United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens GA 30613
Research and Development
EPA/600/S3-85/001 Mar. 1985
Project Summary
Testing
TOX-SCREEN
B. R. Bicknell, S. r
TheTOX-SCREE M model, which was
developed recent! r by the Office of
Toxic Substances of the U.S. Environ-
mental Protection
Agency (EPA), is a
screening level, multimedia model de-
signed to provide i
icals to accumulate
lutant fate and tran
release and dispers
deposition; volatil
leaching; and chem
fer coefficients,
parameters. The
In this study, an
and Evaluation of
Model
. Boutwell, and D. B. Watson
apid assessment of
the long-term tendancy of toxic chem-
n air, surface water.
and soil. TOX-SCF EEN simulates pol-
port by considering
on of chemicals into
the air, water anf pollutants across
media interfaces is specified by atmos-
pheric deposition vi locities, masstrans-
and mass loading
model assumes a
generic positionin j of surface water
bodies and conta ninated land areas
with respect to at nospheric pollutant
sources.
evaluation of TOX-
SCREEN to deteimine its ability to
perform reliable sci eening of chemicals
was conducted. Evaluation included a
review of the TOX-SCREEN processes
and models relative to the current state-
of-the-art methods, a comparison of
TOX-SCREEN results with observed
data and with predictions of other
models, and a sensitivity analysis of
selected input parameters.
This Project Summary was developed
by EPA's Environmental Research
Laboratory. Athens, GA, to announce
key 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 Toxic Substances Control Act of
1976 requires that EPA evaluate the risks
associated with the manufacture and use
of chemicals. Additionally, implementa-
tion of other environmental control
statutes, such as the Clean Water Act, the
Clean Air Act, the Safe Drinking Water
Act, the Federal Insecticide, Fungicide,
and Rodenticide Act, and the Resource
Conservation and Recovery Act, implicitly
require an assessment of health risks
posed to humans and other life by the
manufacture and use of chemicals. One
of the effects of these statutes has been
to focus attention on multimedia (that is,
air, water, soil, and biota) considerations
in chemical risk assessments.
The TOX-SCREEN model was developed
by EPA's Office of Toxic Substances to
provide a multimedia, screening level
model designed to enable rapid assess-
ment of the long-term tendency of toxic
chemicals to accumulate in air, surface
water, soil, and ultimately in biota. Be-
cause TOX-SCREEN was designed as a
screening tool for evaluating chemicals, it
was simplified to minimize input require-
ments and was designed to be overly
predictive or conservative. TOX-SCREEN
estimates pollutant concentrations in the
various media based on chemical property
data, climatic and soil data, simple water
body descriptions, and specific chemical
release scenarios.
TOX-SCREEN simulates pollutant trans-
port and fate in and between air, soil, and
surface water. Point and area sources of
pollution are considered for atmospheric
dispersion. The soil model simulates
hydrology and pollutant transport and
fate in a soil column between the ground
surface and the upper saturated soil zone.
Surface water bodies modeled are rivers,
lakes, estuaries, and ocean outfall areas.
The pollutant fate and transport processes
simulated in the model are shown in
Figure 1.
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Implementation of the TOX-SCREEN
program was accomplished by linking
separate code sections for air, soil, and
aquatic processes. The air model uses a
modified Gaussian plume equation for
dispersion from a point source and a
simple box model for an area source.
Pollutant degradation is considered. The
soil model uses Level 3 of the Seasonal
Soil (SESOIL) model. SESOIL considers
rainfall, infiltration, surface runoff, evapo-
transpiration, ground-water runoff, and
chemical fate-transport processes. The
aquatic models(river, lake, etc.) assume a
completely mixed water body, advection
of pollutants with water, pollutant inter-
actions/decay, and adsorption to sus-
pended sediments.
Physical and chemical processes drive
transport of pollutants across air-soil and
soil-water interfaces. These media inter-
actions occur in TOX-SCREEN via such
processes as volatilization from water
and soil surfaces, wind resuspension,
atmospheric deposition, and surface and
subsurface runoff. Estimation of pollutant
fluxes across media interfaces also re-
quires the locations of water bodies
relative to atmospheric pollutant sources
and contaminated land areas. In TOX-
SCREEN, a generic positioning of media
is assumed in which the contaminated
soil area is always adjacent to any water
body. Water bodies are assumed to
intercept the atmospheric pollutant plume
almost immediately downwind of the
maximum concentration point. If no water
body is simulated, the land area immedi-
ately downwind of this point is assumed
to be the contaminated soil area.
