Sources and Occurrence of Chloroform and 

Other Trihalomethanes in Drinking-Water 

Supply Wells in the United States, 

1986–2001

By Tamara Ivahnenko and John S. Zogorski

Scientific Investigations Report 2006–5015

U.S. Department of the Interior

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

P. Lynn Scarlett, Acting Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

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

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Suggested citation:

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.
iii

FOREWORD

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. (http://www.usgs.

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

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

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

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

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

long-term sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program to support 

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

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

What is the condition of our Nation’s streams and ground water? How are the conditions changing over time? How do natural features and human activities affect the quality of streams and 

ground water, and where are those effects most pronounced? By combining information on 

water chemistry, physical characteristics, stream habitat, and aquatic life, the NAWQA Program 

aims to provide science-based insights for current and emerging water issues and priorities. 

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

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

than 50 of the Nation’s most important river basins and aquifers, referred to as Study Units. 

(http://water.usgs.gov/nawqa/nawqamap.html). Collectively, these Study Units account for more 

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

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

agricultural, urban, and natural sources of contamination.

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

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

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

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

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

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

Nation’s diverse geographic and environmental settings. Comprehensive 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. (http://water.usgs.

gov/nawqa/natsyn.html).

Robert M. Hirsch

Associate Director for Water
iv

Contents

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

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Field Sampling and Laboratory Analytical Methods.......................................................................2

Acknowledgments................................................................................................................................2

Potential Sources of Trihalomethanes to Drinking-Water Supply Wells..............................................3

Occurrence of Trihalomethanes in Drinking-Water Supply Wells ........................................................4

Evidence for an Anthropogenic Origin of Trihalomethanes in Drinking-Water Supply Wells ..........4

Implications for Future Research..............................................................................................................10

Summary........................................................................................................................................................10

References....................................................................................................................................................11

Figures

1. Graph showing detection frequencies of the 10 most frequently detected volatile 

organic compounds in (A) public well samples, and (B) domestic well samples at 

an assessment level of 0.2 microgram per liter. ......................................................................4

2–3. Maps showing:

 2. Total trihalomethane concentrations of 0.2 microgram per liter or greater in 

public well samples...............................................................................................................5

 3. Total trihalomethane concentrations of 0.2 microgram per liter or greater in

domestic well samples. ........................................................................................................6

4–6. Graphs showing:

 4. Concentrations of trihalomethanes in public well samples...........................................7

 5. Concentrations of trihalomethanes in domestic well samples. ....................................8

 6. Detection frequencies of chloroform at 0.2 microgram per liter or greater in

agricultural and urban land-use areas ..............................................................................9

Tables

1. Detection frequencies of trihalomethanes in samples from public 

and domestic wells at an assessment level of 0.2 microgram per liter...............................5

2. Population density and percent of population classified as living in 

urban land use in 500-meter radius buffer around public and domestic wells..................6

3. Number of trihalomethanes detected at an assessment level of 0.2 microgram 

per liter and the percentage of samples with detections in samples from public 

and domestic wells, 1986-2001. ..................................................................................................9

4. Most frequently occurring two-compound mixtures in public well samples at an

assessment level of 0.2 microgram per liter...........................................................................10
Abbreviations and Acronyms:

μg/L micrograms per liter > greater than ≥ greater than or equal to

Br Bromide

CaCl

2

 Calcium chloride

CHBr

2

Cl Dibromochloromethane

CHBr

3

 Tribromomethane (bromoform)

CHBrCl

2

 Bromodichloromethane

CHCl

3

 Trichloromethane (chloroform)

CFCs Chlorofluorocarbons

Cl Chloride

CWSs Community water systems

HCFC-22 Hydrochlorofluorocarbon-22

MCL Maximum Contaminant Level

HOBr Hypobromous acid

HOCl Hypochlorous acid

MTBE Methyl-tert butyl ether

NaCl Sodium chloride

NAWQA National Water-Quality Assessment Program

PWSs Public water systems

RCRA Resource Conservation and Recovery Act

THMs Trihalomethanes

TTHM Total trihalomethane

TRI Toxic Release Inventory

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VOCs Volatile organic compounds

Sources and Occurrence of Chloroform and Other 

Trihalomethanes in Drinking-Water Supply Wells in the 

United States, 1986–2001

By Tamara Ivahnenko and John S. Zogorski

Abstract

Chloroform and three other trihalomethanes (THMs)—

bromodichloromethane, dibromochloromethane, and bromoform—are disinfection by-products commonly produced 

during the chlorination of water and wastewater. Samples 

of untreated ground water from drinking-water supply wells 

(1,096 public and 2,400 domestic wells) were analyzed for 

THMs and other volatile organic compounds (VOCs) during 1986–2001, or compiled, as part of the U.S. Geological 

Survey’s National Water-Quality Assessment Program. This 

report provides a summary of potential sources of THMs and 

of the occurrence and geographical distribution of THMs in 

samples from public and domestic wells. Evidence for an 

anthropogenic source of THMs and implications for future 

research also are presented.

Potential sources of THMs to both public and domestic 

wells include the discharge of chlorinated drinking water and 

wastewater that may be intentional or inadvertent. Intentional 

discharge includes the use of municipally supplied chlorinated 

water to irrigate lawns, golf courses, parks, gardens, and other 

areas; the use of septic systems; or the regulated discharge 

of chlorinated wastewater to surface waters or ground-water 

recharge facilities. Inadvertent discharge includes leakage of 

chlorinated water from swimming pools, spas, or distribution 

systems for drinking water or wastewater sewers. Statistical 

analyses indicate that population density, the percentage of 

urban land, and the number of Resource Conservation and 

Recovery Act hazardous-waste facilities near sampled wells 

are significantly associated with the probability of detection of 

chloroform, especially for public wells. Domestic wells may 

have several other sources of THMs, including the practice of 

well disinfection through shock chlorination, laundry wastewater containing bleach, and septic system effluent.

Chloroform was the most frequently detected VOC in 

samples from drinking-water supply wells (public and domestic wells) in the United States. Although chloroform was 

detected frequently in samples from public and domestic wells 

and the other THMs were detected in some samples, no concentrations in samples from either well type exceeded the U.S. 

Environmental Protection Agency’s Maximum Contaminant 

Level of 80 micrograms per liter for total THMs. Chloroform 

was detected in public well samples almost twice as frequently 

(11 percent) as in domestic well samples (5 percent). The other 

three THMs also were detected more frequently in public well 

samples than in domestic well samples. This detection pattern 

may be attributed to public wells having a higher pumping 

capacity than domestic wells. The higher capacity wells create 

a larger capture zone that potentially intercepts more urban 

and other land uses and associated point and nonpoint sources 

of contamination than the smaller capacity domestic wells.

