER LITER

Tribromomethane

1,1,1-Trichloroethane

Trichloroethene

Trichloromethane

1,2,4-Trimethylbenzene

0.1 1,000 1 10 100

1,2,3-Trichloropropane

1,1,2-Trichloro-1,2,2-trifluoroethane

0.1 10,000 1 10 100 1,000

0.01 1,000 0.1 1 10 100

Trichlorofluoromethane

1,2-Dibromoethane

EXPLANATION

SAMPLE CONCENTRATIONS AND

 CONCENTRATION RANGE 

MAXIMUM CONTAMINANT LEVEL

 (U.S. Environmental Protection

 Agency, 2000a)

MEDIAN CONCENTRATION

LIFETIME HEALTH ADVISORY LEVEL

 (U.S. Environmental Protection

 Agency, 2000a)

Tetrachloromethane
36 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

The data in table 12 illustrate the change in 

detection frequency that can occur as a result of 

applying various assessment levels. Detection frequencies of 54 VOCs were calculated after censoring the 

data at three different assessment levels: (1) no assessment level; (2) 0.05 μg/L; and (3) 0.2 μg/L. Most compounds were detected at one or more assessment levels, 

but 12 VOCs were not detected at any assessment level 

including 2-propenal, 2-propenenitrile, hexachloroethane, bromomethane, 1,1,2-trichloroethane, 1,2,4-

trichlorobenzene, trans-1,3-dichloropropene, cis-1,3-

dichloropropene, hexachlorobutadiene, bromoethene, 

ethyl tert-butyl ether, and 1,2,3-trichlorobenzene. The 

implications of the lack of detections of these compounds are that these compounds are not ground-water 

contaminants in the areas that were sampled, or that 

these compounds are present in ground water in the 

areas sampled but at concentrations less than the 

laboratory’s analytical capability.

Five compounds were not detected at an assessment level of 0.2 μg/L but were detected at an assessment level of 0.05 μg/L and at no assessment level: 1,3-

dimethylbenzene/1,4-dimethylbenzene, 1,2-dimethylbenzene, ethenylbenzene, 1,3-dichlorobenzene, and 

1,2-dichlorobenzene. One compound, n-butylbenzene, 

was not detected at assessment levels of 0.2 or 

0.05 μg/L, but was detected at no assessment level. 

Most compounds that were detected at all three assessment levels were detected more frequently at lower or 

no assessment levels.

VOCs show variations in detection frequencies 

with differing assessment levels. A good way to 

examine this variability is with a ratio of detection frequencies. The ratio of detection frequency of an individual VOC at one assessment level divided by its 

detection frequency at another assessment level is the 

detection-frequency ratio. A detection-frequency ratio 

was computed for detection at no assessment level 

versus detection at an assessment level of 0.2 μg/L 

(table 12). The median detection-frequency ratio for all 

VOCs is 2.3, and most compounds that were detected 

have detection-frequency ratios less than 5 (table 12). 

Several VOCs have substantially higher detectionfrequency ratios than other compounds. Chloromethane and 1,2,4-trimethylbenzene, in particular, 

have detection-frequency ratios (73 and 24, respectively) that are substantially greater than the other 52 

VOCs. The high detection-frequency ratios for these 

two compounds are the result of frequent detection at 

low concentrations. The large number of detections of 

these two compounds at low concentrations may be the 

result of some possible random contamination during 

sampling or may simply reflect typical environmental 

concentrations resulting from either natural or anthropogenic sources. Chloromethane has many natural 

sources, and some of the low concentrations of this 

compound could be natural in origin (Lovelock, 1975; 

Palmer, 1976; Inn and others, 1981; Harper, 1985; 

Wuosmaa and Hager, 1990).

Figure 13. Detection frequency of one or more volatile organic compounds versus assessment level for samples from this study.

0

5

10

15

20

25

30

35

0 1 2 3 4 5 6 7 8 9 10

ASSESSMENT LEVEL, IN MICROGRAMS PER LITER

DETECTION FREQUENCY, IN PERCENT
Detections of VOCs Using No Assessment Level 37

Table 12. Frequencies of detection of individual volatile organic compounds at various assessment levels 

[μg/L, micrograms per liter; --, not applicable]

