Chloroform in the Hydrologic System—Sources,

Transport, Fate, Occurrence, and Effects on

Human Health and Aquatic Organisms

Scientific Investigations Report 2004-5137

U.S. Department of the Interior

U.S. Geological Survey

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

NATIONAL SYNTHESIS ON VOLATILE ORGANIC COMPOUNDS

ClFe or SO4 reducing, 

or methanogenic

conditions 

HOCl

Water chlorination

Other 

disinfection

byproducts

H+ + 2eGround-Water

Abstraction

Unsaturated

Zone

Saturated

Zone

Infiltration,

Transformation,

Dilution

Surface

Water

Ground-Water

Discharge

and

Recharge

Cl

Cl Cl

H

C

Cl Cl

Cl

Cl

C

Production by

marine algae
Front cover photograph 1: Photo of a chlorination contact tank.

Photograph courtesy of Joanne Weycker, Green Bay Metropolitan Sewer District.

Front cover photograph 2: Photo of a gathering storm on the North Atlantic Ocean.

Photograph by Jack Barbash.

Molecular diagrams provided by Mark Sandstrom and Jack Barbash.

Schematic at bottom modified from Cartwright and others (1991).
Chloroform in the Hydrologic 

System—Sources, Transport, Fate, 

Occurrence, and Effects on Human

Health and Aquatic Organisms

By Tamara Ivahnenko and Jack E. Barbash

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2004-5137
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

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

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

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.

Suggested citation:

Ivahnenko, Tammy, 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.
 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. 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. Shaped by and coordinated with ongoing efforts of other Federal, State, and local 

agencies, the NAWQA Program is designed to answer: What is the condition of our Nation’s 

streams and ground water? How are the conditions changing over time? How do natural features 

and human activities affect the quality of streams and ground water, and where are those effects 

most pronounced? By combining information on water chemistry, physical characteristics, stream 

habitat, and aquatic life, the NAWQA Program aims to provide science-based insights for current 

and emerging water issues. NAWQA results can contribute to informed decisions that result in 

practical and effective water-resource management and strategies that protect and restore water 

quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more than 50 

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

these Study Units account for more than 60 percent of the overall water use and population served 

by public water supply, and are representative of the Nation’s major hydrologic landscapes, priority ecological resources, and agricultural, urban, and natural sources of contamination. 

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

analysis. The assessments thereby build local know-ledge 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. 

The USGS places high value on the communication and dissemination of credible, timely, and relevant science so that the most recent and available knowledge about water resources can be 

applied in management and policy decisions. We hope this NAWQA publication will provide you 

the needed insights and information to meet your needs, and thereby foster increased awareness 

and involvement in the protection and restoration of our Nation’s waters. 
iv 

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

all water-resource issues of interest. External coordination at all levels is critical for a fully integrated understanding of watersheds and for cost-effective management, regulation, and conservation of our Nation’s water resources. The Program, therefore, depends extensively on the 

advice, cooperation, and information from other Federal, State, interstate, Tribal, and local agencies, non-government organizations, industry, academia, and other stakeholder groups. The assistance and suggestions of all are greatly appreciated.

Robert M. Hirsch

Associate Director for Water
 v

Contents

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

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

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

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

Production and Use of Chloroform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Sources of Chloroform. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Direct Releases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Formation of Chloroform and Other Trihalomethanes as Disinfection By-Products. . . . . . . . . . 4

Chlorinated Water as a Source of THMs in the Hydrologic System. . . . . . . . . . . . . . . . . . . . . . . 10

Formation of Chloroform from the Dehalogenation of Tetrachloromethane . . . . . . . . . . . . . . . 11

Natural Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Anthropogenic Fluxes in Relation to Global Emissions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Physical and Chemical Properties Controlling the Transport and Fate of Chloroform. . . . . . . . . . . . 13

Chloroform Occurrence in the Hydrologic System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Ground Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Surface Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Drinking Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Human-Health Effects and Drinking-Water Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Inhalation Exposure and its Effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Ingestion Exposure and its Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Dermal Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Toxicity to Aquatic Organisms and Aquatic Water-Quality Criteria. . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Implications for Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Figures

1-4. Graphs showing:

1. Industrial production of chloroform in the United States from 1970 through 1999 . . . .3

2. Industrial emissions of chloroform to the air in the United States from

1988 through 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

3. Industrial discharges of chloroform to surface water in the United States

from 1988 through 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

4. Industrial discharges of chloroform by underground injection and

releases to land in the United States from 1988 through 2001. . . . . . . . . . . . . . . . . . . . . .6

5. Schematic showing hypothesized mechanisms by which a generic 1,3-hydroxyaromatic compound (a model for a humic or fulvic acid molecule containing

an aromatic ring) is transformed during chlorination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
vi 

6. Graph showing effect of pH on the yield of chloroform from the chlorination

of the substrate 1,3-dihydroxybenzene (resorcinol), based on measurements

of chloroform concentrations using either direct aqueous injection or

solvent extraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

7. Schematic showing reactions that may either produce chloroform or

transform it into other compounds in the hydrologic system . . . . . . . . . . . . . . . . . . . . . . . . . . 12

8. Map showing detections of total trihalomethanes at or greater than 

0.2 micrograms per liter in ground and surface waters sampled for the 

American Water Works Association Research Foundation national study . . . . . . . . . . . . . 18

Tables

1. Percentage of large and small drinking-water systems using chlorine and

non-chlorine disinfection methods, in 1998 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2. Chemical and physical properties that influence the transport and fate of

the four trihalomethanes discussed in this report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3. Studies of chloroform and other trihalomethane occurrence in ground

water in the United States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4. Studies of chloroform occurrence in surface waters in the United States. . . . . . . . . . . . . 19

5. Summary of chloroform and other trihalomethane detections in drinking

water in eleven States of the Northeastern and Mid-Atlantic regions

of the United States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

6. Concentrations of chloroform in air (either measured directly or predicted

from models) in shower stalls, near washing machines and dishwashers,

above kitchen sinks, and in indoor swimming pool facilities . . . . . . . . . . . . . . . . . . . . . . . . . . 21

7. Summary of the effects of chloroform and total trihalomethanes on pregnancy

outcomes and the incidence of cancer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

8. Selected studies of the non-lethal effects of chloroform exposure on

aquatic organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
 vii

Acronyms and Abbreviations Used in This Report

AQUIRE AQUatic Information REtrieval system

BCF Bioconcentration factor

CAS Chemical Abstract Service

CCl2F2 Dichlorodifluoromethane

CCl3COOH Trichloroacetic acid

CCl4 Tetrachloromethane

CERCLA Comprehensive Environmental Response, Compensation and Liability Act

CFCs Chlorofluorocarbons

CHBr2Cl Dibromochloromethane

CHBr3 Tribromomethane (bromoform)

CHBrCl2 Bromodichloromethane

CHCl2Br Dichlorobromomethane

CHCl3 Trichloromethane (chloroform)

CHClBr2 Chlorodibromomethane

CHClF2 Chlorodifluoromethane

CI Confidence interval

Cl Chloride

Cl2 Chlorine gas

CPO Chloroperoxidase enzyme

DBPs Disinfection by-products

DOC Dissolved organic carbon

DOM Dissolved organic matter

foc Fraction of organic carbon

HCFCs Hydrochlorofluorocarbons

HCl Hydrochloric acid

HOCl Hypochlorous acid

IGR intrauterine growth retardation

KH Henry’s Law constant

Koc Organic-water sorption partition coefficient

Kow Octanol-water partition coefficient

LOELs Lowest-observed-effect-levels

MCL Maximum Contaminant Level

MCLG Maximum Contaminant Level Goal

NAWQA National Water-Quality Assessment Program

NIOSH National Institute for Occupational Safety and Health

NOM Natural organic matter

OCl Hypochlorite ion

OSHA Occupational Safety and Health Administration

RCRA Resource Conservation and Recovery Act

SUVA Specific ultraviolet absorbance

THMs Trihalomethanes

TRI Toxic Release Inventory
viii 

TTHMs Total trihalomethanes

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VOCs Volatile organic compounds
Chloroform in the Hydrologic System—Sources, 

Transport, Fate, Occurrence, and Effects on

Human Health and Aquatic Organisms

By Tamara Ivahnenko and Jack E. Barbash

Abstract

Chloroform is one of the volatile organic compounds 

(VOCs) detected most frequently in both ground and surface 

water. Because it is also one of the four trihalomethanes 

(THMs) produced in the highest concentrations during the chlorination of drinking water and wastewater, the frequent detection of this compound in ground and surface water of the United 

States is presumed to be caused primarily by the input of chlorinated water to the hydrologic system. Although anthropogenic 

sources of the compound are substantial, they are currently 

estimated to constitute only 10 percent of the total global input 

to the hydrologic system. Natural sources of the compound 

include volcanic gases, biomass burning, marine algae, and soil 

microorganisms. Under most conditions (except in the presence 

of unusually high bromide concentrations), chloroform is the 

THM produced in the highest concentrations during chlorination. Furthermore, in most cases where more than one THM is 

produced from chlorination, the relative concentrations among 

the different compounds usually decrease with increasing bromination (chloroform > dichlorobromomethane > chlorodibromomethane > bromoform). This phenomenon is presumed to be 

responsible for the common observation that when more than 

one THM is detected during investigations of the occurrence of 

these compounds in the hydrologic system, this same trend is 

typically observed among their relative concentrations or, for a 

uniform reporting limit, their relative frequencies of detection. 

This pattern could provide a valuable means for distinguishing 

between chlorinated water and other potential sources of 

chloroform in the environment.

Chloroform has been widely detected in national, regional, 

and local studies of VOCs in ground, surface, source, and 

drinking waters. Total THM (TTHM) concentrations of the 

compound, however, were typically less than the Maximum 

Contaminant Level (MCL) of 80 μg/L (micrograms per liter) 

established by the U.S. Environmental Protection Agency 

(USEPA) for TTHMs. In the studies that compared land-use 

settings, frequencies of detection of chloroform were higher 

beneath urban and residential areas than beneath agricultural or 

undeveloped areas. Because chloroform is a suspected human 

carcinogen, its presence in drinking water is a potential human 

health concern. Liver damage, however, is known to occur at 

chloroform exposures lower than those required to cause 

cancer, an observation that has been considered by the USEPA 

as the basis for setting a new, non-zero Maximum Contaminant 

Level Goal of 70 μg/L for the compound. As part of its National Water-Quality Assessment 

Program, the U.S. Geological Survey has been assembling and 

analyzing data on the occurrence of VOCs (including chloroform) in ground and surface water on a national scale from 

studies conducted between 1991 and the present. This report 

presents a summary of current (2004) information on the uses, 

sources, formation, transport, fate, and occurrence of chloroform, as well as its effects on human health and aquatic 

organisms.

Introduction

Chloroform—known more formally as trichloromethane 

(CHCl3)—has been one of the volatile organic compounds 

(VOCs) detected most frequently in the Nation’s ground water 

by the U.S. Geological Survey’s National Water-Quality 

Assessment (NAWQA) Program (Squillace, Moran, and others, 

1999). Grady (2003) also reported chloroform to be the most 

frequently detected VOC in a national study of both groundand surface-water sources of drinking water.

Most of these detections are presumed to have been a 

consequence of contamination by chlorinated water and wastewater because chloroform and the three other most frequently 

detected trihalomethanes (THMs)—bromodichloromethane 

(CHBrCl2), dibromochloromethane (CHBr2Cl), and bromoform (CHBr3)—are commonly produced during the chlorination of water and wastewater. Although the word trihalomethanes applies to trihalogenated methanes containing any of 

the halogen atoms (chlorine, bromine, iodine, or fluorine), this 

term will be used in this report to refer only to those containing 
2 Chloroform in the Hydrologic System

either chlorine or bromine because these are the ones most 

commonly detected, or analyzed for, in chlorinated water.

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, 2000a). In 1998, the USEPA revised the carcinogenicity risk assessment for chloroform, stating that the compound is likely to be carcinogenic to humans by all routes of 

exposure at concentrations high enough to cause cytotoxicity 

or the formation of regenerative nodules in susceptible tissues 

(U.S. Environmental Protection Agency, 2002a). More 

recently, the USEPA revised the Maximum Contaminant Level 

Goal (MCLG) for chloroform to 70 μg/L (micrograms per liter) 

based on non-cancer effects (U.S. Environmental Protection 

Agency, 2004a). Other Federal legislation that has listed chloroform as a potential health concern includes the Occupational 

and Safety Health Act, the Comprehensive Environmental 

Response, Compensation and Liability Act (CERCLA or 

“Superfund”), the Clean Air Act, the Resource Conservation 

and Recovery Act (RCRA), the Safe Drinking Water Act, 

Clean Water Act, the Toxic Release Inventory (TRI), and the 

Chemical, Food, Drug and Cosmetic Act (Environmental 

Defense Fund, 1999).

Purpose and Scope

Information on chloroform in the hydrologic system was 

compiled by the VOC National Synthesis team of the U.S. 

Geological Survey (USGS) NAWQA Program. A major component of the NAWQA Program, the National Synthesis Investigations use NAWQA and other water-quality data to examine 

and describe the sources, transport, distribution, and fate of 

specific contaminant groups on a national scale. This report 

provides a summary of current knowledge regarding the uses, 

nonpoint sources, formation, transport and fate of chloroform in 

the hydrologic system; its occurrence in ground water, surface 

water, and drinking water; and its effects on human-health and 

aquatic organisms. As such, it is intended to provide a current 

(2004) information resource for the managers of water systems 

where elevated chloroform concentrations may be a source of 

concern. This summary includes data collected by the NAWQA 

program from 1991 to 2002. Because the focus of the discussion 

is on ambient ground and surface waters, rather than on specific 

contamination incidents, data on the occurrence of chloroform 

in water affected by known point sources are not included. The 

report also focuses primarily on chloroform—rather than on all 

four of the THMs produced in the highest concentrations during 

chlorination—both because chloroform is the most frequently 

detected THM, and because far less information is currently 

available on the sources, transport, fate, and biological effects 

of the other three compounds.