THM detection frequencies in domestic well samples 

show a pattern of decreasing frequency with increasing bromide content, that is in the order: chloroform > bromodichloromethane ≥ dibromochloromethane ≥ bromoform. This same 

pattern has been documented in studies of water chlorination, 

indicating that an important source of chloroform and other 

THMs in drinking-water supply wells may be the recycling of 

chlorinated water and wastewater. Mixtures of THMs commonly occur in public well samples, and the most frequently 

occurring are combinations of the brominated THMs. These 

THMs have limited industrial production, few natural sources, 

and small or no reported direct releases to the environment. 

Therefore, industrial, commercial, or natural sources are not 

likely sources of the brominated THMs in public and domestic 

well samples. The THM detection frequency pattern, the cooccurrence of brominated THMs, and other lines of evidence 

indicate that the recycling of water with a history of chlorination is an important source of these compounds in samples 

from drinking-water supply wells.

Introduction

Chloroform—also known by its formal name as trichloromethane (CHCl3

)—was the most frequently detected volatile 

organic compound (VOC) in the Nation’s ground water 

sampled by the U.S. Geological Survey’s (USGS) National 

Water-Quality Assessment (NAWQA) Program (Squillace and 

others, 1999; Moran and others, 2002; Zogorski and others, 

2006). Grady (2003) also reported chloroform as the most frequently detected VOC in a national study of both ground- and 
surface-water sources of drinking water for community water 

systems (CWSs).

Chloroform and three other detected trihalomethanes (THMs)—bromodichloromethane, dibromochloromethane, and bromoform—are disinfection by-products commonly 

produced during the chlorination of water and wastewater. 

Strictly speaking, “trihalomethanes” applies to trihalogenated 

methanes containing any of the halogen atoms (chlorine, 

bromine, iodine, or fluorine), but this term will be used in 

this report to refer only to those containing either chlorine or 

bromine because these are the ones most commonly detected, 

or analyzed for, in chlorinated waters. The practice of treating large amounts of surface and ground water with chlorine 

for public health reasons began in the United States in Jersey 

City, New Jersey, in 1908, and since has become a standard 

procedure for CWSs across the Nation (Sawyer and McCarty, 

1978). The primary purpose of chlorination of drinking water 

is to prevent the spread of waterborne diseases, especially fatal 

diseases such as cholera and typhoid. Although chlorination 

has been used to treat drinking water nationwide for almost 

100 years, chloroform and other THMs were first identified by 

J.J. Rook in 1974 as being produced during the chlorination 

process (Rook, 1974).

The presence of chloroform in drinking water is a 

potential human-health concern. In 1986, the U.S. Environmental Protection Agency (USEPA) classified chloroform as a 

probable human carcinogen (“Group B2” carcinogen), based 

on evidence of its carcinogenicity in animals (U.S. Environmental Protection Agency, 2002). In 1998, the USEPA revised 

the carcinogenicity risk assessment of chloroform, stating 

that the compound is likely to be carcinogenic to humans by 

all routes of exposure at concentrations high enough to cause 

cytotoxicity and formation of regenerative nodules in susceptible tissues (U.S. Environmental Protection Agency, 2002). 

Bromodichloromethane and bromoform also are suspected 

human carcinogens, whereas insufficient data exist to assess 

the human carcinogenic potential of dichlorobromomethane 

(U.S. Environmental Protection Agency, 2004).

Purpose and Scope

This report provides a summary of potential sources of 

THMs and of the occurrence and geographical distribution 

of THMs in samples collected or compiled by the USGS’s 

NAWQA Program from drinking-water supply wells (public 

and domestic wells) during 1986–2001. Samples of untreated 

water (collected at the wellhead and before any treatment or 

blending) from 1,096 public and 2,400 domestic wells were 

collected nationally between 1986 and 2001 or compiled as 

part of the NAWQA Program. A detailed description of the 

public and domestic well data sets is documented in Zogorski and others (2006) and Moran and others (2006). Previously, Squillace and others (1999) reported the occurrence of 

THMs and other VOCs in untreated ambient ground water for 

samples collected during 1985–1995. Evidence of recycling of 

chlorinated water as a source of anthropogenic chloroform and 

other THMs also is presented in this report. Implications for 

future research also are described. A more detailed summary 

of chloroform sources (including information on industrial, 

degradation, and natural sources), formation of THMs as disinfection by-products, transport, fate, occurrence, and effects 

of THMs on human health and aquatic organisms is presented 

in Ivahnenko and Barbash (2004).

Public wells are privately or publicly owned and provide 

ground-water supply for public water systems (PWSs). In this 

report, the discussion of public wells refers to the quality of 

water captured by wells that provide drinking water to PWSs 

that serve at least 25 individuals. Domestic wells are defined 

as privately owned, self-supplied sources for household water 

use (Moran and others, 2002).

Field Sampling and Laboratory Analytical 

Methods

NAWQA data-collection protocols and procedures for 

ground water (described in Koterba and others, 1995) were 

followed for the collection of samples for VOC analysis, 

as well as to characterize the water quality of the sampled 

aquifer. Laboratory analysis, including the description of the 

analytical methods and the associated quality-assurance/quality-control program, for the suite of 55 VOCs (including the 

four THMs) for the NAWQA Program are presented in Rose 

and Schroeder (1995) and Connor and others (1998). A summary of both the field and laboratory methods are described in 

Moran and others (2006).

In this report, all computations, including detection frequencies, were calculated at an assessment level of 0.2 microgram per liter (μg/L). The 0.2-μg/L assessment level represents 

the laboratory reporting value for the USGS prior to 1996 and 

for other data-contributing agencies. Because of improved 

field sampling protocols and analytical technology, the USGS 

laboratory reporting value was lowered after 1996; however, 

for consistency, concentrations of THMs in samples collected 

after 1996 were censored to 0.2 μg/L. The computation of total 

THMs in this report is the sum of all quantified concentrations 

of chloroform, bromodichloromethane, dibromochloromethane, and bromoform, censored to 0.2 μg/L.

Acknowledgments

The author thanks all the PWS managers and private 

homeowners for allowing water samples to be collected from 

their wells. Through their cooperation, a nationwide data set 

of water-quality information has been generated, providing 

researchers with the information to assess the state of the 

Nation’s aquifers and drinking-water supply wells at local, 

regional, and national levels. The author also thanks the many 

USGS field technicians, hydrologists, and laboratory personnel who have collected and analyzed the water samples during 

the first decade of the NAWQA Program.

