1. VOC Study Design

National findings on the occurrence of 55 VOCs in ambient, untreated ground water are 

based on data collected or compiled as part of 98 aquifer studies within the NAWQA Program 

during 1985-2002 from about 3,500 wells that were selected randomly and distributed across 

broad areas. The aquifer studies were located throughout the conterminous United States as 

well as in Alaska and Hawaii. The NAWQA analysis was based on one sample per well. The 

resulting assessment provides an aquifer-scale resource characterization of the occurrence 

of VOCs. Findings can be used to make decisions regarding water-resource protection and 

water-supply management at regional and national levels.

A separate analysis, limited to wells used only for drinking-water supply (domestic and 

public), was conducted to specifically address possible drinking-water issues of concern by 

water utilities, public health agencies, and citizens. Samples were collected at the wellhead before any treatment or blending, and thus the analyses do not necessarily indicate 

the chemical composition of water ingested by consumers using domestic or public wells. 

Domestic wells are defined in this assessment as privately owned, self-supplied sources for 

drinking water and other household uses. Public wells are privately or publicly owned and 

supply ground water for public water systems. This analysis of source waters included data 

from 2,401 domestic wells and 1,096 public wells. It is important to note that about 2,500 of 

the wells included in this analysis of source waters met the objectives of, and were therefore 

also part of, the aquifer-resource assessment described above. The remaining wells used 

in the analysis of source waters (about 1,000) were sampled by scientists associated with 

other USGS Programs or other agencies (all of which met approved USGS and USEPA quality-assured methodology). More information on the approach, planning, and design of this 

assessment can be found at http://water.usgs.gov/nawqa/vocs/national_assessment/

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3048

April 2006

Volatile Organic Compounds in the Nation’s Ground Water 

and Drinking-Water Supply Wells—A Summary

Printed on recycled paper

Volatile organic compounds (VOCs) are 

produced in large volumes and are associated with a myriad of products, such 

as plastics, adhesives, paints, gasoline, 

fumigants, refrigerants, and dry-cleaning fluids. Widespread and long-term use 

of VOCs and their ability to persist and 

migrate in ground water raise questions 

about possible adverse effects on the 

environment, including drinking-water 

quality. A long-term investigation by 

the National Water-Quality Assessment 

(NAWQA) Program of the United States 

Geological Survey (USGS) provides the 

most comprehensive national analysis to 

date of the occurrence of VOCs in ground 

water, based on results from sampling 

between 1985 and 2002. Among the major 

findings are that VOCs were detected in 

most aquifers throughout the Nation, and 

were not limited to a few specific aquifers 

or regions. VOCs were not detected, however, in many of the nearly 3,500 sampled 

wells; for example, about 80 percent had 

no detections above a threshold of 0.2 part 

per billion. The most frequently detected 

VOCs were chloroform, the solvents PCE 

and TCE, and the gasoline oxygenate 

MTBE; 13 of the 55 compounds included in 

the assessment were not detected at all. 

A separate analysis of untreated groundwater samples from drinking-water supply 

wells showed that VOCs were detected 

in domestic well samples (14 percent) 

and public well samples (26 percent), 

but seldom at concentrations likely to 

affect human health (less than 2 percent 

had concentrations greater than Federal 

drinking-water standards). This Fact Sheet 

highlights selected national and regional 

findings on the occurrence of VOCs in 

ground water and in drinking-water supply 

wells. It serves as a companion product to 

a USGS Circular (1292) titled “The Quality 

of Our Nation’s Waters—Volatile Organic 

Compounds in the Nation’s Ground Water 

and Drinking-Water Supply Wells,” by 

Zogorski and others (2006).

Importance of Studying VOCs in 

Ground Water

Protecting and managing ground water 

are critical. Nearly one-half of the Nation’s 

population depends on ground water for 

drinking and other household uses, and 

it is an important natural resource that 

supports many of the Nation’s industrial, 

agricultural, and commercial activities.

Volatile organic compounds (VOCs) 

are an important group of environmental 

contaminants to monitor and manage in 

ground water because of their widespread 

and long-term use. Many VOCs are 

mobile and persistent in ground water. 

