NATIONAL WATER-QUALITY ASSESSMENT PROGRAM, NATIONAL SYNTHESIS ON VOLATILE ORGANIC COMPOUNDS in cooperation with the U.S. Environmental Protection Agency, Office of Ground Water and Drinking Water 

Occurrence and Distribution of Volatile Organic Compounds in Drinking 

Water Supplied by Community Water Systems in the Northeast and 

Mid-Atlantic Regions of the United States, 1993-98 

—Michael Moran, Stephen Grady, and John Zogorski 

Data on volatile organic compounds 

(VOCs) in drinking water supplied by 

community water systems (CWSs) are 

available for 12 Northeast and MidAtlantic States from 1993-98. The data 

are from 2,110 CWSs representing a 

20 percent random selection of the total 

10,749 active CWSs in the region. The 

data were collected for compliance monitoring under the Safe Drinking Water 

Act from both surface- and ground-water 

sources and largely represent samples of 

finished drinking water collected prior to 

distribution. Overall, 39 percent of the 

2,110 randomly selected CWSs reported 

a detection of one or more VOCs at or 

above 1.0 μg/L (micrograms per liter). 

Although differences in analytical 

coverage complicate comparisons, in the 

1,543 CWSs with THM data at or above 

1.0 μg/L, 42 percent reported an occurrence of one or more THMs. The common detection of THMs in finished 

drinking water probably is related to 

their formation through the chlorination 

of drinking-water supplies. Comparatively, solvents, the next most frequently 

detected VOC group, were reported in 

9.8 percent of 2,097 CWSs with solvent 

data at or above 1.0 μg/L, and gasoline 

components were detected in 9.0 percent 

of 2,098 CWSs with data at or above 

1.0 μg/L. 

Individually, the THMs—chloroform, 

bromodichloromethane, chlorodibromomethane, and bromoform—were the 

most frequently detected VOCs ranging 

from 33 to 8 percent. The most frequently detected non-THM compound 

was methyl tert-butyl ether, which was 

identified in 8 percent of CWSs. Of the 

2,110 randomly selected CWSs, 

6 percent had at least one sample with 

one or more VOCs with a concentration 

above a Maximum Contaminant Level, 

Health Advisory, or Drinking-Water 

Advisory. 

VOCs were more frequently detected 

in drinking water from systems that are 

supplied by surface-water sources, or 

both surface- and ground-water sources, 

than in systems that are supplied exclusively by ground water, and from systems 

serving very large and large populations 

(serving >3,300 people) compared to 

systems serving medium and small populations (serving <3,300 people). 

Introduction 

Identifying and understanding the 

occurrence of volatile organic compounds (VOCs) in drinking water is 

important. Contamination of drinkingwater supplies by VOCs is a human 

health concern because many are toxic or 

are known or suspected human carcinogens (U.S. Environmental Protection 

Agency, 2000). The occurrence of 

VOCs in surface- and ground-water 

resources is widespread (Delzer and 

others, 1996; Lopes and Dionne, 1998; 

Squillace and others, 1999; Lopes and 

others, 2000) and the occurrence of 

VOCs in public drinking water has been 

noted for some time (Westrick and 

others, 1984). In a recent national 

survey of community water systems 

(CWSs), 38 percent of CWSs identified 

VOCs as potential contaminants in their 

source waters (U.S. Environmental 

Protection Agency, 1997). In that same 

survey, 23 percent of CWSs with exclusively surface-water sources and 

27 percent of CWSs with exclusively 

ground-water sources indicated that they 

used some type of treatment in at least 

one facility to remove organic 

contaminants (U.S. Environmental 

Protection Agency, 1997). 

Under the Safe Drinking Water Act 

(SDWA), the U.S. Environmental 

Protection Agency (USEPA) has established Maximum Contaminant Levels 

(MCLs) for 21 VOCs in drinking water 

supplied by CWSs. Drinking water 

supplied by CWSs is required to be monitored for compounds with MCLs. The 

data from the compliance monitoring of 

CWSs provide an important resource of 

information on the occurrence and distribution of both regulated and unregulated 

VOCs in drinking water from surfaceand ground-water sources. 

The primary purpose of this fact sheet 

is to describe the occurrence and distribution of VOCs in public drinking water. 

