United States
Environmental Protection
Agency
Office of Health and
Environmental Assessment
Washington DC 20460
Research and Development
EPA/600/S6-87/003 Jan. 1988
Project Summary
Fate and Persistence in Soil of
Selected Toxic Organic
Chemicals
Roxanne Sukol, Edwin Woolson, and William Thompson
The environmental fate and behavior
of several toxic organic materials are
reviewed in the final report on which
this summary is based. This effort has
sought to summarize the chemical and
physical properties of these materials,
and discusses how these properties
affect persistence and behavior of toxic
chemicals in the soil/water/air
systems.
In general, the organic carbon con-
tent of a soil has the greatest effect on
the behavior of hydrophobic toxic
organic compounds. The organic com-
pounds sorb strongly to the organic
matter in the soil. Several equations
have been derived that define water
solubility relationships. These are
partition coefficients between
octanol/water and organic matter/
water.
Persistence of the toxic organic
compounds depends on several envir-
onmental factors, including soil organic
matter, total precipitation and inten-
sity, temperature, sunlight intensity,
and soil texture. Organic chemicals are
subject to one or more of seven possible
fates: (1) sorption, (2) volatilization, (3)
microbial degradation, (4) photode-
composition on the soil surface. (5)
translocation to plants, (6) chemical
degradation, and (7) leaching to ground
water. Some of these fates are directly
related to the degree of sorption; i.e.,
very little of a material that is strongly
sorbed will be in solution and available
for degradation or movement by the
other processes.
Some generalities are presented
regarding the environmental conditions
and chemical/physical properties that
affect persistence and mobility; how-
ever, the reader should bear in mind
that there are always exceptions to the
rule.
Disregarding any interactions
between environmental conditions, the
following effects might be expected:
1. Temperature—The warmer the
temperature is, the greater the
volatility, the lower the organic
matter content of the soil, the
more active the microbial popu-
lation, and the higher the rate of
evapotranspiration. The result is
a decrease in pesticide
persistence.
2. Moisture—There is an optimum
level of soil moisture for microbial
activity. If a soil is too wet or too
dry, activity slows down. Volatil-
ity is also affected by moisture
content; the nature of the effect
depends on the solubility of the
chemical. The total amount, the
intensity, and the frequency of
rainfall or irrigation water
received affect the movement of
chemicals in soil.
3. Light— Photochemical reactions
are directly proportional to the
number of photons absorbed by
a chemical. Nearness to the equa-
tor or an increase in altitude will
accelerate photochemical
reactions.
4. Soil texture—Soil texture is an
important factor. Soil organic
matter is directly influenced by
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the soil texture. Coarse (i.e.,
sandy) soils will normally be low
in organic matter; therefore,
water percolation will be rapid
and the leaching potential of
chemical compounds will be high
regardless of Kow or Koc values.
The opposite is true for heavy
(i.e., clayey) soils.
The property that affects persistence
and mobility most directly is water
solubility. Within a class of compounds
(e.g., dioxins or PCBs), the higher the
degree of chlorination or bromination
is, the lower the water solubility and,
therefore, the greater the persistence.
Low-molecular-weight compounds
with low chlorine content (e.g., chlor-
obenzene, dichlorobenzene, naptha-
lene) will be subject to a greater degree
of biodegradation, photodecomposi-
tion, volatilization, and leaching than
will high-molecular-weight com-
pounds with higher chlorine or bromine
content (e.g., hexachlorobenzene,
dibenzodioxins and dibenzofurans,
PCBs, PBBs, and DDT and its related
compounds).
The literature search revealed a
sparsity of information on many of the
compounds discussed in this docu-
ment, and gaps were numerous. No
information was found on biphenylenes
and azoxybenzenes.
Half-life estimates between com-
pounds were difficult to compare
because of the differences in experi-
mental and/or environmental condi-
tions. If a standard set of conditions
were adopted, and half-life estimates
were developed for a well-studied
compound (e.g., DDT) under each set
of conditions, other compounds could
be studied under these same standard
conditions and half-life estimates could
then be calculated relative to the
standard materials. These relative half-
lives could then be compared and used
to predict behavior based on similarities
and differences among other com-
pounds of interest.
This Project Summary was devel-
oped by EPA's Office of Health and
Environmental Assessment, Washing-
ton, DC, to announce key findings of
the research project that is fully doc-
umented in a separate report of the
same title (see Project Report ordering
information at back).