Acknowledgments

The authors thank the following individuals for their contributions to this report: Professor Edward Bouwer, John 

Hopkins University, for his help in sorting out the details of 

chlorination chemistry; Steve Grady (USGS), for help with the 

trihalomethane data from the 12-State Northeast and MidAtlantic VOC study; and Mark Sandstrom (USGS), for help 

with drawing the chemical structures for the cover. The authors 

also are grateful to Joanne Weycker, Green Bay Metropolitan 

Sewer District, for permission to publish their photo of the 

water-treatment process; Farla Kaufman (California Environmental Protection Agency, Office of Environmental Health 

Hazard Assessment), for reviewing the section on “Human 

Health-Effects and Drinking-Water Criteria;” and Patty Toccalino (Oregon Graduate Institute), John Zogorski, Jessica 

Hopple, and Colleen Rostad (USGS), for their valuable technical comments and suggestions. Finally, the authors would like 

to thank Janet Carter, Ella Decker, and Connie Ross (USGS), 

for their editorial comments, preparation of the report for 

publication, and help with the illustrations and cover art.

Production and Use of Chloroform

The major industrial use of chloroform is for the production of hydrochlorofluorocarbon-22 or HCFC-22 (known more 

formally as chlorodifluoromethane (CHClF2)). A refrigerant in 

home air conditioners and large commercial freezers, HCFC-22 

also has been proposed as a potential replacement for chlorofluorocarbons (CFCs), which are known to deplete stratospheric 

ozone. In 1987, concerns over the potential damage to the 

Earth’s ozone layer caused by CFCs led to the “Montreal 

Protocol on Substances that Deplete the Ozone Layer,” which 

called for restrictions on the production of CFCs (Alternative 

Fluorocarbon Environmental Acceptability Study, 2002). 

While CFCs are fully halogenated compounds, in HCFCs, one 

or more of the halogens are replaced by hydrogen atoms, 

resulting in a reduced potential to react with ozone. HCFC-22 

was therefore proposed as a substitute for the highly ozonedepleting CFC-12 (dichlorodifluoromethane (CCl2F2)) 

(Mannsville Chemical Products Corporation, 1999). 

To accommodate the higher demand for HCFC-22 resulting from its use as a substitute for CFC-12, an estimated 

98 percent of the total chloroform production in the United 

States has been used in the manufacture of HCFC-22 since 

1990 (Mannsville Chemical Products Corporation, 1999). 

HCFC-22 itself, however, is expected to be phased out of production in the United States by the year 2020 (U.S. Environmental Protection Agency, 2002b). The remaining 2 percent of 

chloroform manufactured in the United States since 1990 has 

been used for a variety of commercial purposes, including its 

use as a reagent and extraction solvent for fats, oils, grease, 

resins, lacquers, rubber, alkaloids, gums, and waxes (Budavari, 
Sources of Chloroform 3

1989; Mannsville Chemical Products Corporation, 1999). 

Chloroform also has been used as a fumigant, as an insecticide, 

as a fire extinguishing agent, as a precursor for dyes and pesticides, and for the manufacture and processing of pharmaceuticals (Lucius and others, 1992; Agency for Toxic Substances 

and Disease Registry, 1997). 

Figure 1 displays the history of the industrial production of 

chloroform in the United States from 1970 to 1999. Although 

the importation of chloroform peaked at 33 million pounds in 

1985, no chloroform was imported to the United States from 

1996 to 1998. Exports, however, increased from 48 to 240 million pounds per year between 1990 and 1997 (Mannsville 

Chemical Products Corporation, 1999).

Sources of Chloroform

Chloroform may enter the hydrologic system from direct 

releases, intentional or inadvertent inputs of chlorinated water, 

dehalogenation of tetrachloromethane, and a variety of natural 

sources. All of these sources are examined in greater detail 

below, with a discussion of anthropogenic fluxes of the 

compound in relation to global emissions.

Direct Releases

The USEPA’s TRI documents industrial releases of a 

broad range of anthropogenic compounds to the environment 

on a nationwide basis. In the United States, these releases are 

reported annually and include air emissions (fig. 2), discharges 

to surface water (fig. 3), underground injections, and releases to 

land (fig. 4). According to the TRI, a total of approximately 1.6 

million pounds of chloroform was released by these routes 

across the Nation in 2001 (U.S. Environmental Protection 

Agency, 2003a). The TRI, however, excludes data on direct 

releases of chloroform to the atmosphere from hazardous 

waste-disposal sites and sanitary landfills, as well as from 

municipal and hazardous waste incinerators that either burn 

waste containing chloroform or produce chloroform during 

combustion (Travis and others, 1986; Green and Wagner, 

1993).

Figure 1. Industrial production of chloroform in the United States from 1970 through 1999. (Data for years 1970, 1975, 

1980, 1985, 1990, and 1994-98 were obtained from Mannsville Chemical Products Corporation (2002); those for 1987-89 

and 1991-93 were obtained from the Chemical Manufacturers Association (1998).)

100

0

200

300

400

500

600

700

800

900

1970 1975 1980 1985 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999

YEAR

ANNUAL CHLOROFORM PRODUCTION, IN MILLIONS OF POUNDS
4 Chloroform in the Hydrologic System

Emissions of anthropogenic chloroform to the atmosphere, 

as reported by the TRI (fig. 2), have decreased steadily since 

1989, perhaps as a consequence of improved air pollution 

control measures. In 2001, air emissions represented about 

89 percent of the total releases of chloroform to the hydrologic 

system from reporting industries. The largest air emissions of 

chloroform that year were from the paper (56 percent), chemical (18.8 percent), and food (5.1 percent) industries.

Unintentional releases of chloroform to the atmosphere 

may occur during the manufacture of HCFC-22, fluoropolymers, pharmaceuticals, dyes, or fumigants (Hazardous 

Substances Databank, 1996). Other unintentional chloroform 

releases to the atmosphere result from its formation in, and 

subsequent volatilization from, chlorinated water. Chloroform 

stripping efficiencies from water to air, based on computer simulations, were estimated by Moya and others (1999) to be as 

high as 55 percent and by Corsi and others (1992) to be as high 

as 65 percent. These predictions indicate that airborne chloroform concentrations could be significant in a confined space, 

such as a shower stall. Other local sources of airborne chloroform from chlorinated waters include washing machines, dishwashers, and sinks, where heat and aeration can be expected to 

increase volatilization rates (Shah and Singh, 1988; Corsi and 

others, 1992; Howard and Corsi, 1996; Agency for Toxic Substances and Disease Registry, 1997; Howard and Corsi, 1998; 

Howard-Reed and others, 1999; Levesque and others, 2002).

Discharges and releases of chloroform to surface water 

and land, as reported by the TRI, decreased from 1988 to 2001 

(figs. 3 and 4, respectively). Releases to land, as defined by the 

USEPA, include disposal or burial of chemicals in landfills, 

application farming (in which the chemical is incorporated into 

the soil, a practice also known as land treatment), spills, leaks, 

and leaching from surface impoundments and waste piles (U.S. 

Environmental Protection Agency, 1999). In 2001, the paper 

industry contributed about 34 percent of the anthropogenic 

chloroform releases to surface water and 92 percent of the 

releases to land. Releases of chloroform through industrial 

practices to surface water and land, however, represent approximately 1.2 and 0.5 percent, respectively, of the total releases of 

anthropogenic chloroform to the environment; as noted earlier, 

most of the chloroform released to the hydrologic system by 

human activities is through air emissions. According to the 

RCRA solvent recovery program (Standard Industrial Classification Code 4953—Refuse Systems), which is responsible for 

75 percent of the chloroform injected into the subsurface (that 

is, the vadose and/or saturated zones), underground injections 

Figure 2. Industrial emissions of chloroform to the air in the United States from 1988 through 2001. (Data from 

U.S. Environmental Protection Agency, 2003a).

YEAR

ANNUAL CHLOROFORM EMISSION, IN MILLIONS OF POUNDS

5

0

10

15

20

25

30

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001
Sources of Chloroform 5

of chloroform increased from 25,000 to 227,000 pounds 

between 1997 and 2000 (fig. 4) and decreased to 147,000 

pounds in 2001 (U.S. Environmental Protection Agency, 

2003a). 

Formation of Chloroform and Other

Trihalomethanes as Disinfection By-Products

In 1902, Belgium was the first country to implement the 

continuous use of chlorine to disinfect drinking water (Drinking 

Water and Wastewater, 2002). Chlorination of public water 

supplies in the United States first began in 1908, when the community of Jersey City, New Jersey, began adding hypochlorite 

to water drawn from the Boonton Reservoir. This led to a celebrated court case in which the right of the city to chlorinate the 

water supply was upheld in the best interest of public health 

(Sawyer and McCarty, 1978). As a result, the practice of chlorinating public water supplies soon became a nearly routine 

method of water treatment in the United States. The disinfection 

of drinking water in the United States by chlorination, which 

includes the use of chlorine gas, sodium hypochlorite, calcium 

hypochlorite, and/or chloramines (collectively referred to as 

reactive chlorine), is now a common practice (table 1).

Table 1. Percentage of large and small drinking-water systems 

using chlorine and non-chlorine disinfection methods, in 1998.

[Source: Chlorine Chemistry Council, 2002; <, less than; --, not used]

Disinfection method

Proportion of systems employing method, 

in percent

Large systems

(serving more than

10,000 people)a

Small systems

(serving 10,000 or

fewer people)

Chlorine gas 84 61

Sodium hypochlorite 20 34

Calcium hypochlorite <1 5

Chloramines 29 --

Ozone 6 --

Ultraviolet -- --

Chlorine dioxide 8 --

a

Column does not sum to 100 percent because many larger systems employ 

more than one disinfection method, whereas some employ no disinfection at 

all.

Figure 3. Industrial discharges of chloroform to surface water in the United States from 1988 through 2001. (Data from 

U.S. Environmental Protection Agency, 2003a.)

YEAR

ANNUAL CHLOROFORM DISCHARGE, IN MILLIONS OF POUNDS

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4
6 Chloroform in the Hydrologic System

Figure 4. Industrial discharges of chloroform by underground injection and releases to land in the United States from 1988 

through 2001. (Data from U.S. Environmental Protection Agency, 2003a.)

YEAR

ANNUAL CHLOROFORM DISCHARGE, IN THOUSANDS OF POUNDS

Underground injection

Releases to land

0

50

100

150

200

250

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

One of the principal advantages of chlorination over other 

disinfection methods is that, in addition to the destruction of 

microorganisms that takes place on initial contact (as discussed 

below), the maintenance of a minimum concentration of reactive chlorine in the treated water (commonly referred to as a 

chlorine residual) provides ongoing disinfection within the distribution system (Weber, 1972). Some systems use ozone, ultraviolet light, or potassium permanganate rather than chlorination 

for initial disinfection, but none of these other methods provide 

residual protection within the distribution system. Because of 

the use of these other initial methods of drinking water disinfection, the USEPA now requires (through the Stage 1 Disinfectants and Disinfection By-Products Rule) that each community 

water system maintain an adequate chlorine residual to control 

microorganisms in the distribution system, regardless of the 

initial disinfection method used (U.S. Environmental Protection 

Agency, 2002c). 

The initial step in the chlorination of water or wastewater 

commonly involves the dissolution of chlorine gas (Cl2) in 

the medium of interest. When dissolved in water, the Cl2 

molecule—in which both atoms possess an oxidation state of 

zero—readily undergoes a disproportionation reaction, in 

which one of the chlorine atoms is reduced to chloride (Cl-

), 

and the other is oxidized to Cl+, a highly reactive species that 

rapidly displaces a proton from water to form hypochlorous 

acid (HOCl). The overall result is the hydrolysis of the Cl2

molecule according to the following equilibrium:

Cl2 + 2H2O↔H3O+ + Cl- + HOCl (1)

Hypochlorous acid dissociates at alkaline pH to form hypochlorite ion (OCl-

) according to the following equilibrium: 

HOCl + H2O↔H3O+ + OCl- (2)

At 20°C, the pKa (dissociation constant) value for equilibrium (eq. 2) is 7.6, indicating that the concentration of OCl-

 will 

be higher than that of HOCl at pH values greater than 7.6, while 

the opposite will occur at pH values less than 7.6. Disinfection 

occurs primarily through the oxidation of biological tissues by 

HOCl and OCl-

, as well as by other reactants containing chlorine (or other halogens) with an oxidation state higher than that 
Sources of Chloroform 7

of Cl-. The term “free available chlorine” (or, for the purposes 

of this report, simply free chlorine) refers collectively to the 

reactants HOCl, OCl-

, and Cl2. While the term “combined 

available chlorine” refers to reactive chlorine species such as 

chloramines that include elements other than oxygen or hydrogen. Because HOCl is a much stronger disinfectant than OCl-

, 

the effectiveness of disinfection by chlorinated water increases 

with decreasing pH (Weber, 1972). The bromide ion, if present, 

is rapidly oxidized by the free chlorine to form a variety of 

reactive bromine species such as HOBr, OBr-

, Br2 and BrCl 

(Trussell and Umphres, 1978). The oxidation of bromide by 

HOCl, for example, can produce hypobromous acid (HOBr) 

which, like HOCl, is a powerful disinfectant (Boyce and 

Hornig, 1983):

HOCl + Br-→ HOBr + Cl- (3)

The presence of THMs and other halogenated compounds 

in water disinfected through chlorination was first reported in 

the mid 1970s by Rook (1974) who, in a subsequent publication 

(Rook, 1975), attributed this association to the reaction of free 

chlorine with dissolved organic matter (DOM). (DOM is commonly defined as the fraction of natural organic matter (NOM) 

that passes through a 0.45-micron filter (Thurman, 1985).) The 

reactions of free chlorine with DOM and other precursor compounds in chlorinated waters generate chloroform and a variety 

of “disinfection by-products” or DBPs (Weinberg and others, 

2002), as a result of the haloform reaction. If bromide is present, 

products from the haloform reaction usually include brominated 

compounds, including one or more of the three brominated 

THMs. Similarly, iodinated THMs form if iodide is present.