Pollutant concentrations in the three
media reflect both direct input to any or all
of the media from specified sources, and
subsequent interaction via the processes
listed above. TOX-SCREEN simulates a
1-year period during which pollutant
source terms and environmental driving
forces (wind speed, velocity, rainfall, etc.)
may vary monthly. During a time step
(month), the source terms are assumed to
be constant.
Testing Procedures
The procedures used for evaluating
TOX-SCREEN were generally motivated
by two concepts. First, the multimedia
nature of TOX-SCREEN raises the ques-
tion of testing processes in several media
concurrently or individually. Lack of com-
prehensive multimedia data to test models
of this type is a problem. Separate testing
of component models, including the inter-
media transport models, is the alternative.
Second, the screening nature of TOX-
2
Source
(point, area)
Atmosphere
A dvect ion/Dispersion
Degradation
Deposition
(wet, dry)
Wind Erosion
Deposition
(wet,-dry)
Volatilization Erosion and
Surface Runoff
Volatilization
\
Direct '.Vr'.V- So/7
Source
i|^.'! Advection (leaching) Hydrolysis
:\ Diffusion fair)
':'/ Adsorption
'{,' Degradation
Water
Advection
Direct
Source
Cation Exchange \Ground- Water, Degradation
Complexation '.\ Runoff Adsorption
Acid/base spec/at ion
'•'. [SedimentBed]* \\\\\'•;'•':':':
[Ground Water}*
[ ]* Not Simulated
Figure 1. Pollutant pathways simulated in TOX-SCREEN.
SCREEN and its "conservatism" compli-
cate comparison testing with observed
data. Generally, environmental screening
level models are expected to be accurate
to within approximately one order of
magnitude. In addition to the simplifica-
tions incorporated into TOX-SCREEN,
intentional conservatism also was incor-
porated, which may tend to make its
predictions less accurate when compared
to observed pollutant concentrations.
Thus, validation of the model by standard
field testing methods must consider these
caveats.
As a result of the above problems, TOX-
SCREEN testing procedures were based
primarily on the individual process models
(intramedia and intermedia) rather than
comprehensive multimedia testing and
on review-evaluation of model algorithms.
Comparison with observed data and other
models was performed, but the overall
evaluation relied more on objective eval-
uation of the model components; compar-
isons made in this project to evaluate
TOX-SCREEN were:
• Review the transport-transformation
models in detail for each environmen-
tal medium (air, soil, water) and the
media interaction models (runoff, vola-
tilization, deposition); compare with
other models and state-of-the-art
methods for the processes; identify
major assumptions, and deficiencies.
• Compare predictions of individual
media models with observed data
(where available) and other model
predictions recognizing the temporal
and spatial simplifications of TOX-
SCREEN
• Perform sensitivity testing of selected
input parameters. This was performed
primarily for the atmospheric model
and atmospheric deposition param-
eters.
Conclusions
The evaluation led to a number of
conclusions regarding the capability of
TOX-SCREEN to perform reasonable
screening assessments of chemicals with
limited data available. In this analysis, an
expected accuracy for screening method-
ologies of one order of magnitude was
considered. Although testing included in
this analysis primarily involved individual
media, conclusions regarding the overall
multimedia capabilities can be made from
those tests as well as sensitivity testing
and subjective review of the model. Given
the lack of multimedia data available to
test this type of methodology and the
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deliberate overpredictive nature of TOX-
SCREEN, however, actual validation of
the overall system is difficult.
Specif ic conclusions of this study were:
• The TOX-SCREEN model is currently
an acceptable model for screening
chemicals that do notadsorbtoa large
extent. Both the aquatic and soil
models do not adequately consider
adsorption in their transport simula-
tions. High Kd pollutants in both media
will tend to be advected out of the
system too rapidly because bed sedi-
ments are not modeled in water bodies
and time of travel of soil pollutant to
ground water does not depend on
chemical properties.
