Prepared in cooperation with the Agency for Toxic Substances and Disease Registry, 

U.S. Department of Health and Human Services

Description, Properties, and Degradation of Selected 

Volatile Organic Compounds Detected in Ground Water—

A Review of Selected Literature

Open-File Report 2006-1338

U.S. Department of the Interior

U.S. Geological Survey

Description, Properties, and Degradation of Selected 

Volatile Organic Compounds Detected in Ground Water—

A Review of Selected Literature

By Stephen J. Lawrence

Prepared in cooperation with the Agency for Toxic Substances and Disease Registry, 

U.S. Department of Health and Human Services

Open-File Report 2006–1338

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

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

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Suggested citation:

Lawrence, S.J., 2006, Description, properties, and degradation of selected volatile organic compounds 

detected in ground water— A Review of Selected Literature: Atlanta, Georgia, U. S. Geological Survey, 

Open-File Report 2006-1338, 62 p., a Web-only publication at http://pubs.usgs.gov/ofr/2006/1338/.
Contents

Abbreviations and Acronyms.......................................................................................................... vi

Conversion Factors........................................................................................................................... vii

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

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

Purpose and Scope ................................................................................................................. 1

Available Literature Addressing Volatile Organic Compounds........................................ 1

Naming Conventions and Descriptions of Volatile Organic Compounds in 

Ground Water....................................................................................................................... 3

Sources of Volatile Organic Compounds Detected in Ground Water........................................ 6

Sources of Chlorinated Alkanes............................................................................................ 6

Sources of Chlorinated Alkenes and Benzenes ................................................................. 9

Sources of Gasoline Compounds.......................................................................................... 9

BTEX Compounds (Benzene, Toluene, Ethylbenzene, and Xylene)........................ 10

Methyl Tert-butyl Ether .................................................................................................. 10

Basic Properties of Selected Volatile Organic Compounds ...................................................... 10

Degradation of Selected Volatile Organic Compounds in Ground Water............................... 10

Degradation of the Chlorinated Alkanes.............................................................................. 17

Abiotic Transformation................................................................................................... 17

Aerobic Biodegradation ................................................................................................ 20

Anaerobic Biodegradation ........................................................................................... 20

Degradation of the Chlorinated Alkenes.............................................................................. 21

Aerobic Biodegradation ................................................................................................ 21

Anaerobic Biodegradation............................................................................................ 23

Degradation of the Chlorinated Benzenes .......................................................................... 24

Degradation of the Gasoline Compounds............................................................................ 25

Aerobic Biodegradation of BTEX Compounds .......................................................... 25

Anaerobic Biodegradation of BTEX Compounds ...................................................... 27

Aerobic Biodegradation of Methyl Tert-butyl Ether.................................................. 30

Anaerobic Biodegradation of Methyl Tert-butyl Ether ............................................. 31

References Cited............................................................................................................................... 36

Glossary.............................................................................................................................................. 50

    iii
Figures

1–19. Diagrams Showing—

1. Generic and International Union of Pure and Applied Chemistry naming 

conventions for the same aliphatic compound .......................................................... 5

2. Relation between degree of chlorination and anaerobic reductivedechlorination, aerobic degradation and sorption onto subsurface material ...... 16

3. Laboratory-derived pathway for the abiotic degradation, anaerobic, and 

methanogenic biodegradation of 1,1,2,2-tetrachloroethane; 

1,1,2-trichloroethene; and 1,1,2-trichloroethane ........................................................ 18

4. Laboratory-derived pathway for the abiotic, aerobic, and anaerobic 

biodegradation of 1,1,1-trichloroethane ...................................................................... 19

5. Laboratory-derived pathway for the aerobic biodegradation of 

1,2-dichloroethane........................................................................................................... 20

6. Laboratory-derived pathways for the anaerobic biodegradation of 

tetrachloromethane (carbon tetrachloride)................................................................ 21

7. Laboratory-derived pathways for the aerobic biodegradation 

of trichloroethene ............................................................................................................ 22

8. Laboratory-derived pathway for the anaerobic biodegradation 

of tetrachloroethene ....................................................................................................... 23

9. Laboratory-derived pathways for the aerobic and anaerobic biodegradation 

of 1,2,4-trichlorobenzene................................................................................................ 24

10. Laboratory-derived pathway for the aerobic biodegradation of 

1,4-dichlorobenzene........................................................................................................ 25

11. Laboratory-derived pathway for the aerobic biodegradation of 

chlorobenzene and 1,2-dichlorobenzene. ................................................................... 26

12. Laboratory-derived pathways for the aerobic biodegradation of benzene, 

o-, and m-xylene .............................................................................................................. 28

13. Laboratory-derived pathways for the aerobic biodegradation of toluene ............ 29

14. Laboratory-derived pathways for the aerobic bio­degradation of p-xylene .......... 30

15. Laboratory-derived pathway for the aerobic biodegradation of ethylbenzene.... 31

16. Field and laboratory-derived pathways for the anaerobic biodegradation of 

the BTEX compounds—benzene, toluene, ethylbenzene, and xylene................... 33

17. Laboratory-derived pathway for the aerobic biodegradation of methyl 

tert-butyl ether ................................................................................................................. 34

18. Laboratory-derived pathway for the aerobic biodegradation of m-cresol............ 35

19. Laboratory-derived pathways for the aerobic biodegradation of styrene ............ 35

iv   
Tables

1. List of selected publications providing literature reviews and summaries of 

volatile organic compound degradation and behavior in ground water................ 2

2. Names and synonyms of volatile organic compounds commonly detected in 

ground water.................................................................................................................... 4

3. The first four members of the straight-chain alkane series and associated 

alkyl radical....................................................................................................................... 5

4. Volatile organic compounds ranked by those frequently detected in ground 

water near landfills and hazardous waste dumps in the United States and the 

Federal Republic of Germany ........................................................................................ 6

5. Volatile organic compounds detected in regional and national ground-water 

studies in the United States ........................................................................................... 7

6. Volatile organic compounds detected in ground-water case studies at 

selected U.S. Department of Defense installations ................................................... 8

7. Major organic compounds in a typical gasoline blend............................................. 9

8. Henry’s Law constants for selected volatile organic compounds detected in 

ground water.................................................................................................................... 11

9. Water-solubility data for selected volatile organic compounds detected in 

ground water.................................................................................................................... 12

10. Density of selected volatile organic compounds detected in ground water 

compared to the density of water at 20 degrees Celsius.......................................... 13

11. Octanol-water partition coefficients for selected volatile organic compounds 

detected in ground water............................................................................................... 14

12. Soil-sorption partition coefficients for selected volatile organic compounds 

detected in ground water............................................................................................... 15

13. Common abiotic and biotic reactions involving halogenated 

aliphatic hydrocarbons................................................................................................... 16

14. Laboratory half-lifes and by-products of the abiotic degradation (hydrolysis 

or dehydrohalogenation) of chlorinated alkane compounds detected in 

ground water. ................................................................................................................... 18

15. Mean half-life in days for the anaerobic biodegradation of selected 

chlorinated alkane and alkene compounds ................................................................ 20

16. Laboratory or environmental half-lifes and by-products for the aerobic and 

anaerobic biodegradation of selected chlorinated benzene compounds 

detected in ground water............................................................................................... 27

17. Average half-life for the aerobic biodegradation of the fuel compounds 

BTEX and methyl tert-butyl ether to carbon dioxide in an uncontaminated 

and contaminated matrix of aquifer sediments and ground water......................... 32

18. Mean half-life in days for the anaerobic biodegradation of the fuel 

compounds BTEX, and methyl tert-butyl ether, tert-butyl alcohol under 

various reducing conditions .......................................................................................... 32

   
Abbreviations and Acronyms

ATSDR	 Agency for Toxic Substances and Disease Registry

BTEX 	 benzene, toluene, ethylbenzene, xylenes

CA	 chloroethane

CAA	 Clean Air Act

CB	 chlorobenzene

CERCLA	 Comprehensive Environmental Response, Compensation, and Liability Act

CDC	 Centers for Disease Control

CO2	 carbon dioxide

CSIA	 compound-specific isotope analysis

CTET	 carbon tetrachloride

CVOC	 chlorinated volatile organic compounds

DCA	 dichloroethane

11-DCA	 1,1-dichloroethane

12-DCA	 1,2-dichloroethane

12-DCB	 1,2-dichlorobenzene

13-DCB	 1,3-dichlorobenzene

14-DCB	 1,4-dichlorobenzene

DCE	 dichloroethene

12-cDCE	 cis-1,2-dichloroethene

12-tDCE	 trans-1,2-dichloroethene

DNA	 deoxyribonucleic acid

DO	 dissolved oxygen

FDA	 U.S. Food and Drug Administration

gVOC	 volatile gasoline compounds

H	 Henry’s Law constant

IUC	 International Union of Chemistry

IUPAC	 International Union of Pure and Applied Chemistry

Koc soil organic carbon partition coefficient

Kow octanol-water partition coefficient

MTBE	 methyl tert-butyl ether

NAPL	 non-aqueous phase liquid

ORD	 Office of Research and Development

PCA	 tetrachloroethane

PCE	 tetrachloroethene

PLFA	 phospholipid fatty acid

RCRA	 Resource Conservation and Recovery Act

RFG	 Reformulated Gasoline program

RNA	 ribonucleic acid

SMA	 signature metabolites analysis

TBA	 tert-butyl alcohol

TCA	 trichloroethane

112-TCA	 1,1,2-trichloroethane

TCB	 trichlorobenzene

123-TCB	 1,2,3-trichlorobenzene

124-TCB	 1,2,4-trichlorobenzene

TCE	 1,1,2-trichloroethene

TEA	 terminal electron acceptor

124-TMB	 1,2,4-trimethylbenzene

USEPA	 U.S. Environmental Protection Agency

USGS	 U.S. Geological Survey

VC	 vinyl chloride

VOC(s)	 volatile organic compound(s)

vi   
Conversion Factors

Multiply By To obtain

microgram per liter (μg/L) 6.243108 pound per cubic foot

microgram per liter (μg/L) 1x10–3 milligram per liter (mg/L)

milligram per liter (mg/L) 6.243105 pound per cubic foot

gram mole per cubic meter (g mol/m3) 6.243×105 pound per cubic foot

kiloPascal (kPa) 9.869210–3 standard atmosphere

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C=(°F-32)/1.8

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or 

micrograms per liter (μg/L).

    vii

Description, Properties, and Degradation of Selected

Volatile Organic Compounds Detected in Ground Water—

A Review of Selected Literature

By Stephen J. Lawrence

Abstract

This report provides abridged information describing

the most salient properties and biodegradation of 27 chlorinated volatile organic compounds detected during groundwater studies in the United States. This information is condensed from an extensive list of reports, papers, and literature

published by the U.S. Government, various State governments, and peer-reviewed journals. The list includes literature

reviews, compilations, and summaries describing volatile

organic compounds in ground water. This report crossreferences common names and synonyms associated with

volatile organic compounds with the naming conventions

supported by the International Union of Pure and Applied

Chemistry. In addition, the report describes basic physical

characteristics of those compounds such as Henry’s Law

constant, water solubility, density, octanol-water partition

(log Kow), and organic carbon partition (log Koc) coefficients.

Descriptions and illustrations are provided for natural and

laboratory biodegradation rates, chemical by-products, and

degradation pathways.

Introduction

The presence of volatile organic compounds (VOCs)

in ground water is a major concern to all who use ground

water as a drinking water source because many of these

compounds can adversely affect human health. Likewise,

concern about VOCs in ground water is shared by State and

Federal regulatory agencies responsible for protecting the

ground-water resource from contaminants and for protecting human health. This report is prepared in cooperation

with The Agency for Toxic Substances and Disease Registry,

U.S. Department of Health and Human Services (ATSDR) and

provides under one cover an abridged description of selected

properties, and biodegradation information published in academic and government literature. In addition, the report cross

references commonly used names with a generally accepted

international naming convention for 27 VOCs frequently

detected in ground water.

Purpose and Scope

The purpose of this report is to (1) list 27 VOCs frequently detected in ground water, (2) cross-reference common VOC names and synonyms associated with the naming

conventions supported by the International Union of Pure and

Applied Chemistry (IUPAC),	(2) 

properties	of OCs	by	subclass, 

various ays 

the	degradation OCs	in	ground ater.	The 

of	the	report	is viously 

reviews on VOCs in ground water, but rather to condense

that information, and information from other papers, into a

“digest” or abridged document that describes only the most

salient and generally accepted scientific information regarding

nomenclature, properties, and degradation paths for 27 VOCs

detected in ground water in the United States.

Available Literature Addressing

Volatile Organic Compounds

The information for this report is condensed from

selected academic and government literature published within

the last 30 years (1975–2006) that describe laboratory and

field experiments and ground-water studies of VOC. The

27 VOCs described in this report are among the VOCs commonly detected in aquifers and ground-water sources of drinking water in the United States (Zogorski and others, 2006).

The amount of academic, government, and popular literature addressing volatile organic compounds in ground water is

vast and scattered among paper and electronic venues, some

published and some unpublished. Reviewing this literature

would be a daunting task and certainly beyond the scope for this

report. Fortunately, a number of published papers and reports

are readily available that reviewed, compiled, or summarized

the properties, chemistry, or degradation paths of VOCs in

ground water. Citations for several of these compilations and

the information summarized are listed in table 1. Unless a report

provided newly synthesized information, all facts or interpretations described in summary reports or literature reviews are

cited using the primary source reported in the publication.
Table 1.  List of selected publications providing literature reviews and summaries of volatile organic compound degradation and 

behavior in ground water.

[BTEX, benzene, toluene, ethyl benzene, xylenes; PCE, tetrachloroethylene; VOC, volatile organic compound; MTBE, methyl tert-butyl ether]

Publication citation Subject

Aronson and others, 1999 Aerobic biodegradation rates for BTEX, PCE

Aronson and Howard, 1997 Anaerobic biodegradation rates for BTEX, naphthalene, styrene, chlorinated aliphatic compounds

Azadpour-Keeley and others, 1999 Microbial degradation and natural attenuation of VOCs in ground water

Beek, 2001 Natural degradation processes and rates for VOCs

Christensen and others, 2000 Oxidation-reduction conditions in ground-water contaminant plumes

Howard and others, 1991 Environmental degradation rates of chemical compounds

Vogel and others, 1987 Chemical reactions involved in VOC degradation

Vogel, 1994 Biodegradation of chlorinated solvents

Washington, 1995 Hydrolysis rates of dissolved VOCs

Wiedemeier and others, 1998 Natural attenuation of VOCs in ground water

Wilson and others, 2005 Natural attenuation of MTBE in ground water

Although a large amount of the citations in the academic

or government literature or on the Internet are published

through reliable agencies or entities, some citations reference

obscure sources or sources that are generally inaccessible to

the public. Because of this issue, the literature cited in this

report is confined to the body of work that is available and

easily accessed through mainstream academic journals, State

or Federal agencies using various libraries, or online databases

on the Internet.

In general, the academic literature focuses on VOCs from

two perspectives: (1) analytical and physical chemistry and

(2) environmental occurrence, transport, and fate. The analytical and physical chemistry literature provide information on

the physico-chemical properties of VOCs—such as experimental and computed Henry’s Law constants, fugacity, water

solubility, organic carbon solubility, octanol-water partition

coefficients, partitioning among various physical phases (that

is, gas, liquid, solid), experimentally derived and computersimulated reaction rates, microbial degradation, and reaction

types (that is, hydrolysis, oxidation-reduction, dehalogenation).

The literature describing the environmental occurrence, transport, and fate of VOCs in ground water primarily deals with

site-specific contamination, and the abiotic and microbial transformation, attenuation, and degradation observed in ground

water. Some of those documents attempt to confirm or apply

in vitro (laboratory microcosm) results to contaminated areas

in situ and many are written from a remediation perspective.

The local, State, territorial, and U.S. Government literature on VOCs in ground water typically encompass issues

important to its citizenry in an environmental or regulatory

context. This literature commonly involves larger geographical areas than those of a typical academic paper. With some

exceptions, publications of the U.S. Geological Survey

(USGS) are less attentive to site-specific contamination in

ground water and more attentive to contamination issues of

areal, regional, or national importance (Grady, 2003; Hamlin

and others, 2002, 2005; Moran, 2006; Zogorski and others,

2006). One exception is the USGS Toxic Substances Hydrology Program (http://toxics.usgs.gov/), which routinely publishes USGS reports and scientific articles in refereed journals.

The primary focus of that program is site-specific fate and

transport studies involving trace metal and organic (including

VOCs) contamination in ground water.

In contrast to USGS reports, the literature produced by

the U.S. Environmental Protection Agency (USEPA) primarily

focuses on applying scientific results to regulatory and remediation issues in compliance with the Clean Air and Clean

Water Acts and their amendments, and those statutes underwriting the Resource Conservation and Recovery Act (RCRA)

and the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). The Office of Research

and Development (ORD) is the scientific research arm of the

USEPA that routinely publishes reports addressing the fate

and transport of VOCs in ground water within a regulatory

context. Moreover, each State and territory within the United

States publishes scientific and regulatory literature regarding

the occurrence of VOCs in ground water and its transport, fate,

and impact on human and ecological health that are specific to

those States. Other government agencies such as the Centers

for Disease Control (CDC) and the ATSDR publish printed

and electronic literature relating the potential human-health

effects of VOCs in drinking water.

Paper and electronic literature published by the popular

press, such as sports and outdoor magazines, and publications of environmental groups such as the Sierra Club and the

Nature Conservancy typically use academic and government

publications as sources for their articles. These articles are

intended to educate their readers and members on environmental contamination and regulatory issues. With the exception of

literature published by the popular press, the literature selected

and used in this report spans the venues described in preceding

paragraphs. Most of this literature, particularly the environmental fate and transport literature, focuses on fewer than

30 VOCs in ground water.

    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
Naming Conventions and Descriptions

of Volatile Organic Compounds in

Ground Water

The compounds addressed in this report belong to the

class of organic chemicals called volatile organic compounds

(VOCs). Depending on the source, a VOC has two definitions—one within a physico-chemical context and the other

within a regulatory context. The physico-chemical definition

of a VOC as stated by Australia’s National Pollutant Inventory

is: Any chemical compound based on carbon chains or rings 

(and also containing hydrogen) with a vapor pressure greater 

than 2 mm of mercury (mm Hg) at 25 degrees Celsius (°C). 