   Sources and Occurrence of Chloroform and Other Trihalomethanes in Drinking-Water Supply Wells in the United States, 1986–2001
Potential Sources of Trihalomethanes 

to Drinking-Water Supply Wells

Chloroform and other THMs could originate from 

sources other than as by-products of water disinfection, and 

some of these industrial sources are documented on a nationwide basis in the USEPA’s Toxic Release Inventory (TRI). In 

the United States, these direct releases to the environment have 

been reported annually and include air emissions, surfacewater discharges, underground injections, and releases to land. 

According to the TRI, approximately 1.1 million pounds of 

chloroform were released in 2003 by these routes across the 

Nation (U.S. Environmental Protection Agency, 2005a). The 

paper industry contributed 52 percent of the total emissions 

(mostly as air emissions), the chemical industry contributed 

28 percent (mostly as air emissions), the Resource Conservation and Recovery Act (RCRA) solvent recovery industry contributed 12 percent (nearly all as underground injections), and 

the food industry contributed 8 percent (all as air emissions) 

(U.S. Environmental Protection Agency, 2005a). According 

to the TRI, dichlorobromomethane and bromoform had total 

releases of 89 and 182 pounds, respectively, in 2003 (U.S. 

Environmental Protection Agency, 2005a). For both compounds, the chemical industry was the primary contributor, 

either as waste to landfills or as air emissions. Dibromochloromethane is not listed in the TRI compilations.

The major industrial use of chloroform is for the production of hydrochlorofluorocarbon-22 (HCFC-22), a refrigerant 

to replace the ozone-depleting chlorofluorocarbons (CFCs) in 

home air conditioners and large commercial freezers. In 1999, 

98 percent of the industrially produced chloroform was used 

in the manufacture of HCFC-22. The remaining 2 percent was 

used for a variety of commercial purposes including reagents 

and extraction solvents (Budvari, 1989; Mannsville Chemical 

Products Corporation, 1999), fumigants, insecticides, and a 

precursor for dyes and pesticides (Lucius and others, 1992; 

Agency of Toxic Substances and Disease Registry, 1997). In 

1998, the most recent year with production data, about 46 million gallons of chloroform were produced by industry.

Industrial use of the other THMs (bromodichloromethane, dibromochloromethane, and bromoform) has been as solvents for fats, waxes and resins, fire-extinguishing agents, and 

an intermediate in chemical synthesis. Currently (2005), dibromochloromethane has no commercial use (Agency of Toxic 

Substances and Disease Registry, 2005), and bromodichloromethane primarily is used only as a chemical intermediate for 

organic synthesis (Agency of Toxic Substances and Disease 

Registry, 1989). Because of the limited or non-existent use of 

both dibromochloromethane and bromodichloromethane, no 

information on industrial production volume for these compounds is available. However, total industrial production of 

bromoform in 1997, the most recent year with production data, 

was estimated to be as much as 45,700 gallons (Agency of 

Toxic Substances and Disease Registry, 2005).

An important source of anthropogenic chloroform 

detected in ground water and surface water appears to be 

chlorinated drinking water and wastewater that is discharged 

intentionally into the hydrologic system. This includes wastewater from the bleaching processes in pulp and paper mills, 

wastewater generated from the manufacturing and processing 

of chloroform-containing chemicals and materials, effluent 

from hazardous-waste disposal sites, and rinse water from 

domestic and industrial cleaning and laundry operations. Other 

such sources include regulated discharges of chlorinated water 

from wastewater treatment plants, cooling tower blowdown 

from electric power generating plants, and combined sewer 

overflows (Agency for Toxic Substances and Disease Registry, 1997). Most of these intentional discharges, however, 

are expected to represent only minor contributions to total 

chloroform releases to the hydrologic system, relative to the 

amount of chloroform contributed by inadvertent releases of 

chlorinated drinking water and wastewater (Agency for Toxic 

Substances and Disease Registry, 1997).

Chlorinated drinking water from PWSs may enter the 

subsurface through the irrigation of lawns, gardens, golf 

courses, athletic fields, and parks, as well as leaking swimming pools, spas, or sewers for treated wastewater. In addition, 

the maintenance of a chlorine residual is required in the distribution systems of PWSs and increases the likelihood that leakage from these systems also will lead to the contamination of 

ground water and surface water by chloroform and other disinfection by-products. Solley and others (1998) estimated that 

the loss of treated drinking water through leaking distribution 

pipes and other unknown routes may be as high as 15 percent 

in some systems. Overall, an estimated 0.4 million gallons of 

chloroform are generated per year through the chlorination of 

drinking water in the United States (McCulloch, 2003).

Although most homeowners who obtain their water 

from private wells do not chlorinate their water prior to use, 

chloroform was found to be the most frequently detected VOC 

in domestic well water. Potential sources of chloroform to 

domestic wells include (1) shock chlorination, which is a periodic disinfection process in which a dilute solution of bleach 

is added directly to a well to eliminate bacterial contamination, and (2) laundry wastewater containing bleach. In both 

cases, chloroform is produced through the haloform reaction 

of chlorine (contained in bleach) with organic matter.

Septic systems also may be an important source of 

chloroform to ground water. Previous studies have documented the presence of chloroform in septic-system effluent 

(DeWalle and others, 1985; Ayers Associates, 1993), and 

several States have identified septic systems as a major source 

of ground-water contamination, second only to underground 

storage tanks (U.S. Environmental Protection Agency, 2000). 

In addition, the U.S. Environmental Protection Agency (2000) 

has noted that improperly designed, maintained, or operated 

septic systems can result in ground-water contamination in 

the vicinity of the system, especially if the degradation of the 

organic matter is incomplete. The detection of chloroform or 

other THMs in a domestic well sample could be an indicator 

Potential Sources of Trihalomethanes to Drinking-Water Supply Wells 
Figure 1. Detection frequencies of the 10 most frequently 

detected volatile organic compounds in (A) public well 

samples, and (B) domestic well samples at an assessment 

level of 0.2 microgram per liter.

Chloroform (THM)

Methyl tert-butyl ether

Perchloroethene

1,1,1-Trichloroethane

Dichlorodifluoromethane

Trichloroethene

Bromoform (THM)

Bromodichloromethane (THM)

Dibromochloromethane (THM)

1,1-Dichloroethane

0 2 4 6 8 10 12

DETECTION FREQUENCY, IN PERCENT

Chloroform (THM)

Methyl tert-butyl ether

Perchloroethene

1,1,1-Trichloroethane

Dichlorodifluoromethane

Toluene

Chloromethane

Trichloroethene

Dibromochloropropane

Methylene chloride

A

B

THM = trihalomethane

of other possible health concerns (pathogenic microorganisms, 

for example) that may exist for the domestic well user (especially for those with untreated well water) whose well is in the 

vicinity of an improperly operating septic system.

