United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens GA 30613
Research and Development
EPA-600/S3-84-019 Feb. 1984
&EPA Project Summary
Estimation of Parameters for
Modeling the Behavior of
Selected Pesticides and
Orthophosphate
P. S. C. Rao, V. E. Berkheiser, and L. T. Ou
A 3-year laboratory study was con-
ducted to determine sorption and degra-
dation of selected pesticides as well as
sorption of orthophosphorus on several
U.S. agricultural soils. The data ob-
tained in the study permit the estimation
of sorption and degradation parameters
required in predicting nonpoint source
loadings of pesticides and phosphorus
from croplands. Nine pesticides (six
herbicides and three insecticides) repre-
senting a broad range of chemical
classes were studied. The pesticide
studies were conducted with seven soil
types; P-sorption was measured on 36
soils that are distributed widely across
the United States.
Data collected during this project
suggest that pesticide sorption coeffi-
cient (Koc) based on soil organic carbon
is not only independent of soil type but
also of soil particle sizes within a soil
type. Thus, each pesticide can be as-
signed a single Koc value, and given the
organic carbon enrichment of the sedi-
ment, partitioning of pesticide between
solution and sediment phases during a
given runoff may be estimated. For all
pesticides studied, microbial degrada-
tion rates followed first-order kinetics.
The rate coefficients (or half-lives) were
not correlated with any of the following
soil properties: soil organic carbon
content, clay content, cation exchange
capacity, pH, Eh, and total bacterial and
fungal propagules. Temperature and
soil-water tension (soil-water content)
were the two soil environmental factors
that did show a significant effect on
pesticide degradation rates. For temper-
atures in the range of 15-35°C and soil
water tensions in the range of 0.1 -1 bar,
however, degradation rate coefficients
for a given pesticide were essentially
constant. Considerable amounts of non-
extractable "bound" residues were
formed for all soil pesticide combina-
tions studied. Generally, larger amounts
of bound residues were found in moist
soils (incubated at soil water tensions
less than 1 bar) than in drier soils (at 15
bar tension).
Phosphate sorption was measured
using a standardized protocol. Meas-
ured P-sorption data were fitted to
Freundlich, Langmuir. and Temkin equa-
tions. Phosphate sorption constants in
the Temkin equations were better corre-
lated with several soil properties than
were the coefficients in Langmuir or
Freundlich equations. Ammonium oxa-
late extractable Al, ammonium acetate
extractable Ca, and clay content were
the three soil properties most useful in
estimating Temkin equation parameters.
This Project Summary was developed
by EPA's Environmental Research Labo-
ratory, Athens, GA, to announce key
findings of the research project that are
fully documented in a separate report of
the same title (see Project Report order-
ing information at back).
Introduction
The use of pesticides, especially herbi-
cides, has increased significantly during
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the past three decades. These chemicals
have contributed to increased crop yields
and in many cases have decreased pro-
duction costs. The use of pesticides,
however, poses a potential threat to the
environment and human health. There-
fore, agricultural producers as well as
environmental scientists are concerned
with the fate of pesticides m agro-
ecosystems. Consequently, information
on pathways by which pesticides leave
the site of application (i.e..croplands) and
subsequently cause nonpomt source pol-
lution will continue to be sought. Such
data are essential to the development of
appropriate controls and management
technologies for minimizing nonpomt
source loadings of pesticides from crop-
lands
A major pathway for pesticide removal
from croplands, especially in terms of
pesticide movement to nontarget areas,
is in runoff water and on sediment carried
along with the runoff water The amount
of soil-applied pesticide that leaves the
field in this manner depends primarily on
the intensity and the duration of a rainfall
event and on the time lag between pesti-
cide application and the runoff event. The
depth of soil layer (i.e., mixing zone) from
which pesticides are removed during a
runoff event isdetermined by the intensity
and duration of the rainfall event. The
amount of pesticide available within this
mixing zone is determined by the extent
of pesticide dissipation that occurs be-
tween application and a given runoff
event. Various processes governing the
fate of pesticides in soils are retention
(adsorption-desorption), transformations
(microbial and chemical degradation), and
transport (overland flow, leaching, and
volatilization).