Discussion
The implementation of environmental
programs to clean up organic chemicals
that have been released onto and con-
taminated soils requires an understand-
ing of the long-term risk associated with
leaving the soil in place or transporting
it to an ultimate disposal site. Currently
available information on the persistence
of highly toxic organic chemicals is too
sparse to allow development of exposure
assessments with the degree of confi-
dence needed by the regulator or by the
public. The U.S. Environmental Protec-
tion Agency (EPA) recognizes the need
for guidance materials to assist in the
determination of long-term human
health risks posed by persistent toxic
compounds.
The objective of the guidance material
in this document is to provide information
needed to support procedures for esti-
mating the environmental half-life of
compounds having a high affinity for
soils. Specific compounds discussed
herein were selected from the following
groups:
Chlorinated benzenes
Halogenated biphenyls
Chlorinated azobenzenes
Halogenated biphenylenes
Chlorinated azoxybenzenes
Chlorinated naphthalenes
Chlorinated cyclohexanes
Halogenated dibenzofurans
Halogenated dibenzodioxins
Also identified for investigation were
toxaphene, DDT, and hexachloro-
butadiene.
Selection of representative com-
pounds or mixtures (e.g., toxaphene,
polychlorinated biphenyls) was based on
their potential for human toxicity and the
availability of information on their
behavior in soil (Table 1). Some
compounds (e.g., DDT and y-hexachloro-
cyclohexane) have been studied by many
investigators; therefore, much
information is available. Other
compounds, however, including the
entire groups of biphenylenes and azoxy-
benzenes, have not been studied.
Because information is sparse on these
latter compounds, they are not addressed
here.
Before the primary objective of this
guidance material could be achieved,
several lesser objectives had to be
identified and addressed. For example,
this study addresses how soil character-
istics, physical/chemical processes,
biological processes, chemical structure,
microorganisms, and interactions influ-
ence the persistence of compounds in
soils. It also evaluates the influences of
various environmental factors (including
solar radiation, temperature, moisture,
pH, Eh, and the presence of other
chemicals) on persistence in and affinity
for soils.
Wherever possible, the final report
includes information on the behavior of
toxic organic compounds under varying
soil and climatic conditions throughout
the UnitedStates. Factors responsible for
half-life variability under various condi-
tions are also identified.
Table 1.
Representative Organic Compounds Selected for Review Based on Potential Toxicity
in the Environment
Group
Compound
Chlorinated benzenes
Chlorinated azobenzenes
Chlorinated cyclohexanes
Halogenated dibenzofurans
Halogenated dibenzodioxins
Halogenated biphenyls
Chlorinated naphthalenes
Related Compounds
Hexachlorobenzene
1,2-Dichlorobenzene
3.3'.4.4'-Tetrachloroazobenzene(TCAB)
y-Hexachlorocyclohexane (HCH)
2.3,7,8- Tetrachlorodibenzofuran (TCDF)
2,3,7,8-Tetrachlorodibenzodioxin (TCDDj
Polychlorinated biphenyls {PCBs/
Polybrominated biphenyls (PBBs)
Polychlorinated naphthalenes
Toxaphene
Hexachlorobutadiene
DDT
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A method for estimating the half-life
of compounds for which no such data
are available is discussed. This method
uses the Koc partition coefficient to place
compounds with unknown half-lives in
a position relative to those whose half-
lives are predicted with some degree of
certainty. Finally, the relative soil adsorp-
tive characteristics, persistence, and
toxicity of the selected organic com-
pounds are presented.
Section 2 of the final report provides
a review of soil and its physical/chemical
properties, including composition and
texture, water content, pH, organic
matter, cation exchange capacity, and
temperature. Section 3 describes each
of the major pathways of chemical loss
from the soil. Section 4 presents an
explanation of partitioning in the envi-
ronment and includes information on
determining the likelihood of each
chemical's fate. Section 5 provides
information on the persistence, toxicity,
and half-life of the specific organic
compounds selected for investigation.
The materials used for the final report
were drawn from an existing literature
data base. The sources are cited in the
text of that report.
Roxanne Sukol, Edwin Woolson, and William Thompson are with PEI Associates,
Inc., Cincinnati, OH45246.
Charles H. Nauman is the EPA Project Officer (see below).
The complete report, entitled "Fats and Persistence in Soil of Selected Toxic
Organic Chemicals," (Order No. PB 87-186 433; Cost: $18.95, subject to
change} will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Office of Health and Environmental Assessment
U.S. Environmental Protection Agency
Washington, DC 20460
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
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•IAR-2'88
Official Business
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