These compounds may contain oxygen, nitrogen and other elements. Substances that are specifically excluded are: carbon 

dioxide, carbon monoxide, carbonic acid, carbonate salts, 

metallic carbides and methane (Australian Department of

Environment and Heritage, 2003). A physico-chemical definition of VOC as explicit as that from Australia and originating

in the United States was not found during extensive Internet

searches. In the United States, the regulatory definition of

VOC is provided by the USEPA under the Clean Air Act

and published in the Code of Federal Regulations—Volatile 

organic compound (VOC) means any compound of carbon, 

excluding carbon monoxide, carbon dioxide, carbonic acid, 

metallic carbides or carbonates, and ammonium carbonate, 

which participates in atmospheric photochemical reactions

(U.S. Environmental Protection Agency, 2000a).

Among the academic, government, and popular literature,

it is common to find a confusing variety of names used to

identify VOCs. For example, tetrachloroethene (IUPAC name)

is also known as perchloroethylene, PCE, and tetrachloroethylene (table 2). Furthermore, some compounds are identified

using the various brand names under which they are sold. The

name used to identify any particular compound may depend

on a number of variables. These variables include the area

or region where the compound is used (for example, Europe,

United States, Northeastern United States, and so forth), the

type of publication referring to the compound (journal article,

administrative report, government report, or popular magazine), the popularity of that name in recently published literature, and the profession of the person using the name (that

is, analytical chemist, environmental scientist, environmental

toxicologist, biologist or ecologist, organic chemist, journalist,

farmer, and so forth). Because of the numerous name variations for VOCs, attempts to merge information from a variety

of venues “on-the-fly” for a particular compound are tedious,

confusing, and fraught with error. As early as the late 1800s,

chemists and others recognized the need for a consistent naming convention for all chemical compounds.

In 1889, an international consortium of chemists,

encouraged by the need for a consistent naming convention

for all chemical compounds, formed the International Union

of Chemistry (IUC). During 1892 at a meeting in Geneva,

Switzerland, the IUC was formalized with a goal to create

a system of rules for naming chemical compounds (Geneva

Rules). The Geneva Rules established the foundation, the

framework, and the initial rules for a consistent, international

naming convention for all chemical compounds, including the

complex organic compounds. Since this first meeting, the IUC

has evolved into the International Union of Pure and Applied

Chemistry (IUPAC), an organization responsible for creating

new rules and keeping established rules current (Brown and

LeMay, 1977, p. 723). The history of the IUPAC organization and the Geneva Rules establish provenance for the formal

names given to the VOCs described in this report.

Under the IUPAC naming convention, VOCs are commonly assigned to two general groups: (1) aliphatic hydrocarbons (alkanes, alkenes), and (2) aromatic hydrocarbons

(Brown and LeMay, 1977). An alkane is a straight chain or

cyclic (ring-like; such as cycloalkane) structure that consists of

carbon-carbon and carbon-hydrogen single bonds. A chlorinated alkane also contains at least one chlorine-carbon single

bond. A chemical bond is the electrical attraction between two

atoms, one that has a negative charge and the other a positive

charge. In organic compounds, these chemical bonds are covalent, meaning that two bonded atoms share electrons (Brown

and LeMay, 1977). An alkene is typically a straight-chain

structure that contains at least one carbon-carbon double bond.

A chlorinated alkene also contains at least one chlorine-carbon

single bond. These double bonds indicate stronger covalent

bonds between two carbon atoms and impart more stability to

the compound than the single bond in an alkane compound.

In contrast to the aliphatic compounds, aromatic compounds are those with alternating carbon-carbon single and

double bonds arranged in a ring structure. Benzene is the

most commonly recognized aromatic compound (Brown and

LeMay, 1977). Chlorinated aromatic compounds also contain

one chlorine-carbon single bond (for example, chlorobenzene).

Aromatic compounds are typically more resistant to degradation (more stable) than the alkane and alkene compounds.

The aliphatic and the aromatic hydrocarbons are commonly subgrouped even further based on the presence of

attached halogen atoms (chlorine as chloro, bromine as bromo,

or fluorine as fluoro) or functional groups including, but not

limited to, alkyl radicals. The VOC subgroups include the alkyl

benzenes (such as methylbenzene), chlorinated alkanes (such

as 1,2-dichloroethane), chlorinated alkenes (such as 1,1-dichloroethene), and the chlorinated aromatics (such as 1,2-dichlorobenzene; table 2). The alkyl radicals are the lower molecular

weight alkanes minus one hydrogen atom (table 3) and are

highly reactive compounds that can easily displace a hydrogen

atom on another molecule. Halogenated or alkylated aromatics

such as chlorobenzene or toluene are more easily degraded than

benzene in aerobic and anaerobic ground water because the

stability of the benzene ring is reduced and the ring is weakened (Borden and others, 1997). Adding halides or alkyl groups

to the ring structure disperses the electrical charges from the

carbon-carbon bonds on the ring and weakens that bond.

Naming Conventions and Descriptions of Volatile Organic Compounds in Ground Water    
Table 2. Names and synonyms of volatile organic compounds commonly detected in ground water.

[IUPAC, International Union of Pure and Applied Chemistry; CAS, Chemical Abstract Services; —, not applicable]

IUPAC name1 Common or alternative name

(synonyms)2 Other possible names2 Predominant source

CAS

number1

Alkyl benzenes

1,2-dimethylbenzene o-xylene The X in BTEX, dimethyltoluene, Xylol gasoline 95-47-6

1,3-dimethylbenzene m-xylene 108-38-3

1,4-dimethylbenzene p-xylene 106-42-3

ethylbenzene — The E in BTEX, Ethylbenzol, phenylethanegasoline 100-41-4

methylbenzene toluene The T in BTEX, phenylmethane,

Methacide, Toluol, Antisal 1A

gasoline 108-88-3

1,2,4-trimethylbenzene pseudocumene pseudocumol, asymmetrical

trimethyl-benzene

gasoline 95-63-6

Aromatic hydrocarbons

benzene — The B in BTEX, coal naphtha, 1,3,5-cyclohexatriene, mineral naphthagasoline 71-43-2

naphthalene naphthene — gasoline, organic synthesis91-20-3

stryrene vinyl benzene phenethylene gasoline, organic synthesis100-42-5

Ethers

2-methoxy-2-

methyl­propane

methyl tert-butyl

ether, MTBE

tert-butyl methyl ether fuel oxygenate 1634-04-4

Chlorinated alkanes

chloroethane ethyl chloride,

monochloroethane

hydrochloric ether, muriatic ether solvent 75-00-3

chloromethane methyl chloride — solvent 74-87-3

1,1-dichloroethane ethylidene dichloride — solvent, degreaser 75-34-3

1,2-dichloroethane ethylidene dichloride glycol dichloride, Dutch oil solvent, degreaser 107-06-2

tetrachloromethane carbon tetrachloride perchloromethane, methane tetrachloride solvent 56-23-5

1,1,1-trichloroethane methyl chloroform — solvent, degreaser 71-55-6

Chlorinated alkenes

chloroethene vinyl chloride chloroethylene, monochloroethene,

monovinyl chloride (MVC)

organic synthesis,

degradation product

75-01-4

1,1-dichloroethene 1,1-dichloroethylene, DCE vinylidene chloride organic synthesis,

degradation product

75-35-4

cis-1,2-dichloroethene cis-1,2-dichloroethylene 1,2 DCE, Z-1,2-dichloroethene solvent, degradation

product

156-59-2

trans-1,2-dichloroethene trans-1,2-dichloroethylene 1,2 DCE, E-1,2-dichloroethene solvent, degradation

product

156-60-2

dichloromethane methylene chloride — solvent 74-09-2

Chlorinated alkenes

tetrachloroethene perchloroethylene, PCE,

1,1,2,2-tetrachloroethylene

ethylene tetrachloride, carbon dichloride,

PERC®, PERK®

solvents, degreasers 127-18-4

1,1,2-trichloroethene 1,1,2-trichloroethylene, TCE acetylene trichloroethylene solvents, degreasers

organic synthesis

79-01-6

Chlorinated aromatics

chlorobenzene monochlorobenzene benzene chloride, phenyl chloride solvent, degreaser 108-90-7

1,2-dichlorobenzene o-dichlorobenzene ortho dichlorobenzol organic synthesis 95-50-1

1,2,3-trichlorobenzene 1,2,6-trichlorobenzene — organic synthesis 87-61-6

1,2,4-trichlorobenzene 1,2,4-trichlorobenzol — organic synthesis 102-82-1

1

International Union of Pure and Applied Chemistry, 2006 2U.S. Environmental Protection Agency, 1995

    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
The simplest IUPAC rules for naming organic chemicals are those for the alkane compounds. First, the compound

is named for the longest carbon-carbon (C-C) chain in the

compound. If the compound contains a halogen or a branching

alkyl group, then each carbon in the compound is numbered

starting at the end that is closest to the halogen or alkyl group.

The longest carbon chain may include the original branching

alkyl group and result in a branching alkyl group with a different carbon position and name. After identifying the longest

C-C chain in the compound, the numerical position of the

alkyl group or halogen, if any, on a particular carbon is

determined. The alkyl group is named based on the number of carbon atoms it contains corresponding to a name in

the alkane series (table 3). To illustrate the IUPAC naming

process, consider the following example: before the IUPAC

rules, a seven-carbon alkane (septane) with an ethyl functional

group (table 3) on the second carbon would be named 2-ethyl

septane (fig. 1). Under IUPAC rules, however, the longest

carbon chain in this example begins at the first carbon in the

ethyl group. This new carbon chain contains eight carbon

atoms rather than the original seven and is now called octane.

Moreover, the octane compound now has a methyl group

branching from its third carbon. Therefore, the new IUPAC

name is 3-methyl octane (fig. 1).

Table 3. The first four members of the straight-chain alkane series and associated alkyl radical.1

[IUPAC, International Union of Physical and Applied Chemistry; C, carbon atom; H, hydrogen atom; n, normal; t, tertiary; °C, degrees Celsius]

Alkane series condensed

formula (IUPAC name)2

Number of carbon

atoms in chain

Alkyl group, condensed formula

(IUPAC name)

 2

Boiling point

(°C)

CH4 (methane) 1 CH3

-- [methyl] –161

CH3CH3 (ethane) 2 CH3

CH2-- [ethyl] –89

CH3CH2CH3 (propane) 3 CH3CH2CH2-- [n-propyl]

--CHCH3CH3 [isopropyl]

–44

CH3CH2CH2CH3 (butane) 4 CH3CH2CH2CH2-- [n-butyl]

	 CH3

CH3C-- [t-butyl]

	 CH3

–0.5

Under IUPAC naming conventions, higher molecular weight alkanes are not typically present as alkyl radicals in volatile

organic compounds

1

Brown and LeMay, 1977, tables 24.1, 24.2 2

International Union of Pure and Applied Chemistry, 2006

H

H

C

H

H

C

H

1 2 3 4 5 6 7

2-ethyl septane

ethyl

group

H C C

3 4 5 6 7

3-methyl octane

methyl group

8

H C C C C C C C C C C C C

1

2

H

H

C

H

H

C

H

GENERIC IUPAC

Figure 1. Generic and International Union of Pure and 

Applied Chemistry (IUPAC) naming conventions for the 

same aliphatic compound.

Naming Conventions and Descriptions of Volatile Organic Compounds in Ground Water    
Sources of Volatile Organic Compounds

Detected in Ground Water

A relatively large amount of literature exists that

describes VOCs in ground water at specific, known areas

of contamination. Few documents, however, describe VOC

contamination in a regional or national context. One report

by Arneth and others (1989) lists the top 15 VOCs detected

in ground water near landfills in the United States and in Germany (table 4). This list shows that the VOCs contaminating

ground water near landfills are similar in both countries. Most

of these VOCs are chlorinated solvents (CVOCs) and gasoline

compounds (gVOCs). Furthermore, the frequency of VOCs

detected in representative studies completed on national,

regional, and site-specific scales in the United States show a

remarkable similarity to those in table 4 (table 5; Delzer and

Ivahnenko, 2003; Moran, 2006; Zogorski and others, 2006).

Although the number of VOCs analyzed in ground-water samples is large for national and regional studies, the most commonly detected compounds, primarily CVOCs and gVOCs,

are similar to those at site-specific studies completed at U.S.

Department of Defense installations (table 6). The 10 most

commonly detected VOCs in the studies summarized in

tables 5 and 6 are methyl tert-butyl ether (MTBE), tetrachloroethene (PCE), 1,1,2-trichloroethene (TCE), methyl­benzene

(toluene), 1,1,1-trichloroethane (111-TCA), benzene,

cis-1,2-dichloroethene (12-cDCE), 1,1-dichloroethane

(11-DCA), trans-1,2-dichloroethene (12-tDCE), the dimethylbenzenes (m-, o-, p-xylenes)

Sources of Chlorinated Alkanes

The chlorinated solvents within the alkane group are

listed in table 2. The CVOCs are typically used in the manufacturing of industrial, chemical, electronic, and consumer

goods (Smith and others, 1988; U.S. Environmental Protection

Agency, 2005b). In addition, these compounds are heavily

used as solvents in cleaning and degreasing products. For

example, 111-TCA is used as a solvent for adhesives and in

metal degreasing, pesticides, textile processing, cutting fluids,

aerosols, lubricants, cutting oil formulations, drain cleaners,

shoe polishes, spot cleaners, printing inks, and stain repellents.

Carbon tetrachloride (CTET) was used as feedstock for

the production of chlorofluorocarbon gases, such as dichlorodifluoromethane (F-12) and trichlorofluoromethane (F-11), which

were used as aerosol propellants in the 1950s and 1960s (Holbrook, 1992). During 1974, the U.S. Food and Drug Administration (FDA) banned the sale of CTET in any product used in

the home and the USEPA regulated the use of chlorofluorocarbon gases as aerosols or propellants. By 2000, CTET production

for nonfeedstock purposes was phased-out completely.

Table 4. Volatile organic compounds ranked by those frequently detected in ground water near landfills and hazardous waste dumps 

in the United States and the Federal Republic of Germany.1

[IUPAC, International Union of Physical and Applied Chemistry; —, not applicable]

Rank

United States of America Federal Republic of Germany

IUPAC name 2 Common or alternative name IUPAC name 2 Common or alternative name

1 1,1,2-trichloroethene 1,1,2-trichloroethylene, TCE tetrachloroethene perchloroethylene, tetrachloroethylene, PCE

2 tetrachloroethene perchloroethylene,

tetrachloroethylene, PCE

1,1,2-trichloroethene 1,1,2-trichloroethylene, TCE

3 cis-1,2-dichloroethene cis-1,2-DCE trans-1,2-dichloroethene trans-1,2-DCE

4 benzene benzene trichloromethane —

5 chloroethene vinyl chloride 1,1-dichloroethene 1,1-dichloroethylene, DCE

6 trichloromethane — dichloromethane methylene chloride

7 1,1,1-trichloroethane methyl chloroform 1,1,1-trichloroethane methyl chloroform

8 dimethylbenzene xylene 1,1-dichloroethane ethylene dichloride

9 trans-1,2-dichloroethene trans-1, 2-dichloroethylene 1,2-dichloroethane ethylene dichloride

10 methylbenzene toluene phenol —

11 ethylbenzene ethylbenzene acetone dimethyl ketone, 2-propanone,

and beta-ketopropane

12 dichloromethane methylene chloride toluene methyl benzene

13 dichlorobenzene, total — bis-(2-ethylhexyl)-phthalate —

14 chlorobenzene chlorobenzene benzene benzene

15 tetrachloromethane carbon tetrachloride chloroethene vinyl chloride

1

Arneth and others, 1989, p. 399 2

International Union of Pure and Applied Chemistry, 2006

    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
Table 5. Volatile organic compounds detected in regional and national ground-water studies in the United States.

