United States
Environmental Protection
Agency
Office of Radiation and Indoor
Air(6603J)
Office of Solid Waste and
Emergency Response (5101)
9355.0-53FS
EPA/540/F-94-025
PB 94-963309
January 1996
Fact Sheet: A Technical Guide
to Ground-Water Model
Selection at Sites Contaminated
with Radioactive Substances
Quick Reference Fact Sheet
BACKGROUND
Mathematical models that characterize the source,
transport, fate, and effects of hazardous and radio-
active materials are used to help determine cleanup
priorities and select remedial options at sites con-
taminated with radioactive materials.
A joint Interagency Environmental Pathway Mod-
eling Working Group has been established by the
EPA Offices of Radiation and Indoor Air (ORIA)
and Solid Waste and Emergency Response (OSWER),
the DOE Office of Environmental Restoration and
Waste Management (EM), and the Nuclear Regula-
tory Commission (NRC) Office of Nuclear Material
Safety and Safeguards (NMSS). The purpose of the
Working Group is to promote the more appropriate
and consistent use of mathematical environmental
models in the remediation and restoration of sites
contaminated by radioactive substances.
The Working Group has published reports intended
to be used by technical staff responsible for identify-
ing and implementing flow and transport models to
support cleanup decisions at hazardous and radio-
active waste sites. This fact sheet is one of a series of
fact sheets that summarize the Working Group's
reports.
REPORT
Purpose
This report describes methods for selecting ground-
water flow and contaminant transport models. The
selection process is described in terms of the various
site characteristics and processes requiring model-
ing and the availability, reliability, validity, and
costs of the computer codes that meet the modeling
needs.
Contents of the Report
The report is divided into five sections. Following
the introduction, Section 2 presents an overview of
the types of ground-water modeling decisions fac-
ing the site remediation manager. Section 3 de-
scribes the construction of a site conceptual model
and how it is used in the selection and use of ground-
water flow and contaminant transport codes. Sec-
tion 4 describes the various site characteristics and
ground-water flow and contaminant transport con-
ditions that require specific model capabilities. Sec-
tion 5 describes the review and evaluation process
for screening and selecting computer models that
are best suited to meet site-specific modeling needs.
The report also contains five appendices, including
a glossary (Appendix A). Appendix B provides a list
of electronic ground-water modeling resources. Ap-
pendix C describes the mathematical techniques
used in ground-water model codes. Appendix D
presents site- and code-related features of ground-
water flow and transport codes that should be con-
sidered when selecting a model.
Deciding Whether to Model
Ground-water flow and transport modeling can be
useful in making informed and defensible remedial
decisions. Site remediation managers must deter-
mine whether ground-water modeling is needed
and how modeling will support the remedial deci-
sion-making process.
The ground-water pathway may be considered a
potentially significant exposure pathway if
radionuclide concentrations in the ground water
exceed, or could eventually exceed, acceptable levels.
It is likely that ground-water modeling will be useful
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if the concentrations of radionuclides in ground
water downgradient from the site or in leachate at
the site exceed acceptable levels and the ground
water in the vicinity of the site is being used, or has
the potential to be used, as a source of drinking
water. The drinking-water standards set forth in 40
CFR141 currently guide remedial decision making.
Once it is determined that the ground-water expo-
sure pathway is potentially important, ground-wa-
ter flow and transport modeling can have a wide
range of uses in support of remedial decision mak-
ing. In combination with field measurements, fate
and effects models are used to screen sites that may
need remedial action, support the design of envi-
ronmental measurement/sampling programs, help
understand the processes that affect radionuclide
behavior at a site, and predict the effectiveness of
alternative strategies for mitigating impacts.
However, models are not substitutes for data acqui-
sition and expert judgement. Models should not be
used until the specific objectives of the modeling
exercise are defined and the limitations of the mod-
els are fully appreciated.
Developing a Site Conceptual Model
The first step in the model selection process is the
construction of a conceptual model of the site. The
conceptual model depicts the types of waste and
contaminants, where they are located, and how
they are being transported off site. The conceptual
model helps visualize the source and movement of
contaminants, potential receptors, and the ways in
which receptors may be exposed.