Chloroform was originally considered of solely anthropogenic origins; however, several natural sources of chloroform 

have been identified. These include volcanic gases (Isidorov 

and others, 1990), marine algae (Gribble, 1994; Laturnus and 

others, 2002), and soil fungi (McCulloch, 2003). Although a 

natural substance, salt concentrations (NaCl and CaCl2) have 

increased in the environment because of the use of these compounds for roadway deicing. A recent study (Princeton Geoscience, 2005) indicated that the abundance of chloride ions in 

the runoff of deicing meltwaters may be enhancing the natural 

chloroform formation process in nearby soils. Chloroform 

concentrations as high as 110 μg/L were determined in samples of shallow ground water adjacent to the paved surface. 

Although natural sources contribute approximately 90 percent 

of the total global calculated chloroform flux (McCulloch, 

2003), evidence in the NAWQA-collected drinking-water well 

data is inconclusive about whether natural sources (including 

roadway deicing salts) contribute (and in what proportion) 

to chloroform in ground water. Marine algae also have been 

identified as natural sources for bromodichloromethane, 

dibromochloromethane, and bromoform (Gribble, 1994).

Occurrence of Trihalomethanes in 

Drinking-Water Supply Wells

Of the 55 VOCs analyzed for by the NAWQA Program 

(Bender and others, 1999), chloroform was the most frequently detected VOC in samples from drinking-water supply 

wells. Chloroform was detected about twice as frequently 

in public well samples (11 percent) than in domestic well 

samples (5 percent), whereas other THMs occurred in public 

well samples 7 to 15 times more frequently than in domestic 

well samples (table 1). The other THMs (bromodichloromethane, dibromochloromethane, and bromoform) also were 

among the 10 most frequently detected VOCs in public well 

samples (fig. 1). Because the purpose of the NAWQA Program 

was to sample ambient, untreated ground water from different 

aquifers, the frequent and widespread occurrence of chloroform and the detection of other THMs in some samples was 

not expected.

The locations of the sampled wells and samples with 

total trihalomethane (TTHM) concentrations greater than 

0.2 μg/L are shown in figure 2 for public wells and in figure 3 

for domestic wells. Although chloroform and other THMs 

were detected frequently in public and domestic well samples, 

most TTHM concentrations were an order of magnitude lower 

than the USEPA Maximum Contaminant Level (MCL) of 

80 μg/L. No individual THM concentrations in samples from 

either well type were greater than the USEPA’s TTHM MCL 

of 80 μg/L (figs. 4 and 5). Median concentrations for individual THMs in samples with detections, from both public and 

domestic wells, ranged from 0.4 to 0.8 μg/L (figs. 4 and 5). 

Comparisons of THM concentrations to the USEPA’s MCL are 

for informational purposes only, and do not indicate compliance with the drinking-water standard for water supplied by 

PWSs.

Evidence for an Anthropogenic Origin 

of Trihalomethanes in Drinking-Water 

Supply Wells

The greater frequency of THM detection in public well 

samples than in domestic well samples may be due to public 

wells having greater pumping capacities than domestic wells. 

These higher production capacities in public wells create a 

larger capture zone, greater drawdown, and faster movement 

of water from the top of the water table (including water 

containing surface-derived anthropogenic compounds that 

have migrated to the water table) to the well screen than the 

comparatively low production capacities of domestic wells 

(Bruce and Oelsner, 2001). The larger capture zone of the 

public wells also may include multiple land uses and point (as 

well as nonpoint) sources of VOCs (Stackelberg and others, 

2000, 2001).

 Sources and Occurrence of Chloroform and Other Trihalomethanes in Drinking-Water Supply Wells in the United States, 1986–2001
Figure 2. Total trihalomethane concentrations of 0.2 microgram per liter or greater in public well samples.

Alaska

Hawaii

Total trihalomethane concentration,

 in micrograms per liter

No detection or less than 0.2

0.2 or greater

EXPLANATION

0 200 400 MILES

0 200 400 KILOMETERS

Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1990

Albers Equal-Area Projection

North American Datum of 1983

Table 1. Detection frequencies of trihalomethanes in samples from public and domestic wells at 

an assessment level of 0.2 microgram per liter.

Compound name Chemical 

formula

Number of 

samples

Number of 

detections

Detection frequency, 

in percent

Public wells

Chloroform CHCl3 1,092 125 11.4

Bromodichloromethane CHBrCl2 1,095 46 4.2

Dibromochloromethane CHBr2Cl 1,095 48 4.4

Bromoform CHBr3 1,096 49 4.5

Domestic wells

Chloroform CHCl3 2,400 124 5.2

Bromodichloromethane CHBrCl2 2,400 14 0.6

Dibromochloromethane CHBr2Cl 2,400 12 0.5

Bromoform CHBr3 2,399 8 0.3

Evidence for an Anthropogenic Origin of Trihalomethanes in Drinking-Water Supply Wells 
Figure 3. Total trihalomethane concentrations of 0.2 microgram per liter or greater in domestic well samples.

Alaska

Hawaii

No detection or less than 0.2

0.2 or greater

EXPLANATION

Total trihalomethane concentration,

 in micrograms per liter

0 200 400 MILES

0 200 400 KILOMETERS

Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1990 

Albers Equal-Area Projection 

North American Datum of 1983 

The greater detection frequency of chloroform and other 

THMs in public well samples than in domestic well samples 

also may be due to the larger likelihood of THM sources in 

the capture zone as indicated by the greater population density 

and greater amount of land classified as urban in proximity 

to public wells. Squillace and others (1999) reported that the 

probability of detecting one or more VOCs in ground water 

increased with increasing population density. Spatial analysis of 500-meter radius buffers around public and domestic 

wells using the 1990 Census data (U.S. Bureau of the Census, 

1991) indicates that the buffers for public wells have an average population density about six times greater than buffers 

for domestic wells (table 2). In addition, a threefold greater 

percentage of the population was classified as living in urban 

areas near public wells than near domestic wells.

Chloroform and other THMs were detected in ground 

water beneath a broad range of land-use settings including 

not only urban, residential, and industrial locations, but also 

agricultural, forested, and other undeveloped areas. As might 

have been expected, among those studies that compared different land-use settings, detection frequencies of chloroform 

were higher beneath urban areas than beneath agricultural 

areas (fig. 6) (Stackelberg and others, 1997; Grady and Mullaney, 1998; Squillace and others, 1999). In addition, the 

percentage of urban land and RCRA hazardous-waste facilities 

near sampled wells have been shown to be factors statistically 

associated with the detections of chloroform in public and 

domestic well samples (Zogorski and others, 2006; Squillace 

and Moran, 2006).

Table 2. Population density and percent of population classified 

as living in urban land use in 500-meter radius buffer around 

public and domestic wells.