[μg/L, micrograms per liter; [12], percentage of samples above the analytical reporting limit; <, less than; ND, not detected above analytical reporting level]

Rank Statewide, ground water in Wisconsin1 Ground water in the Santa Ana River Basin, California2

Ground-water and drinking-water supply wells in the United 

(concentrations greater than 0.2 μg/L)3

Aquifer studies4 Domestic water-supply wells5 Pu

1 dichloromethane [16.3] 1,1,2-trichloroethene (TCE) [12] tetrachloroethene (PCE) [3.7] 2-methoxy-2-methylpropane

(MTBE) [2.9]

2-methox

(MTBE

2 1,1-dichloroethane [13.6] 1,1,1-trichloroethane [10.5] 2-methoxy-2-methylpropane

(MTBE) [2.8]

tetrachloroethene (PCE) [2.0] tetrachlor

3 cis-1,2-dichloroethene, 1,1-dichloroethane [13.6] tetrachloroethene (PCE) [9.1] 1,1,2-trichloroethene (TCE) [2.6] 1,1,1-trichloroethane [1.4] 1,1,2-trich4 1,1,2-trichloroethene (TCE) [13.3] 1,1-dichloroethene [5.7] methylbenzene [1.9] methylbenzene [1.0] 1,1,1-trich5 methylbenzene [11.6] 2-methoxy-2-methylpropane

(MTBE) [5.3]

1,1,1-trichloroethane [1.7] chloromethane [.97] 1,1-dichlo

6 tetrachloroethene (PCE) [9.8] cis-1,2-dichloroethene [4.3] chloromethane [1.1] 1,1,2-trichloroethene (TCE) [.92] cis-1,2-di7 benzene [8.5] methylbenzene [3.8] trans-1,2-dichloroethene [0.91] dichloromethane [.67] 1,1-dichlo

8 chloroethene [8.0] 1,1-dichloroethane [2.9] dichloromethane [0.89] 1,2,4-trimethylbenzene [.32] trans-1,2-9 1,3- and 1,4-dimethylbenzenes [7.9] benzene [1.4] 1,1-dichloroethane [0.86] 1,1-dichloroethane [.29] methylbe

10 1,1,1-trichloroethane [7.8] 1,2-dimethylbenzene [1.4] 1,1-dichloroethene [0.66] benzene, 1,2-dichloroethane [.21] tetrachlor11 ethylbenzene [7.6] 1,3- and 1,4-dimethylbenzene [1.4] benzene [.63] tetrachloromethane [.21] 1,3- and 112 1,2,4-trimethylbenzene [7.1] trans-1,2-dichloroethene [<1] 1,2,4-trimethylbenzene [.63] 1,1-dichloroethene [.21] 1,2-dichlo13 1,2-dimethylbenzene [6.8] dichloromethane [<1] 1,2-dichloroethane [.47] total xylenes [0.21] 1,2-dimet

14 chloromethane [6.7] ethylbenzene [<1] cis-1,2-dichloroethene [.42] cis-1,2-dichloroethene [.18] benzene, 

ethylb

15 naphthalene [6.5] naphthalene [<1] total xylenes [.38] naphthalene [.15] chlorome16 chloroethane [6.3] tetrachloromethane [<1] tetrachloromethane [.31] ethylbenzene [.12] 1,2,4-trim

17 chlorobenzene [4.3] 1,2,4-trimethylbenzene [<1] chloroethane [.29] chloroethane [.093] chloroeth

18 1,2-dichloroethane [3.7] chlorobenzene [ND] chloroethene [.26] chloroethene [.083] vinyl ben19 trans-1,2-dichloroethene [3.3] chloroethane [ND] ethylbenzene [.26] trans-1,2-dichloroethene [.045] chloroben20 1,1-dichloroethene (DCE) [2.6] chloromethane [ND] chlorobenzene [.17] chlorobenzene [.042] chloroeth21 1,2-dichlorobenzene [2.4] chloroethene [ND] naphthalene [.16] 1,2-dichlorobenzene [.042] naphthale22 2-methoxy-2-methylpropane

(MTBE) [2.3]

1,2-dichlorobenzene [ND] 1,2-dichlorobenzene [.12] vinyl benzene [ND] 1,2,4-trich

23 tetrachloromethane [1.8] 1,2-dichloroethane [ND] vinyl benzene [ND] 1,2,4-trichlorobenzene [ND] 1,3-dichlo24 vinyl benzene [1.2] 1,2,3-trichlorobenzene [ND] 1,2,3-trichlorobenzene [ND] 1,1,2-trichloroethane [ND] 1,1,2-trich25 1,2,4-trichlorobenzene,

1,1,2-trichloroethane [<1.0]

vinyl benzene [ND] 1,2,3-trichlorobenzene [ND] 1,2,3-trich

11,305–4,086 samples (Wisconsin Department of Natural Resources, 2000)29–112 samples (Hamlin and others, 2002)3Zogorski and others, 200641, 710–3,498 samples51,190–1,208 samples6828–1,096 samples
Chemical manufacturing is the largest use of 11-DCA

and 1,2-dichloroethane (12-DCA). Both compounds serve as

an intermediate during the manufacture of chloroethene (vinyl

chloride, VC), 111-TCA, and to a lesser extent high-vacuum

rubber. Both DCA isomers also are used as a solvent for

plastics, oils, and fats, and in cleaning agents and degreasers

(Agency for Toxic Substances and Disease Registry, 1990c,

p. 51; 2001, p. 160). About 98 percent of the 12-DCA produced in the United States is used to manufacture VC. Smaller

amounts of 12-DCA are used in the synthesis of vinylidene

chloride, TCE, PCE, aziridines, and ethylene diamines, and

in other chlorinated solvents (U.S. Environmental Protection

Agency, 1995).

The compound 111-TCA was initially developed as a safer

solvent to replace other chlorinated and flammable solvents.

The compound is used as a solvent for adhesives (including

food packaging adhesives) and in metal degreasing, pesticides,

textile processing, cutting fluids, aerosols, lubricants, cutting oil

formulations, drain cleaners, shoe polishes, spot cleaners, printing inks, and stain repellents, among other uses (Agency for

Toxic Substances and Disease Registry, 2004, p. 181). The other

TCA isomer, 1,1,2-trichloroethane (112-TCA), has limited use

as a common, general-use solvent but is used in the production

of chlorinated rubbers (Archer, 1979). In some cases, 112-TCA

may be sold for use in consumer products (Agency for Toxic

Substances and Disease Registry, 1989, p. 59).

Before 1979, the single largest use of chloroethane was

in the production of tetraethyl lead. As recently as 1984, the

domestic production of tetraethyl lead accounted for about

80 percent of the chloroethane consumed in the United States;

whereas about 20 percent was used to produce ethyl cellulose,

and used in solvents, refrigerants, topical anesthetics, and in

the manufacture of dyes, chemicals, and pharmaceuticals.

Since the 1979 ban on tetraethyl lead in gasoline and its subsequent phase out in the mid-1980, the production of chloroethane in recent years has declined substantially in the United

States (Agency for Toxic Substances and Disease Registry,

1998, p. 95).

Table 6. Volatile organic compounds detected in ground-water case studies at selected U.S. Department of Defense installations. 

[[43.3], percentage of samples with a detected concentration; ND, not detected above analytical reporting level]

Rank Dover Air Force

Base, Maryland1

U.S. Army Armament Research

and Development Center,

Picatinny, New Jersey, 1958–85 2

U.S. Naval Undersea

Warfare Center,

Washington, D.C. 3

Wright-Patterson Air

Force Base, Ohio,

1993  – 94 4

1 2-methoxy-2-methylpropane

(MTBE) [25.5]

1,1,2-trichloroethene

(TCE) [58.5]

chloroethene [64] 1,1,2-trichloroethene

(TCE) [12.5]

2 cis-1,2-dichloroethene [21.7] tetrachloroethene (PCE) [24.9] cis-1,2-dichloroethene [59] tetrachloroethene

(PCE) [5.8]

3 1,1,2-trichloroethene (TCE) [20.3] trans-1,2-dichloroethene

(DCE) [18.6]

trans-1,2-dichloroethene

[44.8]

1,1,1-trichloroethane

[2.3]

4 tetrachloroethene (PCE) [13.7] 1,1,1-trichloroethane [16.8] 1,1,2-trichloroethene (TCE)

[40.4]

chloromethane [2.3]

5 benzene [10.4] 1,1-dichloroethane [9.6] total BTEX compounds [40.1] cis- and trans-1,2-

dichloroethene [1.2]

6 methylbenzene [6.6] cis-1,2-dichloroethene

(DCE) [9.6]

1,1-dichloroethane [37.2] chloroethene [.9]

7 dimethylbenzenes (m-, p-xylene)

[3.7]

methylbenzene (toluene) [4.4] chloroethane [33.9] dichloromethane [.9]

8 ethylbenzene [2.3] benzene [2.6] 1,1-dichloroethene [31.3] methylbenzene [.6]

9 chloroethene [ND] — tetrachloroethene (PCE) [9.6] benzene [.3]

10 — — 1,1,1-trichloroethane [6.9] chloroethane [.3]

11 — — — tetrachloromethane [.3]

1 212 samples (Barbaro and Neupane, 2001; Guertal and others, 2004)

2 607 samples (Sargent and others, 1986)

3121–179 samples (Dinicola and others, 2002)

4343 samples (Schalk and others, 1996)

    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
Sources of Chlorinated Alkenes and Benzenes

The chlorinated alkenes listed in table 2 include two

of	the	most uted	solvents 

United	States ents, 

also	are	among	the	most 

water	(tables	5	and	6). xtile	industry	uses	the	largest

amount	of inishing w	and	

finished	textiles, 

cleaning	(U.S.	Environmental y,	2005b,	

Web	page:	http://www.epa.gov/opptintr/chemfact/f_perchl.txt,

accessed May 23, 2006). Most of the TCE used in the United

States is for vapor degreasing of metal parts and some textiles

(U.S. Environmental Protection Agency, 2005b, Web page:

http://www.epa.gov/OGWDW/dwh/t-voc/trichlor.html,

accessed May 23, 2006). Other uses of PCE and TCE include

manufacturing of pharmaceuticals, other organic compounds,

and electronic components, and in paint and ink formulations

(Smith and others, 1988).

Four chlorinated benzenes commonly detected in groundwater contamination studies include chlorobenzene (CB),

1,2-dichlorobenzene (12-DCB), and two isomers of trichlorobenzene, 1,2,3-trichlorobenzene (123-TCB) and 1,2,4-trichlorobenzene (124-TCB; tables 5 and 6). Chlorobenzene

is commonly used as a solvent for pesticide formulations, in

the manufacturing of di-isocyanate, as a degreaser for automobile parts, and in the production of nitrochlorobenzene.

Solvent uses accounted for about 37 percent of chlorobenzene

consumption in the United States during 1981 (Agency for

Toxic Substances and Disease Registry, 1990a, p. 45). The

compound 12-DCB is used primarily to produce 3,4-dichloroaniline herbicides (Agency for Toxic Substances and Disease

Registry, 1990b, p. 263). The two trichlorobenzene isomers

are primarily used as dye carriers in the textile industry. Other

uses include septic tank and drain cleaners, the production of

herbicides and higher chlorinated benzenes, as wood preservatives, and in heat-transfer liquids (U.S. Environmental Protection Agency, 2005b, Web page: http://www.epa.gov/OGWDW/

dwh/t-voc/t-124-tric.html, accessed May 23, 2006).

Sources of Gasoline Compounds

At a basic level, gasoline production is simply a process of

sequential distillations that separate, by vaporization, volatile

hydrocarbons from crude oil. Typically, these hydrocarbons are

the lower molecular weight compounds that commonly are the

most volatile compounds in crude oil. More advanced methods

such as heat “cracking” are used to breakdown the complex

aromatic hydrocarbons in crude oil into smaller, more volatile

compounds that are easily distilled. Once the hydrocarbons are

in a vapor form, a condensation process cools the vapor and the

resulting liquid is collected for further refining. The hydrocarbon composition of gasoline depends on the source of the

crude oil used, the refining process, the refiner, the consumer

demand, the geographic location of the refinery, and the distributional area of the gasoline (Harper and Liccione, 1995).

Gasoline is typically a mixture of various hydrocarbons

that include alkanes, cycloalkanes, cycloalkenes, akylbenzenes, and aromatic compounds, and some oxygenated alcohol

additives (table 7). Many of the hydrocarbons in gasoline

are additives and blending agents intended to improve the

performance and stability of gasoline. These additives typically consist of oxygenates such as methyl tert-butyl ether

(MTBE), ethanol, or methanol, antiknock agents, antioxidants,

metal deactivators, lead scavengers, antirust agents, anti-icing

agents, upper-cylinder lubricants, detergents, and dyes. At

the end of the refining process, finished gasoline commonly

contains more than 150 separate compounds; however, some

blends may contain as many as 1,000 compounds (Harper and

Liccione, 1995).

Table 7. Major organic compounds in a typical gasoline blend.1

[n, C5-C13 carbon chain; MTBE, methyl tert-butyl ether;

TBA, tert-butyl alcohol]

Major compounds Percent composition by weight

n-alkanes 17.3

Branched alkanes 32.0

Cycloalkanes 5.0

Olefins 1.8

Aromatic hydrocarbons 30.5

	 Benzene 3.2

	 Toluene 4.8

	 Ethylbenzene 1.4

	 Xylenes 6.6

	 Other benzenes 11.8

	 Other aromatics 2.7

Other possible additives

Octane enhancers: MTBE, TBA, ethanol

Antioxidants: N, N'-dialkylphenylenediamines, di- and trialkylphenols, butylated methyl, ethyl and dimethyl phenols

Metal deactivators: various N, N'-disalicylidene compounds

Ignition controllers: tri-o-cresylphosphate (TOCP)

Detergents/dispersants: alkylamine phosphates, poly-isobutene

amines, long-chain alkyl phenols, alcohols, carboxylic acids,

and amines

Corrosion inhibitors: phosphoric acids, sulfonic acids,

carboxylic acids

1

Harper and Liccione, 1995

Sources of Volatile Organic Compounds Detected in Ground Water   
BTEX Compounds (Benzene, Toluene, 

Ethylbenzene, and Xylene)

About 16 percent of a typical gasoline blend consists of

BTEX compounds (collectively, benzene, toluene, ethlybenzene, and three xylene compounds; table 7). Of the different

components contained in gasoline, BTEX compounds are the

largest group associated with human-health effects. Because

of the adverse impact on human health, BTEX compounds

are typically the fuel components analyzed in ground-water

samples collected from fuel-contaminated aquifers. Furthermore, three minor components of gasoline: naphthalene, vinyl

benzene (styrene), and 1,2,4-trimethylbenzene (124-TMB) are

commonly detected along with BTEX compounds and MTBE

in contaminated ground water (tables 5 and 6). Although the

individual BTEX compounds are widely used as solvents and

in manufacturing (Swoboda-Colberg, 1995), gasoline leaks

from underground storage tanks and distribution pipelines is

the primary contributor of BTEX contamination in ground

water (U.S. Environmental Protection Agency, 2000b; U.S.

Environmental Protection Agency, 2005a).

Methyl Tert-butyl Ether

Methyl tert-butyl ether (MTBE; IUPAC 2-methoxy-2-

methyl­propane) is a gasoline additive within the class of fuel

oxygenates. Oxygenates are organic compounds that enrich

gasoline with oxygen to improve the combustion efficiency

of gasoline and reduce carbon monoxide emissions in vehicle

exhaust. Since the late 1980s, gasoline shipped to areas of

the United States that fall under the Reformulated Gasoline

(RFG) and Oxygenated Fuel (Oxyfuel) Programs of the Clean

Air Act (CAA) and its amendments has contained oxygenates

(Moran and others, 2004). Moreover, 30 percent of the gasoline used in the United States since 1998 contained oxygenates in compliance with RFG requirements while 4 percent

of the gasoline used complied with the Oxyfuel requirements

(U.S. Environmental Protection Agency, 1998). Reformulated

gasoline contains about 11 percent MTBE by volume (Delzer

and Ivahnenko, 2003).

Basic Properties of Selected

Volatile Organic Compounds

Volatile organic compounds have a number of unique

properties that both inhibit and facilitate ground-water contamination. Tables 8 through 12 list basic physical properties of

27 VOCs detected in ground water. Physical properties unique

to each compound typically are governed by the number of

carbons and the covalent bonding in the compound, the number

and location of chlorine atoms, and the number, location and

type of alkyl groups. The physical properties addressed in this

report include the Henry’s Law constant (H), water solubility, density, octanol-water partitioning (Log Kow), and organic

carbon partitioning (Log Koc) of the non-aqueous phase liquid

(NAPL). Models that estimate the fate and transport of VOCs

in ground water depend on the accuracy and reliability of physical property measurements. Some models, such as the fugacity

models, also use these properties to predict a compound’s

rate of movement into and out of environmental compartments (soil, water, air, or biota; Mackay, 2004). Predicting the

environmental fate of a compound in ground water depends on

data that quantifies: (1) the compound’s tendency to volatilize

(gaseous phase), (2) to dissolve in water (aqueous phase), (3) to

float on or sink beneath the water surface, (4) to dissolve in or

sorb to other organic compounds (including natural organic

matter), and (5) the compound’s affinity for ionically charged

surfaces such as clay or soil particles. Fugacity models of

varying complexity are in common use and rely on the physical properties of these compounds to estimate plume migration

and persistence, and to guide the remediation of contaminated

ground water (Mackay and others, 1996; Institute for Environmental Health, 2004; Saichek and Reddy, 2005).

Degradation of Selected Volatile

Organic Compounds in Ground Water

Under specific conditions, most organic compounds

degrade at a particular rate during a given length of time. The

speed of the degradation depends on the presence and activity of

microbial consortia (bacteria and fungi species), environmental

conditions (temperature, aquifer materials, organic matter content), and the availability and concentration of carbon sources

(primary substrate) available to the microbial consortia. The primary substrate can be a VOC or organic carbon found dissolved

in water or sorbed to aquifer sediments. When primary substrate

concentrations are small, the microbial population is small and

biodegradation rates are relatively slow. As the substrate concentrations increase, the microbial population grows and the

degradation rate increases concomitantly (Bradley and Chapelle,

1998). The microbial population will grow until they reach a

maximum growth rate (Aronson and others, 1999).

The degradation of VOCs in ground water is a transformation of a parent compound to different compounds commonly called daughter products, degradates, or degradation

by-products. These transformations can be grouped into two

general classes: (1) those that require an external transfer of

electrons, called oxidation-reduction reactions; and (2) those

that do not involve a transfer of electrons, called substitutions

and dehydrohalogenations (Vogel and others, 1987). Table	13	

summarizes these reactions. Oxidation-reduction reactions

are the dominant mechanisms driving VOC degradation and

most of these reactions are catalyzed by microorganisms

(Wiedemeier and others, 1998; Azadpour-Keeley and others,

1999). Substitution reactions that can remove chlorine atoms,

such as hydrolysis, can degrade some chlorinated alkanes

(trichloroethane) to nonchlorinated alkanes (ethane) with or

without a microbial population catalyzing the reaction (Vogel

10    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
and others, 1987; Olaniran and others, 2004). Typically, the

polychlorinated compounds (for example, PCE and TCE) easily degrade under anaerobic conditions and are less mobile in

soil and aquifer materials than the di- and mono-chlorinated

compounds (fig. 2). Degradation pathways are illustrated in

figures 3 through 19 for a subset of the compounds listed in

table 2. These figures are modifications of pathways described

in the University of Minnesota’s biodegradation/biocatalysis

database (Ellis and others, 2006) accessible via the Internet at

http://umbbd.msi.umn.edu, accessed May 23, 2006.

Table 8. Henry’s Law constants for selected volatile organic compounds detected in ground water.