Some are known or suspected human 

carcinogens and their concentrations in 

drinking water from public water systems 

are regulated by the U.S. Environmental 

Protection Agency (USEPA). VOCs can 

be released to the environment and reach 

ground water through many sources 

and pathways, including exhaust from 

gasoline engines, industrial air emissions, 

leaking storage tanks, landfills, infiltration of urban runoff and wastewater, septic systems, and injection through wells.

The National Water-Quality Assessment (NAWQA) Program investigation 

provides a large-scale resource characterization of VOC occurrence, and was 

not designed to focus on local VOC 

contamination of ground water, such as at 

landfills and leaking underground storage 

tanks (see sidebar 1). The assessment 

was also not designed to relate specific 

sources to VOC detections in wells. Findings do, however, provide preliminary 

insights on several human and natural 

factors associated with the occurrence of 

VOCs in ground water, including aquifer 

characteristics, the physical and chemical characteristics of the compounds, and 

land-use activities.
VOC Occurrence

VOCs were detected in most aquifers 

throughout the Nation, and were not limited to a few specific aquifers or regions. 

VOCs were not detected, however, above 

a threshold of 0.2 part per billion in 80 percent of the sampled wells.

VOCs were detected in most aquifers across the Nation. Specifically, at 

least one of the 55 VOCs was detected in 

at least one well sample in 90 of the 98 

aquifer studies assessed by the NAWQA 

Program from 1985 to 2002. Despite 

their nationwide occurrence, no VOCs 

were detected in samples from about 80 

percent of the nearly 3,500 wells included 

in the 98 aquifer studies. Detection frequencies cited throughout this document 

are based on a threshold level of 0.2 part 

per billion (or microgram per liter). For 

perspective, detection levels for public 

drinking-water compliance monitoring 

for regulated VOCs are set by Federal 

regulations at 0.5 part per billion.

In general, detections of most 

VOCs—primarily solvents, gasoline 

hydrocarbons, trihalomethanes (THMs), 

and refrigerants—were relatively evenly 

distributed across the Nation. Exceptions 

included gasoline oxygenates, such as 

methyl tert-butyl ether (MTBE), which 

were detected primarily in aquifers in the 

New England and Mid-Atlantic States; 

and agricultural fumigants, such as ethylene dibromide (EDB) and dibromochloropropane (DBCP), which were detected 

predominantly in aquifers in the Central 

Valley of California and in Oahu, Hawaii. 

The occurrence of gasoline oxygenates 

and fumigants is, in large part, related to 

regional patterns of their use.

Most (three-fourths) of the 55 VOCs 

were detected at least once in the aquiferstudy assessment; however, about 15 

compounds accounted for most detections (see table 1). The most frequently 

detected VOC—in about 7 percent of 

the samples—was the THM chloroform. 

Chloroform and other THMs are disinfection by-products from the chlorination 

of water. The solvents perchloroethene 

(PCE) and trichloroethene (TCE), the 

gasoline oxygenate MTBE, and the 

gasoline hydrocarbon toluene also were 

among the top five most frequently 

detected compounds. Thirteen of the 55 

VOCs were not detected in any samples.

Mixtures of multiple VOCs were 

detected in some water samples. The 

Table 1. A total of 24 VOCs are listed below because they were either (1) among the most 

frequently detected VOCs in aquifer studies (defined as detected in greater than 1 percent 

of samples), or (2) detected in at least one domestic or public well sample, collected prior to 

treatment or blending, at a concentration greater than or within a factor of 10 of a Maximum 

Contaminant Level (MCL). Such detections may warrant their inclusion in future studies or 

monitoring of ground water.