Because ammonia, ferrous iron, and sulfide are all reduced 

species, if any of them are present, they will provide an additional inorganic demand for the chlorine immediately following 

chlorination, competing with the DOM for reaction with the 

free chlorine. As a result, in the presence of sufficiently high 

concentrations of one or more of these reduced species, only 

traces of THMs will form during the initial stages of the reaction. Once most of the inorganic demand is satisfied, reaction of 

the remaining free chlorine with DOM produces higher yields 

of THMs (Trussell and Umphres, 1978).

A variety of mechanisms have been proposed for the 

haloform reaction. For aromatic substrates (including humic 

and fulvic acids that contain an aromatic ring), this reaction 

(shown in fig. 5, using a generic 1,3-hydroxyaromatic compound as model substrate) is presumed to take place through an 

initial, rapid halogenation by HOCl (although, as noted above, 

in the presence of dissolved bromide or iodide, the attacking 

species also might be HOBr or HOI) of one or more aromatic 

carbon atoms activated by ortho hydroxyl (-OH) or (at pH 

values above the pKa of the phenol), phenoxy (-O-

) substituents 

(reaction 1 in fig. 5). The initial halogenation is then presumed 

to be followed by the hydrolytic cleavage of the ring at a site 

adjacent to a halogenated carbon (reaction 2); further halogenation of the aliphatic products of ring scission (reaction 3); and 

release of the THM following alkaline hydrolysis (reaction 4) 

(Rook, 1977; Boyce and Hornig, 1983). The reactions shown in 

figure 5 also include separate pathways that account for the frequent detection of dichloromethane (methylene chloride) 

(reaction 5) and polyhaloacetic acids (reactions 6 and 7) in chlorinated water.

An extensive amount of research has helped to elucidate 

the factors that affect the production rates and yields of chloroform and other DBPs during chlorination. Of principal importance among these factors are the structure and concentrations 

of the DOM and other precursor compounds that may be 

present, the concentration (or dose) of free chlorine, pH, 

bromide ion concentration, and temperature.

Several studies have shown that the rate of DBP formation 

is directly related to DOM concentration and to chlorine dose. 

Alawi and others (1994) observed the rate of chloroform 

appearance to be linearly related to the concentrations of both 

free chlorine and DOM. Trussell and Umphres (1978) reported 

similar results, but observed this relation only after the inorganic demand was satisfied. Fujii and others (1998) observed a 

linear correlation between the dissolved organic carbon (DOC) 

concentrations in agricultural drainage waters from the Sacramento-San Joaquin Delta, California, and the THM formation 

potential (that is, the total yield of THMs over a fixed reaction 

time under standardized conditions) in samples of these waters. 

The reaction of chlorine with DOM, however, is relatively slow. 

Under conditions typical of water treatment, significant rates of 

formation of chloroform and other THMs may still be observed 

after 24 hours (Babcock and Singer, 1979).

Many different characteristics of NOM have been examined as potential predictors of the yields of THMs and other 

DBPs during chlorination. The precursors that have been found 

to produce DBPs in highest yields include several aliphatic 

ketones, such as 2-butanone (methyl ethyl ketone) and acetone 

(Reckhow and others, 1990). In most cases, however, DBP 

precursors possess aromatic structures, usually with a variety 

of different functional groups, including hydroxyl, aldehyde, 

ketone, carboxyl, and methoxyl moieties. Evidence that most of 

these precursors possess some aromatic character is provided 

by the observation that the DBP formation potential of NOM 

has been found to be correlated with its specific ultraviolet 

absorbance (SUVA), a property that provides an indirect measure of aromatic content. The SUVA is defined as the absorbance of ultraviolet light at 254 nanometers by the material of 

interest, in aqueous solution, normalized by the concentration 

of DOC (Fujii and others, 1998; Liang and Singer, 2003).

DOM is often classified operationally into two types of 

compounds—humic and fulvic acids—based on their solubility 

at low pH. Humic acids precipitate from aqueous solutions at 

pH values less than 1, whereas fulvic acids do not (Thurman, 

1985). Although the relative proportions of the two fractions 

vary considerably among different source materials, climates, 

ecosystems, and locations, fulvic acids generally constitute 

approximately 90 percent of DOM by weight, whereas humic 

acids constitute about 2 to 3 percent (Babcock and Singer, 

1979).
8 Chloroform in the Hydrologic System

Figure 5. Hypothesized mechanisms by which a generic 1,3-hydroxyaromatic compound (a model for a humic or fulvic acid 

molecule containing an aromatic ring) is transformed during chlorination. (Cl+

, Br+, and I+ represent any halogenating electrophiles 

of the form XOH2

+

, X2, HOX, X2O, etc., where X = Cl, Br or I.) Numbers in circles refer to reaction steps listed in text. Bonds broken 

during reactions a-f are indicated by dashed lines, and resulting compounds are labeled a-f, respectively. Curved arrows denote 

movement of electron pairs. Adapted and redrawn from Rook (1977).

OH

R

C

C

C

C

C

C

1

R2

R3

H

OH

HOCl

H O2

O

R

C C

C

C

C

1

R2

R3

Cl

O

Cl

OHδ

+

δ

−

C

Cl

OHO

R

C C

C

C

C

1

R2

R3

Cl

O

Cl

C

Cl

-

OH

H+

O

R

C

C

C

C C

1

R2

R3

Cl

O

Cl

C

Cl

OH

H Cl+

(Br , I )

O

R

C

C

C

C C

1

R2

R3

Cl

O

Cl

C

Cl

OH

Cl H+

OHOH- OHf

c

e

b

d

a

f e c d

a b CH Cl 2 2

C

C C

O

(H,Cl)

Cl

Cl Cl (H,Cl)

Cl

C

C H

O

Cl

Cl

Cl

+

C

C OH

O

(H,Cl)

Cl

Cl

C

C C

O

(H,Cl)

Cl

Cl Cl Cl

Cl

CHCl

Formed if

bromide

or iodide

are

initially

present

3

CHBrCl 2

CHBr 3

CHBrICl

CHBr Cl 2

CHBr I2

+ +

+

+

+

+

+

other THMs

+

C

C H

O

Cl

Cl

Cl

+ C

C OH

O

(H,Cl)

Cl

Cl

1

3

4

2

5

6

7

Cl ,+

Br , I + +
Sources of Chloroform 9

Both humic and fulvic acids may react during chlorination 

to form chloroform and other THMs (Trussell and Umphres, 

1978; Reckhow and others, 1990; Adin and others, 1991; Alawi 

and others, 1994). Although humic acids usually constitute a 

small percentage of the total DOM, Babcock and Singer (1979) 

observed them to react with free chlorine more readily than 

fulvic acids, producing more than three times as much chloroform per gram of DOC as that produced by fulvic acids, and 

consuming 75 percent of the free chlorine under the conditions 

examined. Given the fact that humic acids generally are more 

hydrophobic than fulvic acids (Thurman, 1985), the results 

reported by Babcock and Singer (1979) were corroborated 

during a more recent study by Liang and Singer (2003), who 

observed that the hydrophobic fractions of DOC taken from a 

variety of different sources usually generated higher THM 

yields than did the hydrophilic fractions (at both pH 6 and 8).

The rates of production of chloroform and other THMs 

during the haloform reaction have a complex dependence on pH 

that may vary among different precursor compounds. For a 

series of chlorination experiments conducted between pH 2 and 

12, Boyce and Hornig (1983) observed that a variety of 1,3-

dihydroxyaromatic precursors had similar patterns of chloroform yield as a function of pH—provided the aromatic carbon 

atom between the two hydroxyl groups was either unsubstituted 

or possessed a carboxyl group—with maximum yields occurring between pH 8 and 10 (fig. 6). The authors suggested that 

the lower rate of chloroform production at pH values greater 

than 10 is most likely a result of the diminished rate at which 

OCl-

 engages in the haloform reaction, relative to HOCl (eqs. 1 

and 2). Results from more recent experiments involving the 

chlorination of water from a Florida reservoir also showed 

higher rates of THM formation at pH 8 than at pH 6 (Liang and 

Singer, 2003), consistent with the pattern observed for 1,3-dihydroxybenzene within this pH range (fig. 6).

The substantial rates at which the 1,3-dihydroxyaromatic 

compounds undergo the haloform reaction appear to result from 

the presence of an unsubstituted carbon atom at the ortho 

position shared by the two hydroxyl-bound carbons in these 

Figure 6. Effect of pH on the yield of chloroform from the chlorination of the substrate 1,3-dihydroxybenzene (resorcinol), 

based on measurements of chloroform concentrations using either direct aqueous injection or solvent extraction. (Modified 

from Boyce and Hornig, 1983.)

0

0.2

0.4

0.6

0.8

1.0

1 2 3 4 5 6 7 8 9 10 11 12 13

pH, IN STANDARD UNITS

YIELD OF CHLOROFORM, IN MOLES PER MOLE OF SUBSTRATE

Direct Aqueous Injection

Solvent Extraction

[Substrate] = 1x10-5 M (Molar)

[CI2] = 1x10-4 M
10 Chloroform in the Hydrologic System

molecules (Boyce and Hornig, 1983), since hydroxyl groups, 

like alkoxy groups, are known to be ortho and para directing 

with respect to electrophilic aromatic substitution (Rook, 1977). 

(According to Boyce and Hornig (1983), the elevated reactivity 

of 2,6-dihydroxybenzoic acid appears to be facilitated by the 

decarboxylation of the molecule. While the authors do not state 

this explicitly, structural constraints appear to require that the 

halogenation of the molecule be preceded, rather than followed 

by, the decarboxylation step.) Other studies also have reported 

high yields of chloroform (greater than 85 percent) from the 

chlorination of 1,3-dihydroxybenzenes at pH 7, 10, and 11 

(Rook, 1977; Johnson and Jensen, 1986). In contrast, 1,2- and 

1,4-dihydroxybenzenes produce chloroform yields greater than 

10 percent at pH 10, but not at pH 7 (Stevens and others, 1976; 

Boyce and Hornig, 1983).

As noted previously, when bromide ion is present during 

chlorination, the haloform reaction commonly produces one or 

more of the brominated THMs, in addition to chloroform (for 

example, 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 

(reaction 3) much more rapidly than it reacts with DOM (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 (that is, 

0.03 or less), the yields of the four principal THMs, generated 

during chlorination, decrease with increasing bromination of 

the product, that is, 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 by Fujii and others 

(1998) of the THM formation potential of agricultural drainage 

waters from the Sacramento-San Joaquin Delta, California. 

Results from studies where more than one THM has been 

detected in the hydrologic system commonly indicate 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; Lundgren and Lopes, 1999; Squillace, Moran, 

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 chlorinated waters 

from other potential sources of chloroform in the hydrologic 

system—especially if the concentrations among the four principal THMs (or detection frequencies, if based on the same detection limit for all four compounds) have the trend noted above.

In water chlorinated in the presence of sufficiently high 

concentrations of bromide, bromoform may be the predominant 

THM formed. The bromide concentration threshold above 

which this occurs has been found to range between 0.1 and 

2 mg/L (milligrams per liter) and depends upon several factors, 

including DOM concentration and chlorine dose. For a given 

chlorine dose, the presence of bromide has been found to 

increase both the total yield of THMs and the rate of THM 

formation (Trussell and Umphres, 1978; Luong and others, 

1982).

As is the case for other thermal (that is, non-photochemical) reactions, rates of THM formation increase with temperature according to the Arrhenius relation (see Moore and Pearson 

(1981) for a description of this equation). Estimated activation 

energies, however, show considerable variation among studies. 

While Urano and others (1983) reported an activation energy of 

37 kJ (kilojoules) per mole for the haloform reaction, the value 

estimated by Kavanaugh and others (1980) was 9.6 kJ per mole.

The effect of temperature on the rate of the haloform reaction is probably responsible for the common observation of 

higher chloroform concentrations in chlorinated water during 

the summer, relative to winter. Stevens and others (1976), for 

example, suggested that seasonal variations in water temperature could account for the differences observed in the concentrations of chloroform in finished Cincinnati tap water between 

winter (30 μg/L or less) and summer (as much as 200 μg/L). 

Similarly, in a year-long comparison of raw and treated water 

from three drinking-water treatment plants in Canada, LeBel 

and others (1997) reported lower chloroform concentrations 

during the period from December to April (raw water temperatures less than 5°C) than during the period from June to September (raw water temperatures between 15 and 23°C). For all three 

treatment plants, mean chloroform concentrations were significantly higher (p<0.02) in the summer than in the winter. (A significance level [alpha] of 0.05 will be used for all statistical tests 

described in this report.) These observations also indicate that 

as temperatures increase, the effects of higher haloform reaction 

rates on THM concentrations are generally large enough to 

overwhelm the effect of the enhanced volatilization of these 

compounds that also is likely to occur as a result of the increase 

in their Henry’s Law constants (Nicholson and others, 1984).