[IUPAC, International Union of Physical and Applied Chemistry; kPa, kilopascals; m3, cubic meter; mol, mole; °

C, degrees Celsius; —, not applicable]

IUPAC name1 Common or alternative name2 Henry’s Law3 constant (H)

(kPa m3 mol–1 at 25°C)

tetrachloromethane carbon tetrachloride 2.99

Increasing tendency for a compound to move from the water

phase to the vapor phase when in equilibrium with pure water

chloroethene vinyl chloride, chloroethylene 2.68

1,1-dichloroethene 1,1-dichloroethylene, DCE 2.62

1,1,1-trichloroethane methyl chloroform 1.76

tetrachloroethene perchloroethylene, tetrachloroethylene, PCE 1.73

chloroethane ethyl chloride, monochloroethane 4

1.11

1,1,2-trichloroethene 1,1,2-trichloroethylene, TCE 1.03

trans-1,2-dichloroethene trans-1,2-DCE, trans-1,2-dichloroethylene .960

chloromethane methyl chloride 5.920

ethylbenzene — .843

1,3-dimethylbenzene m-xylene .730

1,4-dimethylbenzene p-xylene .690

methylbenzene toluene .660

1,1-dichloroethane 1,1-ethylidene dichloride .630

benzene — .557

1,2-dimethylbenzene o-xylene .551

1,2,4-trimethylbenzene pseudocumene .524

cis-1,2-dichloroethene cis-1, 2-dichloroethylene, cis-1,2-DCE .460

chlorobenzene monochlorobenzene .320

stryrene vinyl benzene .286

1,2,4-trichlorobenzene 1,2,4-trichlorobenzol .277

1,2,3-trichlorobenzene 1,2,6-trichlorobenzene .242

1,2-dichlorobenzene o-dichlorobenzene .195

1,2-dichloroethane 1,2-ethylidene dichloride, glycol dichloride .140

1,1,2-trichloroethane methyl chloroform .092

2-methoxy-2-methylpropane methyl tert-butyl ether, MTBE .070

naphthalene naphthene .043

1

International Union of Pure and Applied Chemistry, 2006

2U.S. Environmental Protection Agency, 1995

3Lide, 2003

4Gossett, 1987

5National Center for Manufacturing Sciences, 2006

Degradation of Selected Volatile Organic Compounds in Ground Water    11
Table 9.  Water-solubility data for selected volatile organic compounds detected in ground water.

[IUPAC, International Union of Pure and Applied Chemistry; mg/L, milligrams per liter; °C, degrees Celsius;

—, not applicable]

IUPAC name1 Common or alternative name2 Water solubility3 

(mg/L at 25°C)

2-methoxy-2-methylpropane methyl tert-butyl ether, MTBE 36,200

Increasing amount of non-aqueous phase liquid that can dissolve in water

1,2-dichloroethane 1,2-ethylidene dichloride, glycol dichloride 8,600

chloromethane methyl chloride 45,320

chloroethane ethyl chloride, monochloroethane 56,710

cis-1,2-dichloroethene cis-1,2-dichloroethylene 6,400

1,1-dichloroethane 1,1-ethylidene dichloride 5,000

1,1,2-trichloroethane methyl chloroform 4,590

trans-1,2-dichloroethene trans-1,2-dichloroethylene 4,500

chloroethene vinyl chloride, chloroethylene 2,700

1,1-dichloroethene 1,1-dichloroethylene, DCE 2,420

benzene — 1,780

1,1,1-trichloroethane methyl chloroform 1,290

1,1,2-trichloroethene 1, 1, 2-trichloroethylene, TCE 1,280

tetrachloromethane carbon tetrachloride 1,200

methylbenzene toluene 531

chlorobenzene — 495

stryrene vinyl benzene 321

tetrachloroethene perchloroethylene, tetrachloroethylene, PCE 210

1,2-dimethylbenzene o-xylene 207

1,4-dimethylbenzene p-xylene 181

1,3-dimethylbenzene m-xylene 161

ethylbenzene — 161

1,2-dichlorobenzene o-dichlorobenzene 147

1,2,4-trimethylbenzene pseudocumene 57

1,2,4-trichlorobenzene 1,2,4-trichlorobenzol 37.9

naphthalene naphthene 631.0

1,2,3-trichlorobenzene 1,2,6-trichlorobenzene 30.9

1

International Union of Pure and Applied Chemistry, 2006

2U.S. Environmental Protection Agency, 1995

3Lide, 2003

4National Center for Manufacturing Sciences, 2006

5Horvath, 1982

6Lyman, 1982

12    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
Table 10. Density of selected volatile organic compounds detected in ground water compared to the density 

of water at 20 degrees Celsius.

[IUPAC, International Union of Pure and Applied Chemistry; g/cm, grams per centimeter; °C, degrees Celsius; —, not applicable]

IUPAC name1 Common or alternative name2 Density3

(g/cm3, 20°C)

1,2,3-trichlorobenzene 1,2,6-trichlorobenzene 41.690

Increasing density (heavier than water)

tetrachloroethene perchloroethylene, tetrachloroethylene, PCE 1.623

tetrachloromethane carbon tetrachloride 1.594

1,1,2-trichloroethene 1,1,2-trichloroethylene, TCE 1.464

1,2,4-trichlorobenzene 1,2,4-trichlorobenzol 41.45

1,1,2-trichloroethane methyl chloroform 1.44

1,1,1-trichloroethane methyl chloroform 1.339

1,2-dichlorobenzene o-dichlorobenzene 1.306

cis-1,2-dichloroethene cis-1,2-dichloroethylene 1.284

trans-1,2-dichloroethene trans-1,2-dichloroethylene 1.256

1,2-dichloroethane 1,2-ethylidene dichloride, glycol dichloride 1.235

1,1-dichloroethene 1,1-dichloroethylene, DCE 1.213

1,1-dichloroethane 1,1-ethylidene dichloride 1.176

chlorobenzene monochlorobenzene 1.106

pure water at 0°C 1.000

naphthalene naphthene .997

Decreasing density (lighter than water)

chloromethane methyl chloride .991

chloroethane ethyl chloride .920

chloroethene vinyl chloride, chloroethylene 4.910

stryrene vinyl benzene .906

1,2-dimethylbenzene o-xylene .880

benzene — .876

ethylbenzene — .867

1,2,4-trimethylbenzene pseudocumene .876

methylbenzene toluene .867

1,3-dimethylbenzene m-xylene .864

1,4-dimethylbenzene p-xylene .861

2-methoxy-2-methylpropane methyl tert-butyl ether, MTBE .740

1

International Union of Pure and Applied Chemistry, 2006

2U.S. Environmental Protection Agency, 1995

3Lide, 2003

4Chiou and others, 1983

Degradation of Selected Volatile Organic Compounds in Ground Water    13
Table 11. Octanol-water partition coefficients for selected volatile organic compounds detected 

in ground water.

[IUPAC, International Union of Pure and Applied Chemistry; Kow, octanol-water partition coefficient; —, not applicable]

IUPAC name1 Common or alternative name2 Octanol/ water partition

coefficient3 (Log Kow)

1,2,3-trichlorobenzene 1,2,6-trichlorobenzene 44.07

Increasing affinity for organic matter and lipids

1,2,4-trichlorobenzene 1,2,4-trichlorobenzol 44.04

1,2,4-trimethylbenzene pseudocumene 3.65

1,2-dichlorobenzene o-dichlorobenzene 3.46

naphthalene naphthene 3.36

1,3-dimethylbenzene m-xylene 3.20

ethylbenzene ethylbenzene 3.15

1,4-dimethylbenzene p-xylene 3.15

1,2-dimethylbenzene o-xylene 3.12

stryrene vinyl benzene 3.05

tetrachloroethene perchloroethylene, tetrachloroethylene, PCE 2.88

chlorobenzene monochlorobenzene 42.84

methylbenzene toluene 2.73

tetrachloromethane carbon tetrachloride 42.64

1,1,2-trichloroethene 1,1,2-trichloroethylene, TCE 32.53

1,1,1-trichloroethane methyl chloroform 42.49

1,1,2-trichloroethane methyl chloroform 2.38

1,1-dichloroethene 1,1-dichloroethylene, DCE 2.13

benzene — 2.13

trans-1,2-dichloroethene trans-1,2-dichloroethylene 1.93

cis-1,2-dichloroethene cis-1,2-dichloroethylene 1.86

1,1-dichloroethane 1,1-ethylidene dichloride 41.79

1,2-dichloroethane 1,2-ethylidene dichloride, glycol dichloride 41.48

chloroethane ethyl chloride 1.43

chloroethene vinyl chloride, chloroethylene 1.38

2-methoxy-2-methylpropane methyl tert-butyl ether, MTBE .94

chloromethane methyl chloride .91

1

International Union of Pure and Applied Chemistry, 2006

2U.S. Environmental Protection Agency, 1995

3Sangster, 1989

4

Mackay and others, 1992a

14    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
Table 12. Soil-sorption partition coefficients for selected volatile organic compounds detected 

in ground water.

[IUPAC, International Union of Pure and Applied Chemistry; Koc, soil organic carbon partition coefficient; —, not applicable]

IUPAC name1 Common or alternative name2 Soil-sorption coefficient

(Log Koc in soil)

1,2,4-trimethylbenzene pseudocumene 33.34

Increasing affinity for soil organic matter

1,2,3-trichlorobenzene 1,2,6-trichlorobenzene 43.18 –33.42

naphthalene naphthene 32.98

1,2,4-trichlorobenzene 1,2,4-trichlorobenzol 52.94

vinyl benzene styrene 22.72–2.74

1,2-dichlorobenzene o-dichlorobenzene 62.46–52.51

tetrachloroethene perchloroethylene, tetrachloroethylene, PCE 72.37

ethylbenzene — 52.22

1,1-dichloroethene 1,1-dichloroethylene, DCE 22.18

1,3-dimethylbenzene m-xylene 72.11–2.46

1,1,1-trichloroethane methyl chloroform 82.03

1,1,2-trichloroethene 1,1,2-trichloroethylene, TCE 72.00

chlorobenzene monochlorobenzene 51.91

1,1,2-trichloroethane methyl chloroform 71.78–2.03

tetrachloromethane carbon tetrachloride 91.78

methylbenzene toluene 71.75–102.28

chloroethene vinyl chloride, chloroethylene 21.75

1,2-, 1,4-dimethylbenzene o-xylene, p-xylene 21.68–1.83

chloroethane ethyl chloride 41.62

cis-1,2-dichloroethene cis-1,2-dichloroethylene 21.56–1.69

1,2-dichloroethane 1,2-ethylidene dichloride, glycol dichloride 61.52

trans-1,2-dichloroethene trans-1,2-dichloroethylene 21.56–1.69

1,1-dichloroethane 1,1-ethylidene dichloride 121.52

benzene — 51.49–71.73

methyl tert-butyl ether MTBE 111.09

chloromethane methyl chloride 3.778

1

International Union of Pure and Applied Chemistry, 2006

2U.S. Environmental Protection Agency, 1995

3Boyd and others, 1990

4Schwarzenbach and Westall, 1981

5Chiou and others, 1983

6Chiou and others, 1979

7Seip and others, 1986

8Friesel and others, 1984

9Kile and others, 1996

10Garbarini and Lion, 1986

11U.S. Environmental Protection Agency, 1994

12U.S. Environmental Protection Agency, 2005b

Degradation of Selected Volatile Organic Compounds in Ground Water    15
Table 13. Common abiotic and biotic reactions involving halogenated aliphatic hydrocarbons.1

[+, plus; Cl, chloride]

Reactions Potential reaction products

Substitution

abiotic hydrolysis alcohol then an acid or diol

(chloroethanol  chloroacetic acid)

biotic hydrolysis alcohol then an acid or diol via microbial enzymes (hydrolases or glutathione S-transferases;

(chloroethanol  chloroacetic acid)

conjugation or nucleophilic reactions (biotic) free halide plus a new compound with the nucleophile or conjugate

Dehydrohalogenation

dehydrohalogenation halogenated acid (chloroacetic acid),

alkane to alkene (dichloroethane  chloroethane)

Oxidation

α-hydroxylation monochlorinated alkane to a monochlorinated alcohol

(chloroethane  chloroethanol)

halosyl oxidation monohalogenated alkane to a nonhalogenated alkane

(chloroethane  ethane + Cl)

epoxidation halogenated epoxide compound

biohalogenation nonhalogenated alkene to a monohalogenated alcohol

(ethene + Cl  chloroethanol)

Reduction

hydrogenolysis free halide and nonhalogenated compound

(chloroethane  Cl + ethane)

dihaloelimination dihalogenated alkane to a nonhalogenated alkene

(dichloroethane  ethene)

coupling combining of two halogenated compounds into one halogenated compound

1

Vogel and others, 1987, figure 1

Aerobic degradation

Sorption

Reductive dechlorination rate

DEGREE OF CHLORINATION

Monochlorinated Polychlorinated

DEGRADATION RATE

Sorption onto subsurface material

0.25 4

Figure 2. Relation between degree of chlorination 

and anaerobic reductive-dechlorination, aerobic 

degradation and sorption onto subsurface material 

(modified from Norris and others, 1993, p. 10–19). 

Degree of chlorination is number of chloride atoms 

divided by number of carbon atoms.

16    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
Aquifer conditions (aerobic and anaerobic) and microbial

metabolism (respiration, fermentation, and co-metabolism)

control the environmental degradation of VOCs in ground water.

In aerobic environments, oxygen serves as the terminal electron

acceptor (TEA) and compounds such as MTBE and BTEX are

subsequently degraded (oxidized) to other compounds (Azadpour-Keeley and others, 1999). Furthermore, under aerobic conditions CVOCs can be inadvertently degraded (co-metabolized)

via nonspecific enzymes (oxygenases) produced by microorganisms during the metabolism of other compounds serving

as primary substrates (for example, BTEX, methane, propane,

toluene, ammonia, ethene, ethane). Although the aerobic

mineralization of most VOCs ultimately yields carbon dioxide

and water, co-metabolic biodegradation of CVOCs generally

proceeds via an unstable epoxide intermediate that spontaneously decomposes to carbon dioxide, chloride, or other organic

by-products such as acetate (Roberts and others, 1986).

Anaerobic degradation is typically a series of decarboxylations and oxidation-reduction (redox) reactions catalyzed

either by single microorganisms or by a consortium of

microorganisms (Dolfing, 2000). During anaerobic degradation, CVOCs function as terminal electron acceptors in

a process called reductive dechlorination (Vogel and others, 1987). Theoretically, reductive dechlorination is the

sequential replacement of one chlorine atom on a chlorinated

compound with a hydrogen atom. The replacement continues until the compound is fully dechlorinated. For example,

PCE can undergo reductive dechlorination to less-chlorinated

compounds, such as TCE or 12-DCE, or to nonchlorinated

compounds such as ethene, ethane, or methane (methanogenesis). Each successive step in the dechlorination process

is theoretically slower than the preceding step. The dechlorination process slows because as chlorines are removed

the energy costs to remove another chlorine atom increases

(free energy of the reaction decreases; Dolfing, 2000). As

a result, biodegradation may not proceed to completion in

some aquifers leaving intermediate compounds (for example,

dichloroethenes and vinyl chloride) to accumulate in ground

water (Azadpour-Keeley and others, 1999). Other constraints

on biodegradation such as a reduction in or loss of primary

substrate, or microbial suppression also can play a role in the

accumulation of intermediate compounds. This is a particular

concern with VC because it is a known human carcinogen

(Agency for Toxic Substances and Disease Registry, 2005)

and its accumulation may create a health issue that might not

be a concern during the early stages of ground-water contaminated by TCE.

Degradation of the Chlorinated Alkanes

The degradation of chlorinated VOCs is fundamentally

different from that of BTEX compounds (Wiedemeier and

others, 1995). The chlorinated alkanes can be degraded by

abiotic processes through hydrolysis or dehydrohalogenation

or by biotic processes through reductive dechlorination or

dichloroelimination. These degradation processes can proceed

under either aerobic or anaerobic conditions (figs. 3–6; Vogel

and McCarty, 1987a; Vogel, 1994). According to McCarty

(1997), 111-TCA is the only chlorinated compound that can

be degraded in ground water within 20 years under all likely

ground-water or aquifer conditions.

Abiotic Transformation

Hydrolysis and dehydrohalogenation are two abiotic

processes that may degrade chlorinated ethanes under either

aerobic or anaerobic conditions. The tendency for a chlorinated ethane to degrade by hydrolysis depends on the ratio of

chlorine to carbon atoms (fig. 2) or the location of chlorine

atoms on the number 2 carbon in the compound. Chlorinated

alkanes are more easily hydrolyzed when the chlorine-carbon

ratio is less than two or when chlorine atoms are only located

on the number 1 carbon atom (Vogel and McCarty, 1987b;

Vogel, 1994). For example, chloroethane and 111-TCA have

half-lifes that are measured in days or months (Vogel and others, 1987; Vogel, 1994; table 14). Conversely, the more chlorinated ethanes such as 1,1,1,2-tetrachloroethane (PCA) and

those with chlorine atoms on the number 2 carbon tend to have

half-lifes measured in decades or centuries (table 14). Dehydrohalogenation is the removal of one or two halogen atoms

from an alkane (Vogel and McCarty, 1987a). The dehydrohalogenation of two chlorine atoms is called dichloroelimination.

Chen and others (1996) show that PCA can be abiotically

transformed to TCE under methanogenic conditions (fig. 3).

In addition, the abiotic degradation of 111-TCA has been well

studied in the scientific literature (fig. 4; Jeffers and others,

1989; McCarty and Reinhard, 1993; Chen and others, 1996;

McCarty, 1997). McCarty and Reinhard (1993) indicate that

the transformation of 111-TCA by hydrolysis is about four

times faster than by dehydrochlorination. During abiotic degradation, about 80 percent of 111-TCA is transformed to acetic

acid by hydrolysis (McCarty, 1997), and the remaining 20 percent is transformed to 11-DCE by dehydrochlorination (table 7;

Vogel and McCarty, 1987b; McCarty, 1997). The presence of

11-DCE in contaminated ground water is probably the result of

the dehydrochlorination of 111-TCA (McCarty, 1997).