[X, Yes; —, No; NA, Not applicable because these VOCs do not have MCLs]

Volatile organic compound

Detected in at 

least 1 percent 

of samples 

from aquifer 

studies

Detected in samples from drinkingwater supply wells 

at a concentration 

greater than an 

MCL

Detected in samples 

from drinking-water 

supply wells at a 

concentration less 

than, but within a 

factor of 10 of an 

MCL

Fumigants

Dibromochloropropane (DBCP) — X X

1,2-Dichloropropane — X X

Ethylene dibromide (EDB) — X —

Gasoline hydrocarbons

Benzene — — X

Toluene X — —

Gasoline oxygenate

Methyl tert-butyl ether (MTBE) X NA NA

Organic synthesis compounds

1,1-Dichloroethene (1,1-DCE) — X X

Vinyl chloride — X X

Refrigerants

Dichlorodifluoromethane (CFC12) X NA NA

Trichlorofluoromethane (CFC-11) X NA NA

Solvents

Carbon tetrachloride — — X

Chloromethane X NA NA

1,1-Dichloroethane (1,1-DCA) X NA NA

1,2-Dichloroethane (1,2-DCA) — — X

cis-1,2-Dichlorethene (cis-1,2-

DCE) — — X

trans-1,2-Dichloroethene (trans1,2-DCE) X — —

Methylene chloride X X X

Perchloroethene (PCE) X X X

Trichloroethene (TCE) X X X

1,1,1-Trichloroethane (TCA) X — X

Trihalomethanes

Chloroform X — X

Bromodichloromethane X — X

Bromoform X — X

Dibromochloromethane X — X

most common mixtures of compounds 

(at a concentration greater than 0.2 part 

per billion) included PCE, TCE, and 

chloroform. Specific mixtures of these 

compounds were detected in less than 2 

percent of the wells. Mixtures can result 

from multiple compounds sharing the 

same source and from the widespread 

occurrence of VOCs in ground water. 

Mixtures also can result from degradation 

of a parent compound to other compounds. For example, although TCE is 

produced and used as a solvent, its occurrence in ground water can be enhanced as 

the parent compound (PCE) biodegrades 

to TCE under certain environmental conditions. NAWQA data on the occurrence 

of VOC mixtures can help to identify 
Most concentrations of VOCs in domestic 

well samples were below human-health 

benchmarks and about 1 percent of the 

samples had concentrations greater than 

Federal drinking-water standards (MCLs). 

Trihalomethanes (THMs), solvents, 

refrigerants, and gasoline oxygenates 

were the most commonly occurring 

groups of VOCs. Monitoring the quality 

of water from domestic wells is primarily 

the well owner’s responsibility, however, 

most States and some local agencies 

provide guidance to domestic well 

owners through Web sites and printed 

material.

VOC concentrations in this assessment were 

greater than MCLs in about 2 percent of public 

well samples. VOCs were detected more 

frequently in public well samples (26 percent) 

than in domestic well samples (14 percent). 

This is most likely attributed to the location 

of public wells in developed, more urbanized 

areas where VOC sources and releases are 

more common than in rural areas. Higher 

detection frequencies also may relate to greater 

withdrawal rates of public wells compared 

to domestic wells, which can increase the 

movement of contaminated ground water to 

public wells. This finding is consistent with 

results of a companion USGS study that showed 

many VOCs—such as solvents, gasoline hydrocarbons, and refrigerants—were detected 

more frequently and at greater concentrations in ground water that served as a drinkingwater supply for very large community water systems (serving more than 50,000 people) than 

in ground water serving as a supply for smaller systems (Grady, 2003).

those that are most commonly detected in 

the Nation’s ground water and in drinking-water supply wells.

“Detections” of VOCs summarized 

in this document are at low concentrations (0.2 part per billion) and do not 

necessarily indicate adverse effects in 

ground water or potential human-health 

concern if the water is ingested. In fact, 

concentrations of individual VOCs in 

most samples were below Federal drinking-water standards or other humanhealth benchmarks (see next section), and 

the total (or summed) concentration of all 

VOCs in samples typically were less than 

1 part per billion. The NAWQA Program 

uses analytical methods designed to 

measure concentrations of contaminants 

at low levels in order to comprehensively 

assess VOC occurrence in the Nation’s 

ambient ground water, to identify emerging issues related to the presence of 

VOCs in ground water, to track changes 

in concentrations over time, and to gain 

insights about natural and anthropogenic 

factors that explain VOC detections. The 

national occurrence of VOCs has not 

been documented previously because 

most monitoring, such as that for compliance with drinking-water standards, 

does not focus on aquifer-scale resource 

assessments or measure concentrations at 

comparable low levels.