Type of wells

Average population 

density 

(persons per square 

mile)

Urban population as 

percentage of total 

population

Public wells 423 41

Domestic wells 73 14

   Sources and Occurrence of Chloroform and Other Trihalomethanes in Drinking-Water Supply Wells in the United States, 1986–2001
14.8

11.4

4.2

4.4

4.5

163

125

46

48

49

1,096

1,092

1,095

1,095

1,096

The MCL of 80 micrograms per liter is for the sum of the concentrations of four trihalomethanes including chloroform, 

bromodichloromethane, dibromochloromethane, and bromoform (U.S. Environmental Protection Agency, 2004).

1

Detection

frequency,

 in percent

Number

of

detections

Number

of

samples

0.001 0.01 0.1 1 10 100

CONCENTRATION, IN MICROGRAMS PER LITER

14.8

163

1,096

EXPLANATION

VOC concentration for a sample—Results from several analytical methods, with differing sensitivity for a

specific VOC with time, as well as between VOCs, are shown in the bar concentration chart. Multiple

samples of equal concentration will appear as a single symbol

U.S. Environmental Protection Agency Maximum Contaminant Level (MCL) for total trihalomethanes

Detection frequency, in percent, at an assessment level of 0.2 microgram per liter

Median concentration

Number of detections at an assessment level of 0.2 microgram per liter

Number of samples

Assessment level of 0.2 microgram per liter

Total trihalomethanes1

Chloroform

Bromodichloromethane

Dibromochloromethane

Bromoform

Figure 4. Concentrations of trihalomethanes in public well samples.

When the bromide ion is present during chlorination, 

either as a naturally occurring substance or as a commonly 

occurring contaminant in the chlorine used for disinfection 

(U.S. Environmental Protection Agency, 2005b), the haloform 

reaction generally produces one or more of the brominated 

THMs in addition to chloroform (Boyce and Hornig, 1983). 

This occurs even if bromide concentrations are several orders 

of magnitude lower than chlorine concentrations on a molar 

basis (Luong and others, 1982). Formation of brominated 

THMs occurs under such conditions because (1) free chlorine 

oxidizes bromide to free bromine (HOCl + Br- —> HOBr + 

Cl-) much more rapidly than it reacts with dissolved organic 

matter (Luong and others, 1982), and (2) free bromine is a 

more powerful halogenating agent than free chlorine (Rook, 

1974).

At low molar ratios of bromide ion to free chlorine (0.03 

or less), the yields of the four principal THMs generated during chlorination of water decrease with increasing bromination 

of the product in the following order: CHCl3 > CHBrCl2 ≥

CHBr2Cl ≥ CHBr3 (Luong and others, 1982; Summers and 

others, 1993). This trend also was evident among the concentrations of the THMs formed during the study of the THM 

formation potential of agricultural drainage waters from the 

Sacramento-San Joaquin Delta, California (Fujii and others, 

1998). Results from studies where more than one THM has 

been detected in the hydrologic system commonly show the 

same trend among the detection frequencies for these compounds (Rook, 1974; Westrick and others, 1984; Voelker, 

1989; Kolpin and Thurman, 1995; Stackelberg and others, 

1997; Reiser and O’Brien, 1998; Lungren and Lopes, 1999; 

Squillace and others, 1999; Inkpen and others, 2000; Thiros, 

2000; Grady and Casey, 2001; Moran and others, 2002). 

Such observations indicate that the presence of brominated 

THMs might be used as a valuable criterion for distinguishing chlorination-treated waters from other potential sources 

of chloroform in the hydrologic system—especially if the 

concentrations among the four principal THMs (or detection 

Evidence for an Anthropogenic Origin of Trihalomethanes in Drinking-Water Supply Wells 
Figure 5. Concentrations of trihalomethanes in domestic well samples.

5.3

5.2

0.6

0.5

0.3

128

124

14

12

8

2,400

2,400

2,400

2,400

2,399

The MCL of 80 micrograms per liter is for the sum of the concentrations of four trihalomethanes including chloroform, 

bromodichloromethane, dibromochloromethane, and bromoform (U.S. Environmental Protection Agency, 2004).

1

Detection

frequency,

 in percent

Number

of

detections

Number

of

samples

0.001 0.01 0.1 1 10 100

CONCENTRATION, IN MICROGRAMS PER LITER

5.3

128

2,400

EXPLANATION

VOC concentration for a sample—Results from several analytical methods, with differing sensitivity for a 

specific VOC with time, as well as between VOCs, are shown in the bar concentration chart. Multiple

samples of equal concentration will appear as a single symbol

U.S. Environmental Protection Agency Maximum Contaminant Level (MCL) for total trihalomethanes

Detection frequency, in percent, at an assessment level of 0.2 microgram per liter

Number of detections at an assessment level of 0.2 microgram per liter

Number of samples

Median concentration

Assessment level of 0.2 microgram per liter

Total trihalomethanes1

Chloroform

Bromodichloromethane

Dibromochloromethane

Bromoform

frequencies, if based on the same detection limit for all four 

compounds) have the trend noted above.

Detection frequencies of THMs in samples from domestic wells show the trend of decreasing detection frequency 

with increasing bromide, that is, in the following order: 

CHCl3 > CHBrCl2 ≥ CHBr2Cl ≥ CHBr3 (table 1), indicating 

that chlorinated water may be the source. This pattern is less 

evident for public well samples; however, mixtures of two of 

more THMs occurred in 5.4 percent of public well samples 

(table 3), or about 37 percent of the samples that had one or 

more THM present. Of these mixtures, the most frequently 

occurring were combinations of the brominated THMs 

(table 4). In addition, the percentage of public well samples 

with detections of one or more THMs decreases with the number of THMs detected (table 3), whereas this pattern is not as 

evident for domestic well samples. In general, the brominated 

THMs are more predominant in public well samples than in 

domestic well samples, indicating that the capture of chlorinated water may be the primary source of THMs in public 

well samples; however, the detection of all four of the THMs 

in domestic well samples also indicates an anthropogenic 

source for these wells.

The brominated THMs have limited industrial production, few natural sources, and small or no reported direct 

releases to the environment, as noted previously. Therefore, 

industrial, commercial, or natural sources are not likely 

sources of the brominated THMs to public and domestic wells. 

Furthermore, the national distribution (figs. 2 and 3) and 

occurrence of chloroform and the brominated THMs in both 

public and domestic well samples and the occurrence pattern 

(CHCl3 > CHBrCl2 ≥ CHBr2Cl ≥ CHBr3) for domestic well 

samples indicates that the source of the compounds may be the 

result of the nationwide practice of water chlorination and use 

of bleach in households.