Two approaches for estimating the
edge-of-field nonpomt source loadings of
pesticides are available. In the first,
empirical relationships have been devel-
oped by correlating the quantities of
pesticides available in the surface soil (0-
1 cm) and the losses measured at the
edge of the field. Obviously, such empir-
ical relationships are developed based on
extensive and often costly field measure-
ments. Alternately, pesticide retention,
transformation, and transport parameters
can be used in several process-oriented
nonpoint source simulation models that
are presently available.
A 3-year laboratory study was conduc-
ted to measure sorption and degradation
of selected pesticides and sorption of
orthophosphate in U.S. soils. Seven soils
from major agricultural areas in the U.S.
along with nine pesticides were included
in this study. The pesticides were atrazine,
cyanazine, metribuzm, diuron, propachlor,
2,4,5-trichlorophenoxyacetic acid, carba-
ryl, carbofuran and fonofos. These data
were used to provide estimates of sorption
coefficients and degradation rate coeffi-
cients for pesticides. Sorption of ortho-
phosphate was measured on 36 U.S.
soils. These data were used to determine
soil factors influencing P-sorption on
soils. The data may be used to estimate
parameter values required in various
process-oriented simulation models for
describing nonpoint source loadings of
pesticides and orthophosphate from crop-
lands.
Conclusions
Based upon the data collected during
this project, the following justifiable
simplifications for estimating pesticide
sorption parameters are proposed.
1. Pesticide sorption on whole soils
and soil size-separates is deter-
mined primarily by the soil organic
carbon content. That is, the sorption
partition coefficient based on soil
organic carbon (Koc) for a given
pesticide isessentially independent
not only of soil type but also of soil
particle size. Thus, the problem of
predicting pesticide sorption by soils
or the partitioning of pesticides
between the sediment and water
phases of runoff reduces to a more
manageable problem of a single Koc
value for a pesticide and the organic
carbon enrichment for a given
runoff event. This approximation is
more likely to be in error for ionic
and ionizable pesticides such as
2,4,5-T.
2. The extent of sorption isotherm
nonsingularity may be specified
quantitatively using the index ft=
(Na/Nd), where Na and Nd are the
Freundlich isotherm constants, re-
spectively, for the adsorption and
desorption isotherm loops. Large ft
values indicate extensive nonsingu-
larity, while /3=1 suggests singular
isotherms. Based upon 0 values, it
was concluded that isotherm non-
singularity increased with increas-
ing soil particle size. For sand size-
fraction (> 50 pm) ft value was
about 4, while for fine clay fraction
/3=1. Analysis of published sorption
data for sediments supported this
conclusion.
Two types of pesticide degradation
rates were measured: mineralization rate
based on 14CC>2 evolution and disappear-
ance rate based on the loss of solvent-
extractable parent chemical. Using these
data, the following conclusions regarding
pesticide degradation are suggested:
1. Pesticide degradation rates in soils
could be described using the first-
order kinetics. For each soil-pesticide
combination, the degradation rate
coefficient (k, day~1) or half-life (tV2,
days) could be specified.
2. For all pesticides studied, the disap-
pearance rate was greater than the
mineralization rate. The differences
between these two rates are due to
accumulation of intermediate me-
tabolites, assimilation into microbial
biomass, and formation of "bound"
residues. Of these, bound residue
formation appears to be the major
contributing factor.
3. Pesticide degradation rates were
not correlated with soil organic
carbon content, pH, cation exchange
capacity, clay content, total bacterial
propagules, and total fungal propa-
gules. The two soil environmental
factors that did influence pesticide
degradation rates were soil-water
content (or soil-water tension) and
soil temperature.