Degradation of Selected Volatile Organic Compounds in Ground Water    17
1,1,2-trichloroethene (TCE)

1,2-dichloroethene, cis1,2-dichloroethene, trans

chloroethene

(vinyl chloride)

ethene

ethane

chloroethane

1,1,2,2-tetrachloroethane (PCA) 1,1,2-trichloroethane (112-TCA)

1,2-dichloroethane (12-DCA)

(1) Chen and others, 1996

(2) McCarty, 1997

(3) Grostern and Edwards, 2006

(1)

(1)

(1)

(1) (1)

(1) (1)

(1)

(1, 3) (1)

(1, 3)

(1)

(1)

(2)

Microorganism catalyzing 

 reaction Dehalobactor sp.

Dehalococcoides sp.

Dehalococcoides sp. Dehalobactor sp.

Dehalobactor sp.

EXPLANATION

Literature reference describing 

 reaction pathway

Proven microbe-catalyzed

 anaerobic dechlorination

Proven methanogenic 

 dechlorination

Proven biotic dichloro-

 elimination

Proven abiotic dehydro-

 chlorination

Figure 3. Laboratory-derived pathway for the abiotic degradation, anaerobic, and methanogenic biodegradation of 

1,1,2,2-tetrachloroethane; 1,1,2-trichloroethene; and 1,1,2-trichloroethane (modified from Chen and others, 1996).

Table 14. Laboratory half-lifes and by-products of the abiotic degradation (hydrolysis or dehydrohalogenation) of chlorinated alkane 

compounds detected in ground water.

[IUPAC, International Union of Pure and Applied Chemistry; —, not applicable]

Compound

(IUPAC name)1 Degradation by-products Half-life Literature reference

chloroethane ethanol 44 days Vogel and others, 1987

1,1-dichloroethane — 61 years Jeffers and others, 1989

1,2-dichloroethane — 72 years Jeffers and others, 1989

1,1,1-trichloroethane acetic acid; 1,1-dichloroethane 1.1–2.5 years Mabey and Mill, 1978; Jeffers and others, 1989;

Vogel and McCarty, 1987a,b

1,1,2-trichloroethane 1,1-dichloroethane 140 years Jeffers and others, 1989

1,1,1,2-tetrachloroethane trichloroethene 47–380 years Mabey and Mill, 1978; Jeffers and others, 1989

1,1,2,2-tetrachloroethane 1,1,2-trichloroethane; trichloroethene 146–292 days Mabey and Mill, 1978; Jeffers and others, 1989

1International Union of Pure and Applied Chemistry, 2006

18    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
Figure 4. Laboratory-derived pathway for the abiotic, aerobic, and anaerobic biodegradation of 

1,1,1-trichloroethane (modified from Sands and others, 2005; Whittaker and others, 2005).

(1) Egli and others, 1987

(2) Gälli and McCarty, 1989

(3) De Best and others, 1999

(4) Yagi and others, 1999

(5) Oldenhuis and others, 1989

(6) Newman and Wackett, 1997

(7) Motosugi and others, 1982

(8) Jun Oh, 2005

(9) De Wever and others, 2000

(10) McCarty, 1997

1,1,1-trichloroethane

(Clostridium spp., 

Dehalobacter strain TCA1)

1,1-dichloroethane

chloroethane

acetaldehyde

(1, 2)

(2)

(3)

Anaerobic pathway

1,1,1-trichloroethane

(Methylosinus trichosporium OB3b,

Mycobacterium spp. TA5, 27)

2,2,2-trichloroethanol

(4, 5)

2,2,2-trichloroacetate

(Pseudomonas sp.)

carbon oxalate

dioxide

(6)

(8) (7)

Aerobic pathway

2,2-dichloroacetate

(9)

1,1,1-trichloroethane

1,1-dichloroethene acetic acid

(10)

Abiotic pathway

EXPLANATION

Literature reference describing 

 reaction pathway

Proven dehydrochlorination

Hydrolosis

Co-metabolism

Proven microbe-catalyzed

Proven abiotic hydrolysis

Methylosinus

trichosporium Microorganism catalyzing reaction

Degradation of Selected Volatile Organic Compounds in Ground Water    19
Aerobic Biodegradation

According to the degradation pathway constructed by

Sands and others (2005) and Whittaker and others (2005), the

dichloroethanes are not a by-product of 111-TCA or 112-TCA

biodegradation under aerobic conditions (fig. 4). Apparently,

the only source of 11-DCA and 12-DCA via a degradation

pathway is the reductive dechlorination of 111-TCA and

112-TCA, respectively, under anaerobic conditions (figs. 3

and 4). Under aerobic conditions, however, 12-DCA can be

degraded when used as a carbon source by microorganisms.

The intermediate by-product of this degradation is chloroethanol, which is then mineralized to carbon dioxide and water

(fig. 5; Stucki and others, 1983; Janssen and others, 1985;

Kim and others, 2000; Hage and others, ).	

Anaerobic Biodegradation 

While researching the scientific literature for their report,

Wiedemeier and others (1998) did not find published studies

describing anaerobic biodegradation of chlorinated ethanes

in ground water. Since the publication of Wiedemeier and

others (1998), however, numerous published studies describe

the anaerobic biodegradation of chlorinated ethanes. McCarty

(1997) indicates that carbon tetrachloride was transformed

to chloroform under denitrifying conditions and mineralized

to carbon dioxide and water under sulfate-reducing conditions (fig. 6). Adamson and Parkin (1999) show that under

anaerobic conditions, carbon tetrachloride and 111-TCA

tend gradation .	Adamson 

Parkin (1999) also show that carbon tetrachloride was rapidly

degraded by co-metabolism when acetate was the carbon

source.

Chen and others (1996) describe how methanogenic

conditions in a municipal sludge digester	 gradation	of PCA	to	112-TCA, 

dehydrohalogenation	(fig.	3). 

report	that	co-metabolic	transformations -

occur	under	methanogenic	conditions. ,	112-TCA 

was	degraded	tochloroethane ficient	amounts 

carbon	source	were ig.	3). as

inhibited	by	the	presence ut	not	nitrite.

Dolfing (2000) discusses the thermodynamics of reductive dechlorination during the degradation of chlorinated

hydrocarbons and suggests that fermentation of chloroethanes

to ethane or acetate may be energetically more favorable than

“classic” dechlorination reactions. Moreover, polychlorinated

ethanes may degrade preferentially by reductive dechlorination under strongly reducing conditions. Dichloroelimination,

however, may actually be the dominant degradation reaction

for polychlorinated ethanes because more energy is available

to microorganisms than is available during reductive dechlorination (Dolfing, 2000). During an gradation, 

mean half-lifes of the chloroethane compounds can be as short

as three days, in the case of 111-TCA, or as long as 165 days,

in the case of 12-DCA (table 15).

Table 15. Mean half-life in days for the anaerobic biodegradation 

of selected chlorinated alkane and alkene compounds.1

[(27), number of samples used to derive the mean value; —, not available)

Compound All studies Field/in situ studies

chloroethene 0.018 (27) 0.0073 (19)

1,2-dichloroethane 63-165 (2) 63–165 (2)

tetrachloroethene (PCE) 239–3,246 (36) 239 (16)

tetrachloromethane 47 (19) 40 (15)

1,1,1-trichloroethane 2.3–2.9 (28) —

1,1,2-trichloroethane 47–139 (1) —

trichloroethene (TCE) 1,210 (78) 277 (30)

1

Aronson and Howard, 1997, p. 111

1,2-dichloroethane, aerobic

(xanthobacter autotrophicus GJ10,

Ancylobacter aquaticus Ad20, AD25, AD27)

2-chloroethanol

choroacetaldehyde

chloroacetic acid

glycolate

intermediary

metabolism

(1)

(2)

(3)

(4, 5)

(1) Verschueren and others, 1993

(2) Xia and others, 1996

(3) Liu and others, 1997

(4) Janssen and others, 1985

(5) Hisano and others, 1996

EXPLANATION

Literature reference describing 

 reaction pathway

Proven microbe-catalyzed oxidation reaction

Microorganism catalyzing reaction xanthobacter

autotrophicus

Figure 5. Laboratory-derived pathway for the aerobic 

biodegradation of 1,2-dichloroethane (modified from 

Renhao, 2005).

20    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
carbon tetrachloride

Pathway 1 Pathway 2 Pathway 3

(Pseudomonas stutzeri KC, Methanosarcina barkeri,

Desulfobacterium autotrophicum, Moorella thermoacetica,

Methanoabacterium thermoautotrophicum)

chloroform

dichloromethane

methyl chloride

methane

C1 metabolic cycle

carbon

monoxide

(Methylococcus

capsulatus;

Pseudomonas

aeruginosa)

carbon

dioxide

formate

thiophosgene phosgene

carbon

dioxide

(1)

(1)

(1)

(1)

(1)

(1)

(2)

(2) (1)

(3) (4)

(1)

(1) Lee and others, 1999

(2) Krone and others, 1991

(3) Eggen and others, 1991

(4) Lamzin and others, 1992

thiophosgene

Pseudomonas

stutzeri

EXPLANATION

Proven microbe-catalyzed 

 dechlorination

Postulated intermediate compound

Postulated microbe-catalyzed 

 reductive pathway

Literature reference describing

 reaction pathway

Proven microbe-catalyzed

 reductive pathway 

Microorganism catalyzing reaction

Figure 6. Laboratory-derived pathways for the anaerobic biodegradation of tetrachloromethane (carbon tetrachloride; 

modified from Ma and others, 2005; Sands and others, 2005).

Degradation of the Chlorinated Alkenes

The primary degradation of the most common chlorinated

alkenes is microbial reductive dechlorination under anaerobic

conditions. However, biodegradation of certain chlorinated

compounds—such as trichloroethene, the dichloroethenes,

vinyl chloride, or chloroethane—can also proceed via oxidative pathways under aerobic conditions. Two forms (isomers)

of dichloroethene occur in ground water as chemical by-products of PCE and TCE biodegradation (Wiedemeier and others,

1998; Olaniran and others, 2004). Abiotic degradation of PCA

to TCE can occur in PCA-contaminated ground water (fig. 3;

Chen and others, 1996).

Aerobic Biodegradation

Several studies have shown that chlorinated ethenes, with

the exception of PCE, can degrade under aerobic conditions

by oxidation (Hartmans and e Bont, 1992; Klier and others,	

1999; Hopkins and McCarty, 1995; Coleman and others, 2002)

and by co-metabolic processes (Murray and Richardson, 1993;

Vogel, 1994; McCarty and Semprini, 1994). Studies describing the degradation of PCE under aerobic conditions were not

found in the peer-reviewed literature. In one study, aerobic

biodegradation of PCE was not measurable beyond analytical precision after 700 days of incubation (Roberts and others,

1986). Furthermore, Aronson and others (1999) indicate that

Degradation of Selected Volatile Organic Compounds in Ground Water    21
PCE is not degraded when dissolved oxygen (DO) is greater

than 1.5 mg/L, the approximate boundary between aerobic and

anaerobic conditions (Stumm and Morgan, 1996). Chen and

others (1996) suggest the structure and oxidative state of PCE

prevents its aerobic degradation in water.

According to the aerobic biodegradation pathway constructed by Whittaker and others (2005), the dichloroethenes

are not a by-product of TCE degradation under aerobic conditions (fig. 4). Rather, TCE is degraded along three different

pathways by different microorganisms (fig. 7). These pathways

do not form any of the dichlorothene compounds and the only

apparent source of 12-DCE is by the reductive dechlorination

of TCE under anaerobic conditions (figs. 3 and 8). The compounds 12-DCE and VC, however, can be degraded under aerobic conditions by microorganisms utilizing the compounds as a

primary carbon source (fig. 5; Bradley and Chapelle, 1998).

Although PCE is not known to degrade through cometabolism under aerobic conditions, co-metabolism is known

to degrade TCE, the dichloroethenes, and VC. The rate of cometabolism increases as the degree of chlorination decreases

on the ethene molecule (Vogel, 1994). During aerobic cometabolism, the chlorinated alkene is indirectly dechlorinated

by oxygenase enzymes produced when microorganisms use

other compounds, such as BTEX compounds, as a carbon

source (Wiedemeier and others, 1998). The co-metabolic degradation of TCE, however, tends to be limited to low concentrations of TCE because high concentrations in the milligram

per liter range are toxic to microbes catalyzing this reaction

(Wiedemeier and others, 1998). In field studies by Hopkins

and McCarty (1995), VC is shown to degrade by co-metabolism under aerobic conditions when phenol and toluene were

used as a carbon source.

trichloroethene (TCE)

(Methylosinus trichosporium OB3b)

trichloroethene (TCE)

(Burkholderia cepacia G4) 

trichloroethene (TCE)

(Pseudomonas putida F1) 

chloral hydrate

trichloroacetate

(Pseudomonas sp.)

trichloroethene epoxide glyoxylate

formate trichloroethanol

dichloroacetate

(Pseudomonas sp.)

glyoxylate

carbon monoxide

(Methylococcus capsulatus,

Pseudomonas aeruginosa)

carbon dioxide

carbon oxalate

dioxide

(1)

(2)

(2)

(2)

(4) (3)

(1) (8)

(8) (5)

(1) (6)

(6)

(6)

(7)

(1) Rosenzweig and others, 1993

(2) Newman and Wackett, 1991

(3) Motosugi and others, 1982

(4) Jun Oh, 2005

(5) Newman and Wackett, 1995

(6) Newman and Wackett, 1997

(7) Weightman and others, 1985

(8) Li and Wackett, 1992

(9) Eggen and others, 1991

(9)

C1 metabolic 

cycle

EXPLANATION

Literature reference describing 

 reaction pathway

Dechlorination

Hydrolysis

Proven microbe-catalyzed

Methylosinus

trichosporium Microorganism catalyzing reaction

Figure 7. Laboratory-derived pathways for the aerobic biodegradation of trichloroethene 

(modified from Whittaker and others, 2005).

22    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
Anaerobic Biodegradation

Many laboratory and field studies have shown that

microorganisms degrade chlorinated ethenes under anaerobic

conditions (Bouwer and others, 1981; Bouwer, 1994, Dolfing,

2000). Ground water is considered anoxic when the dissolved

oxygen concentration falls below 1.0–1.5 

Morgan, 1996; Christensen and others, 2000). Under anoxic

conditions, anaerobic or facultative microbes will use nitrate

as an electron acceptor, followed by iron (III), then sulfate,

and finally carbon dioxide (methanogenesis; Chapelle and others, 1995; Wiedemeier and others (1998). As the concentration

of each electron acceptor sequentially decreases, the redox

potential of the ground water becomes greater (more negative)

and biodegradation by reductive dechlorination is favored.

Anaerobic conditions in ground water can be determined

by measuring the vertical and spatial concentrations of oxygen, iron (II), manganese (II), hydrogen sulfide, or methane

in ground water and using that data as a qualitative guide to

the redox status (Stumm and Morgan, 1996; Christensen and

others, 2000). Other measurements of anaerobic conditions

involving microorganism biomarkers include volatile fatty

acids, ester-linked phospholipid fatty acid (PLFA), deoxyribonucleic acid (DNA), and ribonucleic acid (RNA) probes, and

TEAP bioassay (Christensen and others, 2000). The reduction

of iron (III) to iron (II), manganese (IV) to manganese (II),

sulfate to hydrogen sulfide, and carbon dioxide to methane

during the microbial reduction of chlorinated VOCs can have a

major influence on the distribution of iron (II), manganese (II),

hydrogen sulfide, and methane concentrations in ground water

(Stumm and Morgan, 1996; Lovley, 1991; Higgo and others,

1996; Braun, 2004).

The highly chlorinated alkenes are commonly used as

electron acceptors during anaerobic biodegradation and are

reduced in the process (Vogel and others, 1987). The primary

anaerobic process driving degradation of CVOCs, except VC,

is reductive dechlorination (figs. 3 and 8; Bouwer and others,

1981; Bouwer, 1994). Tetrachloroethene and TCE are the most

susceptible to reductive dechlorination because they are the

most oxidized of the chlorinated ethenes; however, the more

reduced (least oxidized) degradation by-products such as the

dichloroethenes and vinyl chloride are less prone to reductive

dechlorination. The main by-product of anaerobic biodegradation of the polychlorinated ethenes is VC (fig. 8), which

is more toxic than any of the parent compounds (Agency

for Toxic Substances and Disease Registry, 2004). The rate

of reductive dechlorination tends to decrease as the reductive dechlorination of daughter products proceeds (Vogel

and McCarty, 1985; Bouwer, 1994). Murray and Richardson

(1993) suggest that the inverse relation between the degree

of chlorination and the rate of reductive dechlorination may

explain the accumulation of 12-DCE and VC in anoxic ground

water contaminated with PCE and TCE. In addition, the

anaerobic reduction of VC to ethene is slow and inefficient

under weak reducing conditions, which favors the persistence

of VC in anoxic ground water (Freedman and Gossett, 1989).

Reductive dechlorination has been demonstrated under

nitrate- and iron-reducing conditions (Wiedemeier and others,

1998). Reductive dechlorination of the CVOCs, however, may

be more rapid and more efficient when oxidation-reduction

(redox) conditions are below nitrate-reducing levels (Azadpour-Keeley and others, 1999). Sulfate-reducing and methanogenic ground-water conditions create an environment	

that facilitates not only biodegradation for the greatest	

number of CVOCs, but also more rapid biodegradation rates

(Bouwer, 1994). Reductive dechlorination of DCE and VC is

most apparent under sulfate reducing and methanogenic conditions (Wiedemeier and others, 1998). Anaerobic biodegradation

rates for the chlorinated alkenes can be as short as 45 minutes,

in the case of VC, to as long as 9 years for PCE (table	15).