   Sources and Occurrence of Chloroform and Other Trihalomethanes in Drinking-Water Supply Wells in the United States, 1986–2001
Figure 6. Detection frequencies of chloroform at 0.2 microgram 

per liter or greater in agricultural and urban land-use areas 

(The different letters (A, B) indicate statistical difference at the 

0.05 significance level between the land-use areas; each dot 

represents a separate land-use study.

0

10

20

30

40

50

60

70

80

DETECTION FREQUENCY, IN PERCENT

Agricultural Urban

Median

Median

A B

N=30 N=33

Table 3. Number of trihalomethanes detected at an assessment level of 0.2 microgram per liter and the percentage of 

samples with detections in samples from public and domestic wells, 1986-2001.

Category

 Number of trihalomethanes detected

None One Two Three Four Two or more

Public Wells

Number of samples 935 103 25 17 16 58

Percentage of samples with detections 85.3 9.4 2.3 1.6 1.5 5.4

Domestic Wells

Number of samples 2,274 111 4 5 6 15

Percentage of samples with detections 94.8 4.6 .2 .2 .2 .6

Evidence for an Anthropogenic Origin of Trihalomethanes in Drinking-Water Supply Wells   
Table 4. Most frequently occurring two-compound mixtures in public well samples at an 

assessment level of 0.2 microgram per liter.

[VOC, volatile organic compound; MTBE, methyl-tert butyl ether]

Rank

Mixture Number of 

detections

Detection 

frequency, in 

Trihalomethane Second VOC percent

1 Bromodichloromethane Dibromochloromethane 37 3.4

2 Chloroform Bromodichloromethane 35 3.2

3 Dibromochloromethane Bromoform 32 2.9

4 Chloroform Dibromochloromethane 27 2.5

5 Bromodichloromethane Bromoform 22 2.0

6 Chloroform Bromoform 19 1.7

7 Chloroform Trichloroethene 17 1.6

8 Chloroform Perchloroethene 15 1.4

9 Chloroform 1,1,1-Trichloroethane 14 1.3

10 Chloroform MTBE 13 1.2

Implications for Future Research

The widespread occurrence of chloroform in samples 

from drinking-water supply wells may be especially important 

with regards to the presence of other disinfection by-products, 

for example, haloacetic acids, which have a USEPA MCL of 

60 μg/L for the total concentration of five compounds (U.S. 

Environmental Protection Agency, 2001). Studies to assess the 

extent of the occurrence, transport pathways, and concentration ranges of haloacetic acids and other disinfection by-products are needed because little national, regional, or aquiferscale data exist for these compounds, especially for domestic 

wells. Other areas of possible future research include more 

detailed information regarding the spatial distributions and 

temporal variability of anthropogenic sources of THMs, especially chlorinated drinking water and wastewater derived from 

leaking water distribution lines and wastewater sewers, as well 

as chlorinated water used to irrigate lawns, golf courses, parks, 

gardens, and other urban and residential settings.

Summary

Chloroform and three other trihalomethanes (THMs)—

bromodichloromethane, dibromochloromethane, and bromoform—are disinfection by-products commonly produced 

during the chlorination of water and wastewater. As part of the 

U.S. Geological Survey’s National Water-Quality Assessment 

(NAWQA) Program, samples of untreated ground water from 

drinking-water supply wells (1,096 public and 2,400 domestic 

wells) were analyzed for THMs and other volatile organic 

compounds (VOCs) during 1986–2001, or compiled from 

other studies. This report provides a summary of potential 

sources of THMs and of the occurrence and geographical 

distribution of THMs in these samples and presents evidence 

for an anthropogenic source of chloroform and other THMs. 

Implications for future research also are presented.

Potential sources of THMs to both public and domestic 

wells include the use and discharge of chlorinated drinking 

water and wastewater that may be intentional. These potential 

sources include the use of municipally supplied chlorinated 

water to irrigate lawns, golf courses, parks, gardens, and other 

areas; the use of septic systems; or the regulated discharge 

of chlorinated wastewater to surface waters or ground-water 

recharge facilities. Other potential sources of THMs to public 

and domestic wells also may be inadvertent including leakage 

of chlorinated water from swimming pools, spas, or distribution systems for drinking water or wastewater sewers.

Although most water from domestic wells is not chlorinated prior to use, chloroform was found to be the most 

frequently detected VOC in domestic well samples. Several 

potential additional sources of chloroform include shock chlorination, in which a dilute solution of bleach is added directly 

to the well to eliminate bacterial contamination, and laundry 

wastewater containing bleach. In both cases, chloroform is 

produced through the haloform reaction of chlorine (contained in bleach) with organic matter. Also, chloroform has 

10 Sources and Occurrence of Chloroform and Other Trihalomethanes in Drinking-Water Supply Wells in the United States, 1986–2001
been documented in septic-system effluent, and improperly 

designed, maintained, or operated septic systems can result 

in ground-water contamination in the vicinity of the system. 

The detection of chloroform or other THMs in a domestic well 

sample could be an indicator of other possible health concerns 

(pathogenic microorganisms, for example) that may exist for 

the domestic well user (especially for those with untreated 

well water) whose well is in the vicinity of an improperly 

operating septic system.

Chloroform also was the most frequently detected 

VOC in public well samples collected or compiled by the 

U.S. Geological Survey’s NAWQA Program. The other 

THMs—bromodichloromethane, dibromochloromethane, and 

bromoform—were among the 10 most frequently detected 

VOCs in the NAWQA data sets for public well samples. The 

widespread occurrence and frequency of THMs in public 

and domestic well samples, however, was not expected, as 

the purpose of the NAWQA Program was to sample ambient, 

untreated ground water from a large number of aquifers across 

the Nation. Although chloroform was detected frequently in 

public and domestic well samples and other THMs were present in some samples, no total THM concentrations in samples 

from either well type exceeded the U.S. Environmental Protection Agency’s Maximum Contaminant Level of 80 micrograms per liter (μg/L).

Chloroform was detected about twice as frequently in 

public well samples than in domestic well samples, which may 

be due to the generally higher capacity public wells generating 

a larger capture zone in land-use areas classified as urban. The 

larger capture zone for public wells could incorporate multiple 

land uses and a greater number of point and nonpoint contamination sources than the smaller capture zone of domestic 

wells. Chloroform was detected in ground water in a broad 

range of land-use settings; however, the detection frequencies 

of chloroform were higher beneath urban areas than beneath 

agricultural areas. Statistical analyses indicate that population 

density, percentage of urban land, and the number of Resource 

Conservation and Recovery Act hazardous-waste facilities 

near the sampled wells have a significant association with the 

detection of chloroform.