Compound

Percent detection Detection

frequency

ratio

No assessment 1

level 

Assessment level

of 0.05 μg/L

Assessment level

of 0.2 μg/L

Trichloromethane 11.9 6.7 4.3 2.8

1,2,4-Trimethylbenzene 9.6 2.6 0.4 24

Chloromethane 7.3 1.7 0.1 73

Methylbenzene 5.6 3.5 1.1 5.1

Tetrachloroethene 5.5 2.9 1.7 3.2

1,1,1-Trichloroethane 3.4 1.5 1.1 3.1

Methyl tert-butyl ether 3.3 3.0 2.2 1.5

1,3-Dimethylbenzene/1,4-Dimethylbenzene 2.8 0.4 0 --

Dichloromethane 2.1 1.0 0.6 3.5

Benzene 2.0 0.5 0.3 6.7

Dichlorodifluoromethane 1.9 1.7 1.2 1.6

Trichloroethene 1.8 1.3 0.8 2.3

1,2-Dibromo-3-chloropropane 1.3 1.2 1.1 1.2

Bromodichloromethane 1.1 0.7 0.4 2.8

Tribromomethane 1.0 0.3 0.2 5.0

1,2-Dichloropropane 1.0 0.9 0.8 1.3

1,2-Dimethylbenzene 0.7 0.2 0 --

Trichlorofluoromethane 0.7 0.5 0.4 1.8

Diisopropyl ether 0.7 0.5 0.2 3.5

1,2,3-Trichloropropane 0.7 0.7 0.6 1.2

Ethenylbenzene 0.7 0.1 0 --

1,4-Dichlorobenzene 0.7 0.1 0.1 7.0

Chlorobenzene 0.7 0.1 0.1 7.0

1,1-Dichloroethane 0.7 0.4 0.3 2.3

Tetrachloromethane 0.6 0.4 0.3 2.0

Chlorodibromomethane 0.6 0.4 0.4 1.5

Ethylbenzene 0.5 0.2 0.1 5.0

1,3-Dichlorobenzene 0.4 0.1 0 --

1,1-Dichloroethene 0.4 0.2 0.1 4.0

(1-Methylethyl) benzene 0.3 0.1 0.1 3.0
38 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

tert-Amyl methyl ether 0.3 0.3 0.1 3.0

Chloroethane 0.2 0.2 0.1 2.0

cis-1,2-Dichloroethene 0.2 0.2 0.1 2.0

1,2-Dichloroethane 0.2 0.2 0.2 1.0

n-Propylbenzene 0.2 0.1 0.1 2.0

1,2-Dibromoethane 0.2 0.2 0.2 1.0

1,2-Dichlorobenzene 0.2 0.1 0 --

n-Butylbenzene 0.1 0.0 0 --

Naphthalene 0.1 0.1 0.1 1.0

1,1,2-Trichloro-1,2,2-trifluoroethane 0.1 0.1 0.1 1.0

trans-1,2-Dichloroethene 0.1 0.1 0.1 1.0

Chloroethene 0.1 0.1 0.1 1.0

2-Propenal 0 0 0 --

2-Propenenitrile 0 0 0 --

Hexachloroethane 0 0 0 --

Bromomethane 0 0 0 --

1,1,2-Trichloroethane 0 0 0 --

1,2,4-Trichlorobenzene 0 0 0 --

trans-1,3-Dichloropropene 0 0 0 --

cis-1,3-Dichloropropene 0 0 0 --

Hexachlorobutadiene 0 0 0 --

Bromoethene 0 0 0 --

Ethyl tert-butyl ether 0 0 0 --

1,2,3-Trichlorobenzene 0 0 0 --

1Computed by dividing percent detection at no assessment level by percent detection at an assessment level of 0.2 μg/L.

Table 12. Frequencies of detection of individual volatile organic compounds at various assessment levels–Continued

[μg/L, micrograms per liter; --, not applicable]

Compound

Percent detection Detection

frequency

ratio

No assessment 1

level 

Assessment level

of 0.05 μg/L

Assessment level

of 0.2 μg/L
Detections of Mixtures of VOCs Using No Assessment Level 39

Detections of Mixtures of VOCs Using No 

Assessment Level

An analysis was performed to determine the 

most common mixtures of VOCs in ground water from 

domestic wells. This analysis was performed using no 

assessment level because uncensored data contains the 

most information on low VOC concentrations. 

Harmful human health effects due to low concentrations of many chemicals include chemical sensitivity, 

endocrine disruption, and cancer (Hasegawa and 

others, 1994; Ashford and Miller, 1998). Many studies 

have demonstrated the toxicity and carcinogenicity of 

individual VOCs with respect to human health (Sadik 

and Witt, 1999), but few studies have been performed 

to date that examine multi-compound mixtures of 

VOCs and their effects on health (Yang, 1994). However, mixtures are a common mode of occurrence for 

many VOCs.

One or more VOCs occurred in 32 percent of 

samples when no assessment level was applied. Mixtures of two or more VOCs occurred in 16 percent of 

samples when no assessment level was applied. Therefore, about one-half of all detections of VOCs, using no 

assessment level, occurred as a mixture of two or more 

VOCs. Figure 14 illustrates detection frequency and 

number of compounds detected using no assessment 

level. Note the increased detection frequencies when 

no assessment level is applied compared to an assessment level of 0.2 μg/L (fig. 2).

