United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens GA 30613
Research and Development
EPA-600/S3-82-028 Sept. 1982
Project Summary
Modeling the Fate of Toxic
Organic Materials in
Aquatic Environments
'fl. A. Park, C. I. Connolly, J. R. Albanese, L S. Clesceri, G. W. Heitzman, H. H.
Herbrandson, B. H. Indyke, J. R. Loehe, S. Ross, D. D. Sharma, and W. W.
Shuster
In the project report, documentation
is given for PEST, a dynamic simulation
model for evaluating the fate of toxic
organic materials (TOM) in freshwater
environments. PEST represents the
time-varying concentration (in ppm)
of given TOM in each of as many as 16
carrier compartments; it also com-
putes the percent distribution and half
life of the TOM in each of the carriers.
Possible carriers include phytoplank-
ton, macrophytes, zooplankton, wat-
erbugs, zoobenthos, fish, paniculate
organic matter, floating organic mat-
ter, clay, and water (with TOM in the
dissolved phase).
PEST simulates TOM degradation
by hydrolysis, oxidation, photolysis,
microbial metabolism, and biotrans-
formation by higher organisms; it
simulates TOM transfer by solution,
volatilization, sorption, absorption
onto gills, consumption, excretion,
defecation, biodeposition, mortality,
and throughflow. These processes are
subject to time-varying environmental
factors such as pH, temperature,
dissolved oxygen, wind, solar radiation,
and biomass and condition of organ-
isms.
The model has been verified with
process-level laboratory data and with
ecosystem-level site data. The site
data for fish ponds in Missouri and
Israel and a reservoir in Iowa constitute
prototype data sets that can be used to
evaluate other compounds.
PEST is an interactive, user-oriented
model with ten commands. The user
can edit parameters and driving
variables, display process-response
curves for all combinations of pro-
cesses and driving variables, run a
simulation for any length of time, print
any or all state-variable results, debug
loadings and rates during the simula-
tion, tabulate the results, obtain line-
printer and graphics-device plots,
dump common block contents, and
access an extensive HELP file.
The model is written in standard
FORTRAN IV and will run in 22k on a
POP 11/03 with overlaying. It has
also been tested on an IBM 3033. The
program is well structured and is easy
to understand. System-dependent
features are restricted to two optional
subroutines: one that handles opera-
tions such as file numbering and time
calls and one that provides an interface
to graphics terminals and plotters.
This Project Summary was devel-
oped by EPA's Environmental Research
Laboratory, Athens. GA, to announce
research findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The PEST model has been under
development for the past four years in
response to the need for a detailed,
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chemically and biologically realistic
model to predict the fate of toxic organic
materials in natural aquatic environ-
ments. As such, its development has
paralleled that of several other fate
models; however, each model has its
particular emphasis, and PEST fulfills a
need for detail and biologic realism that
is not addressed by other models.
PEST can be considered an evaluative
model; it is intended to be used primarily
to indicate the relative importance of the
various processes under well-defined
environmental conditions and to deter-
mine the environmental compatibility of
particular organic materials. Many of
the demands placed on the EPA relative
to evaluating new materials can be
answered through the expediency of
such a process-oriented evaluative
model. The model can also assist in the
extrapolation of data from laboratory
experiments and microcosms to natural
environments.
PEST combines detailed chemical
kinetics and bioenergetics to permit
examination and evaluation of the
behavior of toxic organic materials in
the context of the entire aquatic
ecosystem. Of course, use of such a
complex model requires an understand-
ing of the many assumptions and
parameters, as well as a knowledge of
the mechanics of the program. The
purpose of this report is to acquaint the
potential user with the details of PEST
so that the model can be used both
easily and wisely.
PEST is capable of simulating the
time-varying concentration of a toxic
organic material (TOM) in each of as
many as 16 carrier compartments. The
16 state variables can be parameterized
to represent a variety of TOM-carrier
associations typical of aquatic ecosys-
tems (for example. Figure 1).