The components that make up the initial conceptual
model of the site include contaminant characteris-
tics, site characteristics (hydrogeology, land use,
demography), and exposure scenarios and path-
ways. As information about a site accumulates, the
site conceptual model is continually revised and
refined. The figure on the following page is an
example of a conceptual model (from EPA, Guid-
ance/or Conducting Remedial Investigations and Feasi-
bility Studies Under CERCLA, EPA/540/G-89/004,
OSWER Directive 9355.3-01, October, 1988).
Contaminant Characteristics. The site conceptual
model should address the characteristics of the
waste and contaminants, including the types and
chemical composition of the radionuclides, waste
form and containment, and source geometry (vol-
ume, area, depth, homogeneity). These characteris-
tics are used to model the source term, which is the
rate at which radionuclides are mobilized from the
source and enter the unsaturated and saturated
zones of a site.
Site Characteristics. The site conceptual model
should begin to address the complexity of the envi-
ronmental and hydrogeological setting. Complex
settings, such as complex lithology, a thick unsatur-
ated zone, or streams on site, generally indicate that
ground-water flow and radionuclide transport at
the site can be reliably simulated only by the use of
complex models.
The site conceptual model also should identify
where ground water currently is being used, or
may be used in the future, as a private or municipal
water supply. At sites with multiple user locations,
an understanding of ground-water flow in two or
three dimensions is needed to predict the likeli-
hood that the contaminated plume will affect active
wells.
Exposure Scenarios and Pathways. The site
conceptual model also should define exposure
scenarios and pathways at the site. Depending on
the regulatory requirements and the phase in the
remedial process, exposure scenarios to be modeled
can include any one or a combination of: the no
action alternative, trespassers, inadvertent
intruders, routine emissions, accidents, and
alternative remedies. For each scenario, an
individual or group of individuals may be exposed
by direct (dermal) contact, inhalation, or ingest ion.
The number of possible scenarios is virtually un-
limited. Scenarios that reasonably bound what
may occur at the site must be determined. The
scenarios selected for consideration define the re-
ceptor locations and exposure pathways that need
to be modeled.
Establishing Modeling Objectives
Modeling objectives are determined by site manag-
ers based largely on existing regulatory require-
ments and potential exposure scenarios at the site.
Exposure pathways that initially need to be mod-
eled are determined during the planning phase
based on judgement regarding the likelihood that a
given pathway may be an important contributor to
risk.
Modeling endpoints must be clearly defined because
the type of endpoint will help to determine the
ground-water model selected. In general, endpoints
are expressed as a concentration, such as pCi/L, in
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Example of a Site Conceptual Model
MUMMY
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fteUAU
MCCMMMM
•ecoNMMV
SOUACC
KCOMMMV
NCLCASC
MCCHANNM
MTHWAV
EXPOSURE
mure
RECEP1OR
Drum*
•no
Ttnkt
>,
1
\
Dull motor
VoMM
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INOESTKDN
INHALATION
DERMAL CONTACT
\
ground water at a specific location. Radionuclide
concentrations also can be expressed as a function of
time or as a time-averaged value. Some computer
codes convert ground-water radionuclide
concentrations to individual risk expressed in mrem/
yr or lifetime risk of cancer. Other codes present
results in terms of cumulative population impacts
expressed in person-rems/yr or total number of
cancers induced per year.
A baseline risk assessment at a site contaminated
with radioactive material is used to determine the
annual radiation dose to an individual drinking
water from a potentially contaminated well. The
endpoint in this case is the dose to an individual
expressed in mrem/yr. To estimate this dose, it is
necessary to estimate the average concentration of
radionuclides in the well water over a year. Mod-
eling objectives at each stage of the remedial inves-
tigation must be well defined early in the project.
The modeling objectives must consider the avail-
able data and the remedial decisions that the model
results are intended to support. The selected mod-
eling approach should not be driven by the data
available. If the modeling objectives demand more
sophisticated models and input data, the necessary
data should be obtained.
A mathematical model translates the conceptual
model into a series of equations that simulate the
fate and effects of the contaminants and displays
the results in a manner convenient to support
remedial decision making. The next step in the
model selection process requires detailed analysis
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of the conceptual model to determine the degree to
which specific contaminant and site characteristics
need to be modeled. Once these are determined,
the model selection process becomes a matter of
identifying the models that meet the defined
modeling objectives.
Remedial Phase
The greatest difficulty faced during model selection
is determining which capabilities are required to
support remedial decision making during each re-
medial phase at a specific site. Successful ground-
water modeling requires the selection of a com-
puter code that is consistent with the site character-
istics and modeling objectives, which are strongly
dependent on the phase of the remedial process.