The 10 most frequently occurring mixtures consisted of two VOCs (table 13). Three-compound mixtures also were fairly common and occurred in about 

4 percent of samples. Mixtures of four and more VOCs 

occurred in less than 1 percent of the samples. VOCs 

used primarily as solvents comprised the majority of 

compounds in the 10 most frequently occurring mixtures (table 13). However, one refrigerant, one VOC 

used in organic synthesis, and several gasoline hydrocarbons also were present in the 10 most frequently 

occurring mixtures (table 13). 

The method for determining mixtures and their 

occurrence consisted of counting the number of times 

that two or more unique combinations of VOCs 

occurred in each sample. Any unique mixture of two or 

more VOCs in a sample was counted as an occurrence 

of that mixture. For example, if a sample contained

Figure 14. Detection frequency versus number of volatile organic compounds detected

using no assessment level.

0

2

4

6

8

10

12

14

16

18

1 2 3 4 5 6 7 8 9 11 24

NUMBER OF VOLATILE ORGANIC COMPOUNDS DETECTED

DETECTION FREQUENCY, IN PERCENT
40 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

Table 13. Top 10 most frequently occurring mixtures of volatile organic compounds using no assessment level

Compounds in mixture Predominant use

Number

of occurrences

of mixture

Frequency

of detection

(percent)

Trichloromethane1 Trihalomethane 33 1.7

Methylbenzene Gasoline hydrocarbon

Trichloromethane1 Trihalomethane 32 1.7

Tetrachloroethene Solvent

Trichloromethane1 Trihalomethane 32 1.7

1,2,4-Trimethylbenzene Organic synthesis

Trichloromethane1 Trihalomethane 32 1.7

Chloromethane Refrigerant

Methylbenzene Gasoline hydrocarbon 30 1.6

Tetrachloroethene Solvent

Trichloromethane1 Trihalomethane 29 1.5

1,1,1-Trichloroethane Solvent

Chloromethane Refrigerant 29 1.5

1,2,4-Trimethylbenzene Organic synthesis

Trichloromethane1 Trihalomethane 24 1.2

Methyl tert-butyl ether Oxygenate

Bromodichloromethane1 Trihalomethane 21 1.1

Trichloromethane1 Trihalomethane

Tetrachloroethene Solvent 19 1.0

1,1,1-Trichloroethane Solvent

1Also classified as a solvent or as a VOC used in organic synthesis.

three VOCs, referred to as VOC1, VOC2, and VOC3, 

the four mixtures in this sample would be:

1. VOC1 + VOC2,

2. VOC1 + VOC3,

3. VOC2 + VOC3, and 

4. VOC1 + VOC2 + VOC3. 

Each combination of VOCs in each sample was 

counted as a different mixture. Frequency of occurrence of the mixture (in percent) was determined by 

dividing the number of times a mixture occurred by the 

number of samples, multiplied by 100. 

None of the compounds in the mixtures in 

table 13 are known to be direct transformation products 

of one another. Tetrachloroethene and 1,1,1-trichloroethane are both solvents and have some similar physical/chemical properties and thus might have similar 

environmental behavior characteristics. Likewise, the 

THMs bromodichloromethane and trichloromethane 

share similar physical/chemical properties, and thus 

also might have similar environmental behavior characteristics. The occurrence of bromodichloromethane 

and trichloromethane together could be the result of 
Potential Sources of VOCs and Factors Related to VOC Occurrence 41

treatment of domestic wells using shock chlorination, 

which is explained in more detail later. 

Some of the VOCs in table 13 might occur 

together because of their occurrence in on-site septic 

systems. As will be discussed later, an important possible point source of VOCs near the home is the septic 

system. Many household products contain VOCs and 

some VOCs have been found in septic systems.

Potential Sources of VOCs and Factors 

Related to VOC Occurrence

There are many potential sources of VOCs to 

ground water in domestic wells. It is likely that some 

VOCs occur in domestic wells because of use in 

domestic products, which then become sources of 

VOCs to shallow ground water near the domestic well. 

Relational analyses can provide insight into the hydrogeologic factors and processes that affect and control 

the transport and fate of VOCs in ground water. They 

also may provide insight into the geographic distribution of VOC occurrences, and possible anthropogenic 

sources of VOCs to ground water. 

Potential Sources of VOCs in Ground Water from 

Domestic Wells

The USEPA, in conjunction with a State workgroup, has investigated potential point sources of contaminants (U.S. Environmental Protection Agency, 

2000b). A list of potential sources of ground-water contamination was developed by querying States about the 

major sources of contamination to ground water (U.S. 

Environmental Protection Agency, 2000b). Some of the 

major sources of contaminants identified by the USEPA 

could be point sources near the home including underground storage tanks (USTs), septic systems, fertilizer 

applications, spills, pesticide applications, aboveground storage tanks (ASTs), pipelines, and sewer 

lines. USTs were the most commonly cited potential 

source of contaminants to ground water (U.S. Environmental Protection Agency, 2000b).