• Testing of individual media models
indicated that reasonable choice of
input data resulted in predictions that
satisfied the "one order of magnitude"
criterion for screening accuracy in
most cases. TOX-SCREEN may be on
the boundary of acceptability based on
this criterion.
• The air model requires careful atten-
tion because of its increased overpre-
dictive potential (single wind direction).
Testing results indicate less confidence
that the one order of magnitude
criterion will be satisfied.
• Predicted pollutant concentrations in
rivers and lakes will generally be
acceptable, based on testing results
and analysis. Highly adsorbed chemi-
cals will be questionable because of
lack of a sediment bed and inappro-
priate simulation of suspended sedi-
ment concentrations.
• The soil model acceptability largely
remains to be determined. Compari-
sons between SESOIL predictions and
observed data were generally accep-
table, but testing and analysis suggest
deficiencies in soil layering and the
pollutant transport cycle
• Individual process models, although
necessarily simplified, are generally
based on current state-of-the-art
mechanistic methods forfate-transport
modeling. Empirical methods for some
soil hydrologic processes and pollutant
erosion processes are exceptions.
• All individual media models are rela-
tively convenient to use, that is, input
data requirements are minimal and
typically available. The SESOIL model
considers relatively more complex
chemical processes, but these pro-
cesses and associated input can gen-
erally be ignored, if desired.
• The code is somewhat confusing, and
confirmation of "correct" operation is
difficult. Mass conservation is a critical
question in TOX-SCREEN; verification
of mass balance should be incorpo-
rated. The documentation is appropri-
ate.
Recommendations
A number of improvements in TOX-
SCREEN were identified in this study and
are recommended for further considera-
tion. Many of these recommended im-
provements would require significant
effort to implement, and in some cases,
would slightly increase required user
input. If TOX-SCREEN is to be applied to a
wide spectrum of chemicals, however,
more consideration of adsorption (aquatic
and soil) should be included. Recommend-
ed changes considered to be most im-
portant include:
Atmospheric Dispersion Model
• Implement modification to buoyancy
flux calculations.
• Provide guidance to model users for
avoiding errors caused by parameter
sensitivity. This could include recom-
mendations to test sensitivity of key
parameters in all model applications.
So/7 Model
• Add additional layers to decrease
mathematical dispersion problems.
• Provide guidance to the user for
avoiding errors caused by sensitivity to
the soil disconnectivity parameters.
• Eliminate the threshold that prevents
"computation" of pollutant processes
at low concentrations.
• Modify the pollutant cycle to include
retardation m the computation of "time
of travel" to ground water.
Aquatic Models
• Account for key water column-sedi-
ment bed interaction processes in
lakes, rivers, and estuaries.
• Improve computation of suspended
sediment concentration by accounting
for low stream velocities.
Intermedia Transfer Models
• Provide additional user guidance for
estimating deposition parameters in-
cluding table of values for representa-
tive and classes of chemicals.
D o cumen t ation
• Improve user guidance for defaults
because screening involves the use of
uncertain data for new and poorly
characterized chemicals.
• Improve code structure (shorter sub-
routines, less complicated branching,
indentation).
• Modify output of information contained
in "Monthly Pollutant Concentrations
and Interaction Terms" to more closely
resemble SESOIL output tables.
General
• Investigate the modifications and level-
of-effort required to incorporate mass
balance computations.
• Incorporate a ground-water model.
• Provide guidance for performance of
sensitivity analysis as part of any
assessment. Provide list of key sensi-
tive parameters and typical sensitivity
ranges. This could also be incorporated
into the code by allowing the user to
entera range of values fora parameter
and have the model compute the
corresponding range of predictions.
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B. R. Bicknell, S. H. Boutwell. andD. B. Watson are with Anderson-Nichols & Co.,
Inc., Palo A/to, CA 94303.
L. A. Mulkey is the EPA Project Officer (see below).
The complete report, entitled "Testing and Evaluation of TOX-SCREEN Model,"
(Order No. PB 85-160 356/AS; Cost: $14.50. 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
•t, U.S. GOVERNMENT PRINTING OFFICE: 1985-559-016/27004
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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