United States
Environmental Protection
Agency
Office of Health and
Environmental Assessment
Washington DC 20460
Research and Development
EPA/600/S8-87/042 May 1988
Project Summary
Selection Criteria for
Mathematical Models Used in
Exposure Assessments:
Surface Water Models
Tom J. McKeon and John J. Segna
Prior to the issuance of the Guide-
linos for Estimating Exposures in 1986,
the U.S. Environmental Protection
Agency (EPA) published proposed
guidelines in the federal Register for
public review and comment. The pur-
pose of the guidelines is to provide a
general approach and framework for
carrying out human and nonhuman
exposure assessments for specific
pollutants. As a result of the review
process, four areas were identified that
required further research. One of these
was the area of selection criteria for
mathematical models used in exposure
assessment.
The purpose of this document is to
present criteria that provide a means
for selecting the most appropriate
mathematical model(s) for conducting
an exposure assessment related to
surface water contamination.
A concerted effort was made to
provide general background informa-
tion regarding surface water flow and
contaminant transport and to charac-
terize the important assumptions and
limitations of existing models. Included
in this document is a detailed summary
matrix and descriptions of 10 runoff
models, 12 surface water flow models,
and 12 contaminant transport models
that have been used previously by EPA
to study surface water quality prob-
lems. General guidelines and principles
for model selection are presented, such
as the overview of the modeling pro-
cess and important issues related to
model selection (e.g., familiarity,
model reliability, model selection vs.
model application). Following the
general guidelines is a step-by-step
approach for identifying the appropri-
ate model(s) to use in a specific
application.
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).
Background
In the exposure assessment field,
many of the assessors involved in either
the development or review of exposure
assessment documents containing
modeling results do not necessarily
understand the mathematical equations
that influence model values. The purpose
of this report is not to make the reader
an "instant" expert in surface water
modeling, but instead to give the reader
a basic understanding of the theory that
underlines the modeling process.
Introduction
In the last three decades there has
been a dramatic increase in the produc-
tion and use of chemicals in our society.
These chemicals have been developed
and applied to a variety of beneficial uses
in domestic, industrial, and agricultural
applications. In some cases these chem-
icals have had unexpected adverse
effects. As a result, concern has grown
over the effects of some chemicals, both
at the point of use or application and in
distant areas to which the chemicals may
be transported via various environmental
pathways.
As the process of regulating and
controlling the release of these poten-
tially hazardous chemicals into the
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environment becomes a more complex
task, the EPA has focused its energy
towards a risk assessment/risk reduc-
tion framework for making regulatory
decisions. Part of this risk assessment
process has been the development and
publication of the Guidelines for Estimat-
ing Exposures. These guidelines "provide
the Agency with a general approach and
framework for carrying out human or
nonhuman exposure assessments for
specific pollutants."
Estimating Exposure Areas
The five major areas to be evaluated
when estimating exposure to environ-
mental contaminants are:
1. Source Assessment—a characteri-
zation of a source of contamination;
2. Pathways and Fate Analysis—des-
cription of how a contaminant may
be transported from the source to the
potentially exposed population;
3. Estimation of Environmental Con-
centration—an estimate using mon-
itoring data and/or modeling of
contaminant levels away from one
source where the potentially
exposed population is located;
4. Population Analysis—a description
of the size, location, and habits of
potentially exposed human and
environmental receptors; and
5. Integrated Exposure Analysis—the
calculation of exposure levels and
the evaluation of uncertainty.
The process of estimating the environ-
mental concentration of a contaminant
plays a significant role in exposure
assessment. Often the most critical
element is the estimation of a pollutant
concentration at an exposure point. This
estimation is usually carried out by
means of a combination of field data and
mathematical modeling results. In the
absence of field sampling data, this
process relies primarily on the results of
mathematical models
An ideal exposure assessment model
would account for multiple emission
sources, estimate contaminant concen-
' trations in all media (air, water, food, and
soil) resulting from emission sources,
define multiple pathways of exposure,
and estimate exposure to humans,
plants, and animals. An ideal model
would also contain methods for estima-
ting the simultaneous exposure of
humans, plants, andanimalstoa number
of contaminants so that synergistic or
antagonistic effects could be estimated
as part of the risk assessment. Although
the ideal model is not available,
component models that address various
aspects of exposure assessment are
available. Therefore, decisions based on
modeling analysis may require the use
of multiple models to account for pollu-
tant transport through different media.
This potential need for multiple model
usage in exposure assessments was a
major issue during the development of
the exposure assessment guidelines. An
important concern of the exposure
assessment guidelines was to ensure
that the most appropriate mathematical
models are considered for each exposure
scenario.