Although USTs are most frequently found in 

urban areas, some rural homes have USTs for storing 

heating oil, gasoline, or diesel fuel for domestic or agricultural purposes (Wisconsin Department of Natural 

Resources, 2000). Leakage from these domestic UST 

systems could be a significant source of ground-water 

contamination to domestic wells (Wisconsin Department of Natural Resources, 2000). The most common 

causes of leaks from USTs are faulty installation and 

corrosion of tanks and pipelines (U.S. Environmental 

Protection Agency, 2000b). Some homes in rural areas 

also have ASTs for storing gasoline, diesel fuel, or 

heating oil for domestic or agricultural purposes. 

ASTs and USTs used for storing heating oil and/or 

motor fuels have been identified as sources of petroleum hydrocarbon VOCs to domestic water wells 

(Wisconsin Department of Natural Resources, 2000).

Septic systems were listed by States as the 

second most important potential source of groundwater contamination. Septic systems are common in 

rural areas. For example, in Montana there are 

126,000 individual on-site septic systems used by 

252,000 people (U.S. Environmental Protection 

Agency, 2000b). U.S. households discharge 3.5 billion 

gallons of liquid into septic systems each day (U.S. 

Environmental Protection Agency, 2000b).

Septic systems include a buried tank for holding 

wastewater from the home (influent) and a fluid distribution system or leachfield for distributing liquid from 

the septic tank (effluent). The primary purpose of the 

septic tank is to hold influent coming from the home, 

to allow settlement or flotation of suspended solids 

and to allow for digestion and disintegration of solid 

matter by bacterial actions. Effluent from the septic 

tank is distributed to the leachfield, where it is discharged into relatively permeable leach-bed material. 

The effluent then comes into contact with the surrounding soil where microorganisms or other natural 

degradation processes degrade the organic matter. 

Improperly designed, maintained, or operated 

septic systems can result in ground-water contamination in the vicinity of the system, especially if effluent 

from the leachfield reaches shallow ground water 

before the organic materials have completely 

degraded (U.S. Environmental Protection Agency, 

2000b). Ground-water contamination resulting from 

incomplete degradation of organic material from 

septic-tank effluent can occur as a result of a variety of 

factors such as leaking tanks, poorly or improperly 

constructed tanks or leachfields, inadequate depth to 

water table, or highly permeable soils in the area of the 

leachfield.

Several studies have examined the presence of 

VOCs in the fluid in septic tanks (septage), in effluent 

from septic tanks (effluent), or in shallow ground 

water in areas where numerous septic systems are 

located. Viraraghavan and Warnock (1976) found that 

the concentrations of soluble organic carbon (SOC) in 

shallow ground water below existing septic systems 
42 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

were lower than in the septic-tank effluent. However, 

the concentrations were substantially higher than background concentrations indicating input of SOC to 

shallow ground water from the septic system. DeWalle 

and others (1985) found five VOCs present—methylbenzene, dichloromethane, trichloromethane, tetrachloroethene, and ethylbenzene—in samples of septictank influent and effluent from a community septic tank 

serving 91 homes in a subdivision of Tacoma, Washington. Viraraghavan and Hashem (1986) found bromodichloromethane, benzene, and dichloromethane in 

the septage (liquids and/or solids within the septic tank) 

and septic-tank effluent from a house in Regina, 

Canada, but methylbenzene only was found in the 

septage. 

Ayres Associates (1993) analyzed VOCs in samples of septage and septic-tank effluent beneath the 

leachfield of many homes in several Florida counties 

and in samples of shallow ground water beneath a 

newer subdivision in Florida where all of the homes 

have septic systems. The samples were analyzed for 35 

VOCs, with laboratory reporting levels ranging from 

1 to 50 μg/L. Eight VOCs were detected above laboratory reporting levels in septage: methylbenzene, total 

xylenes, 1,4-dichlorobenzene, trichloromethane, 

dichloromethane, bromodichloromethane, 1,1-dichloroethane, and 1,1,1-trichloroethane. Only four VOCs 

were detected above laboratory reporting levels in 

septic-tank effluent: methylbenzene, trichloromethane, 

dichloromethane, and 1,4-dichlorobenzene. Methylbenzene was detected in the septic-tank effluent of 

every home sampled and in nearly every sample. 

Trichloromethane was the next most commonly 

detected VOC in septic-tank effluent, followed by 

dichloromethane. Bromodichloromethane and 1,4-

dichlorobenzene were detected in the septic-tank 

effluent of only one home. Of the shallow ground water 

analyzed in the Florida study, only one sample from 

one well contained a VOC, trichloromethane, which 

was detected at a concentration of 1.8 μg/L.