Potential for Effects on Human 

Health

VOCs were detected in samples from 

drinking-water supply wells, but seldom 

at concentrations likely to affect human 

health.

2. Human-Health Benchmarks Used in this Assessment

When volatile organic compounds (VOCs) are detected in untreated samples from drinking-water supply wells, it is important to understand what these results may mean to human 

health. As a first step toward understanding VOC occurrence in the context of human health, 

a screening-level assessment was conducted by comparing VOC concentrations to humanhealth benchmarks. The screening-level assessment is intended to identify VOC concentrations that may be of potential concern and to prioritize needs for further investigation; it is 

not a substitute for comprehensive risk assessment, such as done by the U.S. Environmental 

Protection Agency (USEPA). Concentrations of VOCs in samples from drinking-water supply 

wells were compared to USEPA Maximum Contaminant Levels (MCLs) for 27 of the 44 VOCs 

detected in this assessment. USGS worked with USEPA, New Jersey Department of Environmental Protection, and the Oregon Health & Science University to develop additional humanhealth benchmarks, referred to as “Health-Based Screening Levels” or HBSLs, for VOCs that 

do not have MCLs. HBSLs were developed on the basis of standard USEPA Office of Water 

methodologies and the most current USEPA peer-reviewed, publicly available human-health 

toxicity information. HBSLs were calculated for 8 of the 17 unregulated VOCs that were 

detected in this assessment, but were not calculated for the remaining unregulated VOCs 

because of a lack of toxicity information. Additional information about HBSLs and ongoing 

research is available at http://water.usgs.gov.nawqa/HBSL/.

The U.S. Geological Survey (USGS) 

conducted a separate analysis of VOCs 

detected only in drinking-water supply 

wells to understand what VOC concentrations may mean to human health. Specifically, concentrations of VOCs in samples 

collected from domestic and public wells 

prior to any treatment or blending were 

compared to Federal drinking-water 

standards (USEPA Maximum Contaminant Levels, or MCLs) and to USGS 

Health-Based Screening Levels (HBSLs), 

another human-health benchmark (see 

sidebar 2).

VOCs were detected in 14 percent of 

samples from 2,401 domestic wells and 

in 26 percent of samples from 1,096 public wells. VOC concentrations typically 

were below human-health benchmarks. 

Specifically, concentrations were greater 

than the USEPA MCLs in only about 1 

percent of samples from domestic wells 

(or 29 wells), and in about 2 percent of 

samples from public wells (or 16 wells). 

Concentrations greater than MCLs were 

represented, in large part, by the agricultural fumigant DBCP (in domestic wells 

only) and the solvents PCE and TCE (see 
table 1). No samples contained concentrations greater than HBSLs.

The USGS study evaluated drinking-water supplies prior to any treatment 

or blending. This initial screening-level 

assessment helps to evaluate what VOC 

concentrations captured in domestic- and 

public-well water may mean to human 

health. Results can be used to identify 

and prioritize specific compounds to 

consider in long-term monitoring and in 

other future studies (see table 1). Table 

1 also lists VOCs detected at concentrations less than, but within a factor of 10 

of human-health benchmarks. Nine VOCs 

were detected at concentrations less than, 

but within a factor of 10 of an MCL; 

no sample contained concentrations of 

VOCs that were less than, but within a 

factor of 10 of an HBSL.

It is important to note some complexities in the evaluation of VOCs 

within a human-health context (Toccalino 

and others, 2006). For example, the 

presence of VOCs in drinking water at 

concentrations greater than human-health 

benchmarks, such as MCLs or HBSLs, 

does not necessarily indicate that adverse 

human-health effects will occur, even if 

the water is ingested; toxicity-based models used to develop MCLs and HBSLs 

assume lifetime exposure, whereas actual 

exposure may be less than a lifetime. 