 THM detection frequencies for public and domestic well 

samples at an assessment level of 0.2 μg/L showed a general 

pattern of decreasing frequency with increasing bromide content, that is chloroform > bromodichloromethane ≥ dibromochloromethane ≥ bromoform. This pattern is more evident for 

the domestic well samples than for the public well samples. 

Because this same detection frequency pattern of chloroform > 

bromodichloromethane ≥ dibromochloromethane ≥ bromoform has been reported previously for chlorinated waters, it 

might be used as a criterion for distinguishing THMs resulting 

from water with a history of chlorination from other potential sources of chloroform and other THMs in the hydrologic 

system.

Chloroform in drinking-water supply wells could 

originate from many sources. However, the observation that 

chloroform commonly occurred in mixtures with the other 

THMs, especially in public well samples, and that the detection frequency of the THMs was in the following order: chloroform > bromodichloromethane ≥ dibromochloromethane ≥

bromoform in many of the public and domestic well samples, 

indicate that chlorinated drinking water and/or wastewater are 

important sources of THMs in samples from supply wells. 

Additionally, the brominated THMs have limited industrial 

production, few natural sources, and small or no reported 

direct releases to the environment, and are not likely sources 

of the brominated THMs in public and domestic well samples.

References

Agency for Toxic Substances and Disease Registry, 1989, 

Toxicological profile for bromodichloromethane: U.S. 

Department of Health and Human Services, Public Health 

Service, Agency for Toxic Substances and Disease Registry, 

Research Triangle Institute, 77 p.

Agency for Toxic Substances and Disease Registry, 1997, 

Toxicological profile for chloroform: U.S. Department of 

Health and Human Services, Public Health Service, Agency 

for Toxic Substances and Disease Registry, Research Triangle Institute, 293 p.

Agency for Toxic Substances and Disease Registry, 2005, 

Toxicological profile for bromoform and dibromochloromethane: accessed October 20, 2005, at http://www.atsdr.

cdc.gov/toxprofiles/tp130.pdf

Ayers Associates, 1993, Onsite sewage disposal system 

research in Florida—an evaluation of current onsite sewage 

disposal system (OSDS) practices in Florida: Tampa, Fla., 

Ayers Associates, 191 p.

Bender, D.A., Zogorski, J.S., Halde, M.J., and Rowe, B.L., 

1999, Selection procedure and salient information for volatile organic compounds emphasized in the National WaterQuality Assessment Program: U.S. Geological Survey 

Open-File Report 99–182, 32 p.

Boyce, S.D., and Hornig, J.F., 1983, Reaction pathways of 

trihalomethane formation from the halogenation of dihydroxyaromatic model compounds for humic acid: Environmental Science & Technology, v. 17, no. 4, p. 202–211.

Bruce, B.W., and Oelsner, G.P., 2001, Contrasting water quality from paired domestic/public supply wells, Central High 

Plains: Journal of American Water Resources Association, 

v. 37, no. 5, p. 1389–1404.

Budvari, Susan, ed., 1989, The Merck Index: Rahway, N.J., 

Merck and Company, Inc. [variously paged].

References 11
Connor, B.F., Rose, D.L., Noriega, M.C., Murtagh, L.K., and 

Abney, S.R., 1998, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory—determinations of 86 volatile organic compounds in water by gas 

chromotography/mass spectrometry, including detections 

less than reporting limits: U.S. Geological Survey Open-File 

Report 97–829, 78 p.

DeWalle, F.B., Kalman, D.A., Norman, D., Sung, J., and 

Plews, G., 1985, Determination of toxic chemicals in 

effluent from household septic tanks: U.S. Environmental 

Protection Agency Technical Report 600/2–85/050, 25 p.

Fujii, Roger, Ranalli, A.J., Aiken, G.R., and Bergamaschi, 

B.A., 1998, Dissolved organic carbon concentrations and 

compositions, and trihalomethane formation potentials in 

waters from agricultural peat soils, Sacramento-San Joaquin 

Delta, California—implications for drinking-water quality: 

U.S. Geological Survey Water-Resources Investigations 

Report 98–4147, 75 p.

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 MidAtlantic regions of the United States, 1993–98: 

U.S. Geological Survey Water-Resources Investigations 

Report 00–4228, 123 p.

Grady S.J., and Mullaney, J.R., 1998, Natural and human factors affecting shallow water quality in surficial aquifers in 

the Connecticut, Housatonic and Thames River Basins: 

U.S. Geological Survey Water-Resources Investigations 

Report 98–4042, 81 p.

Gribble, G.W., 1994, The natural production of chlorinated 

compounds: Environmental Science & Technology, v. 28, 

no. 7, p. 310A–319A.

Inkpen, E.L., Tesoriero, A.J., Ebbert, J.C., Silva, S.R., and 

Sandstrom, M.W., 2000, Ground-water quality in regional, 

agricultural, and urban settings in the Puget Sound Basin, 

Washington and British Columbia, 1996–1998: U.S. Geological Survey Water-Resources Investigations 

Report 00–4100, 66 p.

Isidorov, V.A., Zenkevich, I.G., and Ioffe, B.V., 1990, Volatile 

organic compounds in solfataric gases: Journal Atmospheric 

Chemistry, v. 10, no. 3, p. 329–340.

Ivahnenko, Tamara, and Barbash, J.E., 2004, Chloroform in 

the hydrologic system—sources, transport, fate, occurrence 

and effects on human health and aquatic organisms: 

U.S. Geological Survey Scientific Investigations 

Report 2004–5137, 34 p.

Kolpin, D.W., and Thurman, E.M., 1995, Postflood occurrence 

of selected agricultural chemicals and volatile organic compounds in near-surface unconsolidated aquifers in the Upper 

Mississippi River Basin, 1993: U.S. Geological Survey 

Circular 1120–G, 20 p.

Koterba, M.T., Wilde, F.D., and Lapham, W.W., 1995, 

Ground-water data-collection protocols and procedures for 

the National Water-Quality Assessment Program—collection and documentation of water-quality samples and related 

data: U.S. Geological Survey Open-File Report 95–399, 

113 p.

Laturnus, Frank; Haselmann, K.F.; Borch, Thomas; and Gron, 

Christian, 2002, Terrestrial natural sources of trichloromethane (chloroform, CHCl3

)—an overview: Biochemistry, 

v. 60, no. 2, p. 121–139.

Lucius, J.E., Olhoeft, G.R., Hill, P.L., and Duke, S.K., 1992, 

Properties and hazards of 108 selected substances—1992 

edition: U.S. Geological Survey Open-File Report 92–527, 

p. 191–195.

Lungren, R.F., and Lopes, T.J., 1999, Occurrence, distribution, 

and trends of volatile organic compounds in the Ohio River 

and its major tributaries, 1987–96: U.S. Geological Survey 

Water-Resources Investigations Report 99–4257, 89 p.