The state-variable equations are
ordinary differential equations with
source and sink terms for the various
processes that result in additions to, and
losses from, the carriers. Broad cate-
gories include TOM in: plants, such as
phytoplankton and macrophytes; ani-
mals, such as zooplankton, waterbugs,
zoobenthos, and fish (different species
and/or age classes); dissolved phase,
either in the water column or in
interstitial water; particulate organic
matter, either suspended or as bottom
sediment; floating organic matter,
usually as a surface film; and clay,
either suspended or as bottom sediment.
The source and sink terms for the
state variables are represented by
\ \ CCH^OOR
Figure 1. Compartments in the PEST model. FMACRO = floating macrophyte.
MACRO = macrophyte, FOM = floating organic matter, POM = particulate
organic matter, WBUG = water bug. ZOOB=zoobenthos, ZOOP = zooplank-
ton, PHYTO = phytoplankton.
process equations. Most of the process
equations are non-linear and involve
several environmental factors (Figure
2).
Output from the model includes: (1)
the time-varying concentration of the
toxic material in each carrier (ppm), (2)
the percent distribution of the toxic
material among the carriers, and (3) the
half lives of the toxic material in each
carrier. One can also obtain plots of the
degradation rates, both as they vary
through time and as a function of
environmental factors.
The model has been verified with
process-level laboratory data for several
compounds and with ecosystem data
from fish ponds in Missouri and Israel
and from a reservoir in Iowa. The site
constants and environmental driving
variables for these ecosystems constitute
useful "prototype" data sets that
enhance the value of the model for
evaluative purposes.
Data Requirements
Data requirements depend on the
intended use of the model. If PEST is to
be used as an evaluative model, as
originally intended, then default data on
prototype sites (such as verification
sites) may be sufficient to characterize
the behavior and fate of a toxic organic
material; therefore, site data would be
unnecessary. If the model is to be
applied as a diagnostic tool in order to
better understand thefate of a compound
at a particular site, then an accurate
characterization of the site is required. If
the problem involves bioconcentration
in a particular group of organisms, then
it will be necessary to accurately
characterize the metabolic requirements
and feeding preference of the organism.
Verification
The philosophy of verification was to
use available parameter values, confirm
the validity of the process equations by
inspecting the process-response curves,
and then apply the model to the
particular site without calibration. If the
fit to the observed data was not
acceptable, the formulations were re-
examined and improved, but the param-
eter values were not changed. This
approach was taken because it was
believed that there would not be
opportunity or rationale for "fine-
tuning" the parameter values in PEST
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Defecation
Paniculate
Organic
Matter
Figure 2. PEST process flow chart.
using observed data when PEST was
used as an evaluative model for new
compounds.
One of the verification sites was the
Fish and Wildlife Service's National
Fisheries Research Laboratory, Colum-
bia, Missouri. Pentachlorophenol was
introduced in 4 applications of 0.2, 0.2,
0.4, and 0.4 ppm in each of 3 low-
nutrient ponds without macrophytes.
The ponds were 297.28 square meters
in area and had an average depth of
1.48 meters. Data were given for
concentrations in water, bluegills,
largemouth bass, and channel catfish.
Even without calibration, PEST gave
reasonable results in comparison with
observed results.
« US.GOVERNMENTPRINTM3OFFICE't«2-559-017/08il
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ft. A. Park, C. I. Connolly. J. ft. Albanese, L. S. Clesceri. G. W. Heitzman. H. H.
Herbrandson, B. H. Indyke, J. R. Loehe, S. Ross, D. D. Sharma, and W. W.
Shuster are with Rensselaer Polytechnic Institute, Troy, NY 12181.
L. A. Burns is the EPA Project Officer (see below).
The complete report, entitled "Modeling the Fate of Toxic Organic Materials in
Aquatic Environments," (Order No. PB 82-254 079; Cost: $16.50, 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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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