In addition, water-treatment processes 

can lower VOC concentrations below 

human-health benchmarks. Additional 

information on what VOC concentrations 

may mean in a human-health context is 

provided elsewhere (Toccalino and others, 2006).

Relations to Sources, Transport, 

and Fate

Each VOC has a unique pattern of occurrence depending on its sources, patterns 

of use, and factors that control its transport and fate in ground water.

Many small-scale studies have been 

conducted to investigate VOC sources 

and the large number of chemical and 

physical properties that control their 

transport and fate in the environment. 

These studies indicate that the occurrence of VOCs in ground water depends 

on multiple factors, including, but not 

limited to the following:

Geographic and temporal extent 

of production and use of VOCs in 

industry, commerce, and household 

products;

•

Types and locations of VOC sources 

relative to sampled wells;

Extent of urban land use in the vicinity 

of the well;

Chemical and physical characteristics 

of the compounds themselves, such as 

their mobility and persistence;

Aquifer characteristics, including 

the presence or absence of dissolved 

oxygen, hydrogeology, and water-table 

depth;

Pumping stress on the aquifer;

Soil characteristics and climate;

Timing and amount of recharge to the 

ground-water system; and

Timing of VOC releases to the environment and the age of ground water.

Although this NAWQA assessment 

was not designed to examine specific 

sources, transport processes, and chemical and physical factors to VOC concentrations in samples from individual wells, 

general patterns and associations emerged 

from national-scale analyses of VOCs 

commonly detected in aquifer studies. 

Two patterns relate to human activities 

and are associated with urban land use 

and production, and two relate to natural 

features and are associated with levels of 

dissolved oxygen in the ground water and 

lithology.

Urban Land Use and Sources—The 

occurrence of VOCs commonly corresponded to percentage of urban land 

use within a one-half kilometer radius 

of sampled wells (see sidebar 3). VOC 

occurrence also was commonly associated with the presence of Resource 

Conservation and Recovery Act 

(RCRA) hazardous waste facilities, 

•

•

•

•

•

•

•

•

•

underground gasoline storage tanks, 

and septic systems.

Production—Estimated production 

rates of VOCs did not correspond 

directly to VOC detection frequencies, 

which can be explained in part because 

production rates may not reflect when, 

where, and which VOCs actually are 

released to the environment. In addition, production data for some VOCs 

were sparse and could only be estimated.

Dissolved Oxygen—Detections of 

VOCs differed with dissolved-oxygen 

conditions in ground water, in large 

part because rates of biodegradation 

of VOCs vary under different dissolved-oxygen conditions. In general, 

the detection frequencies for VOCs in 

the NAWQA assessment are consistent 

with published biodegradation rates 

associated with dissolved oxygen. 

Some VOCs, such as chloroform and 

PCE, were detected more frequently in 

oxygenated ground water than in nonoxygenated ground water. In contrast, 

methylene chloride and chloromethane 

were detected more frequently in nonoxygenated ground water.

Lithology—The frequency of VOC 

detections varied widely in aquifers, 

regardless of lithology, which is the 

physical characteristics of the rock 

comprising an aquifer (such as sandstone, carbonate, crystalline, basalts, 

and sand and gravel).

Occurrence of Selected VOCs

The occurrence of individual VOCs 

in ground water is governed by patterns 

of use, chemical and physical properties, 

•

•

•

3. VOCs and Urban Land Use

VOC detections generally were higher in shallow ground water underlying urban areas than 

in shallow ground water underlying other land-use areas. Specifically, 38 percent of shallow 

wells sampled by NAWQA in residential and commercial areas contained at least one VOC. 

In contrast, about 11 percent of shallow wells in agricultural areas contained at least one 

VOC. Urban areas have more sources and releases of VOCs compared to agricultural areas 

because VOCs are used extensively in industrial, commercial, and household activities. In 

addition, recharge of VOCs to ground water may be enhanced in urban areas by manmade 

structures, such as recharge basins and shallow injection wells. It is worth noting that local 

variations in VOCs in ground water do occur. For example, VOC contamination in Hawaii is 

greater in unconfined ground water underlying agricultural areas in central Oahu, where 

fumigants have been intensively applied, than in shallow ground water underlying urban 

Honolulu. Attention to potential VOC contamination in shallow ground water is warranted 

because this water can move downward into deeper aquifers used for drinking-water supply. 