Some of the VOCs detected in the previously 

cited studies also were found in ground water from 

domestic wells sampled for this report. Trichloromethane, which was found in shallow ground water 

beneath a newer subdivision in Florida (Ayres Associates, 1993), was the most frequently detected VOC in 

this analysis (fig. 11) and has been commonly found in 

septic-tank effluent. There are several other possible 

sources of trichloromethane in domestic well water, 

some are natural and some are anthropogenic.

Natural sources of trichloromethane include biomass burning (Lobert and others, 1999) and formation 

in spruce forest soils (Haselmann and others, 2000). 

Areas of wood degradation and soils with humic layers 

can emit, possibly from degradation of organic substances, up to 1,000 nanograms of trichloromethane 

per square meter per hectare (Hoekstra and others, 

2001). If not volatilized from soil, trichloromethane is 

expected to leach to ground water due to its poor 

adsorption to soils. However, the quantitative contribution of natural sources to measured concentrations of 

trichloromethane in ground water is unknown. In addition, trichloromethane is a transformation by-product 

of tetrachloromethane, which is primarily used as a 

solvent. Major processes of the transformation of tetrachloromethane to trichloromethane include anaerobic 

biodegradation (Parsons and others, 1985; Curtis and 

Reinhard, 1994) and abiotic transformation in an anaerobic environment (hydrogenolysis) (Criddle and 

McCarty, 1991). 

Some sources of trichloromethane probably are 

anthropogenic. Household bleach, of which sodium 

hypochlorite is an active ingredient, often is used to disinfect a domestic well that has been contaminated by 

bacteria (U.S. Environmental Protection Agency, 

1993). A solution of bleach and water is added directly 

to the well to destroy the bacteria in a process often 

referred to as shock chlorination. The addition of 

bleach to ground water adds free chlorine, which then 

can result in the formation of THMs through reactions 

between free chlorine and naturally occurring organic 

material in the water (Trussell and Umphres, 1978; 

Alawi and others, 1994). Trichloromethane might form 

through the same reactions during the use of household 

bleach in washing machines. Wastewater discharged 

from the washing machine to a septic system may enter 

shallow ground water near the domestic well. 

Methylbenzene, tetrachloroethene, dichloromethane, bromodichloromethane, 1,1,1-trichloroethane, 1,1-dichloroethane, and benzene also were found 

in septage or septic-tank effluent. These VOCs also 

were among the 20 VOCs most frequently detected in 

samples in this analysis and many are solvents (fig. 11). 

Chlorinated solvents that are used as degreasing agents 

in cleaning septic-tank pipes and leachfield lines 

(Pankow and Cherry, 1996) also may be a contaminant 

source to domestic well water. 
Potential Sources of VOCs and Factors Related to VOC Occurrence 43

Although found frequently in septage and septictank effluent, the presence of many of the same VOCs 

in ground water from domestic wells does not prove 

that the source of the VOCs is the septic system. However, many types of household products containing 

VOCs are disposed into septic systems. If a septic 

system is improperly designed, maintained, or operated, there is an increased likelihood that these VOCs 

could end up in shallow ground water near the well, 

especially if the well is located close to, or is hydraulically connected to, the septic system.

All of the seven most frequently detected VOCs 

(fig. 11) are ingredients in consumer products that 

could be used in and around households and have been 

found in septic systems. Three of these seven VOCs 

(methylbenzene, tetrachloroethene, and 1,1,1-trichloroethane) are ingredients in a wide variety of products. 

For example, tetrachloroethene is predominantly used 

as a solvent (table 6), but also is used in products such 

as automotive cleaners and degreasers, household 

cleaners, lubricants, pesticides, and products used for 

personal hygiene (table 14).

One important observation regarding the use of 

VOCs in household products is that the 18 VOCs not 

detected in domestic wells either are not known to be 

ingredients in any consumer product category or are 

ingredients in only a few of them (table 14). For 

example, 5 of these 18 VOCs (bromoethene, 2-propenal, ethenylbenzene, 1,2,3-trichlorobenzene, and 

1,1,2-trichlorethane) are not known to be ingredients in 

any consumer product (table 14) although the information concerning VOCs in consumer products given here 

may be incomplete. On the other hand, the seven most 

frequently detected VOCs have many uses in a variety 

of household products (table 14).

Relations of VOCs to Hydrogeologic and 

Anthropogenic Variables

Several studies have examined relations between 

VOCs and hydrogeologic and anthropogenic variables. 

A review of some previous studies and results on this 

subject is provided, and possible relations that would be 

anticipated or expected based on hydrogeologic principles and chemical and environmental processes are 

discussed. The examination of possible relations was 

performed for VOCs by use groups because compounds 

with similar uses likely will have similar sources. When 

more VOC chemical data and ancillary data from 

domestic wells become available, relational analyses 

will be performed on this data set to provide additional 

insight into the distribution, transport, and sources of 

VOCs in ground water that supplies domestic wells.