Chlorinated Water as a Source of THMs in the 

Hydrologic System

A major source of anthropogenic chloroform detected in 

ground 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 wastedisposal 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 (California 

Department of Health, 1990; Agency for Toxic Substances and 

Disease Registry, 1997). These intentional discharges, however, are expected to represent only minor contributions to the 

total chloroform releases of anthropogenic chloroform to the 
Sources of Chloroform 11

hydrologic system, relative to the amount of the compound 

contributed by inadvertent releases of chlorinated drinking 

water and wastewater (Agency for Toxic Substances and 

Disease Registry, 1997). Chlorinated drinking water from public water-supply systems may enter ground and surface water 

through the irrigation of lawns, gardens, golf courses, athletic 

fields, and parks, as well as leaking swimming pools, spas, or 

distribution lines for treated wastewater. In addition, because 

the maintenance of a chlorine residual is now required in community water supplies and distribution systems, this measure 

increases the likelihood that leakage from these systems also 

will discharge chloroform and other DBPs to ground and 

surface water. Solley and others (1998) have 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, McCulloch (2003) reported that an estimated 

2 gigagrams (Gg, where 1 Gg = 1x109 g), or 4.41 million 

pounds of chloroform are generated per year through the chlorination of drinking water in the United States.

Chloroform has been detected in rainwater (Kawamura 

and Kaplan, 1983; Adachi and Kobayashi, 1994). Precipitation 

therefore represents a potential source of the compound to the 

hydrologic system. However, in studies of chloroform contamination of the shallow ground water in Indianapolis, Indiana 

(Fenelon and Moore, 1996), and Glassboro, New Jersey (Baehr 

and others, 1999), the maximum concentrations of chloroform 

estimated (through equilibrium partitioning calculations) to be 

present in rainwater at the two locations (0.1 and 0.0025 μg/L, 

respectively) were lower than those measured in the ground 

water at those locations. This indicates that precipitation was 

unlikely to have been the sole source of the compound detected 

at either study site. Fenelon and Moore (1996) suggested that 

the chloroform detected in ground water during their study may 

have been derived from chlorinated water from irrigation of 

turfgrass, septic systems or industrial settling ponds, or leakage 

of treated water and wastewater from distribution lines.

Baehr and others (1999) hypothesized that the chloroform 

detected in ground water at the study site in Glassboro, New 

Jersey, was likely to have emanated from a nearby point source. 

However, the fact that CHCl3, CHBrCl2, CHBr2Cl, and CHBr3

were all detected in ground water within the study area—and in 

the order of decreasing detection frequency above (Stackelberg 

and others, 1997; Baehr and others, 1999)—indicates that the 

ultimate source of the chloroform detected during this investigation may have been chlorinated water, regardless of its spatial 

configuration (that is, point or nonpoint).

Concentrated hydrochloric acid (HCl) is commonly added 

as a preservative to inhibit biological activity (by lowering the 

pH) in water samples collected for VOC analysis. Because 

concentrated HCl may contain trace levels of free chlorine, it is 

possible that residual free chlorine in the HCl may react with 

DOM to produce chloroform in such samples. An investigation 

by Squillace, Pankow, and others (1999), however, indicated 

that the preservation of ground-water samples by the addition of 

HCl did not form significant concentrations of THMs when 

samples were stored at 4°C and analyzed within 14 days.

Formation of Chloroform from the 

Dehalogenation of Tetrachloromethane

Tetrachloromethane (CCl4, also known as carbon tetrachloride) has been widely used for several decades as an industrial solvent, degreaser, and grain fumigant. This compound 

also is one of the most common contaminants present in the 

chlorine gas used for chlorination (Christman, 1980), perhaps 

because it is frequently used to rinse out the cylinders in which 

chlorine gas is supplied (Krasner and others, 2001). Several 

studies have demonstrated that chloroform may be produced 

from the transformation of tetrachloromethane by microorganisms under iron- or sulfate-reducing conditions (Criddle and 

others, 1990; Assaf-Anid and others, 1994; Curtis and Reinhard, 1994; Matheson and Tratnyek, 1994). The replacement of 

a chlorine atom with hydrogen in this manner is an example of 

a type of reaction known as hydrogenolysis (fig. 7), and occurs 

in this example by way of a trichloromethyl radical intermediate

(•CCl3). Although this transformation, which is an example of 

a broader class of reactions known as reductive dechlorination, 

has been shown to occur abiotically in the laboratory (

and Gonsior, 1984), it is unlikely to occur to any appreciable 

extent in the hydrologic system without either direct or indirect 

microbial assistance.

Natural Sources

Chloroform and other chlorinated compounds are often 

considered to be of solely anthropogenic origin. However, 

many studies have indicated that some of these compounds, 

including chloroform, also may be produced by natural processes (fig. 7). For chloroform, marine and terrestrial sources 

appear to contribute in roughly equal amounts to natural global 

production (Laturnus and others, 2002). A source that is common to both marine and terrestrial settings is volcanic gas, especially that derived from solfataric volcanoes, which produce 

high concentrations of sulfur compounds (Isidorov and others, 

1990).

Khalil and others (1983) indicated that the tropical oceans 

may be a major source of chloroform, with an estimated production rate of approximately 350 Gg of chloroform per year. 

Marine micro and macroalgae have been found to be a significant source of chloroform in the oceans, contributing about 

7 percent of the total annual emission of the compound from 

marine sources (Laturnus and others, 2002). Gribble (1994) has 

suggested that marine algae produce chloroform as a defense 

against predation. Other possible, but undocumented marine 

sources include other organisms, cold vents, marine volcanoes, 

and other geologic phenomena associated with tectonic activity 

(Laturnus and others, 2002).

Klecka v
12 Chloroform in the Hydrologic System

Figure 7. Reactions that may either produce chloroform or transform it into other compounds in the hydrologic system.

C

H

Cl

Cl

Cl

C

Cl

Cl

Cl

Cl

ClIron or sulfate reducing, 

or methanogenic

conditions 

Humic and

fulvic acids

HOCl

Water chlorination

Marine algae, 

peatlands,

soil microbes,

volcanos, 

human sources

Other 

disinfection

byproducts

Haloform

reaction

(oxidation)

Natural and other

anthropogenic sources1 Hydrogenolysis2,3,4,5

Hydrolysis7

(negligible rate)

Hydrogenolysis2,3,4

C

H

Cl

H

Cl

H + 2eClIron or sulfate reducing, or

methanogenic conditions 

Bio-oxidation8

5H + 3Cl

2H2O

O O C

Methanogenic

conditions 

+

+

H + 2e + -

HO.

- -

2H + Cl + -

(Tetrachloromethane)

(Chloroform)

(Dichloromethane)

(Formate ion)

Cl Cl

C

O

C

O

H O

+ C O

-

3H2O

7H + 6Cl +

Indirect 

photolysis6

(Phosgene)

[

1McCulloch, 2003; 2Klecka and Gonsior, 1984; 3Egli and others, 1988; 4 Gälli and McCarty, 1989; 5Picardal and others, 1995; 6Howard, 1991; 7Fells and Moelwyn-Hughes, 1959; 8Bouwer and McCarty, 1983]

v
Physical and Chemical Properties Controlling the Transport and Fate of Chloroform 13

While paper manufacturing and water treatment are the 

largest sources of anthropogenic chloroform (Laturnus and 

others, 2002), soils, termite mounds, and rice fields are estimated to be the largest natural terrestrial sources of the compound. In soils, the production of chloroform, and chlorinated 

soil acids such as trichloroacetic acid (CCl3COOH) has been 

found to be a result of the chlorination of NOM by naturally 

produced HOCl—a transformation analogous to the haloform 

reaction discussed previously (fig. 5). In soils, HOCl is produced by the oxidation of chloride ion by hydrogen peroxide 

under acidic conditions. This reaction is catalyzed by the chloroperoxidase enzyme (CPO), which is commonly detected in 

soils, and most likely derived from soil fungi (McCulloch, 

2003):

CPO

H2O2 + H3O+ + Cl- → HOCl + 2H2O

NOM + HOCl → chlorinated NOM + CHCl3

+ CCl3COOH + other chlorinated products (4)

Rates of chloroform production by natural soils show considerable variability, depending upon soil characteristics. Consistent with the mechanism described above, higher rates are 

observed in soils containing larger amounts of NOM. The 

highest rates of chloroform production (1 microgram per square 

meter per hour (μg/m2

/hr)) have been measured in Douglas fir 

forest soils, in soils covered with decaying wood chips, and in 

soils with large amounts of humic material. In spruce forest 

soils, chloroform production rates have been estimated to be as 

high as 0.5 μg/m2/hr (Haselmann and others, 2000). Substantially lower rates have been observed in less organic rich soils, 

such as those encountered in dry swamps, grasslands, sandy 

areas, and pine forests (Hoekstra and others, 2001). 

Anthropogenic Fluxes in Relation to

Global Emissions

Laturnus and others (2002) reported that of the estimated 

700 Gg of chloroform released annually across the planet, 

9.5 percent is attributable to anthropogenic sources. Similarly, 

McCulloch (2003) reported a global chloroform emission rate 

of 660 Gg per year, with 10 percent derived from anthropogenic 

sources. Khalil and others (1983), however, estimated a global 

anthropogenic chloroform production rate of 250 Gg per year, 

considerably higher than the estimates provided by Laturnus 

and others (2002) and McCulloch (2003). The burning of 

biomass, of which about 90 percent is induced by humans 

(Laturnus and others, 2002), releases approximately 2 Gg of 

chloroform annually worldwide, representing less than 

1 percent of total global production of the compound (Lobert 

and others, 1999). The chloroform emission rate from the 

Northern Hemisphere is considerably higher than that from the 

Southern Hemisphere (Laturnus and others, 2002; McCulloch, 

2003), perhaps as a consequence of the higher levels of industrial activity and development in the north.

Physical and Chemical Properties 

Controlling the Transport and Fate of 

Chloroform

Based on its large Henry’s Law constant compared to other 

VOCs (table 2), chloroform is expected to be present mostly in 

the vapor phase following its release to the atmosphere 

(Howard, 1993). However, because the compound also is relatively water soluble, some removal of atmospheric chloroform 

is expected to occur during rainfall events, as demonstrated by 

the fact (noted previously, p. 12) that it has been detected in 

precipitation (Kawamura and Kaplan, 1983; Adachi and Kobayashi, 1994). The relatively large Henry’s Law constant for this 

compound also implies that much of the chloroform in surface 

waters is likely to volatilize soon after its release. As might be 

anticipated from fundamental principles of mass transfer (for 

example, Weber, 1972), the rate of chloroform volatilization 

from streams increases with increasing water velocity, as well 

as with decreasing stream depth. Also in accord with theory is 

the observation noted previously that chloroform volatilization 

rates, like the Henry’s Law constant, increase with increasing 

temperature (O’Connor and Dobbins, 1958; Churchhill and 

others, 1962; Owens and others, 1964).

The relatively low organic carbon-water partition coefficients (Koc) values for chloroform and the other THMs (table 2) 

indicate that these compounds have relatively low affinities for 

soil organic carbon. As discussed further below, chloroform 

also is quite persistent under oxic conditions (that is, in the presence of detectable concentrations of dissolved oxygen) in the 

hydrologic system. As a result, chloroform that is not volatilized in near-surface soils or taken up by plants following its 

release is expected to migrate substantial distances through the 

subsurface, particularly in aquifers containing materials with a 

low mass fraction of organic carbon, or foc (that is, foc < 0.001 

(McCarty and others, 1981)).

Similarly, field experiments involving the injection of 

chloroform below the water table have demonstrated that the 

migration of the compound through the saturated zone is only 

moderately retarded by low-carbon aquifer materials. Roberts 

and others (1982) observed retardation factors varying 

between 2.5 and 3.8 for the migration of chloroform through a 

relatively carbon-poor aquifer (foc about 0.01) beneath Palo 

Alto, California. (A retardation factor (R) represents the ratio 

between the rate of migration of a conservative solute (that is, 

a non-sorbing, non-reactive species such as chloride under oxic 

conditions) through ground water and that of the compound of 

interest (Freeze and Cherry, 1979). The R values given above, 

for example, imply a rate of chloroform migration through the 

subsurface that is between 25 and 38 percent of the speed of a 

conservative solute.)
14 Chloroform in the Hydrologic System

Table 2. Chemical and physical properties that influence the transport and fate of the four trihalomethanes discussed in this report.

[BCF, bioconcentration factor; CAS, Chemical Abstract Service; °C, degrees Celsius; g/cm3, grams per cubic centimeter; g/mol, grams per mole; KH, Henry’s 

Law constant; Koc, organic carbon-water partition coefficient; Kow, octanol-water partition coefficient; mg/L, milligrams per liter; mL/g, milliliters per gram; mm 

Hg, millimeters of mercury; NL, not listed; Pa m3/mol, Pascal meter cubed per mole]

Property Chloroform Bromodichloromethane Dibromochloromethane Bromoform

CAS number 67-66-3 75-27-4 124-48-1 75-25-2

Molecular weight 119.39a 163.83a 208.28a 252.73b

Density (g/cm3 at 20°C)c 1.4799 1.980 2.451 2.890

Boiling temperature (°C) 61.7 (at 760 mm Hg)a 90 (at 760 mm Hg)a 119-120 (at 748 mm Hg)a 149.5 (at 15 mm Hg)b

Water solubility (mg/L)d 7,950a 4,700 at 22°Ca 4,000c 3,100b

Vapor pressure (Pa)d 26,241c 6,665 at 20°Cc 1,014c 719.9b

Henry’s Law constant (H)

(Pa m3/mol)c

 314.1 162.0 88.2 62.0

Kow 93.3a 125.9c 173.8a 234.4b

Koc (mL/g soil organic carbon)c 43.7 61.0 NL 117.5

BCF (mL/g tissue)c 2.88 - 691.8 5.25 - 34.7 NL 63.1

a

From Howard (1991).

b

From Howard (1993).

c

From Mackay and others (1993).

dAt 25° Celsius, unless otherwise noted.