(1) Neumann and others, 1996

(2) Maymo-Gatell and others, 1997

(3) Magnuson and others, 1998

tetrachloroethene (PCE)

(Dehalococcoides ethenogenes 195, 

Dehalospirillum multivorans, Sporomusa ovata,

Dehalobacter restrictus TEA, Desulfitobacterium sp. PCE-S)

trichloroethene (TCE)

(Dehalococcoides ethenogenes 195)

cis-1,2-dichloroethene trans-1,2-dichloroethene

chloroethene

(vinyl chloride)

ethene

(1)

(2) (2)

(2) (2)

(2)

(3)

EXPLANATION

Literature reference describing 

 reaction pathway

Proven microbe-catalyzed dechlorination

Microorganism catalyzing reaction Dehalococcoides

ethenogenes

Figure 8. Laboratory-derived pathway for the anaerobic 

biodegradation of tetrachloroethene (modified from Ellis and 

Anderson, 2005).

Degradation of Selected Volatile Organic Compounds in Ground Water    23
Degradation of the Chlorinated Benzenes

Several studies have shown that chlorinated benzene compounds containing up to four chlorine atoms can be degraded

by microorganisms under aerobic conditions (Reineke and

Knackmuss, 1984; Spain and Nishino, 1987; Sander and others, 1991). Under aerobic conditions, 1,2,4-trichlorobenzene

(124-TCB; Haigler and others, 1988) and chlorobenzene (CB;

Sander and others, 1991) are used as a primary carbon source

during biodegradation by microorganisms such as Burkholdia

and Rhodococcus species (Rapp and Gabriel-Jürgens, 2003).

During biodegradation, these compounds are completely

mineralized to carbon dioxide (CO2) (van de Meer and others,

1991). Rapp and Gabriel-Jürgens (2003) also indicate that all

of the dichlorobenzene isomers were biodegraded by the

Rhodococcus bacterium. The biodegradation pathways for

124-TCB, 14-DCB, 12-DCB, and CB, under aerobic conditions are shown in figures 9 to 11, respectively. These pathways

are similar to that of benzene, except that one chlorine atom is

eventually eliminated through hydroxylation of the chlorinated

benzene to form a chlorocatechol, then ortho cleavage of the

benzene ring (van der Meer and others, 1998).

Calculated and published degradation half-lifes for the

chlorobenzenes under aerobic conditions are shown in table 16.

The compounds 124-TCB, 12-DCB, and CB lose 50 percent of

their initial mass within 180 days (table 16). Conversely,

Dermietzel and Vieth (2002) show that chlorobenzene was

rapidly mineralized to CO2 in laboratory and in situ microcosm

studies, with complete mineralization ranging from 8 hours

to about 17 days. In addition, the compound 14-DCB was

completely mineralized within 25 days. Nevertheless, under

the aerobic conditions of Dermietzel and Vieth (2002) study,

124-TCB, 12-DCB, and 13-DCB were only partially degraded

after 25 days. In another laboratory-microcosm study by Monferran and others (2005), all isomers of DCB were mineralized

to CO2 within 2 days by the aerobe Acidovorax avenae.

Although Wiedemeir and others (1998) indicate that few

studies existed that described the anaerobic degradation of the

chlorobenzene compounds, a study by Ramanand and others

(1993) did suggest that 124-TCB could be biodegraded to chlorobenzene with 14-DCB as an intermediate compound under

anaerobic conditions. Moreover, Middeldorp and others (1997)

show that 124-TCB was reductively dechlorinated to 14-DCB,

then to chlorobenzene in a methanogenic laboratory microcosm

in which chlorobenzene-contaminated sediment was enriched

with lactate, glucose, and ethanol. These compounds served as

carbon sources. Furthermore, the microbial consortia facilitating the dechlorination of 124-TCB also was able to degrade

isomers of tetrachlorobenzene to other isomers of TCB and

12-DCB. More recent studies show	that	a	strain	of 

Dehalococcoides, can reductively dechlorinate 124-TCB under

anaerobic conditions (Holscher and others, 2003; Griebler and

others, 2004a). In addition, Adrian and others (1998) suggest

that fermentation is the primary degradation process for the

chlorobenzenes under anaerobic conditions. This study also

showed that the co-metabolism of 124-TCB was inhibited by

the presence of sulfate, sulfite, and molybdate.

1,2,4-trichlorobenzene

(Burkholderia sp. P51, PS12)

 3,4,6-trichlorocatechol 

2,5-dichloro-carboxymethylene

but-2-en-4-olide

 2-chloro-3-oxoadipate 

(1)

(1)

(3)

(1)

(2)

(1) van der Meer and others, 1991

(2) Kasberg and others, 1995

(3) Reineke and Knackmuss, 1988

(4) Middeldorp and others, 1997

(5) National Institute for Resources

 and Environment, 2001

3,4,6-trichloro-cis-1,2-dihydroxycyclohexa-3,5-diene

2,3,5-trichloro-cis,

cis-muconate

(1)

2,5-dichloro-4-oxohex2-enedioate

succinate chloroacetate

1,2-dichloroethane

pathway (fig. 5)

1,2,4-trichlorobenzene

methanogenic microbial consortium

1,4-dichlorobenzene1,2-dichlorobenzene

(4) (5)

(5)

60 percent 10 percent

chlorobenzene

(4)

chlorobenzene

pathway

(fig. 11)

EXPLANATION

Literature reference describing 

 reaction pathway

Proven microbe-catalyzed 

Burkholderia sp. Microorganism catalyzing reaction

Aerobic pathway Anaerobic pathway

Oxidation reaction

Dechlorination

Hydrolysis

Figure 9. Laboratory-derived pathways for the aerobic 

and anaerobic biodegradation of 1,2,4-trichlorobenzene 

(modified from Yao, 2006).

24    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
1,4-dichlorobenzene

(Xanthobacter flavus 14pl ;

Alcaligenes sp. strain A175;

 Pseudomonas sp.)

 3,6-dichloro-cis-

 1,2-dihydroxycyclohexa-3,5-diene

 3,6-dichlorocatechol

2,5-dichloro-cis,cis-muconate

trans-2-chlorodienelactone

(1)

(1)

(1)

(2)

(1) Spiess and others, 1995

(2) Sommer and Görisch, 1997

EXPLANATION

Literature reference describing 

 reaction pathway

Proven microbe-catalyzed oxidation reaction

Alcaligenes sp. Microorganism catalyzing reaction

Figure 10. Laboratory-derived pathway for the 

aerobic biodegradation of 1,4-dichlorobenzene 

(modified from Liu, 2006).

Furthermore, Ramanand and others (1993) show that

124-TCB had declined by 63 percent within 30 days under

anaerobic conditions. Dermietzel and Vieth (2002) show

that the anaerobic biodegradation of 14-DCB was markedly

slower under iron-reducing conditions than under aerobic

conditions. In general, it appears that the biodegradation of

the chlorinated benzenes is slower under anaerobic than

under aerobic conditions.

Degradation of the Gasoline Compounds

Laboratory and field studies have shown that microorganisms mediate the degradation (biodegradation) of the common

gasoline compounds (MTBE and BTEX) under both aerobic

and anaerobic conditions. Aerobic microorganisms readily oxidize BTEX compounds while using them as primary

substrates. The biodegradation of BTEX compounds under

various redox conditions is well documented in the scientific

literature (Vogel and Grbić-Galić, 1986; Kuhn and others,

1988; Lovley and Lonergan, 1990; Evans and others, 1991a,b;

Hutchins and others, 1991; Rabus and others, 1993; Edwards

and Grbić-Galić, 1994; Fries and others, 1994; Rabus and

Widdel, 1995; Anderson and others, 1998).

Although early studies concluded that MTBE was

recalcitrant to aerobic biodegradation (Squillace and others,	

1997), w	that,	once	initiated,	the	

aerobic biodegradation of MTBE is relatively rapid (Deeb and

others, 2000), but markedly slower than BTEX degradation.

In addition, the anaerobic biodegradation of MTBE is known

to proceed, although slowly, under a variety of redox conditions. Until recently, however, little was known about specific

pathways involved in the anaerobic degradation of MTBE.

Aerobic Biodegradation of BTEX Compounds 

Laboratory and field studies show that microorganisms mediate the biodegradation of BTEX compounds under

aerobic conditions (table 17; Aronson and others, 1999). The

microbially catalyzed oxidation reaction between dissolved

oxygen and BTEX is thermodynamically favored because

BTEX compounds are in a highly reduced state and the preferred terminal electron acceptor (TEA) is oxygen (Brown and

others, 1996). The microbially catalyzed oxidation of BTEX

compounds requires 3.1 milligrams per liter (mg/L) of dissolved oxygen (DO) to 1 mg/L of a BTEX compound (Aronson and Howard, 1997). Some studies show that the rate of

biodegradation tends to slow when DO concentrations are less

than about 1–2 parts per million (ppm; equal to milligrams

per liter, mg/L; Chiang and others, 1989; Salanitro, 1993).

During laboratory studies in which the initial DO concentration was at least 8 mg/L, individual BTEX compounds or a

BTEX mixture biodegraded rapidly to low concentrations until

the DO concentration was less than 2 mg/L; At this threshold,

biodegradation was rate limited, rather than substrate limited,

because of the low DO concentration (Salanitro, 1993).

Degradation of Selected Volatile Organic Compounds in Ground Water    25
Chlorobenzene

(Pseudomonas sp. 

P51, JS150, RHO1, JS6)

1,2-dichlorobenzene

(Pseudomonas sp.)

3-chloro-cis-1,2-dihydroxycyclohexa-3,5-diene

 3-chlorocatechol

 2-chloro-cis,cis-muconate 3-chloro-2-

hydroxymuconic

semialdehyde

trans-4-carboxymethylenebut-2-en-4-olide

 maleylacetate

 3-oxoadipate

(1) o-dichlorobenzine dihydrodiol

(8)

3,4-dichlorocatechol

(8)

2,3-dichloro-cis-cis-muconic acid

(8)

5-chloro-4-carboxymethylene-but-2-en-4-olide

(8)

5-chloromaleylacetic acid

(8)

2-chloromaleylacetate trans-2-chlorodienelactone

(8)

(9) (8)

(2)

(3) (4)

(5)

(6)

(7)

(1) Werlen and others, 1996

(2) Mason and Geary, 1990

(3) Broderick and O’Halloran, 1991

(4) Riegert and others, 1998

(5) Hammer and others, 1993

(6) Pathak and Ollis, 1990

(7) Kaschabek and Reineke, 1995

(8) Haigler and others, 1988

(9) Vollmer and others, 1993

EXPLANATION

Literature reference describing 

 reaction pathway

Proven microbe-catalyzed 

Microorganism

 catalyzing reaction

Pseudomonas sp.

Oxidation reaction

Dechlorination

Hydrolysis

Hydration

Figure 11. Laboratory-derived pathway for the aerobic biodegradation of chlorobenzene 

and 1,2-dichlorobenzene (modified from McLeish, 2005).

Aerobic microorganisms readily oxidize BTEX compounds while using them as primary substrates. The oxidation of BTEX compounds can proceed via several pathways

(figs. 12–16). In one laboratory column study, methanol was

added to a BTEX mixture to identify possible co-metabolic

pathways. The methanol was not used as the primary substrate

and appeared to depress the biodegradation of BTEX compounds (Hubbard and others, 1994). This study showed that

BTEX degradation was not a result of co-metabolism. Benzene has been shown to degrade completely to carbon dioxide

(mineralization; Gibson and others, 1968; Gibson and Subramanian, 1984; Edwards and Grbić-Galić, 1992). More recent

laboratory experiments show that catechol is an intermediate

compound in the benzene pathway (fig. 12). Five separate

degradation pathways have been identified for toluene under

aerobic conditions (fig. 13). One of these pathways shares a

common intermediate compound (3-methylcatechol) with the

degradation of o- and m-xylene (figs. 12 and 13). The pathway

for p-xylene follows a similar pattern, but differs in the intermediate compounds formed. This difference is caused by the

position of the methyl group on the benzene ring of p-xylene

(fig. 14). The aerobic biodegradation of ethylbenzene follows

three pathways depending on the microorganism using ethylbenzene as its carbon source (fig. 15). Based on laboratory and

field microcosm studies, biodegradation of BTEX under aerobic conditions is more rapid in gasoline-contaminated aquifer

sediments than in uncontaminated aquifer sediments (Aronson

and others, 1999; table 17).

26    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
A nationwide survey of VOCs in ground water showed

that toluene, representing BTEX compounds, was detected

more frequently in oxic rather than in anoxic ground water

(Squillace and Moran, 2006). In other studies, the loss

of BTEX compounds along ground-water flowpaths was

inversely related to dissolved-oxygen concentration, indicating that microbial activity (respiration) was related to BTEX

degradation (Donaldson and others, 1990; Huesemann and

Truex, 1996).

Morasch and others (2001, 2002), using stable isotope

fractionation data, concluded that quantifying aerobic microbial degradation of BTEX in oxic environments may not

be possible. Moreover, laboratory studies have shown that

ethylbenzene can inhibit the microbial degradation of benzene,

toluene, and the xylenes and does so until all of the ethylbenzene is degraded (Deeb and Alvarez-Cohen, 2000).

Anaerobic Biodegradation of 

BTEX Compounds

During anaerobic biodegradation, BTEX compounds are

used metabolically as electron donors (carbon source, primary

substrate) by select microbial populations to produce the

energy for cell growth (Aronson and Howard, 1997). BTEX

degradation can be limited by the availability of terminal

electron acceptors such as nitrate, sulfate, carbon dioxide, or

iron (III) in the aquifer (Lovley and others, 1989; Lovley and

others, 1995). These electron acceptors, however, commonly

exist in ground water at sufficient levels for these reactions to

proceed (Lovley and Lonergan, 1990; Kuhn and others, 1988).

Figure 16 shows the anaerobic biodegradation pathways for

BTEX compounds.

Anaerobic biodegradation of benzene appears to be more

aquifer specific than that for the other monoaromatic hydrocarbons. Current data indicates that biodegradation may not

occur at all sites (Aronson and Howard, 1997). Some of these

studies show that benzene resists anaerobic metabolism in the

field (Reinhard and others, 1984; Barbaro and others, 1992)

and in laboratory enrichments established with sewage sludge,

aquifer sediments, and contaminated soils (Krumholz and others, 1996; Barbaro and others, 1992).

Conversely, several ground-water studies have shown that

BTEX degradation rates decline in a sequence from mildly

reducing conditions (nitrate reduction zone, Hutchins and others, 1991) to strongly reducing conditions (methanogenesis)

in shallow aquifers (Kazumi and others, 1997; Lu and others,

1999; Roychoudhury and Merrett, 2005). Furthermore, other

studies show that when conditions are favorable, benzene (and

other BTEX compounds) can be oxidized to carbon dioxide

under highly reducing conditions. For example, benzene

was rapidly mineralized under sulfate-reducing conditions in

marine and freshwater sediments, and in aquifer sediments

(Lovley and Lonergan, 1990; Edwards and Grbić-Galić, 1992;

Lovley and others, 1995; Phelps and others, 1996; Coates and

others, 1996a,b; Weiner and Lovley, 1998). 

The rate of anaerobic biodegradation of BTEX compounds was quickest under sulfate-reducing conditions in

laboratory and field/in situ studies (Beller and others, 1992a,b;

table 18). The anaerobic biodegradation of BTEX compounds

in ground water was conclusively shown in situ by Griebler

and others (2004b) using compound-specific isotope analysis and signature metabolites analysis. Figure 16 shows the

anaerobic pathway for BTEX compounds developed from the

intermediate compounds identified in ground-water samples

by Griebler and others (2004b).

Table 16. Laboratory or environmental half-lifes and by-products for the aerobic and anaerobic biodegradation of selected 

chlorinated benzene compounds detected in ground water.

[IUPAC, International Union of Pure and Applied Chemistry; CO2, carbon dioxide; DCB, dichlorobenzene]

Compound

(IUPAC name)1

Degradation

by-products

Half-life

(days)

Literature

reference

Aerobic conditions

chlorobenzene 3-chlorocatechol, CO2 69–150 Rathbun, 1998; McLeish, 2005

1,2-dichlorobenzene chlorobenzene 28–180 Rathbun, 1998

1,4-dichlorobenzene chlorobenzene 28–180 Rathbun, 1998

1,2,4-trichlorobenzene succinate, chloroacetate 28–180 Rathbun, 1998; Renhao, 2005; Yao, 2006

Anaerobic conditions

chlorobenzene CO2 280–580 Rathbun, 1998; Monferran and others, 2005

1,2-dichlorobenzene CO2 119–722 Rathbun, 1998

1,4-dichlorobenzene chlorobenzene 112–722 Rathbun, 1998; Yao, 2006

1,2,4-trichlorobenzene 1,4-DCB, chlorobenzene 112–722 Rathbun, 1998; Yao, 2006

1International Union of Pure and Applied Chemistry, 2006

Degradation of Selected Volatile Organic Compounds in Ground Water    27
3-methylcatechol +

carbon dioxide (CO2)

cis,cis-2-hydroxy-6-oxohept-2,4-dienoate

cis-2-hydroxypenta-2,4-dienoate 4-hydroxy-2-oxovalerate

 acetaldehyde

o-xylene

(Burkholderia cepacia MB2)

2-methylbenzyl alcohol

2-methylbenzaldehyde

o-methylbenzoate

1,2-dihydroxy-6-methycyclohexa3,5-dienecarboxylate

m-xylene

(Sphingomonas yanoikutae B1,

Pseudomonas putida mt-2)

3-methylbenzyl alcohol

3-methylbenzaldehyde

m-methylbenzoate

1,2-dihydroxy-3-methycyclohexa3,5-dienecarboxylate

(7)

(7)

(7)

(8)

(8)

(9)

(6)

(6)

(10)

(11)

(12)

(13)

(14)

(8) Higson and Focht, 1992

(9) Kukor and Olsen, 1991

(10) Suzuki and others, 1991

(11) Katagiri and others, 1967

(12) Chalmers and Fewson, 1989

(13) Harayama and others, 1986

(14) Neidle and others, 1992

EXPLANATION

Literature reference describing reaction pathway

benzene

cis-dihydrobenzenediol

catechol

2-hydroxy-cis,cismuconate semialdehyde

(1)

(2)

formate

cis,cis-muconate

(3)

(4)

(5)

(5)

pyruvate

(1) Zamanian and Mason, 1987

(2) Mason and Geary, 1990

(3) Ngai and others 1990

(4) Cerdan and others 1994

(5) Horn and others, 1991

(6) Lau and others, 1994

(7) Jorgensen and others, 1995

(6) (6)

Proven microbe-catalyzed

Burkholderia

cepacia

Microorganism catalyzing 

 reaction

Oxidation reaction

Hydrolysis

Hydration

Reaction, unspecified 

Figure 12. Laboratory-derived pathways for the aerobic biodegradation of benzene, o-, and m-xylene 

(modified from Hyatt and Jun Oh, 2005; Jun Oh, 2005).