Fumigants

The probability of detecting a fumigant in 

ground water likely decreases with increasing depth to 

the top of the screened interval in the well, or well 

depth. Barbash and Resek (1996) report that results 

from a variety of studies of pesticide detections in 

ground water indicated that well depth was the factor 

most commonly correlated with pesticide detections. 

In most of these pesticide studies, well depth was negatively correlated with pesticide detection frequency. 

This negative correlation between pesticide detection 

and well depth is the result of the increased time 

required for a pesticide applied at the surface, such as 

a fumigant, to reach deeper screened intervals. As 

time increases, processes like hydrodynamic dispersion, sorption, transformation, and volatilization 

increase the probability that the pesticide concentration will decrease below detectable concentrations.

The probability of detecting a fumigant likely is 

higher in ground water from unconsolidated aquifers 

compared to consolidated aquifers. Barbash and 

Resek (1996) report that results from most studies 

showed higher detection frequencies of pesticides in 

unconsolidated aquifers compared to bedrock or consolidated aquifers. In general, consolidated aquifers, 

where overlain by unconsolidated aquifers, tend to be 

deeper and less vulnerable to contamination from 

surface sources (Barbash and Resek, 1996). However, 

consolidated aquifers at or near the land surface, especially if composed of karst limestone or fractured bedrock, can be just as vulnerable or more vulnerable than 

unconsolidated aquifers.

The probability of detecting a fumigant in 

ground water likely is higher in areas of predominant 

agricultural land use relative to urban land use in the 

area surrounding the well. Barbash and Resek (1996) 

reported that studies have indicated higher pesticide 

detection frequencies in ground water underlying agricultural areas compared to non-agricultural areas, and 

the occurrence of heavily used fumigants has been 

correlated with use pattern. Because fumigants are 

most extensively used in select agricultural practices, 

the likelihood of occurrence of VOCs in ground water 

underlying agricultural areas is higher.
44 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells Table 14. Consumer usage of 18 volatile organic compounds not detected and 7 volatile organic compounds most frequently detected in rural, untreated, self- supplied domestic well water [VOCs, volatile organic compounds; NA, not applicable] VOC Predominant use Archi- tectural Automotive Fuel House- hold cleanser Insecticide/ pesticide/ fumigant Lubri- cant Personal hygiene Protectant/ sealant Refrig- erant Solvent Cleaner Coating Degreaser 18 VOCs Not Detected in Rural, Self-Supplied Domestic Well Water Bromomethane1 Fumigant

X

cis-1,3-

Dichloropropene2 Fumigant

X

trans-1,3-

Dichloropropene2 Fumigant

X

n-Butylbenzene1 Gasoline 

hydrocarbon

X

X

1,2-Dimethylbenzene1 Gasoline 

hydrocarbon

X X

1,3-Dimethylbenzene1/

 1,4-Dimethylbenzene

Gasoline 

hydrocarbon

X

Bromoethene2,3 Organic 

synthesis

Hexachlorobutadiene1,2 Organic 

synthesis

X

2-Propenal3 Organic 

synthesis

2-Propenenitrile3 Organic 

synthesis

X

Ethenylbenzene3 Organic 

synthesis

1,2,3-Trichlorobenzene3 Organic 

synthesis

Ethyl tert-butyl ether Oxygenate X1,2-Dichlorobenzene2 Solvent X X

1,3-Dichlorobenzene2 Solvent

X
Potential Sources of VOCs and Factors Related to VOC Occurrence 45

18 VOCs Not Detected in Rural, Self-Supplied Domestic Well Water—Continued

Hexachloroethane2 Solvent

X

1,2,4-Trichlorobenzene2 Solvent X X

1,1,2-Trichloroethane2 Solvent

7 VOCs Most Frequently Detected in Rural, Self-supplied Domestic Well Water

1,2-Dibromo-3- chloropropane1,4 Fumigant

X

Methylbenzene4,5 Gasoline 

hydrocarbon

XX XX X X X X X

Methyl tert-butyl ether Oxygenate XDichlorodiflouro- methane1,4 Refrigerant

X

Tetrachloroethene5 Solvent X X X X X X X X X X X

1,1,1-Trichloroethane5 Solvent X X X X X X X XTrichloromethane1,6 Trihalomethane X X X X

1Vershueren, 1983.2Lewis, 1993. 3Prager, 1995.4Montgomery, 1991.5California Air Resources Board, 2000.6Trichloromethane also can be classified as a solvent or as a VOC used in organic synthesis.