The relatively low affinity of chloroform and other THMs 

for organic carbon also is responsible for the limited tendency 

of these compounds to build up in the tissues of living organisms, as indicated by their low bioconcentration factor (BCF) 

values (table 2). (The BCF represents the predicted ratio 

between the concentration of a given compound in the tissues 

of an organism and the concentration of that compound in the 

surrounding water (Nowell and others, 1999).)

Transformation of chloroform and the other THMs in the 

hydrologic system takes place primarily through photolysis or, 

under hypoxic conditions (that is, in the absence of detectable 

levels of dissolved oxygen), reductive dehalogenation. THMs 

also are subject to hydrolysis, but at rates that are effectively 

negligible under most environmental conditions.

Phototransformations in the hydrologic system occur 

through one of two principal mechanisms—direct photolysis, 

in which the compound undergoes transformation as a result of 

its absorption of solar energy of the appropriate wavelength 

and sufficient intensity, and indirect photolysis, in which the 

compound reacts with another chemical species that have been 

energized, either directly or indirectly, in this manner. Direct 

photolysis of chloroform is not considered a significant transformation process in the atmosphere because chloroform does 

not show significant light absorbance within the solar spectrum, 

that is, at wavelengths greater than 290 nanometers (Harris, 

1990; Agency for Toxic Substances and Disease Registry, 

1997). Thus, when chloroform undergoes phototransformation 

in the atmosphere, it does so primarily through indirect, rather 

than direct photolysis (fig. 7). 

One of the most significant mechanisms by which chloroform undergoes indirect photolysis is its reaction with hydroxyl 

radical (•OH), which occurs with a half-life of 26 to 260 days 

(Howard, 1991; Mackay and others, 1993). This reaction occurs 

through a trichloromethyl radical intermediate to form products 

that include phosgene (Cl2C=0) and •OCl (Hanst, 1978):

CHCl3 + •OH → H2O + •CCl3

•CCl3 + O2 → Cl2C=0 + •OCl (5)

The potential importance of this process is suggested by 

the observation that the reactivity of airborne chloroform 

increases in the presence of photochemical smog, which often 

contains substantial concentrations of hydroxyl radical (Baulch 

and others, 1980). During irradiation experiments conducted 

under artificial light in the presence of 0.2 parts per million of 

NOx, Dimitriades and Joshi (1977) observed an average chloroform transformation rate of 0.8 percent per hour, or a half-life 

of about 11 days. Given its large Henry’s Law constant and 

generally low photochemical reactivity in the absence of smog, 

chloroform is likely to be transported relatively long distances 

in the atmosphere before undergoing transformation or being 

removed by precipitation. Although photodegradation may lead 

to some removal of chloroform from the atmosphere, it appears 
Chloroform Occurrence in the Hydrologic System 15

to be too slow in water to represent a significant transformation 

process in most aquatic systems (Howard and others, 1991).

Under most conditions, hydrolysis (fig. 7) also is not considered to be a significant degradation process for chloroform in 

the hydrologic system. Half-lives estimated at 25°C and pH 7 

for this reaction range from 1,760 to 31,600 years (Rathbun, 

1998). Chloroform hydrolyzes more slowly than the other three 

THMs, reflecting the greater ease with which bromide is 

displaced from halogenated organic compounds, relative to 

chloride (March, 1985). At 25°C and pH 7, half-lives for the 

hydrolysis of CHBrCl2, CHBr2Cl, and CHBr3 have been estimated to be approximately 128, 274, and 686 years, respectively (Washington, 1995). 

Because they are highly oxidized compounds, THMs 

undergo more rapid transformation under hypoxic conditions 

than under oxic conditions. Thus, in contrast to the relatively 

slow transformation rates described previously, half-lives for 

the biotransformation of chloroform in water under hypoxic 

(sulfate reducing) conditions have been observed to range from 

3 to 21 days at 15°C between pH 6 and 8. At pH values less than 

6, half-lives increase to between 28 and 173 days (at the same 

temperature) (Saunders and others, 1996). As might be 

expected from the higher oxidation potential of chlorine relative 

to bromine (March, 1985), chloroform undergoes microbial 

transformation in hypoxic aqueous solution more rapidly than 

either CHBr2Cl or CHBr3, the rates decreasing with increasing 

bromination (CHCl3 > CHBr2Cl > CHBr3) (Howard and others, 

1991). (No data appear to be available for CHBrCl2.) Few data 

are reported for chloroform biotransformation under oxic conditions; however, using an initial chloroform concentration of 

500 μg/L under oxic conditions, Tabak and others (1981) 

reported chloroform biotransformation to occur in the laboratory with half-lives ranging from 5 to 7 days at 25°C.

Chloroform Occurrence in the 

Hydrologic System

The detection of chloroform in the hydrologic system has 

been documented in several national, regional, and local studies. Most of these studies have found chloroform to be the most 

frequently detected VOC examined, although its concentrations 

in water are usually low enough that the USEPA’s current 

(2004) Maximum Contaminant Level (MCL) of 80 μg/L for the 

sum of all THM concentrations (also referred to as the total 

trihalomethanes or TTHM concentration) is not often exceeded.

Atmosphere

In a study of VOCs in ambient air in 13 rural and urban 

locations in the United States, chloroform concentrations were 

found to increase with the degree of urbanization; however, 

chloroform was one of the few compounds that did not appear 

to have seasonal trends in concentration (Pankow and others, 

2003). By contrast, in the Arctic region of Norway, average 

concentrations of chloroform in air were found to be lower in 

July 1982 (16.5 parts per trillion by volume (pptv)) than in the 

spring of 1983 (26.6 pptv) (Howard, 1991). Data on chloroform 

concentrations in precipitation appear to be scarce, but concentrations as high as 125 μg/L in rainfall have been reported 

(Kawamura and Kaplan, 1983; Adachi and Kobayashi, 1994).

Ground Water

Table 3 summarizes data on chloroform detections in 

untreated ground water from a variety of studies ranging in 

scale from individual urban areas (for example, Mullaney and 

Grady, 1997) to the entire United States (Squillace, Moran, and 

others, 1999; Moran and others, 2002; Grady, 2003). For all of 

the studies listed, chloroform was the VOC detected most 

frequently in ground water. However, with the exception of the 

investigation by Squillace, Moran, and others (1999), all of the 

sampled ground water contained TTHM concentrations that 

were less than the USEPA MCL of 80 μg/L for TTHMs. 

Figure 8 presents data on THM detections in ground and surface 

water from the national study by Grady (2003).

Chloroform was detected in ground water beneath a broad 

range of land-use settings during these studies, 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, frequencies of detection of the compound 

were higher beneath urban and residential areas than beneath 

agricultural or undeveloped areas (Grady and Mullaney, 1998; 

Squillace, Moran, and others, 1999; E.A. Frick, U.S. Geological 

Survey, written commun., 2003). During the study of VOC 

occurrence in shallow ground water by Grady and Mullaney 

(1998), the differences in chloroform detection frequencies 

among urban, agricultural, and undeveloped areas (table 3) 

were found to be statistically significant (p=0.0110).

As noted previously, the widespread detections of chloroform in many of the urban and suburban areas are likely to have 

been a result of contamination from a variety of sources, including chlorinated public-supply water used to irrigate lawns and 

gardens, leakage from distribution systems for treated water and 

wastewater, spas, pools, and leachate from septic-system drainfields (Thiros, 2000). In addition, results from a study of publicsupply wells in the Coastal Los Angeles and Santa Ana Basins 

indicate that the detection of chloroform and other VOCs in 

ground water downgradient from artificial recharge facilities 

might be a result of contamination from such facilities. Observations from this study also attest to the comparatively high 

mobility of chloroform in the subsurface, because the compound was detected at or greater than 0.024 μg/L more frequently and at greater distances from the recharge facilities than 

any of the other VOCs analyzed (Shelton and others, 2001). 

Similarly, in ground water downgradient from an artificial 

recharge facility receiving chlorinated wastewater in Palo Alto, 

California, Roberts and others (1982) observed chloroform to 

show the highest mobility among all of the VOCs examined 

(including CHBr2Cl and CHBr3).
16 Chloroform in the Hydrologic System

Table 3. Studies of chloroform and other trihalomethane occurrence in ground water in the United States.

[Ag, Agricultural; Com, Commercial; Dom, Domestic; Dp, Deep; Irr, Irrigation; LV, Las Vegas; MCL, Maximum Contaminant Level; Mon, Monitoring; 

Mul, Multi-level; Mx, Mixed; PS, Public-Supply; Res, Residential; Ru, Rural (less than 386 persons per square kilometer); Sh, Shallow; SP, Spring; TTHM, total 

trihalomethanes; Un, Undeveloped; Urb, Urban; USGS, U.S. Geological Survey; μg/L, micrograms per liter; <, less than; --, data not available]

Study 

location, 

by State

Land-use 

setting

Reference

Number of 

sites 

sampled

Site

type

 Frequency of 

chloroform 

detection

at or greater

than 0.2 μg/L

(percent)

Maximum 

chloroform 

concentration

(μg/L)

Frequency of 

exceedance 

of MCL of 

80 μg/L for

TTHMs 

(percent)

Maximum 

TTHM 

concentration

(μg/L)

National 

study

Ru Moran and others (2002) 1,926 Dom 4.3 74 0 76.1

National 

study

Ru Squillace, Moran, and others 

(1999)

2,538 Mon, PS, 

Dom

5.1 210 0.04 210

National 

study

Urb Squillace, Moran, and others 

(1999)

405 Mon, PS, 

Dom

26 400 0.25 400

National 

study

-- Grady (2003) 579 PS 12 22.4 0 70.6

Conn., 

Mass., 

N.H., Vt.

Ag Grady and Mullaney (1998) 24 Mon 8.3 0.4 0 0.4

Ala., Fla., 

Ga.

Ag Frick, E.A. (USGS, written 

commun., 2003)

30 Mon 0 <0.2 0 <0.2

Conn., 

Mass., 

N.H., Vt.

Un Grady and Mullaney (1998) 22 Mon 4.5 2.6 0 2.8

N.J. Un Stackelberg and others 

(1997)

13 Mon, Sh 7.7 1.3 0 1.3

Conn., 

Mass., 

N.H., Vt.

Urb Grady and Mullaney (1998) 40 Mon 33 3.1 0 3.1

Fla., Ga. Urb, Ag, 

Un

Berndt and others (1998) 32 Mon, Sh, 

Dp 

12.5 2.2 0 2.2

Ariz. Ag Cordy and others (2000) 9 Mon, Sh 33 0.5 0 0.5

Ind. Ag Fenelon and Moore (1996) 72 Mon, Sh, 

Dp

3 1.7 0 1.7

Tenn. Ag Johnson and Connell (2001) 30 Mon 3.3 0.4 0 0.4

N.J. Ag Stackelberg and others 

(1997)

15 Mon, Sh 13 0.3 0 0.3

Ariz. Mx Cordy and others (2000) 77 PS, Dom, 

Irr, Com

16 9.9 0 10.2

Tenn. Mx Johnson (2002) 35 Sp 11 1.3 0 1.4

Utah Res, 

Com

Thiros (2000) 30 Mon 60 2.4 0 2.7

Calif. Urb Domagalski and others 

(2000)

19 Mon, Sh 11 5.1 0 5.2
Chloroform Occurrence in the Hydrologic System 17

Ind. Urb Fenelon and Moore (1996) 28 Mon, Sh, 

Dp

40 21 0 21

Ga. Urb Frick, E.A. (USGS, written 

commun., 2003)

40 Mon, Sp 35 4 0 4

Tex. Urb, Mx Bush and others (2000) 119 Mon, PS 4.2 1.3 0 1.5

Conn. Urb Mullaney and Grady (1997) 10 Mon, 

Mul, Sh

71 13 0 13.8

N.J. Urb Stackelberg and others 

(1997)

50 Mon, Sh 30 5.6 0 5.7

Conn. -- Grady (1997) 103 Mon, PS 23 13 0 13.8

Nev. -- Bevans and others (1998) 57 PS 50 LV/ 28 Reno 23 0 70.5

Del. -- Ferrari (2002) 30 PS 40 3.9 0 4.2

Calif. -- Shelton and others (2001) 178 PS 10 11 0 17

N.J. -- Stackelberg and others 

(2000)

 30 PS 40 2.5 0 2.53

Utah -- Thiros (2003) 31 PS 26 19.7 0 26.1

Table 3. Studies of chloroform and other trihalomethane occurrence in ground water in the United States.—Continued

[Ag, Agricultural; Com, Commercial; Dom, Domestic; Dp, Deep; Irr, Irrigation; LV, Las Vegas; MCL, Maximum Contaminant Level; Mon, Monitoring; 

Mul, Multi-level; Mx, Mixed; PS, Public-Supply; Res, Residential; Ru, Rural (less than 386 persons per square kilometer); Sh, Shallow; SP, Spring; TTHM, total 

trihalomethanes; Un, Undeveloped; Urb, Urban; USGS, U.S. Geological Survey; μg/L, micrograms per liter; <, less than; --, data not available]

Study 

location, 

by State

Land-use 

setting

Reference

Number of 

sites 

sampled

Site

type

 Frequency of 

chloroform 

detection

at or greater

than 0.2 μg/L

(percent)

Maximum 

chloroform 

concentration

(μg/L)

Frequency of 

exceedance 

of MCL of 

80 μg/L for

TTHMs 

(percent)

Maximum 

TTHM 

concentration

(μg/L)

The likely reasons for the detection of chloroform in shallow ground water beneath agricultural areas, however, are more 

elusive. In some locations, such as the West Salt River Valley 

of Arizona (Cordy and others, 2000), the occurrence of chloroform in shallow ground water may be attributable to the use of 

treated effluent from wastewater treatment plants for irrigation. 