To achieve this, the U.S. Geological 

Survey (USGS), in cooperation with the 

USEPA, Office of Ground Water and 

Drinking Water, conducted an assessment of the quality of drinking water 

from CWSs in 12 Northeast and MidAtlantic States. The Northeast and MidAtlantic regions were selected for this 

study because they are generally highly 

populated, have a long-term history of 

urbanization, and are areas with high use 

of public water supply. The samples 

were collected during 1993-98 to meet 

the requirements of the SDWA and primarily represent finished drinking water 

collected prior to distribution. Information provided here on the occurrence and 

distribution of VOCs in drinking water 

within the study area summarizes a more 

comprehensive report by Grady and 

Casey (2001). 

Design Approach and Data Set 

The number, size, source, and location (by State) of CWSs selected for this 

study represented the actual distribution 

of CWSs in the region. A stratified 

random design was chosen to select the 

systems to be included in the data inventory, review, and analysis (Grady and 

Casey, 1999). This design allowed for a 

statistical comparison of data by State, 

U.S. Geological Survey USGS FS-089-01 

U.S. Department of the Interior December 2001 
source of water, and number of people 

served by the utility. As of December 1, 

1997, a total of 10,749 CWSs provided 

drinking water to more than 58 million 

people in the 12 Northeast and MidAtlantic States. Of these CWSs, 2,110, 

or about 20 percent, were randomly 

selected for this study. The stratified 

random design distributed the CWSs 

among the 12 States in direct proportion 

to their actual distribution with respect to 

the number and size of the systems, and 

source of water. 

Screening data from State health and 

water-supply agencies for the 2,110 

selected CWSs resulted in more than 

21,000 VOC analyses available for this 

assessment. The locations of 5,022 

surface- and ground-water sources for 

the 2,110 selected CWSs are shown in 

figure 1. Identification of the sources 

and their locations were obtained from a 

variety of State and Federal data bases. 

Although not every source for every 

system could be spatially located, the 

distribution shown in figure 1 includes 

the majority of sources and accurately 

reflects the spatial distribution of 

sources. 

The number of compounds analyzed 

varied from State to State and thus the 

number of analyses for each compound 

varied similarly. Chemical analyses for 

21 VOCs regulated under the SDWA and 

21 unregulated VOCs are determined 

routinely for most drinking water samples and an additional 14 VOCs may be 

included at the discretion of the individual States. In the overall data set, information for 84 individual VOCs was 

reported. The laboratory minimum 

reporting level for VOCs was variable 

among the different State and private 

laboratories that performed the analyses. For most VOCs the laboratory 

minimum reporting level was about 

0.5 μg/L (micrograms per liter). 

Occurrence and Distribution 

Chemical data were censored to a 

level of 1.0 μg/L to determine detection 

frequency for any VOC, groups of 

VOCs, and individual VOCs. This 

strategy helped to equalize variability in 

laboratory minimum reporting levels and 

allowed for the most direct comparison 

of detection frequencies among groups 

of VOCs, and among individual VOCs. 

Occurrence of VOCs by Group 

For purposes of comparison, individual VOCs were grouped into categories 

generally reflecting the predominant use 

of the compounds. The five groups 

based on predominant use are: 

(1) solvents, (2) gasoline components, 

(3) refrigerants, (4) fumigants, and 

(5) VOCs used in the synthesis of other 

chemicals. Another group also was 

included—trihalomethanes (THM) 

including chloroform, bromodichloromethane, chlorodibromomethane, and 

bromoform. Although the name of this 

group does not reflect a predominant use 

of the chemicals, THMs commonly are 

detected in finished drinking water. 

THMs are known to form in water as a 

result of reactions between disinfection 

agents and organic matter in the water. 

For CWSs that have some or all of their 

water supplied by surface-water sources, 

the frequent occurrence of THMs in 

treated water may be attributed to reactions between disinfection agents and 

organic matter in the water. The fuel 

oxygenate methyl tert-butyl ether 

(MTBE) was included in the gasoline 

components group. MTBE has recently 

emerged as a contaminant of concern 

-78 ̊ -74 ̊ 

that is commonly found at low concentrations in surface and ground water 

(Zogorski and others, 1998; Moran and 

others, 1999) and now must be monitored in some CWSs as an unregulated 

contaminant (U.S. Environmental 

Protection Agency, 1999). 

Of the 2,110 randomly selected 

CWSs, 39 percent had a detection of one 

or more VOCs at or above 1.0 μg/L. The 

distribution ofsurface- and ground-water 

sources from CWSs with and without a 

detection of any VOC is shown in 

figure 1. The sources indicated as 

having a detection of any VOC in 

figure 1 include all of the sources for a 

system with a detection and do not indicate the specific source that had the 

detection. 