The following figure presents an overview of how
the approach to modeling a site differs as a function
of the phase of the remedial process.
The most common model selection mistakes are
selecting codes that are more sophisticated than
appropriate for the available data or result desired,
and the application of a simpler code that does not
account for the dominant flow and transport
processes. The simplest code appropriate to the
problem should be used first and more sophisticated
models should be applied later until the modeling
objectives are achieved.
The remedial process generally parallels this progres-
sion. The data available in the early phases of the
remedial process may limit the modeling to one or
two dimensions, which may be sufficient to support
remedial decision making. Generally, it is during the
later phases of the investigation mat sufficient data
havebeen obtained to meetmore ambitious objectives
through complex three-dimensional modeling.
Source Characteristics
Computer codes can accommodate the spatial distri-
bution of contaminant source in a number of ways.
The most common are point sources (drums or tanks),
line sources (trenches), and area sources (ponds, la-
goons, landfills). How the spatial distribution of the
source term should be modeled is dependent on a
number of factors, the most important of which is the
scale at which the site will be investigated and mod-
eled. If the region of interest is very large compared to
the contaminant source area, even sizable lagoons or
landfills could be considered point sources.
Modeling objectives also are important in determin-
ing how the source term should be modeled. For
General Modeling Approach as
Modal Tratt
Accuracy
Temporal Representation of
Flow and Transport Processes
Dimensionality
Boundary and
Initial Conditions
Assumptions Regarding Row
and Transport Processes
Lithology
Methodology
Data Requirements
Scoping
Conservative
Approximations
Steady-State Flow and
Transport Assumptions
One Dimensional
Uncomplicated Boundary
and Uniform Initial
Conditions
Simplified Flow and
Transport Processes
Homogeneous/1 sotropic
Analytical
Limited
a Function of Remedial Phase
Characterization
Site-Specific
Approximations
Steady-State Flow/Transient
Transport Assumptions
1 ,2- Dimensional/
Quasi - 3-di mensional
Non-Transient Boundary
and Nonuniform
Initial Conditions
Complex Row and
Transport Processes
Heterogeneous/ Anisotropic
Semi-Analytical/Numerical
Moderate
Remediation
Remedial Action Specific
Transient Row and
Transport Assumptions
Fully 3-Dimensional/
Quasi-3-Dimensional
Transient Boundary and
Nonuniform
Initial Conditions
Specialized Row and
Transport Processes
Heterogcne ous/Am sotropic
Numerical
Extensive
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example, if simple scoping calculations are being
performed, modeling the source as a point will yield
conservative approximations of contaminant con-
centrations because of limited dispersion. How-
ever, if more realistic estimates of concentrations
and plume geometry are required, generally it will
be necessary to simulate the source term more accu-
rately, especially if the receptor is close to a rela-
tively large source.
Another consideration in code selection is whether
the source is to be modeled as an instantaneous,
continuous, or time-varying release. The need to
model the source as a continuous or time-varying
release primarily depends on the half-life of the
radionuclide relative to the time period of interest
and whether average or time-varying impacts of a
release are of interest. In general, the simplest
calculations, which assume a continuous release,
Model Selection Criteria
Administrative Data
Author
Objective (research, general use, education)
Organizations distributing the code
Organizations supporting the code
Date of first release
Current version number
Documentation
Hardware requirements
Accessibility of source code
History of use
Cost
Programming language
Scoping
Characterization
Phase of Remedial Process
Remediation
Site-Related Criteria
Multiple sources
Source geometry (line, point, area)
Release type (constant, variable)
Confined aquifers
Unconfined aquifers (water-table)
Aquitards
Multiple aquifers
Convertible (aquifer systems)
Two-phase: water/NAPL
Two-phase: water/air
Three-phase: water/NAPL/air
Flow: fully saturated
Flow: variably saturated
Temporal discretization (steady-state or transient) Speciation
Code-Related Criteria
Heterogeneity
Anisotropy
Fractures
Macropores
Layered soils
Dispersion
Advection
Diffusion
Density dependent
Partitioning: solid-gas
Partitioning: solid-liquid
Equilibrium isotherm
Radioactive decay and chain decay
Code usability
Quality assurance: code documentation
Quality assurance: code testing
Code support
Code output
Code dimensionality
Solution methodology
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are sufficient when determining the average annual
doses to ground-water users.