Other agricultural areas, such as those in Connecticut (Grady 

and Mullaney, 1998) or New Jersey (Stackelberg and others, 

1997), may have frequent chloroform detections because of the 

proximity of residential or commercial development. Support 

for the latter hypothesis is provided by the frequent detection of 

predominantly agricultural pesticides in the shallow ground 

water beneath the urban areas examined during the Connecticut 

and New Jersey studies. Alternatively, the detection of chloroform in ground water beneath agricultural areas may be a consequence of its use either as an adjuvant compound in commercial pesticide formulations (Bishop, 1987) or as a fumigant 

(Meister, 2000).

Surface Water

As in ground water, chloroform often is the most frequently detected VOC in surface water (Lundgren and Lopes, 

1999). Table 4 summarizes results from several studies of chloroform occurrence in rivers and streams across a broad range of 

spatial scales in the Nation. The highest concentrations of chloroform in surface water are most commonly detected downstream from discharges of chlorinated urban wastewater. Locations where this observation has been documented include sites 

along the Raritan and Passaic Rivers in New Jersey (Reiser and 

O’Brien, 1998), Rowlett Creek in Texas (Buszka and others, 

1994), the San Antonio River near Elmendorf, Texas (Bush and 

others, 2000), and the Las Vegas Wash in Nevada (Bevans and 

others, 1998). Lundgren and Lopes (1999) also reported higher 

frequencies of chloroform detection in the Ohio River among 

samples collected from stations downstream from large cities 

relative to sites sampled upstream from major metropolitan 

areas. 
Figure 8. Detections of total trihalomethanes at or greater than 0.2 micrograms per liter in ground and surface waters sampled for the Am

Research Foundation national study (modified from Grady, 2003).

Alaska Hawaii

EXPLANATION

Ground water—No detectionGround water—DetectionSurface water—No detectionSurface water—Detection

0 0 
Chloroform Occurrence in the Hydrologic System 19

Table 4. Studies of chloroform occurrence in surface waters in the United States.

[MCL, Maximum Contaminant Level; TTHMs, total trihalomethanes; μg/L, micrograms per liter; <, less than; --, data not available]

Study 

location,

by State

Reference

River or

stream

Reporting 

limit for

chloroform

(μg/L)

Number of 

samples

Frequency 

of 

chloroform 

detection

in samples

(percent)

Chloroform 

concentration

range

(μg/L)

Frequency of 

exceedance 

of MCL of 

80 μg/L for

TTHMs

(percent)

Maximum 

TTHM 

concentration

(μg/L)

National 

study

Grady (2003) 163 River Sites 0.2 375 14 <0.2 to 84.7 0.27 102

 Ill., Ind., 

Ohio, Pa., 

Tenn., 

Va., 

W. Va., 

Lundgren and Lopes (1999) Ohio River .1 44,837 26 <0.1 to 125 3.8 137

N.J., N.Y. Ayers and others (2000) 42 stream sites .05 112 63 <0.05 to 1.5 0 3.1

Calif. Shelton and others (2001) Santa Ana River 0.024; 0.05 40 100; 100 0.13 to 0.86 0 1

Nev. Bevans and others (1998) Las Vegas Wash .05 4 100 0.6 to 1.4 0 1.9

N.J. Reiser and O’Brien (1998) Passaic River .05 9 100 0.13 to 1.5 0 3.1

N.J. Reiser and O’Brien (1998) Raritan River .05 11 100 0.1 to 4 0 5.6

Tex. Buszka and others (1994) Rowlett Creek .2 4 75 1.9 to 3.8 0 4.8

Tex. Bush and others (2000) San Antonio 

River

.1 13 100 0.4 to 4.6 0 6.5

Drinking Water

The term drinking water refers to water at its point of use 

for human consumption. Table 5 summarizes data on the occurrence of chloroform in treated drinking water from a regional 

study by Grady and Casey (2001), who reported that chloroform 

was more frequently detected at or greater than 1.0 μg/L in randomly selected community water systems than any other VOC 

examined. As might be expected, drinking water provided by a 

water utility is more likely to have been chlorinated than water 

drawn from a surface-water body, spring, or domestic well by a 

private homeowner. Thus, high concentrations of chloroform 

measured in some surface waters (table 5) were likely to have 

been a consequence of some of the utilities having used chlorination to treat the water.

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 in 20 States across the Nation (Moran and 

others, 2000) (table 3). Concentrations of chloroform reported 

in well water were generally low (with a median of 0.5 μg/L), 

with no exceedances of the USEPA MCL for TTHMs 

(80 μg/L). One potential source of chloroform to domestic 

wells is shock chlorination, a disinfection process in which a 

dilute solution of bleach is added directly to a well to eliminate 

bacterial contamination. Septic systems also may be a significant source of chloroform in 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, 

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

(2000b) 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 within the system is incomplete. Chloroform also may be present in septic systems as a result of the 

reaction between organic matter and bleach used for laundering 

operations.
20 Chloroform in the Hydrologic System

Table 5. Summary of chloroform and other trihalomethane detections in drinking water in eleven Statesa of the Northeastern 

and Mid-Atlantic regions of the United States. 

[Data from Grady and Casey, 2001; S.J. Grady, U.S. Geological Survey, written commun., 2003. GW, ground water; MCL, Maximum Contaminant Level; 

SW, surface water; TTHM, total trihalomethanes; μg/L, micrograms per liter; --, data not available]

Number and type of public-supply 

systems sampled

Frequency of chloroform 

detection at or greater

than 1.0 μg/L

(percent)

Maximum chloroform 

concentration

(μg/L)

Frequency of 

exceedance of MCL of 

80 μg/L for TTHMs

(percent)

Maximum TTHM 

concentration

(μg/L)

1,190 GW-derived systems (public 

supply wells)

19.4 200 0.06 208

199 SW-derived systems 87.4 410 10.9 425

112 combined systems (GW + 

SW)

82.1 200 5.2 215

a

Connecticut, Delaware, Maryland, Maine, New Hampshire, Massachusetts, New Jesey, New York, Rhode Island, Virginia, and Vermont.

Human-Health Effects and DrinkingWater Criteria

Human exposure to chloroform occurs primarily through 

two routes—ingestion and inhalation. Most of these exposures 

appear to occur through the ingestion of chlorinated drinking 

water, consumption of foods processed with chlorinated water, 

or inhalation of the compound while showering or washing 

dishes with treated water (Agency for Toxic Substances and 

Disease Registry, 1997). Dermal absorption represents another 

potential route of exposure, especially in the shower (Jo and 

others, 1990; Gordon and others, 1998; Miles and others, 2002) 

and in chlorinated swimming pools (Levesque and others, 1994; 

Lindstrom and others, 1997). Current (2004) drinking-waterquality criteria, however, focus on ingestion as the sole route of 

exposure.

Inhalation Exposure and its Effects

Dating back as far as the 1840s, chloroform was commonly administered as an anaesthetic. This practice was discontinued at the beginning of the twentieth century when it was 

discovered that many patients anesthetized with chloroform 

developed complications of the liver and kidneys (BBC Education, 2003). Although no increases in mortality occurred when 

patients undergoing surgery were exposed to vapor-phase chloroform concentrations less than 22,500 parts per million by 

volume (ppmv), concentrations of 40,000 ppmv or greater were 

considered an overdose (Agency for Toxic Substances and 

Disease Registry, 1997).

During inhalation studies involving laboratory animals, 

rats and mice exposed to concentrations of chloroform ranging 

from 2 to 300 ppmv for as many as 90 days (6 hours per day, 5 

to 7 days per week) developed lesions in the liver, kidney, and 

nasal passages. Histological effects in the liver and kidneys 

were only observed at exposure levels greater than 30 ppmv, 

although atrophy of the nasal tissues was observed in rats at 

2 ppmv (U.S. Environmental Protection Agency, 2001). 

The long-term health effects upon workers exposed to 

chloroform for 3 to 10 years at vapor-phase concentrations as 

high as 237 ppmv have included nausea, vomiting, dry mouth, 

dizziness, and headaches (Agency for Toxic Substances and 

Disease Registry, 1997). The National Institute for Occupational Safety and Health (NIOSH) has recommended a shortterm (15-minute) exposure limit of 2 ppmv for the compound, 

whereas the Occupational Safety and Health Administration 

(OSHA) has set a maximum ceiling value of 50 ppmv for an 

8-hour workday (U.S. Department of Health and Human 

Services, 1997). Twenty-four-hour limits for exposure to chloroform in ambient air vary among the States that have established such standards, with values (in ppmv) of 0.003 in 

Arizona, 0.005 in Florida, 0.027 in Massachusetts, 0.020 in 

Oklahoma, 0.051 in South Carolina, and 0.10 in Virginia 

(Sittig, 1994).

Table 6 summarizes data on the vapor-phase concentrations of chloroform in equilibrium with chlorinated water. The 

values listed in the table represent either measurements or 

model predictions in air in residences, shower stalls, or indoor 

swimming pool facilities; in air vented from washing machines 

and dishwashers; and in air near a kitchen sink. None of the concentrations listed in table 6 exceed the NIOSH short-term exposure limit of 2 ppmv or the OSHA ceiling limit of 50 ppmv. 

However, the chloroform concentrations measured in the air 

above indoor swimming pools and within shower stalls 

exceeded the 24-hour exposure limits set by some of the States 

listed previously. (Note that the aqueous concentrations of chloroform measured or predicted during several of the investigations exceeded the MCL of 80 μg/L for TTHMs.) Studies by Jo 

and others (1990), Weisel and Jo (1996), and Gordon and others 

(1998) also have documented the vapor-phase concentrations to 

which people are exposed while showering or bathing with 

chlorinated water.
Human-Health Effects and Drinking-Water Criteria 21

Table 6. Concentrations of chloroform in air (either measured directly or predicted from models) in shower stalls, near washing 

machines, dishwashers, above kitchen sinks, and in indoor swimming pool facilities.

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

Location of 

measurement

Concentrations 

measured [M] or

predicted [P]

Concentration of 

chloroform in

air, in parts

per million,

by volume

Assumptions or relevant

experimental conditions

(where applicable)

Reference

Residence M 0.00002 to 0.004 Aqueous concentration, 100 μg/L Howard and Corsi (1996)

Residence M 0.002 -- Levesque and others (2002)

Shower stall M 0.030 -- Levesque and others (2002)

Washing machine M 0.006 -- Corsi and others (1992)

Washing machine P 0.001 Aqueous concentration, 100 μg/L Howard and Corsi (1998)

Kitchen sink P 0.002 Aqueous concentration, 100 μg/L Howard and Corsi (1996)

Dishwasher P 0.002 20-cubic-meter room; aqueous 

concentration, 10 μg/L

Howard-Reed and others (1999)

Indoor swimming pool M 0.6 to 1.63 Aqueous concentration, 159–533 μg/L Levesque and others (1994)

Indoor swimming pool M 0.054 -- Mannschott and others (1995)

Indoor swimming pool M 0.0297 Aqueous concentration, 68-73 μg/L Lindstrom and others (1997)

Ingestion Exposure and its Effects

Ingestion of chloroform may be through several routes. Of 

principal importance in this regard is the direct consumption of 

chlorinated water, but some foods—many of which are processed with chlorinated water—also contain chloroform. For 

example, chloroform has been found to be a contaminant in fish, 

butter, cheddar cheese, cereals, and margarine (California 

Department of Health, 1990; Agency for Toxic Substances and 

Disease Registry, 1997). It also has been detected in fresh 

vegetables, tea leaves, and frozen vegetables at average concentrations of 30.9, 62.9, and 15.7 micrograms per kilogram of 

biomass, respectively (Adachi and others, 2001). Little information on mortality in humans following ingestion of pure 

chloroform is available, but fatal doses as low as 212 milligrams 

per kilogram (mg/kg) of body mass have been reported (Agency 

for Toxic Substances and Disease Registry, 1997).

In most laboratory studies of animal mortality from chloroform ingestion, males had lower survival rates than females, 

although higher female mortality was observed in pregnant 

animals. Consistent with the acute effects of chloroform exposure by inhalation, the liver and kidney also are the primary 

target organs in humans, rats, and mice following ingestion 

(Agency for Toxic Substances and Disease Registry, 1997). 

Available evidence indicates that chloroform (as well as tetrachloromethane) undergoes reductive dehalogenation by mixedfunction oxidases in the liver, resulting in the formation of toxic 

metabolites that cause damage by binding covalently to hepatic 

and renal tissues (Hook and Hewitt, 1986).

Recent studies have explored some of the ways in which 

oral exposure to THMs in drinking water may affect human 

reproduction, although many of these investigations did not 

report separate results for chloroform alone. Results from some 

of these studies are summarized in table 7. Many of these results 

are discussed in terms of an odds ratio (OR), which is a parameter used to compare the probabilities of a certain event occurring in each of two different study groups. For two hypothetical 

groups A and B, an odds ratio of unity (A/B=1) implies that the 

event is equally likely in both groups, whereas odds ratios 

greater or less than unity imply that the event is more likely in 

group A or B, respectively (Ott and Longnecker, 2001). For the 

studies listed in table 7, the numerator in the OR (group A from 

the example above) is the frequency with which the targeted 

outcome is encountered in the group exposed to the chloroform 

or THM concentration range given, while the demoninator 

(group B) is the frequency of occurrence of the same outcome 

in either the reference (for pregnancy studies) or the control 

group (for cancer studies) group.
22 Chloroform in the Hydrologic System

Table 7. Summary of the effects of chloroform and total trihalomethanes on pregnancy outcomes and the incidence of cancer.