The THMs were by far the most 

commonly occurring VOC group and 

were detected in 42 percent of the 1,543 

CWSs with data (table 1). The common 

detection of THMs in finished drinking 

water probably is related to their formation through the chlorination of drinkingwater supplies. Chlorination of 

drinking-water supplies for disinfection 

has been known to result in the formation of THMs through reactions with 

-70 ̊ -66 ̊ -62 ̊ 

48 ̊ 

44 ̊ 

40 ̊ 

36 ̊ 

Base from U.S. Geological Survey digital data 1:2,000,000 

Albers projection 

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EXPLANATION

SOURCES WITH VOC DETECTIONS 

Ground-water source 

Surface-water source 

SOURCES WITHOUT VOC DETECTIONS 

Ground-water source 

Surface-water source 

Maine 

New Hampshire 

New York 

Vermont 

Connecticut

Rhode Island 

New Jersey 

Pennsylvania 

Maryland 

Virginia 

Delaware 

Massachusetts 

0 100 200 Miles 

Figure 1. Drinking-water sources for randomly selected community water systems 

in the study area, showing sources with analytical data and sources with reported 

detectable concentrations of any volatile organic compound (VOC) in any source in 

the system at or above 1.0 microgram per liter. 
Bromodichloromethane

1,1-Dichloroethane

naturally occurring organic material in 

the water. Solvents were detected in 

9.8 percent of CWSs, and gasoline components were detected in 9.0 percent of 

CWSs. The pattern of detection of 

VOCs by group was statistically significant at the 95-percent confidence interval (p <0.0001). In general, VOCs in 

groups that are more widely used, such 

as solvents and gasoline components, 

were detected more frequently than 

VOCs in groups that are not as widely 

used, such as organic synthesis compounds, refrigerants, and fumigants 

(table 1). 

Occurrence of Individual VOCs 

Of the 84 individual VOCs analyzed, 

54 were detected in at least one sample at 

or above 1.0 μg/L. Detection frequencies for the 20 individual VOCs that 

were reported in more than 1 percent of 

CWSs are shown in figure 2. Individually, the four THMs—chloroform, bromodichloromethane, chlorodibromomethane, and bromoform—were the 

most frequently detected VOCs, ranging 

from 33 to 8 percent. When the concentrations of the four THMs were summed, 

the resultant total THMs were detected 

in 42 percent of the CWSs with data. 

The most frequently detected non-THM 

compound was MTBE, which was identified in 8 percent of CWSs. The frequency of MTBE detections in drinking 

water is significantly related to high 

Table 1. Number of systems with analyses of one or more compounds in groups of volatile 

organic compounds, number of systems with a detection of one or more compounds in groups of 

volatile organic compounds at or above 1.0 micrograms per liter, and frequency of detection of 

groups of volatile organic compounds in drinking water from randomly selected community water 

systems in the study area 

[VOCs, volatile organic compounds; μg/L, micrograms per liter] 

Number of systems with Number of systems with Frequency 

VOC group analyses of one or more detection of one or more of 

VOCs in group VOCs in group at 1.0 μg/L detection 

Trihalomethanes 1,543 

Solvents 2,097 

Gasoline components 2,098 

Refrigerants 1,669 

Organic synthesis 2,098 

Fumigants 2,102 

644 41.7 

206 9.8 

188 9.0 

36 2.2 

24 1.1 

18 .9 

MTBE-use patterns. Detections of 

MTBE were about five times more likely 

in areas where MTBE is or has been used 

in gasoline at greater than 5 percent by 

volume as part of the oxygenated or 

reformulated (OXY/RFG) fuels program. 

Thirty-four other VOCs also were 

detected at or above 1.0 μg/L, but all 

were detected in less than 1 percent of 

CWSs. 

Concentrations of VOCs 

Concentrations of VOCs in drinkingwater samples ranged from 0.01 μg/L for 

ethylene dibromide and dibromochloropropane to 930 μg/L for trichloroethene. Of the 2,110 randomly selected 

CWSs, 6 percent had one or more samples with one or more VOCs with a concentration above the MCL, Health 

Advisory (HA), or Drinking-Water 

Advisory (DWA) (table 2). Concentrations of total THMs at or above 100 μg/L 

in 46 CWSs were the most common 

cause for exceeding the MCL (table 2), 

but concentrations of the solvents tetrachloroethene and trichloroethene at or 

above 5 μg/L also were reported in more 

than 1 percent of CWSs. 