Table 14. Consumer usage of 18 volatile organic compounds not detected and 7 volatile organic compounds most frequently detected in rural, untreated, selfsupplied domestic well water–Continued[VOCs, volatile organic compounds; NA, not applicable]

VOC Predominant 

use

Architectural Automotive Fuel House- hold cleanser Insecticide/ pesticide/ fumigant Lubri- cant Personal hygiene Protectant/ sealant Refrig- erant Solvent Cleaner Coating Degreaser
46 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

The probability of detecting a fumigant in 

ground water likely increases with increasing soil particle size. A study by Teso and others (1996) demonstrated a relation between increased soil particle size 

and increased probability of detecting 1,2-dibromo-3-

chloropropane. Particle size affects soil permeability 

and other hydrologic properties of the soil including 

saturated hydraulic conductivity and water-holding 

capacity, which in turn controls transport of fumigants 

like 1,2-dibromo-3-chloropropane to ground water 

(Teso and others, 1996).

Gasoline Hydrocarbons

The probability of detecting a gasoline hydrocarbon compound in ground water likely decreases 

with increasing depth to the top of the screened interval 

in the well. As a group, gasoline hydrocarbons have a 

relatively low average half-life (fig. 11) and may biodegrade relatively quickly in the environment, especially 

in aerobic conditions (Howard and others, 1991). A 

negative correlation between gasoline hydrocarbon 

detection frequency and depth to the top of the screened 

interval likely would result from the increased time 

required for a compound to reach deeper screened 

intervals. As time increases, physical and chemical processes, especially transformation, increase the probability that the concentration of gasoline hydrocarbons 

will decrease below detectable concentrations.

The probability of detecting a gasoline hydrocarbon compound in ground water likely is higher in 

areas of predominant urban land use relative to agricultural land use in an area surrounding the well. Vowinkel 

and Battaglin (1989) and Vowinkel (1991) found that 

the detection frequency of purgeable organic compounds was higher in urban land-use areas compared to 

agricultural land-use areas. Moran and Davis (1998) 

found that the detection frequency of one of nine 

selected VOCs, two of which were gasoline hydrocarbons (benzene and toluene), was higher in urban areas 

compared to non-urban areas. 

Organic Synthesis

The probability of detecting a compound used 

for organic synthesis in ground water likely decreases 

with increasing depth to the top of the screened interval 

in the well. The increased time required for an organic 

synthesis compound to reach a deeper screened interval 

would allow processes like hydrodynamic dispersion, 

sorption, transformation, and volatilization to increase 

the probability that the concentration would decrease 

below detectable concentrations.

Gasoline Oxygenates

The probability of detecting an oxygenate likely 

increases as population density near the well increases 

and likely is higher in areas of predominant urban land 

use relative to agricultural land use in an area near the 

well. Most use of ether oxygenates today is in larger 

metropolitan areas in the northeast United States and in 

California, and the most frequently used oxygenate is 

MTBE. The areas of high oxygenate use also are areas 

with higher population densities. Because oxygenate 

use is highest in urban areas and areas of high population density, there are many more potential sources in 

these areas and a higher probability of occurrence in 

ground water. Studies have shown a positive correlation 

between MTBE occurrence in ground water and both 

MTBE use and population density (Moran, Halde, and 

others, 1999; Squillace and Moran, 2000; Moran and 

others, 2002). Because MTBE is highly soluble in 

water, does not substantially partition to organic carbon 

in aquifer material, and biodegrades slowly relative to 

gasoline hydrocarbons, it would be expected to occur 

more frequently in ground water than other gasoline 

components.

Refrigerants

The probability of detecting a refrigerant in 

ground water likely decreases with increasing depth to 

the top of the screened interval in the well. Halogenated 

aliphatic compounds like refrigerants, although generally resistant to biodegradation and other removal processes like volatilization and hydrolysis, will degrade 

via various biotic and abiotic processes (McCarty and 

others, 1992). The increased time required for a refrigerant to reach a deeper screened interval in a well 

would allow processes like hydrodynamic dispersion, 

sorption, transformation, and volatilization to increase 

the probability that the concentration would decrease 

below detectable levels.

Solvents

The probability of detecting a solvent in ground 

water likely decreases with increasing depth to the top 

of the screened interval in the well. As depth to the top 

of the screened interval increases, the time required 

for a contaminant to reach the screened interval also 

increases. This increased time factor allows for 

increased attenuation of the concentrations due to 

natural loss processes such as biodegradation, sorption, 

dispersion, and volatilization. Halogenated aliphatic 
Summary and Conclusions 47

compounds like solvents, although generally resistant 

to removal processes like biodegradation, volatilization, and hydrolysis, will degrade via various biotic and 

abiotic processes (McCarty and others, 1992). 

The probability of detecting a solvent in ground 

water is likely higher in areas of predominant urban 

land use than in agricultural land use in an area overlying the well. Large quantities of solvents have been 

produced and used by a wide variety of sectors in our 

modern society (Pankow and Cherry, 1996). Most of 

the industrial and commercial activities that use solvents are performed in urban areas. Because solvents 

are used more widely in urban activities compared to 

agricultural activities, more sources of solvents to 

ground water are expected in urban areas.