[CHCl3, chloroform; CI, confidence interval; IGR, intrauterine growth retardation; IGR5, live birth weights (adjusted for race, sex, and gestational week) at or 

below the fifth percentile; IGR10, live birth weights (adjusted for race, sex, and gestational week) at or below the tenth percentile; Low birth weight, infants born 

with weights less than 2.5 kilograms; Low term birth weight, infants born after 37 or more weeks of gestation and weighing less than 2.5 kilograms; M, male; 

NR, not reported; OR, odds ratio; SW, surface water; TTHMs, total trihalomethanes; μg/L, micrograms per liter; %, percent; <, less than; >, greater than; 

>, greater than or equal to]

Target outcome Study

location

Number of 

target 

subjects

Number of 

reference

or control 

subjects

Chloroform or TTHM threshold 

concentration examined

 OR Reference

Target 

population

Reference

or control 

population

Pregnancy Studies

Low birth weight Iowa 187 935 >10 μg/L 

CHCl3

<10 μg/L 

CHCl3

1.3; 

95% CI 0.8-2.2

Kramer and others, 

1992

Low birth weight North 

Carolina 

74 118 63.4-82.7 μg/L 

TTHMs

40.8-63.3 μg/L 

TTHMs

1.5

95% CI 1.0-2.3

Savitz and others, 

1995

Low birth weight Nova Scotia 440 693 >100 μg/L 

TTHMs

<50 μg/L 

TTHMs

1.04

95% CI 0.92-1.2

Dodds and others, 

1999

Low birth weight Massachusetts 195,506 91,227 >20 μg/L 

CHCl3

<20 μg/L 

CHCl3

1.05;

95% CI 1.02-1.09

Wright and others, 

2004

Low term birth weight Northern New 

Jersey

37,053 22,098 >100 μg/L 

TTHMs

<20 μg/L 

TTHMs

1.4;

90% CI 1.0-2.0

Bove and others, 

1995

Low term birth weight Colorado 6 11 >50 μg/L 

TTHMs

<20 μg/L 

TTHMs

5.9;

95% CI 2.0-17.0

Gallagher and 

others, 1998

Low term birth weight Massachusetts 1,325 NR >80 μg/L 

TTHMs

<60 μg/L 

TTHMs

1.05;

95% CI 0.85-1.3

Wright and others, 

2003

IGR5 Iowa 187 935 >10 μg/L 

CHCl3

<10 μg/L 

CHCl3

1.8;

95% CI 1.1-2.9

Kramer and others, 

1992

IGR5 Northern New 

Jersey

2,727 1,355 >100 μg/L 

TTHMs

<20 μg/L 

TTHMs

1.5;

90% CI 1.2-1.9

Bove and others, 

1995

IGR10 Massachusetts 5,310 NR >80 μg/L 

TTHMs

<60 μg/L 

TTHMs

1.13;

95% CI 1.03-1.24

Wright and others, 

2003

IGR10

(genetically 

susceptible 

newborns)

Montreal, 

Canada

45 37 >29.4 μg/L

TTHMs

<29.4 μg/L

TTHMs

13.2;

95% CI 1.2-146.7

Infante-Rivard, 

2004

Stillbirth Nova Scotia 44 43 >100 μg/L 

TTHMs

<50 μg/L 

TTHMs

1.66;

95% CI 1.09-2.25

Dodds and others, 

1999

Spontaneous abortion California 121 565 >75 μg/L 

TTHMs and 

>5 glasses of 

water per day 

for first 

trimester of 

pregnancy 

<75 μg/L 

TTHMs and 

>5 glasses of 

water per 

day for first 

trimester of 

pregnancy 

2.0;

95% CI 1.1-3.6

Waller and others, 

1998
Human-Health Effects and Drinking-Water Criteria 23

Table 7. Summary of the effects of chloroform and total trihalomethanes on pregnancy outcomes and the incidence of 

cancer.—Continued

Target outcome Study

location

Number of 

target 

subjects

Number of 

reference

or control 

subjects

Chloroform or TTHM threshold 

concentration examined

 OR Reference

Target 

population

Reference

or control 

population

Pregnancy Studies—Continued

Neural tube defects New Jersey 112 248 >40 μg/L 

TTHMs

<40 μg/L 

TTHMs

2.1;

95% CI 1.1-4.0

Klotz and Pyrch, 

1999

Central nervous system 

defects

Northern New 

Jersey

82 36 >80 μg/L 

TTHMs

<20 μg/L 

TTHMs

2.6;

90% CI 1.5-4.3

Bove and others, 

1995

Cancer Studies

Bladder Finland M 79 M 89 NR (Nonsmokers; 

>30 years 

exposure)

NR (Nonsmokers; 

<30 years 

exposure)

2.59;

95% CI 1.13-5.94

Koivusalo and 

others, 1998

Rectal Iowa 13 40 NR (Ingested 

chlorinated SW 

for >60 years)

NR (Ingested 

chlorinated 

SW for 

<60 years)

2.50;

95% CI 1.3-4.9

Hildesheim and 

others, 1998

Nieuwenhuijsen and others (2000) and Bove and others 

(2002) recently reviewed pregnancy outcome studies (including 

some of the investigations listed in table 7) that evaluated the 

effect of THMs on adverse birth outcomes such as low birth 

weight, preterm births, birth defects, spontaneous abortions, 

and fetal deaths. These studies provided evidence for associations between the consumption of drinking water containing 

elevated concentrations of THMs (>10 μg/L) and intrauterine 

growth retardation, neural tube defects, and spontaneous 

abortions (Bove and others, 2002). However, because Bove 

and others (1995) reported the results from their study as 

90-percent confidence intervals for their OR values—rather 

than 95-percent confidence intervals—the statistical significance of the conclusions from their investigation cannot be 

compared directly with those from the other studies discussed 

in this report (for which, as noted earlier, a significance level of 

0.05 has been used).

Many of the studies that examined associations between 

the ingestion of THMs and adverse birth outcomes, however, 

were inconclusive. In particular, one of the most substantial 

limitations to the pregnancy outcome studies conducted to date 

is the considerable uncertainty associated with the assessment 

of exposure (Nieuwenhuijsen and others, 2000). Accurate 

assessments of THM exposure often are difficult to obtain 

because THM concentrations vary among different locations 

within the water distribution system, seasonally, and with 

variations in subjects’ behavior at home and in the workplace. 

Another limitation is that many of the studies did not include 

inhalation or dermal contact as routes of exposure during 

showering or swimming. Ideally, large studies that focus on 

well-defined endpoints and include a more complete characterization of exposure—including exposure to mixtures of THMs, 

and all routes of exposure—are needed to either confirm or 

refute the findings of previous investigations (Nieuwenhuijsen 

and others, 2000).

Although few conclusive studies are available, epidemiological investigations to date have reported more frequent 

occurrences of colon, rectal, and bladder cancer with regular 

consumption of chlorinated drinking water (Agency for Toxic 

Substances and Disease Registry, 1997). Complicating factors 

such as the presence of other THMs (which also appear to be 

carcinogenic) and other chemicals in drinking water make 

definitive conclusions elusive, but the consumption of chlorinated drinking water appears to be most strongly associated 

with bladder and rectal cancers (table 7). However, the USEPA 

has concluded that the available data are currently insufficient 

to establish a causal relation between exposure to chloroform 

and an increased risk of cancer (U.S. Environmental Protection 

Agency, 2002a). Furthermore, recent summaries of the genotoxicity of chloroform indicate that chloroform has a negligible 

tendency to cause genetic mutations or directly damage DNA 

(Fawell, 2000; U.S. Environmental Protection Agency, 2002a). 
24 Chloroform in the Hydrologic System

According to a summary by the USEPA (2002a), several 

animal bioassay studies have reported significant increases in 

the incidence of liver tumors in male and female mice, as well 

as increases in the incidence of kidney tumors in male rats and 

mice, following the ingestion of chloroform at or greater than 

60 mg/kg per day of body mass per day. In other studies, tumors 

were only observed at chloroform concentrations high enough 

to cause cytotoxicity and regenerative cell proliferation in the 

target organs. Tumor production, however, also was found to be 

dependent upon the species, strain, and gender examined. For 

example, in many studies summarized by the USEPA (2002a), 

the incidence of tumors in female laboratory animals did not 

significantly increase upon exposure to chloroform (Roe and 

others, 1979; Jorgenson and others, 1985; Matsushima, 1994). 

This observation appears to be consistent with the lower overall 

mortality rate for females relative to males following chloroform ingestion.

Current (2004) evidence indicates that chloroform is likely 

to be carcinogenic to humans by all routes of exposure (inhalation, ingestion, and dermal absorption) at high concentrations, 

but is unlikely to be carcinogenic to humans by any route of 

exposure at low concentrations (U.S. Environmental Protection 

Agency, 2002a). According to the USEPA—and consistent 

with the results from the laboratory animal studies—chloroform 

appears to be carcinogenic to humans at aqueous concentrations 

that lead to cytotoxicity and cell regeneration in susceptible tissues. However, chloroform is not deemed likely to be carcinogenic to humans at concentrations less than those that cause cell 

regeneration, although cytotoxicity may still be evident at concentrations less than those that are apparently required to cause 

cancer (U.S. Environmental Protection Agency, 2001). Based 

on the available evidence, chloroform has been classified by the 

USEPA as a Group B2 compound—a probable human carcinogen for which there is sufficient evidence of carcinogenicity in 

animals, but for which data on carcinogenicity in humans are 

either absent or inadequate.

As noted earlier, the current (2004) MCL for TTHMs is 

80 μg/L. A considerably more stringent water-quality guideline 

is the USEPA’s MCLG, which is not a regulatory standard, but 

is defined by the USEPA as the highest concentration at which 

a compound is deemed to pose a negligible health hazard in 

drinking water. The USEPA has historically established an 

MCLG value of zero for all carcinogens—including chloroform—based on the presumption that any exposure to carcinogens represents a non-negligible health risk. In addition, it has 

traditionally been assumed that the risk of contracting cancer 

from a particular carcinogen increases linearly with exposure to 

that compound (Hogue, 2001). However, given the assumption 

(noted previously) that chloroform is not likely to be carcinogenic to humans at concentrations less than those that cause cell 

regeneration, the USEPA decided to remove the zero MCLG for 

chloroform from its National Primary Drinking Water Regulations in May 2000 (U.S. Environmental Protection Agency, 

2000a). More recently, the USEPA revised the MCLG for the 

compound to 70 μg/L (U.S. Environmental Protection Agency, 

2004a). Thus, despite the usual assumption that cancer risk 

increases linearly with exposure, such a change has involved the 

adoption of a non-linear approach for the extrapolation of health 

risk to low doses for chloroform. Such an approach is used by 

the USEPA for compounds that have adverse non-cancer 

effects at concentrations lower than those required to cause 

cancer (U.S. Environmental Protection Agency, 2001).

Dermal Absorption

Several studies have indicated that dermal absorption may 

be another significant route of chloroform uptake, either 

through showering or bathing in treated water or from swimming in chlorinated swimming pools. Studies by Jo and others 

(1990), Weisel and Jo (1996), and Gordon and others (1998) 

have indicated that humans absorb chloroform by both dermal 

absorption and inhalation while showering with chlorinated 

water, and that the mean internal chloroform dose resulting 

from dermal contact is approximately equal to that from inhalation. Furthermore, the amounts of chloroform and other THMs 

that the body absorbs increase with increasing temperature; 

concentrations of chloroform measured by Gordon and others 

(1998) in the exhaled breath of subjects taking showers 

increased from 0.00004 ppmv at 28 to 32°C to 0.0014 ppmv at 

38 to 41°C. However, breath concentrations of chloroform from 

all exposure temperatures—and in all subjects—were observed 

to decrease to about one-half of their peak value within 

10 minutes after showering (Gordon and others, 1998). Using a 

physiologically based, pharmacokinetic model, Corley and others (2000) concluded that the contribution of dermal uptake of 

chloroform to the total body burden after bathing in chlorinated 

water for 30 minutes and drinking 2 liters (L) of chlorinated 

water (tested through exhaled breath) is likely to range from 1 

to 28 percent, depending on the temperature (30–40°C) of the 

bath. The model used by Corley and others (2000), however, 

did not account for contributions from inhalation exposure.

Levesque and others (1994) and Lindstrom and others 

(1997) observed an increase in chloroform levels in the body 

following immersion in treated water in chlorinated public 

swimming pools. The results from these two studies indicate 

that the proportion of the total body burden of chloroform contributed by dermal uptake under such circumstances may range 

from approximately 24 percent (Levesque and others, 1994) to 

approximately 80 percent (Lindstrom and others, 1997). 

Table 6 lists the aqueous concentrations of chloroform and 

resulting chloroform concentrations in air to which subjects 

were exposed during these two investigations.

Other studies have attempted to compare body burdens of 

chloroform from all three principal routes of exposure to chlorinated water—inhalation, ingestion, and dermal contact. 

Weisel and Jo (1996) measured the body burdens of chloroform 

that developed in 11 subjects as a consequence of showering 

and bathing in 40°C water, and drinking 0.5 L of tap water 

(20 μg/L chloroform in both shower and drinking water). The 

authors observed approximately equivalent amounts of chloroform entering the body by inhalation, ingestion, and dermal 
Toxicity to Aquatic Organisms and Aquatic Water-Quality Criteria 25

contact. Their experiments also revealed that the route of exposure to chloroform may affect the persistence of the compound 

in the body. Ingested chloroform was not detected in the 

exhaled breath of the test subjects, presumably because it had 

been metabolized in the liver. Chloroform was detected in 

breath, however, for more than 3 hours following either dermal 

contact or inhalation during 10-minute showers, reflecting the 

more widespread dispersion of the compound throughout the 

body following these routes of exposure. Thus, chloroform may 

persist longer in the body following inhalation or dermal uptake 

than following ingestion (Weisel and Jo, 1996).