28    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
toluene

(Pseudomonas mendocina)

toluene

(Pseudomonas pickettii)

toluene

(Pseudomonas putida)

benzyl alcohol 2-hydroxy-toluene 3-hydroxy-toluene toluene-cis-dihydrodiol

(m-cresol)

4-hydroxy-toluene

3-methylcatechol + carbon dioxide (CO2)

cis, cis-2-hydroxy-6-

oxohept-2,4-dienoate

cis-2-hydroxypenta2,4-dienoate

4-hydroxy-2-oxovalerate

 acetaldehyde + pyruvate

benzaldehyde

benzoate

benzoate

pathway

toluene

(Pseudomonas mendocina)

4-hydroxybenzaldehyde

4-hydroxybenzoate

vanillin pathway

m-cresol pathway

(fig. 17)

(1)

(2)

(3)

(4)

(4)

(5)

(5)

(6)

(7)

(8)

(8)

(9)

(10)

(11)

(12)

(13)

(1) Shaw and Harayama, 1992

(2) Shaw and others, 1993

(3) Inoue and others, 1995

(4) Shields and others, 1995

(5) Olsen and others, 1994

(6) Kukor and Olsen, 1991

(7) Menn and others, 1991

toluene

EXPLANATION

Proven microbe-catalyzed Literature reference describing reaction pathway

(8) Lau and others, 1994

(9) Wackett and others, 1988

(10) Simpson and others, 1987

(11) Yen and others, 1991

(12) McIntire and others, 1986

Microorganism catalyzing (13) Bossert and others, 1989

 reaction

Pseudomonas

mendocina

Oxidation reaction

Hydrolysis

Hydration

Figure 13. Laboratory-derived pathways for the aerobic biodegradation of toluene 

(modified from Wackett and Zeng, 2004).

Degradation of Selected Volatile Organic Compounds in Ground Water    29
Aerobic Biodegradation of 

Methyl Tert-butyl Ether

The compound methyl tert-butyl ether contains ether

bonds and branched hydrocarbon skeletons (tert-butyl branch)

that are common to compounds that persist in the environment

(Smith and others, 2003; Alexander, 1973). Although early

studies concluded that MTBE was recalcitrant to biodegradation (Squillace and others, 1997), more recent studies show

that, once initiated, the aerobic biodegradation of MTBE is

relatively rapid (Deeb and others, 2000), but markedly slower

than BTEX degradation (table 17). MTBE may appear to

persist in contaminated ground water if ground-water studies

are concluded too quickly, especially in areas where MTBE is

a new contaminant. This time lag before degradation begins

is the time it takes the microorganisms in the aquifer to adapt

and begin to use MTBE as a carbon source (Drogos and Diaz,

2000; Wilson and others, 2000, 2005). Wilson and others

(2005), using data from various studies, show that microorganisms capable of degrading MTBE take from 10 to 500 times

longer to double their population than do those microbes

that degrade BTEX compounds. Therefore, the capacity for

the natural attenuation of MTBE depends on the age of the

contamination and the presence of microorganisms capable of

assimilating MTBE.

Although some studies concluded that MTBE degrades

slowly in aerobic environments (Squillace and others, 1997),

other more recent studies show that MTBE is easily degraded

under the proper conditions. For example, in a laboratory

study of lake and streambed sediments collected from 

across graded 

50	days y	and	others,	1999,	2001). 

meyer ws as

recalcitrant under anaerobic conditions during 2 weeks of

monitoring, but rapidly degraded when oxygen was added to a

small, discrete flowpath in shallow ground water.

Although three different bacteria are able to aerobically

degrade MTBE via two different pathways (fig. 17; Pedersen

and Essenberg, 2005), the exact mechanisms driving MTBE

degradation are not well known. Tert-butyl alcohol (TBA) is a

commonly detected by-product of aerobic MTBE degradation

(Steffan and others, 1997). Aerobic biodegradation rates for

MTBE are difficult to find in the literature, but those that are

published indicate that the rates are substantially slower than

those for BTEX compounds (table 17). Some studies indicate

that the degradation of MTBE may be inhibited by the presence of BTEX compounds (Deeb and Alvarez-Cohen, 2000);

however, others indicate that BTEX compounds do not inhibit

MTBE degradation (Aronson and others, 1999; Drogos and

Diaz, 2000; Kane and others, 2001; Sedran and others, 2002).

p-xylene

(Pseudomonas putida mt-2)

p-methylbenzyl alcohol

p-tolualdehyde

p-toluate

4-methycyclohexa-3, 5-diene1, 2-cis-diol-1-carboxylic acid

4-methylcatechol

2-hydroxy-5-methyl-cis,

cis-muconic semialdehyde

(1) Shaw and Harayama, 1992

(2) Biegert and others, 1995

(3) Shaw and Harayama, 1990

(4) Walsh and others, 1983

(5) Whited and others, 1986

(6) Cerdan and others, 1994

3-methyl-cis, cishexadienedioate

2-oxohex-trans-4-enoate 4-methylmucono-lactone

4-hydroxy-2-oxohexanoate

pyruvate

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

propanol

(9)

EXPLANATION

Literature reference describing reaction pathway

(7) Diaz and Timmis, 1995

(8) Harayama and others, 1989

(9) Platt and others, 1995

(10) Murakami and others, 1997

(11) Helin and others, 1995

Pseudomonas

putida Microorganism catalyzing reaction

Proven microbe-catalyzed

Oxidation reaction

Hydrolysis

Hydration

Reaction, unspecified 

Figure 14. Laboratory-derived pathways for the 

aerobic bio­degradation of p-xylene (modified from 

Mili and Stephens, 2006).

30    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
ethylbenzene

(Peudomonas putida 39/D,

Pseudomonas sp. NCIB 10643)

ethylbenzene

(Pseudomonas sp. NCIB 9816-4)

cis-2,3-dihydroxy-2,3-

dihydroethylbenzene

2,3-dihydroxyethylbenzene

2-hydroxy-6-oxoocta-2,4-dienoate

propanoate cis-2-hydroxypenta2,4-dienoate

4-hydroxy-2-oxovalerate

acetaldehyde pyruvate

styrene (S)-1-phenylethanol

acetophenone

2-hydroxyaceto-phenone

(1)

(1)

(2)

(2) (2)

(3)

(3) (3)

To the

styrene

pathway

(fig. 19)

(1) Gibson and others, 1973

(2) Jindrova and others, 2002

(3) Lau and others, 1994

(4) Lee and Gibson, 1996

(4) (4)

(4)

(4)

EXPLANATION

Literature reference describing 

 reaction pathway

Peudomonas putida Microorganism catalyzing reaction

Proven microbe-catalyzed

Oxidation reaction

Hydrolysis

Hydration

Figure 15. Laboratory-derived pathway for the aerobic biodegradation of 

ethylbenzene (modified from McLeish, 2005).

In a six-state ground-water study, Kolhatkar and others

(2000) observed the degradation of MTBE and TBA in anoxic

zones near 76 gas stations. Using data from four of those sites,

degradation rates were calculated for MTBE and TBA. These

degradation rates ranged from 0.0011 to 0.0271 day–1 for

MTBE and 0.0151 to 0.0351 day–1 for TBA. MTBE and TBA

degradation were observed only at sites that were methanogenic (dissolved methane >0.5 mg/L). Furthermore, these sites

were depleted in sulfate relative to background concentrations. Kolhatkar and others (2002) confirmed the anaerobic

degradation of MTBE and TBA in anoxic ground water using

stable carbon isotope analysis. This study also concludes that

the anaerobic biodegradation rates of MTBE and TBA may

exceed those estimated for aerobic biodegradation.

Anaerobic Biodegradation of 

Methyl Tert-butyl Ether

The anaerobic biodegradation of MTBE is known to

proceed, although slowly, under methanogenic (Wilson and

others, 2000; Wilson and others, 2005), sulfate-reducing

(Somsamak and others, 2001), iron-reducing (Finneran and

Lovley, 2001), and nitrate-reducing (Bradley and others, 2001)

conditions. Little is presently known, however, about the pathway of MTBE biodegradation under any of these conditions,

although it has been suggested that anaerobic biodegradation

could be initiated by a hydrolytic mechanism (O’Reilly and

others, 2001; Kuder and others, 2005).

Degradation of Selected Volatile Organic Compounds in Ground Water    31
Table 17. Average half-life for the aerobic biodegradation of the fuel compounds BTEX and methyl tert-butyl ether to carbon dioxide 

in an uncontaminated and contaminated matrix of aquifer sediments and ground water.1

[—, no studies referenced; <, less than]

Compound

Median1 primary

degradation rate

(day–1)

Average half-life in

uncontaminated matrix (days)

Average half-life in

contaminated matrix (days)

Field

setting2

Laboratory

column3

Laboratory

microcosm

Field

setting

Laboratory

column4

In situ

microcosm

benzene 0.096 238 1.5 408 558 31–2.3 6,10 3–31

toluene .20 8135–238 4	–7 8,9 40–60 75 2.3 64.5–7

ethylbenzene .113 238 — 960–139 — 2.3 13,1111

m-, p-xylene .054 238 — 8,931–60 — .350 12,113.5–11

o-xylene .054 238 1–4 912–25 — 2.3 6,1014–83

methyl tert-butyl ether 12.0039 — — — — — 13<365

1Aronson and others, 1999

2American Petroleum Institute, 1994

3Alvarez and others, 1998

4Anid and others, 1993

5Kemblowski and others, 1987

6Nielsen and others, 1996

7McCarty and others, 1998

8Barker and others, 1987

9Hubbard and others, 1994

10Holm and others, 1992

11Thomas and others, 1990

12Laboratory microcosm

13Fenner and others, 2000

Table 18. Mean half-life in days for the anaerobic biodegradation of the fuel compounds BTEX, and methyl tert-butyl ether, 

tert-butyl alcohol under various reducing conditions.1

[(46), number of samples used to derive the mean value; MTBE, methyl tert-butyl ether; TBA, tert-butyl alcohol; —, not available]

Environmental condition Benzene Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene MTBE TBA

Field/in situ studies 210 (41) 12 (46) 46 (37) 33 (33) 43 (34) 46 (26) — —

Nitrate-reducing studies 97 (38) 13 (42) 104 (28) 108 (46) 113 (35) 108 (29) — —

Iron-reducing studies 140 (11) 516 (10) 1,828 (4) 1,822 (8) 1,822 (8) 1,822 (8) — —

Sulfate-reducing studies 50 (9) 61 (14) 197 (7) 109 (9) 141 (8) 198 (5) — —

Methanogenic studies 61 (16) 50 (24) 229 (8) 304 (14) 317 (10) 406 (7) 2,330–7,302 2,315–502

1Aronson and Howard, 1997, p. 16

2Kolhatkar and others, 2000

3Wilson and others, 2005

32    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
benzene toluene

(Azoarcus sp. strain T,

Thauera aromatica )

o-xylene m-xylene p-xylene

phenol

benzylsuccinate

 E-phenylitaconylCoA

2-methylbenzylsuccinic acid3-methylbenzylsuccinic acid4-methylbenzylsuccinic acid

1-phenylethylsuccinic acid1-phenylethanol

4-phenylpentanoic

acid

acetophenone

2-methylphenylitaconic acid3-methylphenylitaconic acid4-methylphenylitaconic acid

o-toluic acid m-toluic acid p-toluic acid

phthalic

acid

isophthalic

acid

terraphthalic

acid

3-carboxybenzylsuccinic acid

Stable co-metabolic end members

o-toluic acid m-toluic acid p-toluic acid

benzoyl-CoA

benzoyl acetate

acetyl-CoA

(2)

(3)

ethylbenzene

(unclassified Protobacteria 

strain EB1)

benzoyl acetate Postulated compound from other experiments

Postulated microbe-catalyzed

 reductive pathway

(1) Beller and Spormann, 1997

(2) Kniemeyer and Heider, 2001

(3) Ball and others, 1996

(4) Ulrich and others, 2005

(1)

(1)

(1)

 benzylsuccinylCoA

(1)

(3) (3)

(4)

(4)

EXPLANATION

Literature reference describing 

 reaction pathway Reductive pathway

Proven microbe-catalyzed

Azoarcus sp. Microorganism catalyzing reaction

Figure 16. Field and laboratory-derived pathways for the anaerobic biodegradation 

of the BTEX compounds—benzene, toluene, ethylbenzene, and xylene (modified from 

Edwards and Grbić-Galić, 1994; and Griebler and others, 2004b). 

Degradation of Selected Volatile Organic Compounds in Ground Water    33
methyl tert-butyl ether

(Mycobacterium vaccae JOB5,

Mycobacterium austroafricanum IFP 2012)

hydroxymethyl tert-butyl ether

tert-butyl formate tert-butyl alcohol

2-methyl-2-hydroxy1-propanol

2-hydroxyisobutyrate

2-propanol

methacrylate 2,3-dihydroxy2-methyl propionate

2-hydroxy-2-methyl1,3-dicarbonate

l-lactate

carbon dioxide

formate

C1 metabolic

cycle

methyl tert-butyl ether

(Norcardia sp. ENV425)

hydroxymethyl tert-butyl ether

formaldehyde

C1 metabolic

cycle

carbon dioxide

(1)

(1)

(1)

(1)

(1)

(2)

(2)

(2)

(2) (2) (2)

(2) (2)

(2)

(2)

(1) Smith and others, 2003

(2) Steffan and others, 1997

spontaneous reaction

EXPLANATION

Literature reference describing 

 reaction pathway

Mycobacterium Microorganism catalyzing reaction vaccae

Proven microbe-catalyzed

Oxidation reaction

Hydrolysis

Hydration

Reaction, unspecified 

Figure 17. Laboratory-derived pathway for the aerobic biodegradation of methyl tert-butyl ether 

(modified from Pedersen and Essenberg, 2005).

34    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
(1) Keat and Hopper, 1978a

(2) Keat and Hopper, 1978b

(3) Hopper and Taylor, 1975

(4) Poh and Bayly, 1980

(5) Nakazawa and Hayashi, 1978

m-cresol (3-hydroxytoluene)

(Pseudomonas alcaligenes NCIB 9867,

 Pseudomonas putida NCIB 9869)

 3-hydroxybenzyl alcohol 

 3-hydroxybenzaldehyde

 3-hydroxybenzoate

 3,4-dihydroxy-benzoate 

 (Protocatechuate)

 2,5-dihydroxy-benzoate

(Gentisate)

2,4-dichlorobenzoate pathway

(1)

(2)

(3)

(4) (5)

toluene pathway

(fig. 13)

Literature reference describing 

 reaction pathway

Oxidation reaction 

Hydroxylation reaction

Reaction, unspecified 

Proven microbe-catalyzed

EXPLANATION

Microorganism catalyzing reaction Pseudomonas

alcaligenes

Figure 18. Laboratory-derived pathway for the 

aerobic biodegradation of m-cresol (modified 

from Sakai and others, 2005).

styrene

(Exophiala jeanselmei,

Pseudomonas putida CA-3)

styrene oxide

 phenylacetaldehyde

phenylacetate

 2-hydroxyphenylacetate

homogentisate

 4-maleylacetoacetate

 fumarylacetoacetate

fumarate acetoacetate

(1)

(1)

(1)

(2)

(2)

(3)

(4)

(5)

styrene

(Rhodococcus rhodochrous 

NCIMB 13259)

styrene cis-glycol

3-vinylcatechol

2-hydroxy-6-oxoocta-trienoate

acrylate 2-hydroxypenta2,4-dienoate

4-hydroxy-2-oxovalerate

acetaldehyde pyruvate

(6)

(6)

(6)

(7)

(7)

(1) Cox and others, 1996

(2) Olivera and others, 1994

(3) Fernandez-Canon and Penalva,1995

(4) Hagedorn and Chapman,1985

(5) Crawford,1976

(6) Warhurst and others, 1994

(7) Lau and others, 1994

EXPLANATION

Literature reference describing 

 reaction pathway

Microorganism catalyzing reaction Exophiala

jeanselmei

Proven microbe-catalyzed

Oxidation reaction

Hydroxylation reaction

Hydrolysis

Hydration

Figure 19. Laboratory-derived pathways for the 

aerobic biodegradation of styrene (modified from Kraus 

and others, 2005).

Degradation of Selected Volatile Organic Compounds in Ground Water    35
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References Cited    49
A

abiotic  Not associated with living organisms. Synonymous

with abiological.1

abiotic transformation  Process in which a substance in the

environment is modified by nonbiological mechanisms.1

absorption  The penetration of atoms, ions, or molecules into

the bulk mass of a substance.2

adsorption The retention of atoms, ions, or molecules onto the

surface of another substance.2

aerobe  An organism that needs oxygen for respiration and

hence for growth.1

aerobic  An environment or process that sustains biological

life and growth, or occurs only when free (molecular) oxygen

is present.2

aerobic conditions  Conditions for growth or metabolism in

which the organism is sufficiently supplied with oxygen.1

alcohols  Compounds in which a hydroxy group, –OH,

is attached to a saturated carbon atom R3COH. The term

hydroxyl refers to the radical species, HO.1

aldehydes  Compounds RC(=	O)H, in which a carbonyl

group is bonded to one hydrogenatom and to one R group1.