Because ground-water movement is usually slow, contamination may take many years to 

disperse.
and the characteristics of the hydrologic system. The role of these factors 

is illustrated by highlighting findings 

for selected VOCs, most of which were 

commonly detected in the NAWQA 

assessment. For each compound, findings 

are presented from the (1) broad-scale 

resource characterization of 98 aquifers, 

and (2) separate analysis of drinkingwater supply wells.

Chloroform and Other THMs

Chloroform, a trihalomethane, was the 

most frequently detected VOC in aquifers 

and in drinking-water supply wells.

Chloroform was the most frequently 

detected VOC in ground water—in about 

7 percent of samples collected in the 98 

aquifer studies. Chloroform has many 

industrial uses, including the production 

of refrigerants for home air conditioners and large commercial freezers, and it 

also has been used in reagents, extraction 

solvents, fumigants, insecticides, and 

as a precursor for dyes and pesticides. 

Chloroform and other THMs are commonly produced during the chlorination 

of drinking waters and wastewaters, 

and in the disinfection of domestic and 

public wells (Ivahnenko and Zogorski, 

2006). These compounds also may be 

present in the effluent of domestic septic 

systems from the use of bleach and other 

household cleaning products that contain 

chlorine.

The extensive and relatively long 

period of use of chlorine has allowed 

ample time for recharge of waters containing chloroform to reach many of the 

sampled aquifers. Chloroform persists in 

aquifers with relatively high concentrations of dissolved oxygen and has a relatively low tendency to attach to soil and 

aquifer material. Chloroform can thereby 

move substantial distances with ground 

water and remain in the aquifer system 

for many years.

Chloroform was the most frequently 

detected VOC in samples from domestic 

and public wells—in 5 and 11 percent of 

domestic and public wells, respectively. 

Concentrations of chloroform and the 

three other THMs (bromodichloromethane, dibromochloromethane, and bromoform) were relatively low in domestic and 

public wells; concentrations were below 

the USEPA drinking-water standard 

(MCL) of 80 parts per billion, which is 

established for the summed concentration 

of all four compounds. Prolonged exposure to THMs is associated with chronic 

human-health concerns, including cancer 

and damage to the liver, kidneys, and 

central nervous system.

Artificial recharge of water containing chloroform and other THMs 

to aquifers that are used as drinkingwater supplies is becoming more common, especially in western States. The 

relatively high detection frequencies of 

chloroform in samples from domestic 

and public wells indicates that continued 

routine monitoring of these chemicals is 

needed over the long term, especially in 

areas where artificial recharge of chlorinated waters is commonly practiced.

MTBE

MTBE was among the most frequently 

detected VOCs in samples from many 

aquifers and drinking-water supply wells, 

despite its relatively recent history of use.

MTBE was the third most frequently 

detected VOC in samples from aquifer 

studies—in about 3 percent of samples 

collected in the 98 aquifer studies. The 

distribution of MTBE detections was 

not uniform among aquifers across the 

Nation. The greatest detection frequency 

of MTBE was in areas: (1) with high 

population density, (2) where MTBE 

was used as an oxygenate in reformulated gasoline, and (3) with high rates 

of ground-water recharge, such as in the 

highly populated New England and MidAtlantic States.

MTBE is an oxygenate added to 

gasoline to improve combustion and to 

reduce harmful vehicle emissions. From 

the mid-1990s, the production of MTBE 

increased dramatically as the use of 

oxygen-containing compounds in gasoline was required in areas of the Nation 

where certain air-quality standards were 

not attained. In 2005, the U.S. Congress 

passed the Energy Policy Act that eliminated the oxygen requirement in gasoline 

by mid-2006. Removal of the oxygen 

requirement is expected to decrease the 

use of MTBE in gasoline in the future.