Toxicity to Aquatic Organisms and 

Aquatic Water-Quality Criteria

Like other VOCs, chloroform and other THMs have relatively low bioconcentration factor (BCF) values (table 2) compared to those for most other organic contaminants in the hydrologic system. Consequently, the bioaccumulation of THMs in 

aquatic organisms is anticipated to be relatively small. Results 

from several investigations of THM bioaccumulation in aquatic 

organisms are consistent with this expectation. A summary of 

studies of chloroform concentrations in the tissues of marine 

organisms by McCulloch (2003) reported ranges of 0.02 to 

5 nanograms of chloroform per gram of tissue (ng/g) chloroform in plankton, 15 to 180 ng/g in crabs, and 7.6 to 22 ng/g in 

grey seal blubber. No significant correlation was observed 

between chloroform concentrations in animal tissue and the 

trophic levels of the animals examined. During a study by Toussaint and others (2001), the livers of Medaka fish exposed to 

aqueous chloroform concentrations of 1,463 μg/L for 9 months 

contained between 23 to 219 milligrams of chloroform per 

gram of tissue—substantially less than those that would have 

been expected from the aqueous concentrations to which the 

fish had been exposed based on the BCF for the compound. 

These results indicate that chloroform did not bioaccumulate in 

the livers of the fish examined during the study. (Although it is 

possible that the fish may have detoxified the chloroform in vivo

by reductive dechlorination to form phosgene—a known chloroform metabolite in mammals—that latter compound was not 

detected in their hepatic tissues during the study.)

Several studies have examined a variety of non-lethal 

effects of chloroform on aquatic organisms, most commonly in 

terms of the EC50—that is, the median effective concentration 

required to produce a targeted effect in 50 percent of the test 

organisms. The principal features of some of these studies are 

summarized in table 8. With respect to lethal effects, Rowe and 

others (1997) provided a summary of published values of the 

median lethal concentration (LC50) for chloroform (the concentration resulting in the death of 50 percent of the test organisms), based on results abstracted from the USEPA’s AQUatic 

toxicity Information REtrieval (AQUIRE) system. The authors 

also summarized data from the AQUIRE system on waterquality criteria for several species of aquatic animals and plants.

In 1980, the USEPA issued an acute freshwater criterion of 

28,900 μg/L and a chronic freshwater criterion of 1,240 μg/L 

for chloroform (U.S. Environmental Protection Agency, 1980); 

however, these values are no longer published (U S. Environmental Protection Agency, 1998). According to the USEPA, the 

development of formal criteria for the protection of aquatic life 

is hindered by a paucity of adequate data; the values of 28,900 

and 1,240 μg/L for freshwater acute and chronic exposures, 

respectively, are the lowest-observed-effect-levels (LOELs) for 

the compound (U S. Environmental Protection Agency, 1986). 

The Canadian interim guideline for the protection of freshwater 

aquatic life is currently 1.8 μg/L for chloroform (Canadian 

Council of Ministers of the Environment, 2002), nearly three 

orders of magnitude lower than the USEPA’s LOEL for the 

compound. According to the Canadian Council of Ministers of 

the Environment (2002), however, insufficient data exist for 

establishing a corresponding guideline for the protection of 

marine life.

Table 8. Selected studies of the non-lethal effects of chloroform exposure on aquatic organisms.

[From Rowe and others (1997). EC50, Median Effective Concentration—Concentration at which a specific non-lethal effect is observed in 50 percent of the 

organisms tested; >, greater than]

Species

Chloroform EC50

concentration

(micrograms

per liter)

Duration of 

exposure

(hours)

Observed effect Reference

Spring peeper (Hyla crucifer) 270 168 Birth defects Birge and others (1980)

Fowler’s toad (Bufo woodhousei fowleri) >40,000 72 Birth defects Birge and others (1980)

Water flea (Daphnia magna) 51,000 48 Immobilization Abernathy and others (1986)

Water flea (Ceriodaphnia dubia) 340,000 240 Impaired reproduction Cowgill and Milazzo (1991)

Green algae (Scenedesmus subspicatus) 560,000 48 Inhibited growth Kuhn and Pattard (1990)
26 Chloroform in the Hydrologic System

The aquatic criteria that currently (2004) are recommended by the USEPA for the protection of human health from 

the potentially carcinogenic effects of ingesting chloroform in 

contaminated water and aquatic organisms (particularly fish) is 

5.7 μg/L in water for people who consume both organisms and 

water, and 470 μg/L in water for those who consume organisms 

only (U.S. Environmental Protection Agency, 2003b). Both of 

these values incorporate the use of the BCF to estimate the concentrations of the compound that are likely to accumulate in the 

tissues of aquatic organisms as a function of the trophic levels 

that they occupy (U.S. Environmental Protection Agency, 

2003b). In 2003, the USEPA proposed revised values for these 

criteria—68 μg/L for the consumption of water and organisms 

and 2,400 μg/L for the consumption of aquatic organisms only. 

These values incorporate more recent information, including a 

nationwide fish consumption rate of 17.5 grams per person per 

day, the USEPA Integrated Risk Information System reference 

dose of 0.01 mg/kg of body mass per day, and the BCF (U.S. 

Environmental Protection Agency, 2004b).

Implications for Future Research

Several topics related to the sources, transport, fate, and 

biological effects of chloroform and other THMs in the hydrologic system merit additional investigation. More detailed 

information is needed regarding the spatial distributions and 

temporal variability of anthropogenic sources of the compound, 

especially chlorinated drinking water and wastewater derived 

from leaking distribution lines, as well as chlorinated water 

used to irrigate lawns, golf courses, gardens, and other urban 

and residential settings. Assumptions regarding fluxes of the 

compound from natural sources should also be tested using field 

observations in order to more precisely determine their contributions to the global production of chloroform, relative to 

anthropogenic sources. 

In addition, more data on the rates and mechanisms of 

transformation of chloroform and other THMs under varying 

redox conditions might help to improve the accuracy of predictions regarding the persistence of the compound in major aquifers and its concentrations in drinking-water wells. Finally, 

systematic epidemiological data on the incidence of cancers, 

reproductive anomalies, and other health effects associated with 

chronic exposure to chlorinated drinking water would help regulatory agencies charged with the protection of public health 

and aquatic ecosystems make more informed decisions regarding maximum acceptable levels of chloroform and other THMs 

in the hydrologic system.

Summary

Chloroform (CHCl3) is one of the most frequently detected 

volatile organic compounds (VOCs) in both ground and surface 

water in the United States. Although anthropogenic sources of 

the compound are substantial, they are currently estimated to 

constitute only 10 percent of the total global input to the hydrologic system. Natural sources of the compound include volcanic 

gases, biomass burning, marine algae, and soil microorganisms.

In the United States, known industrial releases of chloroform to the atmosphere, surface water, ground water (through 

underground injections), and the land surface (through spills, 

leaks, disposal in landfills, or incorporation into soil) totaled 

approximately 1.6 million pounds in 2001. In chemical manufacturing, chloroform is used primarily for the production of 

hydrochlorofluorocarbon-22 (HCFC-22), a refrigerant used in 

air conditioners and freezers, and a proposed substitute for the 

more highly ozone-depleting CFC-12. However, the largest 

documented releases of industrial chloroform to the environment in 2001—56 percent of the total—were reported by the 

paper industry. Chloroform also may be produced from the in 

situ dehalogenation (under iron- or sulfate-reducing conditions) 

of another anthropogenic contaminant, tetrachloromethane 

(carbon tetrachloride). Although this transformation has been 

observed to occur by both biotic and abiotic pathways, the 

former is likely to be the predominant mechanism in most 

environments where this reaction takes place.

The widespread detections of chloroform contamination in 

the hydrologic system, however, appear to be largely a consequence of the use of chlorination for the disinfection of drinking 

water and wastewater. Although chlorination has been used 

to treat drinking water since the beginning of the twentieth 

century (having first begun in Europe in 1902 and in the United 

States 6 years later), it was only in the mid-1970s that chloroform and other trihalomethanes (THMs)—primarily dichlorobromomethane (CHCl2Br), chlorodibromomethane (CHClBr2), 

and bromoform (CHBr3)—were first found to be produced by 

the process. The discharge of chlorinated drinking water and 

wastewater may be either intentional (through such mechanisms as the use of municipally supplied chlorinated water to 

irrigate lawns, golf courses, gardens, and other areas, the use of 

septic systems, or the regulated discharge of chlorinated wastewater to surface waters or ground-water recharge facilities) or 

inadvertent (through leakage of chlorinated water from swimming pools, spas, or distribution systems for drinking water or 

wastewater).

The production of chloroform and other disinfection byproducts (DBPs) during chlorination occurs by the haloform 

reaction, which involves the reaction of free chlorine (that is, 

dissolved chlorine as HOCl, OCl-

, and Cl2) with dissolved 

organic carbon (DOC). Factors that affect the rate and amount 

of chloroform production during the haloform reaction include 

pH, bromide concentration, the concentration and composition 

of the DOC, and temperature. Although the overall effectiveness of disinfection during chlorination increases with decreasing pH, the rate of chloroform production during the haloform 

reaction has a complex dependence on pH that varies with the 

structure of the precursor molecules. Increasing bromide concentrations may reduce the concentration of chloroform in 

treated waters by forming higher proportions of brominated 
References 27

THMs (CHCl2Br, CHClBr2, CHBr3) and other brominated 

DBPs. Except in the presence of unusually high bromide concentrations, chloroform is the THM produced in the highest 

concentrations during chlorination. Furthermore, when more 

than one THM is produced from chlorination, the relative concentrations among the different compounds usually decrease 

with increasing bromination (CHCl3 > CHCl2Br > CHClBr2 >

CHBr3). This phenomenon is presumed to be primarily responsible for the common observation that when more than one 

THM is detected during investigations of the occurrence of 

these compounds in the hydrologic system, this same trend is 

typically observed among their relative concentrations or, for a 

uniform detection limit, their relative frequencies of occurrence. This pattern could provide a valuable means for 

distinguishing between chlorinated water and other potential 

sources of chloroform in the environment.

Because of its higher Henry’s Law constant, most chloroform is expected to exist primarily in the vapor phase following 

its release to the atmosphere, or to volatilize when released to 

surface water or soil. The compound is relatively non-reactive 

with respect to phototransformation, however, so it is likely to 

persist long enough to undergo long-range transport in the 

atmosphere. Furthermore, because of its relatively low affinity 

for soil organic carbon (that is, its low Koc value) and considerable persistence under oxic conditions, chloroform that reaches 

ground water has been observed to migrate substantial distances 

in the subsurface as well.

Chloroform has been widely detected in national, regional, 

and local studies of VOCs in ground, surface, and drinking 

water. In many of these studies, chloroform was the most frequently detected VOC examined. Total THM (TTHM) concentrations of the compound, however, were typically below the 

Maximum Contaminant Level (MCL) of 80 μg/L (micrograms 

per liter) established by the U.S. Environmental Protection 

Agency (USEPA) for TTHMs. In the studies that compared 

land-use settings, frequencies of detection of chloroform were 

higher beneath urban and residential areas than beneath agricultural or undeveloped areas.

Human health may be affected by chloroform primarily 

through two routes of exposure—inhalation and ingestion. 

Inhalation of chloroform occurs most commonly while showering or washing dishes with chlorinated water. However, ingestion of chlorinated drinking water—or foods processed with 

chlorinated water—is the most common route of exposure. 

Studies of birth outcomes for pregnant women consuming 

chlorinated drinking water with elevated TTHM concentrations 

have shown an increased risk of low birth weight, central 

nervous system defects, and first trimester miscarriages and 

stillbirths relative to reference populations consuming water 

with lower TTHM concentrations. The value of many pregnancy outcome studies, however, has been limited by complications with the methods used to assess exposure. Epidemiological investigations indicate an increase in the incidence of cancer 

with consumption of chlorinated drinking water, although complicating factors, such as the presence of other carcinogenic 

DBPs in the drinking water, often produce inconclusive results 

from such studies.

Despite these uncertainties, chloroform has been classified 

as a probable human carcinogen (Group B2 compound) based 

on what is considered to be sufficient evidence of carcinogenicity in animals. The USEPA has determined that the compound 

is likely to be carcinogenic to humans by all routes of exposure 

at concentrations high enough to cause cytotoxicity and lead to 

cell regeneration in susceptible tissues. Liver damage, however, 

is known to occur at chloroform exposures lower than those 

required to cause cancer, an observation that has been considered by the USEPA as the basis for setting a new, non-zero 

Maximum Contaminant Level Goal of 70 μg/L for the compound.

In 1980, the USEPA issued acute and chronic waterquality criteria of 28,900 and 1,240 μg/L, respectively, to protect aquatic organisms from exposure to chloroform in freshwater. These criteria were no longer published after 1998, however, owing to what was considered by the USEPA to be insufficient data to support them. Currently (2004), values of 28,900 

and 1,240 μg/L represent the acute and chronic lowestobserved-effect-levels (LOELs) respectively, for aquatic organisms. In Canada, the interim water-quality guideline for the protection of freshwater aquatic life from chloroform contamination currently (2004) is 1.8 μg/L, nearly three orders of 

magnitude lower than the USEPA’s chronic freshwater LOEL. 

According to the Canadian Council of Ministers of the Environment, however, the available data are currently insufficient to 

establish a corresponding guideline for marine biota. The 

USEPA’s aquatic criteria for the protection of human health 

from ingestion of chloroform-contaminated aquatic organisms 

and water are 5.7 μg/L for people consuming both organisms 

and water, and 470 μg/L for those consuming aquatic organisms only.

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Edited by Ella M. Decker.

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