Distribution of VOCs 

The frequency of detection of any 

VOC at or above 1.0 μg/L in one or more 

samples from CWSs varied considerably 

from State to State. Every State had a 

detection frequency greater than zero 

with a range from 22 to 68 percent. The 

highest detection frequencies occurred in 

States with high population densities and 

extensive urban development. The 

lowest VOC detection frequencies occurred in States with substantial rural regions. 

Overall, VOCs were detected 

at or above 1.0 μg/L three times 

more frequently in drinking 

water from systems that are 

supplied by surface-water 

sources, or both surface- and 

ground-water sources, than in 

systems that are supplied exclusively by ground water. The 

detections of VOCs also were 

more frequent in very large and 

large systems (serving > 3,300 

people) compared to medium 

and small systems (serving 

<3,300 people). The detection 

frequency of any VOC in very 

large systems supplied partly by 

surface water was driven by the 

frequent occurrence of THMs. 

Other VOCs also were detected 

more frequently in drinking 

water from the larger systems 

than in drinking water from 

smaller systems. 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

Gasoline components 

Refrigerants 

Trihalomethanes 

Solvents 

Figure 2. Frequency of detection of volatile organic compounds at concentrations equal to or exceeding 

1.0 micrograms per liter in drinking water from randomly selected community water systems (CWSs) in the 

study area for 20 volatile organic compounds with detection frequencies greater than 1 percent. PERCENT OF CWSs 

Total trihalomethanes

Chloroform

Chlorodibromomethane

Bromoform 

Methyl tert-butyl ether

1,1,1-Trichloroethane

Trichloroethene

Tetrachloroethene

Total xylenes

Toluene

Dichloromethane

cis-1,2-Dichloroethene

-Xylenes

m- & p

Ethylbenzene

o-Xylene

1,1-Dichloroethene

Chloromethane

Dichlorodifluoromethane 
Table 2. Volatile organic compounds that equaled or exceeded regulated or recommended 

maximum concentrations in drinking water from randomly selected community water systems 

in the study area, 1993-98 

[μg/L, micrograms per liter; MCL, Maximum Contaminant Level; HA, Health Advisory; DWA, Drinking-Water Advisory; 

CWSs, community water systems] 

Drinking-water regulation or Number of CWSs 

Volatile organic compound recommended maximum concentration reporting concentrations 

that equaled or 

Value, in μg/L Type exceeded value 

Total trihalomethanes 100 MCL 46 

Tetrachloroethene 5 MCL 32 

Trichloroethene 5 MCL 25 

Chloromethane 3 HA 11 

Methyl tert-butyl ether 20 DWA (lower limit) 10 

Dichloromethane 5 MCL 8 

1,1-Dichloroethene 7 MCL 6 

1,2-Dibromoethane 0.05 MCL 5 

Chloroethene 2 MCL 4 

Dibromochloropropane 0.2 MCL 3 

1,2-Dichloropropane 5 MCL 3 

Benzene 5 MCL 2 

1,2-Dichloroethane 5 MCL 2 

cis-1,2-Dichloroethene 70 MCL 2 

1,1,2,3,4,4-Hexachloro-1,3-butadiene 1 HA 1 

Tetrachloromethane 5 MCL 1 

Conclusions and Implications	 trichloroethene exceeded standards fairly 

frequently. Because health standards or 

The detection of VOCs at or above criteria have been exceeded in some 

1.0 μg/L in water supplied by CWSs was drinking water, a better understanding of 

fairly common. The frequent detection the sources, transport, and receptors of 

of THMs as a group and as individual VOCs that exceed these values would


compounds is not surprising considering help in developing plans for better 


that many CWSs use chlorination for protecting source waters from contami­
 disinfection of drinking water. However, nation. 


other anthropogenic or natural sources


could be responsible for at least some of Acknowledgments


the concentrations of these compounds


detected in drinking water. Further work The authors acknowledge the help


may help to clarify the potential contri- and cooperation provided by State and


bution of THMs from other sources and local health and environmental agencies


lead to a better understanding of their in the 12 States, without which this study


transport to drinking-water wells. Com- could not have been completed. This


pounds in groups that are commonly study was done in cooperation with the 


used such as solvents and gasoline com- U.S. Environmental Protection Agency.


ponents were frequently detected. The 


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