Trihalomethanes

The probability of detecting a trihalomethane in 

ground water is likely higher in areas of predominant 

urban land use relative to agricultural land use in an 

area surrounding the well. Trihalomethanes are known 

to be by-products of disinfection of drinking water with 

chlorine via haloform reactions with naturally occurring organic materials in water (Trussell and Umphres, 

1978; Alawi and others, 1994). Disinfection procedures 

are most commonly performed on drinking water 

supplied by community water systems in urban areas. 

Thus, urban areas would be associated with more 

sources of disinfection-treated water. Since trihalomethanes commonly have been detected in septic tank 

effluent, there may be a correlation between the distance from domestic wells to septic systems, or groundwater flow direction, and the presence of trihalomethanes in domestic wells.

SUMMARY AND CONCLUSIONS

Samples of untreated ground water from 1,926 

rural, self-supplied domestic wells were analyzed for 

volatile organic compounds (VOCs) during 1986-99. 

This information was used to characterize the occurrence and status of VOCs in domestic well water. The 

samples were either collected as part of the U.S. 

Geological Survey’s National Water-Quality Assessment (NAWQA) Program occurrence-assessment 

studies or were compiled by NAWQA from existing 

ambient ground-water or source-water-quality monitoring programs conducted by local, State, and other 

Federal agencies. Water samples were collected at the 

wellhead prior to treatment or storage. In most samples, 55 target VOCs were analyzed, and occurrence 

and status information generally was computed at an 

assessment level of 0.2 μg/L. 

Samples of ground water from rural, selfsupplied domestic wells often did not contain VOCs. 

Only 12 percent of the 1,926 samples had a detection 

of at least one VOC at an assessment level of 0.2 μg/L. 

Samples with detections of at least one VOC were 

collected from wells located in 31 of 39 States.

When grouped by predominant use, solvents 

were the most frequently detected VOC group with 

detections in 4.6 percent of samples at an assessment 

level of 0.2 μg/L. The THMs were the second most 

frequently detected VOC group with detections in 

about 4 percent of samples. With the exception of 

compounds used in organic synthesis, detection frequencies of VOCs by groups are proportional to the 

average half-life of compounds in the group. When the 

organic synthesis group is excluded from the analysis, 

there is a good correlation between the detection frequency of VOCs by groups and average half-life of 

compounds in the group. The geographic distribution 

of detections of some VOC groups, such as fumigants 

and oxygenates, appears to be related to the use 

pattern of compounds in that group.

Of the 54 VOCs with data, 36 had concentrations at or above the assessment level of 0.2 μg/L in at 

least one sample. The seven VOCs that had the highest 

detection frequencies included, in order of decreasing 

detection frequency, trichloromethane, methyl tertbutyl ether, tetrachloroethene, dichlorodifluoromethane, 1,1,1-trichloroethane, methylbenzene, and 

1,2-dibromo-3-chloropropane. Only 1.4 percent of 

samples had one or more VOC concentrations that 

exceeded a drinking-water standard, a health criterion, 

or both. Only 0.1 percent of samples had VOC 

concentrations that exceeded a taste/odor threshold. 

The six VOCs that exceeded an MCL at least once 

included: 1,2-dibromoethane, 1,1-dichloroethene, 

1,2-dichloropropane, trichloroethene, 1,2-dibromo-3-

chloropropane, and tetrachloroethene. Sixteen of the 

28 total exceedances of an MCL were by the fumigant 

1,2-dibromo-3-chloropropane. The MCL for 1,2-

dibromo-3-chloropropane is relatively low when compared to other VOCs. Most of the exceedances of 1,2-

dibromo-3-chloropropane were in the Central Valley 

of California where this compound was used extensively as a soil fumigant prior to its ban in 1985.
48 Occurrence and Status of Volatile Organic Compounds in Ground Water from Domestic Wells

Mixtures of two or more compounds were a 

common mode of occurrence of VOCs. Mixtures 

occurred in one-half of all samples where at least one 

VOC was detected when no assessment level was 

applied. The most frequently detected mixtures were 

two-compound mixtures. Solvents were the most 

common VOCs found in mixtures, but compounds 

from the gasoline hydrocarbon, organic synthesis, and 

refrigerant groups also were found. The source of most 

mixtures is not clearly understood, but septic systems 

could be a source for many VOCs.

Potential point sources of VOCs near domestic 

wells are numerous and include underground storage 

tanks (USTs), aboveground storage tanks (ASTs), 

septic systems, fertilizer applications, spills, pesticide 

applications, pipelines, and sewer lines. Releases from 

USTs and ASTs that hold gasoline, diesel fuel, or 

heating oil have the potential to be major point sources 

of contaminants to domestic wells. Shock chlorination 

may be a source of trichloromethane and other THMs 

in some domestic wells. VOCs frequently are ingredients in household products, many of which have been 

found in septic systems. Septic systems are believed to 

be an important source of contaminants to domestic 

wells, but extensive research on this subject does not 

exist.

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