Aquifer Characteristics
The most common aquifer characteristics that influ-
ence code selection include confined aquifers, water-
table (unconfirmed) aquifers, convertible aquifers,
multiple aquifers/aquitards, heterogeneous aquifers,
anisotropic aquifers, fractures/macropores, and lay-
ered soils/ rocks. Recognizing when and if these pro-
cesses need to be modeled is critical to code selection.
Fate and Transport Processes
The transport of radionuclides is affected by various
physical and chemical processes, including advec-
tion, dispersion, matrix diffusion, retardation, and
radioactive decay. Geochemical processes are im-
portant primarily because they reduce the velocity
of the radionuclides relative to the ground water,
which increases transit time and results in addi-
tional radioactive decay.
Evaluating Models
This section presents the basic procedure that should
be followed in evaluating ground-water flow and
transport codes. Given mat an investigator under-
stands the various contaminant and site characteris-
tics that need to be modeled, there often will be
several suitable models in the scientific literature.
Ideally, each candidate should be evaluated in detail
to identify the one most appropriate for the particu-
lar site and modeling objectives.
The first aspect of the review concentrates on the
appropriateness of the particular code to meet mod-
eling objectives. The data requirements of the code
also should be consistent with the quantity and
quality of data available from the site. Next, the
reviewer must determine whether the code has been
properly tested for its intended use. Finally, the
code should have some history of use on similar
projects, be generally accepted within the modeling
community, and readily be available to the public.
The model evaluation process involves the follow-
ing steps:
1. Contact the author of the code and obtain docu-
mentation, other model-related publications, list
of users, and information on code validation.
2. Read all publications related to the model, includ-
ing documentation, technical papers, and testing
reports.
3. Contact code users to find out their opinions.
4. Complete a written evaluation using the criteria
shown in the list of Model Selection Criteria (see
table on page 5).
Much of the information needed for a thorough
evaluation can be obtained from the author or
distributor of the code. Inability to obtain the
necessary publications can be an indication that the
code is not well documented or is proprietary.
Inaccessibility of the documentation and related
publications should be grounds for considering the
code unacceptable.
Most of the items in the table should be described in
the code documentation, although excessive use of
modeling jargon may make some items difficult to
find. Some assistance from an experienced mod-
eler may be required to complete the evaluation.
Conversations with users also can help decipher
cryptic aspects of the documentation.
The evaluation process must rely on user opinions
and published information. User opinions are es-
pecially valuable in determining whether the code
functions as documented or has significant prob-
lems. In some instances, users have performed
extensive testing or are familiar with published
papers documenting the use of the code. In essence,
the evaluation process substitutes user experience
for hands-on testing to shorten the time to perform
a review.
CONTACTS
If you have any questions or want a copy of this or
other reports, contact:
Beverly Irla, Project Manager
Office of Radiation and Indoor Air (6603J)
U.S. Environmental Protection Agency
401 M St., S.W.
Washington, DC 20460
(202) 233-93%
Paul Beam
U.S. Department of Energy
Office of Environmental Restoration
EM-451/CLOVBLDG
19901 Germantown Road
Germantown, MD 20874-1290
(301)903-8133
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Sam Nalluswami
U.S. Nuclear Regulatory Commission
Office of Nuclear Material Safety and Safeguards
(T-7F27)
Washington, DC 20555
(301)415-6694
Superfund Hotline
U.S. Environmental Protection Agency
Office of Solid Waste and Emergency Response
401 M Street, SW (5203G)
Washington, DC 20460
(800) 424-9346
REPORTS
Computer Models Used to Support Cleanup Decision-
Making at Hazardous and Radioactive Waste Sites, EPA
402-R-93-005, March 1993. Also available from the
National Technical Information Center (NTIS), (703)
487-4650, PB93-183333/XAB.
Environmental Characteristics of EPA, NRC, and DOE
Sites Contaminated with Radioactive Substances, EPA
402-R-93-011, March 1993. NTIS,PB93-185551/XAB.
Environmental Pathway Models — Ground-Water
Modelling in Support of Remedial Decision-Making at
Sites Contaminated with Radioactive Material, EPA 402-
R-93-009, March 1993. NTIS, PB93-196657/XAB.
Technical Guide to Ground-Water Model Selection at
Sites Contaminated with Radioactive Substances, EPA
402-R-94-012, September 1994. NTIS, PB94-205804/
XAB.
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