Because of its physical properties, 

MTBE has been frequently detected in 

ground water despite its relatively short 

use history. These properties, including 

high solubility in water and low sorption to organic matter in soil and aquifer 

material, allow MTBE to reach ground 

water and to travel faster and farther than 

many other common gasoline components. In addition, the biodegradation 

of MTBE in ground water is relatively 

slow; MTBE can therefore persist longer 

in aquifer systems relative to many other 

gasoline compounds, such as benzene 

and toluene.

MTBE was detected in 3 and 5 percent of domestic and public wells, respectively. Concentrations were typically 

less than the lower limit of the USEPA 

drinking-water consumer advisory 

concentration range of 20–40 parts per 

billion, which is based on taste and odor 

thresholds. In fact, only one drinkingwater sample, which was from a domestic 

well, had a concentration of MTBE equal 

to the lower limit of the drinking-water 

consumer advisory. No Federal drinkingwater standard (MCL) exists for MTBE 

at this time. A USEPA quantitative risk 

assessment is in process; studies indicate 

that MTBE may affect the kidneys and 

liver and may have carcinogenic effects.

MTBE has been used intensively in 

gasoline for only about 10 years. Because 

of the complex nature of ground-water 

flow, and the high solubility and mobility 

of MTBE, the future distribution patterns of MTBE are unknown. Continued 

routine monitoring of MTBE is needed, 

especially in areas of high use of MTBE 

in gasoline, such as in the New England 

and Mid-Atlantic States (see sidebar 4).

PCE and TCE

PCE and TCE were among the VOCs that 

most frequently had concentrations in 

drinking-water supply wells greater than 

Federal drinking-water standards.

The chlorinated solvents PCE and 

TCE were the second and fourth most 

frequently detected VOCs in samples 

from the 98 aquifer studies—detected 

in about 4 and 3 percent of samples, 

respectively. Chlorinated solvents are 

organic compounds containing chlorine 

and are used in a variety of industrial, 

commercial, and domestic applications. 

For example, PCE is used as a solvent 

by more than 80 percent of commercial 

dry cleaners. Although TCE is commonly used as a solvent, it also can be 

formed from the biodegradation of its 

parent compound, PCE, especially in 

non-oxygenated ground-water conditions. 

Although large quantities of these solvents are still used today, their production 

began declining as early as the 1970s.
Chlorinated solvents have physical 

properties that allow them to be transported to, and be persistent in, ground 

water. In general, these solvents have relatively high densities, which allow them 

to penetrate the water table, and a high 

solubility, which allows them to move 

readily with ground water. The half-lives 

of solvents in ground water generally 

are longer than those of other commonly 

used VOCs, like gasoline hydrocarbons. 

This means that solvents can persist for 

relatively long periods of time in certain 

aquifers.

PCE and TCE were detected in 2 

and 1 percent of samples from domestic wells, respectively, and in 5 percent 

and 4 percent of samples from public 

wells, respectively. These two solvents 

accounted for a large majority of concentrations greater than Federal drinkingwater standards. Specifically, PCE and 

TCE ranked first among VOCs with concentrations greater than MCLs in samples 

collected from public wells, and second 

and third for domestic wells. Chlorinated 

solvents have been associated with cancer 

and other human-health concerns. The 

USEPA is expecting final reassessments 

of health effects of PCE, TCE, and other 

solvents during 2006–2008.

Widespread and long-term use, 

mobility and persistence in ground water, 

and a relatively large number of concentrations greater than the MCLs for 

PCE and TCE indicate that continued 

monitoring of these chlorinated solvents 

in ground water is warranted. In addition, because the production of both PCE 

and TCE has been declining over the 

past several decades, monitoring over the 

long-term will help to track any changes 

in their concentrations in ambient ground 

water and drinking-water supply wells.

DBCP and Other Fumigants

Most detections of fumigants were limited 

to areas of intensive fumigant use.

Detections of fumigants in samples 

generally were limited to areas of known 

and intense fumigant use, such as in 

California and Hawaii. Specifically, 

fumigants were detected in 30 percent of 

samples from aquifers underlying Oahu, 

Hawaii, and in 10 percent of samples 

from aquifers underlying the Central 

Valley of California. These detection frequencies compare to an overall national 

detection frequency of fumigants in the 

98 aquifer studies of about 2 percent. 