Luong, T.V., Peters, C.J., and Perry, Roger, 1982, Influence 

of bromide and ammonia upon the formation of trihalomethanes under water-treatment conditions: Environmental 

Science & Technology, v. 16, no. 8, p. 473–479.

Mannsville Chemical Products Corporation, 1999, Chemical 

products synopsis, chloroform: Adams, N.Y., Mannsville 

Chemical Products Corporation, 2 p.

McCulloch, A., 2003, Chloroform in the environment—occurrence, sources, sinks and effects: Chemosphere, v. 50, 

no. 10, p. 1291–1308.

Moran, M.J., Lapham, W.W., Rowe, B.L., and Zogorski, J.S., 

2002, Occurrence and status of volatile organic compounds 

in ground water from rural, untreated, self-supplied, domestic wells in the United States, 1986–1999: U.S. Geological 

Survey Water-Resources Investigations Report 02–4085, 

51 p.

Moran, M.J., Zogorski, J.S., and Rowe, B.L., 2006, Approach 

to an assessment of volatile organic compounds in the 

Nations’s ground water and drinking-water supply wells: 

U.S. Geological Survey Open-File Report 2005–1452, 36 p.

12 Sources and Occurrence of Chloroform and Other Trihalomethanes in Drinking-Water Supply Wells in the United States, 1986–2001
Princeton Geoscience, 2005, Road salting as a cause of chloroform in ground water?: accessed November 18, 2005, at 

http://princetongeoscience.com/geonew.html

Reiser, R.G., and O’Brien, A.K., 1998, Occurrence and 

seasonal variability of volatile organic compounds in 

seven New Jersey streams: U.S. Geological Survey WaterResources Investigations Report 98–4074, 11 p.

Rook, J.J., 1974, Formation of haloforms during chlorination 

of natural waters: Water Treatment and Examination, v. 23, 

p. 234-243.

Rose, D.L., and Schroeder, M.P., 1995, Methods of analysis by 

the U.S. Geological Survey National Water Quality Laboratory—determination of volatile organic compounds in 

water by purge and trap capillary gas chromatography/mass 

spectrometry: U.S. Geological Survey Open-File Report 

94–708–W, 26 p.

Sawyer, C.N., and McCarty, P.L., 1978, Chemistry for environmental engineering: New York, McGraw-Hill Book 

Company, 532 p.

Solley, W.B., Pierce, R.R., and Perlman, H.A., 1998, Estimated use of water in the United States in 1995: U.S. Geological Survey Circular 1200, 71 p.

Squillace, P.J., and Moran, M.J., 2006, Factors affecting 

sources, transport, and fate of volatile organic compounds in 

aquifers of the United States and implications for groundwater management and assessments: U.S. Geological Survey Scientific Investigations Report 2005–5269, 40 p.

Squillace, P.J., Moran, M.J., Lapham, W.W., Price, C.V., 

Clawges, R.M., and Zogorski, J.S., 1999, Volatile organic 

compounds in untreated ambient groundwater of the United 

States, 1985–1995: Environmental Science & Technology, 

v. 33, no. 23, p. 4176–4187.

Stackelberg, P.E., Hopple, J.A., and Kauffman, L.J., 1997, 

Occurrence of nitrate, pesticides and volatile organic compounds in the Kirkwood-Cohansey aquifer system, southern New Jersey: U.S. Geological Survey Water-Resources 

Investigations Report 97–4241, 8 p.

Stackelberg, P.E., Kauffman, L.J., Ayers, M.A., and Baehr, 

A.L., 2001, Frequently co-occurring pesticides and volatile 

organic compounds in public supply and monitoring wells, 

southern New Jersey, USA: Environmental Toxicology and 

Chemistry, v. 20, no. 4, p. 853–856.

Stackelberg, P.E., Kauffman, L.J., Baehr, A.L., and Ayers, 

M.A., 2000, Comparison of nitrate, pesticides and volatile 

organic compounds in samples from monitoring and publicsupply wells, Kirkwood-Cohansey aquifer system, southern New Jersey: U.S. Geological Survey Water-Resources 

Investigations Report 00–4123, 78 p.

Summers, R.S., Benz, M.A., Shukairy, H.M., and Cummings, 

Laura, 1993, Effect of separation processes on the formation 

of brominated THMs: Journal of the American Water Works 

Association, Research and Technology, January 1993, v. 85, 

no. 1, p. 88–95.

Thiros, S.A., 2000, Analysis of nitrate and volatile organic 

compound data for ground water in the Great Salt Lake 

Basins, Utah, Idaho, and Wyoming 1980–98, National 

Water-Quality Assessment Program: U.S. Geological Survey Water-Resources Investigations Report 00–4043, 4 p.

U.S. Bureau of the Census, 1991, Census of population and 

housing, 1990: Washington, D.C., Public Law 94-171 data 

(United States) [machine readable data files on CD-ROM].

U.S. Environmental Protection Agency, 2000, National water 

quality inventory, 1998 Report to Congress: Washington, 

D.C., EPA 841-R-00-001, 413 p.

U.S. Environmental Protection Agency, 2001, Stage 1 disinfectants and disinfection byproducts rule—a quick reference guide: U.S. Environmental Protection Agency, EPA 

816–F–01–010, 2 p.

U.S. Environmental Protection Agency, 2002, Integrated Risk 

Information System: accessed August 28, 2002, at 

http://www.epa.gov/iris/

U.S. Environmental Protection Agency, 2004, 2004 Edition of 

the drinking water standards and health advisories: Washington, D.C., Office of Water, U.S. Environmental Protection Agency, EPA 822–R–04–005, 20 p.

U.S. Environmental Protection Agency, 2005a, Toxics Release 

Inventory: accessed July 20, 2005, at 

http://www.epa.gov/tri/

U.S. Environmental Protection Agency, 2005b, Disinfection by-product chemistry—formation and determination: 

accessed December 5, 2005, at http://www.epa.gov/ORD/

NRMRL/pubs/600r01110/600r01110chap3.pdf

Voelker, D.C., 1989, Quality of water from public-supply wells in principal aquifers of Illinois, 1984–87: U.S. 

Geological Survey Water-Resources Investigations Report 

88–4111, 29 p.

Westrick, J.J., Mello, J.W., and Thomas, R.F., 1984, The 

groundwater supply survey: Journal American Water Works 

Association, May, v. 76, p. 52–59.

Zogorski, J.S., Carter, J.M., Ivahnenko, Tamara, Lapham, 

W.W., Moran, M.J., Rowe, B.L., Squillace, P.J., and Toccolino, P.L., 2006, Volatile organic compounds in the Nation’s 

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

References 13