4.' Soil temperatures below 25°C had
a greater influence on pesticide
degradation rates than did tempera-
tures above 25°C. Hence activation
energies (AEa) for pesticide degra-
dation between 15°-25°C were
greater than those between 25°-
35°C.
5. Pesticide degradation rates in soils
maintained at 15 bar soil water
tension were substantially lower
than at 1 bar or lower. At tensions
between 0.1 and 1 bar, degradation
rates were not significantly differ-
ent.
6. Soil-water tension may influence
oxygen tension and redox potentials
in soils. As a result, the pesticide
degradation pathway may be altered
with changes in soil-water tension,
and different types of metabolites
may accumulate.
7. Bound residues, i.e., nonextractable
14C-residues, were found during
degradation of all pesticides in all
soils studied. The extent of bound
residue formation, however, was
different for each soil-pesticide
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combination. Generally, more
bound residues were formed in
moist soils (at 0.1, 0 33, and 1 bar
tension) than in drier soils (at 1 5 bar
tension).
Phosphate sorption isotherms were
measured on 36 soils that were distri-
buted widely across the United States
and represented seven soil orders. The
sorption isotherms were measured using
a standard protocol and certain modifica-
tions, including the use of 0.03 M KCI as
the background electrolyte (to more
closely simulate natural conditions) rather
than 0.01 M CaC\z and no disinfectant.
Regressions analyses of the soil proper-
ties with the phosphate sorption param-
eters showed that soil properties were
better correlated with the constants in
the Temkin equation than the Freundlich
or Langmuir equation constants The
equations that were used to estimate the
Temkin constants with the best correla-
tion coefficients were functions of am-
monium oxalate Al and clay content or
ammonium acetate Ca and clay Calcu-
lated confidence limits around each cal-
culated isotherm based on propagation of
errors using the Temkin equation indica-
ted that ammonium acetate, extractable
Ca content, and clay content could be
used successfully to predict 72% of the
measured P sorption isotherms. Calcu-
lated isotherms were based solely on soil
chemical and physical properties.
project were for soils collected from a
single random location within a single
field for each soil type. This was justified
because the principal objective here was
to characterize sorption and degradation
of pesticides in a broad spectrum of soil
types When it is of interest to predict
pesticide losses at the edge of a single
agricultural field or an entire watershed,
however, the spatial and temporal vari-
ability of key properties (organic carbon
content, sorption coefficient, degradation
rate coefficients, etc.) need to be meas-
ured. The impact of the variable model
parameters on the model outputs (con-
centration with depth, mass emissions
below the root zone, etc ) can then be
evaluated. Thus, we recommend that
studies be initiated at several locations in
the U.S. to characterize the spatial and
temporal variability of sorption and degra-
dation parameters in a single field
P. S. C. Rao, V. E. Berkheiser, and L. T. Ou are with the University of Florida
Gainesville, FL 32611.
C. N. Smith is the EPA Project Officer (see below).
The complete report, entitled "Estimation of Parameters for Modeling the
Behavior of Selected Pesticides and Orthophosphate," (Order No. PB 84-148
774; Cost: $ 17.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
College Station Road
Athens. GA 30613
AUS GOVERNMENT PRINTING OFFICE 1984-759-015/7295
Recommendations
In this project a considerable a mount of
data were collected to characterize the
nonsingularity in pesticide sorption iso-
therms for whole soils and soil size-
separates. A number of earlier workers
also have focused on an experimental
investigation of isotherm nonsingularity.
Despite such an extensive data base, a
sound theoretical or conceptual basis for
this phenomenon is still lacking. Efforts
should be directed, therefore, towards a
more systematic study of sorption iso-
therm nonsingularity using well-character-
ized sorbate-sorbent systems. While such
information may not always be essential
for predicting nonpoint source loadings of
pesticides, these data will contribute to a
better understanding of pesticide sorption
on soils and sediments.
While a number of recent studies have
focused on the spatial and temporal
variability of soil physical properties, such
data for various pesticide sorption and
degradation parameters are essentially
nonexistent. The data we obtained in this
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