R	represents	a	functional yl	group

(methyl	or yl 1

aliphatic compounds  A broad category of hydrocarbon

compounds distinguished by a straight, or branched, open

chain arrangement of the constituent carbon atoms, excluding

aromatic compounds. The carbon-carbon bonds may be either

single or multiple bonds. Alkanes, alkenes, and alkynes are

aliphatic hydrocarbons.2

alkanes  The homologous group of linear (acyclic) aliphatic

hydrocarbons having the general formula CnH2n+2. Alkanes can

be straight chains, branched chains, or ring structures, sometimes called paraffins.1

alkenes  Acyclic branched or unbranched hydrocarbons having one carbon–carbon double bond and the general formula

CnH2n, sometimes called olefins.1

alkyl groups  Univalent groups derived from alkanes by

removal of a hydrogen atom from any carbon atom with the

general form of –CnH2n+1. The groups derived by removal of

a hydrogen atom from a terminal carbon atom of unbranched

alkanes form a subclass called normal alkyl (n-alkyl) groups

H[CH2]

n

.

1

alkyl radicals  Carbon-centered radicals derived formally

by removal of one hydrogen atom from an alkane, e.g.,

CH3CH2—(ethyl radical).1

alkynes  The group of acyclic branched or unbranched

hydrocarbons having a carbon-carbon triple bond that have the

general formula CnH2n-2.

1

ambient  The surrounding environment and

prevailing conditions.2

anaerobe  An organism that does not need free-form oxygen

for growth. Many anaerobes are even sensitive to free oxygen.1

anaerobic  A biologically-mediated process or condition not

requiring molecular or free oxygen.1

analyte  The component of a system to be analyzed.1 For

example, chemical elements or ions in ground-water sample.2

anoxic  An environment without oxygen.2

aquifer  A water-bearing layer of soil, sand, gravel, rock

or other geologic formation that will yield usable quantities

of water to a well under normal hydraulic gradients or

by pumpage.3

aromatic  A group of organic compounds that are cyclic,

contain resonant carbon-carbon double bonds in the form of

at least one 6-carbon benzene ring.2 In the traditional sense,

“having a chemistry typified by benzene.”1

attenuation  The set of human-made or natural processes

that either reduce or appear to reduce the amount of a chemical compound as it migrates away from one specific point

towards another point in space or time. For example, the

apparent reduction in the amount of a chemical in a groundwater plume as it migrates away from its source. Degradation,

dilution, dispersion, sorption, or volatilization are common

processes of attenuation.2

B

biodegradation  Transformation of substances into new

compounds through biochemical reactions or the actions of

microorganisms, such as bacteria. Typically expressed in terms

of a rate constant or half-life.2

biota  Living organisms.2

breakdown product  A compound derived by chemical,

biological, or physical action on a chemical compound. The

breakdown is a process which may result in a more toxic or a

less toxic compound and a more persistent or less persistent

compound than the original compound.2

Glossary

Sources: 1International Union of Pure and Applied Chemistry, 2006; 2U.S. Environmental Protection Agency, 2004; 3

Wiedemeier and others, 1998; 4U.S. Geological Survey, 2006; 5Brown and LeMay, 1977

50    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
C

carbon  Element number 6 in the periodic table of elements.

For a description of the various types of carbon as a solid, the

term carbon should be used only in combination with an additional noun or a clarifying adjective (that is, organic carbon).1

catabolism  The breakdown of complex molecules into

simpler ones through the oxidation of organic substrates to

provide biologically available energy (for example, ATP,

adenosine triphosphate).1

catalysis  The process where a catalyst increases the rate of

a chemical reaction without modifying the overall standard

Gibbs energy change in the reaction.1

catalyst  Substances that increases the rate of a chemical reaction. The catalyst is both a reactant and product of

the reaction. The words catalyst and catalysis should not be

used when the added substance reduces the rate of reaction

(see inhibitor).1

chemical bond  The forces acting among two atoms or

groups of atoms that lead to the formation of an aggregate

with sufficient stability to make it convenient for the chemist

to consider it as an independent “molecular species.”1

chemical induction (coupling)  When one reaction accelerates another in a chemical system there is said to be chemical

induction or coupling. Coupling is caused by an intermediate or by-product of the inducing reaction that participates in

a second reaction. Chemical induction is often observed in

oxidation–reduction reactions.1

chemical reaction  A process that results in the interconversion of chemical species. Chemical reactions may be elementary reactions or stepwise reactions.1

chlorinated solvent  A volatile organic compound containing chlorine. Some common solvents are trichloroethylene,

tetrachloroethylene, and carbon tetrachloride.2

cis, trans isomers  The difference in the positions of atoms

(or groups of atoms) relative to a reference plane in an organic

molecule. In a cis-isomer, the atoms are on the same side of

the molecule, but are on opposite sides in the trans-isomer.

Sometimes called stereoisomers, these arrangements are common in alkenes and cycloalkanes.1

co-metabolism  The simultaneous metabolism of two compounds, in which the degradation of the second compound

(the secondary substrate) depends on the presence of the first

compound (the primary substrate). For example, in the process

of degrading methane, some bacteria can degrade chlorinated

solvents that would otherwise not be degraded under the

same conditions.2

concentration  Composition of a mixture characterized in

terms of mass, amount, volume or number concentration with

respect to the volume of the mixture.1

conservative constituent or compound  One that does not

degrade, is unreactive, and its movement is not retarded within

a given environment (aquifer, stream, contaminant plume, and

so forth).4

constituent  An essential part or component of a system

or group (that is, an ingredient of a chemical mixture). For

instance, benzene is one constituent of gasoline.4

covalent bond  A region of relatively high electron

density between atomic nuclei that results from sharing of

electrons and that gives rise to an attractive force and a

characteristic internuclear distance. Carbon-hydrogen bonds

are covalent bonds.1

D

daughter product  A compound that results directly from the

degradation of another. For example cis-1,2-dichloroethene

(12-cDCE)is commonly a daughter product of trichloroethene

(TCE) degradation. This is a term that has currently (2006)

fallen out of general use. See metabolic by-product.3

dehydrohalogenation  Removal of hydrogen and halide ions

from an alkane resulting in the formation of an alkene.3

denitrification  Bacterial reduction of nitrate to nitrite to gaseous nitrogen or nitrous oxides under anaerobic conditions.4

density (ρ)  The ratio of the mass of a substance to the mass

of an equal volume of distilled water at 4 degrees Celsius.

Since the mass of one milliliter (ml) of water at 4 degrees

Celsius is exactly 1 gram, the specific gravity (unitless) is

numerically equivalent to its density (in grams per ml).1

detection limit (in analysis)  The minimum single result that,

with a stated probability, can be distinguished from a representative blank value during the laboratory analysis of substances

such as water, soil, air, rock, biota, tissue, blood, and so forth.1

dichloroelimination  Removal of two chlorine atoms from an

alkane compound and the formation of an alkene compound

within a reducing environment.4

dihaloelimination  Removal of two halide atoms from an

alkane compound and the formation of an alkene compound

within a reducing environment.3

diols  Chemical compounds that contain two hydroxy (--OH)

groups, generally assumed to be, but not necessarily, alcoholic.

Aliphatic diols are also called glycols.1

downgradient  In the direction of decreasing static hydraulic

head (potential).4

E

electron acceptor  A compound capable of accepting electrons during oxidation-reduction reactions. Microorganisms

obtain energy by transferring electrons from electron donors

such as organic compounds (or sometimes reduced inorganic

compounds such as sulfide) to an electron acceptor. Electron

acceptors are compounds that are relatively oxidized and

include oxygen, nitrate, iron (III), manganese (IV), sulfate,

carbon dioxide, or in some cases chlorinated aliphatichydroReferences Cited    51
carbons such as perchloroethene (PCE), TCE, DCE, and

vinyl chloride.2

electron donor  A compound capable of supplying (giving

up) electrons during oxidation-reduction reactions. Microorganisms obtain energy by transferring electrons from electron

donors such as organic compounds (or sometimes reduced

inorganic compounds such as sulfide) to an electron acceptor.

Electron donors are compounds that are relatively reduced and

include fuel hydrocarbons and native organic carbon.3

electronegativity  Concept introduced by Nobel Laureate

Linus Pauling as the power of an atom to attract electrons

to itself.1

elimination  Reaction where two groups such as chlorine and

hydrogen are lost from adjacent carbon atoms and a double

bond is formed in their place.3

endergonic reaction  A chemical reaction that requires

energy to proceed. A chemical reaction is endergonic when the

change in free energy is positive.3

enzyme  Macromolecules, mostly proteins or conjugated proteins produced by living organisms, that facilitate the degradation of a chemical compound (catalyst). In general, an enzyme

catalyzes only one reaction type (reaction specificity) and

operates on only one type of substrate (substrate specificity).1, 4

epoxidation  A reaction wherein an oxygen molecule is

inserted in a carbon-carbon double bond and an epoxide

is formed.3

epoxides  A subclass of epoxy compounds containing a saturated three-membered cyclic ether. See epoxy compounds.1

epoxy compounds  Compounds in which an oxygen atom is

directly attached to two adjacent or nonadjacent carbon atoms

in a carbon chain or ring system; thus cyclic ethers.1

F

facultative anaerobes  Microorganisms that use (and

prefer) oxygen when it is available, but can also use alternate

electron acceptors such as nitrate under anaerobic conditions

when necessary.3

fermentation  Microbial metabolism in which a particular

compound is used both as an electron donor and an electron

acceptor resulting in the production of oxidized and reduced

daughter products.3

functional group  An atom, or a group of atoms attached to

the base structure of a compound that has similar chemical

properties irrespective of the compound to which it is a part. It

defines the characteristic physical and chemical properties of

families of organic compounds.1

G

H

half-life (t1⁄2 )  The time required to reduce the concentration of

a chemical to 50 percent of its initial concentration. Units are

typically in hours or days.2

halide  An element from the halogen group. These include

fluorine, chlorine, bromine, iodine, and astatine.5

halogen  Group 17 in the periodic table of the elements.

These elements are the reactive nonmetals and

are electronegative.5

Henry’s Law  The relation between the partial pressure of

a compound and the equilibrium concentration in the liquid

through a proportionality constant known as the Henry’s

Law constant.4

Henry’s Law constant  The concentration ratio between a

compound in air (or vapor) and the concentration of the compound in water under equilibrium conditions.4

heterogeneous  Varying in structure or composition at different locations in space.4

heterotrophic  Organisms that derive carbon from organic

matter for cell growth.4

homogeneous  Having uniform structure or composition at

all locations in space.4

hydration  The addition of a water molecule to a compound

within an aerobic degradation pathway.5

hydrogen bond  A form of association between an electronegative atom and a hydrogen atom attached to a second,

relatively electronegative atom. It is best considered as an

electrostatic interaction, heightened by the small size of hydrogen, which permits close proximity of the interacting dipoles

or charges.1

hydrogenation  A process whereby an enzyme in certain

microorganisms catalyzes the hydrolysis or reduction of a

substrate by molecular hydrogen.2

hydrogenolysis  A reductive reaction in which a carbonhalogen bond is broken, and hydrogen replaces the

halogen substituent.3

hydrolysis  A chemical transformation process in which

a chemical reacts with water. In the process, a new carbonoxygen bond is formed with oxygen derived from the water

molecule, and a bond is cleaved within the chemical between

carbon and some functional group.1

hydroxylation  Addition of a hydroxyl group to a chlorinated

aliphatic hydrocarbon.3

I

inhibition  The decrease in rate of reaction brought about

by the addition of a substance (inhibitor), by virtue of its

effect on the concentration of a reactant, catalyst, or

reaction intermediate.1

in situ  In its original place; unmoved; unexcavated; remaining in the subsurface.4

J

K

52    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
L

lag phase  The growth interval (adaption phase) between

microbial inoculation and the start of the exponential growth

phase during which there is little or no microbial growth.1

M

measurement  A description of a property of a system by

means of a set of specified rules, that maps the property onto

a scale of specified values, by direct or “mathematical”

comparison with specified reference(s).1

metabolic by-product (by-product)  A product of the reaction

between an electron donor and an electron acceptor. Metabolic

by-products include volatile fatty acids, daughter products of

chlorinated aliphatic hydrocarbons, methane, and chloride.3

metabolism  The entire physical and chemical processes

involved in the maintenance and reproduction of life in which

nutrients are used to generate energy and in the process degrade to simpler molecules (catabolism), which by themselves

may be used to form more complex molecules (anabolism).1

methanogens  Strictly anaerobic archaebacteria, able to

use only a very limited spectrum of substrates (for example,

molecular hydrogen, formate, methanol, methylamine, carbon

monoxide or acetate) as electron donors for the reduction of

carbon dioxide to methane.1

methanogenic  The formation of methane by certain

anaerobic bacteria (methanogens) during the process of

anaerobic fermentation.4

microcosm  A diminutive, representative system analogous to

a larger system in composition, development, or configuration.4

microorganisms  Microscopic organisms that include bacteria, protozoans, yeast, fungi, mold, viruses, and algae.4

mineralization  The release of inorganic chemicals from

organic matter in the process of aerobic or anaerobic decay.4

monoaromatic  Aromatic hydrocarbons containing a single

benzene ring.4

N

nucleophile  A chemical reagent that reacts by forming covalent bonds with electronegative atoms and compounds.4

nutrients  Major elements (for example, nitrogen and

phosphorus) and trace elements (including sulfur, potassium,

calcium, and magnesium) that are essential for the growth

of organisms.3

O

octanol-water partition coefficient (KOW)  The equilibrium

ratio of a chemical’s concentration in octanol (an alcoholic

compound) to its concentration in the aqueous phase of a

two-phase octanol/water system, typically expressed in log

units (log KOW). KOW provides an indication of a chemical’s

solubility in fats (lipophilicity), its tendency to bioconcentrate

in aquatic organisms, or sorb to soil or sediment.2

order of reaction  A chemical rate process occurring in

systems for which concentration changes (and hence the rate

of reaction) are not themselves measurable, provided it is

possible to measure a chemical flux.1

organic carbon (soil) partition coefficient (KOC)  The proportion of a chemical sorbed to the solid phase, at equilibrium in

a two-phase, water/soil or water/sediment system expressed on

an organic carbon basis. Chemicals with higher KOC values are

more strongly sorbed to organic carbon and, therefore, tend to

be less mobile in the environment.2

oxidation  In general, a reaction in which electrons are

transferred from a chemical to an oxidizing agent, or where a

chemical gains oxygen from an oxidizing agent.2

P

Q

R

rate  Derived quantity in which time is a denominator

quantity. Rate of x is dx/dt.1

rate constant, k  See order of reaction.1

rate-controlling step (rate-limiting step, rate-determining

step)  The elementary reaction having the largest control factor exerts the strongest influence on the rate (v). A step having

a control factor much larger than any other step is said to be

rate-controlling.1

recalcitrant  Unreactive, nondegradable, refractory.4

redox  Reduction-oxidation reactions. Oxidation and reduction occur simultaneously; in general, the oxidizing agent

gains electrons in the process (and is reduced) while the reducing agent donates electrons (and is oxidized).2

reduction  In general, a reaction in which electrons are transferred to a chemical from a reducing agent, or where oxygen is

removed from a chemical.2

respiration  The process of coupling oxidation of organic

compounds with the reduction of inorganic compounds, such

as oxygen, nitrate, iron (III), manganese (IV), and sulfate.2

S

solvolysis  Generally, a reaction with a solvent, involving

the rupture of one or more bonds in the reacting solute. More

specifically the term is used for substitution, elimination,

or fragmentation reactions in which a solvent species is the

nucleophile (hydrolysis, if the solvent is water or alcoholysis,

if the solvent is an alcohol).1

stable  As applied to chemical species, the term expresses

a thermodynamic property, which is quantitatively measured

by relative molar standard Gibbs energies. A chemical species

A is more stable than its isomer B if ∆rGo > 0 for the (real or

hypothetical) reaction A → B, under standard conditions.1

References Cited    53
substrate  Component in a nutrient medium, supplying

microorganisms with carbon (C-substrate), nitrogen (N-substrate) as “food” needed to grow.1

T

terminal electron acceptor (TEA)  A compound or molecule

that accepts an electron (is reduced) during metabolism (oxidation) of a carbon source. Under aerobic conditions molecular oxygen is the terminal electron acceptor. Under anaerobic

conditions a variety of terminal electron acceptors may be

used. In order of decreasing redox potential, these TEAs

include nitrate, manganic manganese, ferric iron, sulfate, and

carbon dioxide. Microorganisms preferentially utilize electron acceptors that provide the maximum free energy during

respiration. Of the common terminal electron acceptors listed

above, oxygen has the highest redox potential and provides the

most free energy during electron transfer.4

U

unsaturated zone  The zone between land surface and the

capillary fringe within which the moisture content is less than

saturation and pressure is less than atmospheric. Soil pore

spaces also typically contain air or other gases. The capillary

fringe is not included in the unsaturated zone.4

upgradient  In the direction of increasing potentiometric

(piezometric) head.4

V

vadose zone  The zone between land surface and the water

table within which the moisture content is less than saturation

(except in the capillary fringe) and pressure is less than atmospheric. Soil pore spaces also typically contain air or other

gases. The capillary fringe is included in the vadose zone.4

vapor pressure (Pv)  The force per unit area exerted by a

vapor in an equilibrium state with its pure solid, liquid, or

solution at a given temperature. Vapor pressure is a measure of

a substance’s propensity to evaporate. Vapor pressure increases

exponentially with an increase in temperature. Typical units

are millimeter of mercury (mm Hg), torr, or inches of mercury

(in. Hg).2

W

water solubility (S)  The maximum amount of a chemical that

can be dissolved in a given amount of pure water at standard

conditions of temperature and pressure. Typical units are milligrams per liter (mg/L), gallons per liter (g/L), or pounds per

gallon (lbs/gal).2

X

Y

Z

54    Description, Properties, and Degradation of Selected VOCs Detected in Ground Water
Manuscript approved for publication, October 23, 2006

Prepared by USGS Georgia Water Science Center

Edited by Patricia L. Nobles

Graphics by Caryl J. Wipperfurth

For more information concerning the research in this report, contact 

USGS Georgia Water Science Center, Atlanta, 

telephone: 770-903-9100