Model Selection Process
In general, the mathematical model
selection criteria are written for the
exposure assessor whose goal is to
understand the exposure-related situa-
tion, to consider the media impacted by
the chemical (surface water, ground
water, and/or air) and then to under-
stand the important physical mecha-
nisms (absorption, volatilization, etc.)
that are the controlling factor(s) for that
situation. Once this is achieved, the
assessor can then determine which
model(s) best describes the problem that
needs to be resolved. To achieve this
model selection goal could be a complex
task; however, basic model selection
criteria were developed to sort out the
necessary level of complexity of the
analysis and the model(s) to choose in
the selection process.
This report outlines five general steps
which can be identified in the modeling
process:
1. Problem Characterization—the
exposure assessor clearly identifies
the exposure assessment study
objectives and constraints;
2. Site Characterization—the assessor
reviews available data, identifies the
processes of interest, determines if
a modeling study is necessary, and
if so, identifies the data needs and
fills those needs. The results of this
step will determine the technical
specifications for the model
selection;
3. Model Selection Criteria—the selec-
tion of the most appropriate model(s)
is the most important step in the
process;
4. Code Installation—if the mod'
selected is a computer code, the co
is installed on the computer and
tested for its ability to reproduce
accepted solutions to standard prob-
lems; and
5. Model Application—the model uses
site characterization data as inputfor
the exposure assessment simu-
lation.
These five general steps are not the
model selection criteria but rather the
overall process by which a problem is
identified and a model selected tc
perform an exposure assessment study.
Model selection is listed as the third step
in this process. The two previous steps
Problem Characterization and Site Char-
acterization are crucial in the selectior
of an appropriate model(s). While the
steps can be considered sequential ir
nature, it is important to recognize
interactions and feedback mechanism!
between them. For instance, knowledg<
of the model selection criteria is impor
tant to assure that site characterizatior
is adequate and properly formatted. Ar
understanding of the code installatioi
procedures is required for proper sche
duling and resource allocation. Familiar
ity with candidate models is needed ti
assure that site characterization provide
necessary input data.
Model Selection Criteria
The proper selection of a model i
essential to the successful simulation c
an exposure assessment. The third ste
of the five general steps defines th
modeling process. Model Selectio
Criteria. This criterion has three factor
which dictate the level of complexity t
the model chosen in the selectio
process:
1. Objective Criteria—specify th
nature and intent of the analysis 1
be performed;
2. Technical Criteria—specify the sit(
specific processes to be simulated t
the model; and
3. Implementation Criteria—speci
the quality assurance and documei
tation requirements. •
The first level of consideration wh«
selecting a mathematical model
related to the objectives of the stud
Based on the objectives, the analyst cs
limit the choices to either simple an
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lytical solutions/models or more com-
plex numerical models.
The Ojbective Criteria refer to the level
of modeling detail required to meet the
objectives of the study. There are many
different objectives of modeling studies;
however, in the context of model selec-
tion, all objectives can be classified in
two broad categories: (1) to perform a
screening-level study (generic or general
analysis with minimal data require-
ments) or (2) to perform a detailed level
study (site specific with usage of complex
models and more data intensive require-
ments).
The second level of consideration
when selecting a mathematical model is
the Technical Criteria. Technical criteria
are those model criteria related to the
mathematical model's ability to simulate
the site-specific contaminant transport
and fate phenomena of importance.
These criteria are based on the physical,
chemical, and biological characteristics
of the site and the contaminant of
interest. The characteristics of the site
and the processes that need to be
simulated are determined from the
hydrogeologic and contaminant data and
the conceptual model of the site.
The third level of consideration when
selecting a mathematical model is
related to the Implementation Criteria.
The implementation criteria are those
criteria dependent on the ease with
which a model can be obtained and its
acceptability demonstrated. Whereas the
technical criteria identify models capable
of simulating the relevant phenomena
within the specified environmental
setting, the implementation criteria
identify documentation, verification, and
validation requirements, and ease of use
so that the model selected provides
accurate, meaningful results.
Other general factors related to model
selection which should never override
technical or implementation criteria
include data availability, schedule,
budget, staff and equipment resources,
and level of complexity of the system(s)
under study.
The model selection criteria provide
the general framework for a series of
specific questions concerning the model
selection goal. The results to the ques-
tions would then be matched against a
model summary matrix table. Models
which then appear to best reflect the
exposure problem should then be con-
sidered a likely candidate in the exposure
assessment analysis.
The EPA author, John J. Segna (also the EPA Project Officer, see below),
is with Office of Health and Environmental Assessment, U.S. Environmental
Protection Agency, Washington, DC 20460; and Tom J. McKeon is with ICF
Northwest. Richland, WA 99352.
The complete report, entitled "Selection Criteria for Mathematical Models Used
in Exposure Assessments: Surface Water Models," (Order No. PB 88-139
928/AS; Cost: $19.95, 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:
Office of Health and Environmental Assessment
U.S. Environmental Protection Agency
Washington, DC 20460
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