Fumigants in ground water in the Central 

Valley of California are believed to be 

associated with the historical application of DBCP to vineyards and almond 

orchards. The fumigant detections on 

Oahu have resulted, in large part, from 

fumigant applications containing EDB, 

1,2-dichloropropane, and 1,2,3-trichloropropane to pineapple fields. In 1970, 

for example, about 1.8 million pounds 

of fumigants were applied to pineapple 

fields on Oahu to combat rootworms.

Fumigant formulations containing 

DBCP, EDB, 1,2-dichloropropane, and 

1,2,3-trichloropropane were banned in 

the United States during the late 1970s 

to early 1980s after about 20 to 30 years 

of use. Fumigants continue to persist in 

ground water underlying Oahu, areas of 

California, and elsewhere because of their 

prior extensive use combined with slow 

biodegradation of the compounds.

Fumigants accounted for about twothirds of concentrations larger than drinking-water standards (MCLs) in samples 

from domestic wells. About one-half of 

these concentrations were accounted for 

by the fumigant DBCP, which was used 

from the late 1950s to the late 1970s. 

Most of the elevated concentrations were 

in samples collected in California.

References

Grady, S.J., 2003, A national survey 

of methyl tert-butyl ether and other 

volatile organic compounds in drinking-water sources—Results of the Random Survey: U.S. Geological Survey 

Water-Resources Investigations Report 

02-4079, 85 p.

Grady, S.J. and Casey, G.D., 2001, 

Occurrence and distribution of methyl 

tert-butyl ether and other volatile 

organic compounds in drinking water 

in the Northeast and Mid-Atlantic 

regions of the United States, 1993-

98: U.S. Geological Survey WaterResources Investigations Report 

00-4228, 123 p.

Ivahnenko, Tamara, and Zogorski, J.S., 

2006, Sources and occurrence of 

chloroform and other trihalomethanes 

in drinking-water supply wells in the 

United States, 1986-2001: U.S. Geological Survey Scientific Investigations 

Report 2006-5015, 13 p.

Toccalino, P.L., Rowe, B.L., and Norman, 

J.E., 2006, Volatile organic compounds 

in the Nation’s drinking-water supply 

wells—What findings may mean to 

human health: U.S. Geological Survey 

Fact Sheet 2006-3043, 4 p.

Zogorski, J.S., and others, 2006, The 

Quality of Our Nation’s Water—Volatile organic compounds in the Nation’s 

ground water and drinking-water 

supply wells. U.S. Geological Survey 

Circular 1292, 101 p.

By Michael J. Moran, Pixie A. Hamilton, and 

John S. Zogorski 

Contact for additional information 

or questions

John S. Zogorski, Project Chief, 

VOC National Synthesis

U.S. Geological Survey

1608 Mountain View Road

Rapid City, SD 57702

Email: jszogors@usgs.gov

(605) 394-3214

4. MTBE was Detected in 

Drinking Water in New 

England and Mid-Atlantic 

States

MTBE was detected in drinking water 

collected from community water systems 

in 10 New England and Mid-Atlantic 

States in a study conducted by the USGS 

in cooperation with the USEPA Office of 

Ground Water and Drinking Water (Grady 

and Casey, 2001). MTBE was detected 

in about 9 percent of the water systems.

Consistent with other assessments, 

concentrations were low; less than 

1 percent of the samples had concentrations of MTBE greater than the USEPA 

drinking-water advisory concentration 

of 20–40 parts per billion. Most concentrations (82 percent of detections) in 

drinking water were less than 5 parts per 

billion, which has been established as an 

aesthetic standard by some States.

USGS Promotes Public Access 

to Water-Quality Information

This Fact Sheet and VOC Circular are free 

of charge and are on the World Wide 

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

vocs/national_assessment. Included at 

this Web site are downloadable maps on 

VOC occurrence, information on sampling 

designs and methodology, background 

on data analyses, and frequently asked 

questions.