United States
Environmental Protection
Agency
Office of Radiation and Indoor
Air(6603J)
Office of Solid Waste and
Emergency Response (5101)
9355.0-52FS
EPA/540/F-94-024
PB 94-963308
January 1996
Fact Sheet: Environmental Pathway
ModelsGround-Water Modeling in
Support of Remedial Decision
Making at Sites Contaminated with
Radioactive Material
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 Resto-
ration and Waste Management (EM), and the
Nuclear Regulatory 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 math-
ematical environmental models in the remediation
and restoration of sites contaminated by radioac-
tive substances.
The Working Group has published reports intended
to be used by technical staff responsible for identi-
fying and implementing flow and transport models
to support cleanup decisions at hazardous and
radioactive waste sites. This fact sheet is one of a
series of fact sheets that summarize the Working
Group's reports.
REPORT
Purpose
This report identifies the role of, and need for,
modeling in support of remedial decision making
at sites contaminated with radioactive materials. It
addresses all exposure pathways, but emphasizes
ground-water modeling at EPA National Priority
List (NPL) and NRC Site Decommissioning Man-
agement Program (SDMP) sites.
The primary objective of the report is to describe
when modeling is needed and the various processes
that need to be modeled. In addition, the report
describes when simple versus more complex models
may be needed to support remedial decision making.
Contents of Report
Following the introductory section. Section 2 pre-
sents a generic discussion of the role and purpose of
modeling in support of remedial decision making.
Section 3 describes the various ground-water flow
and transport processes that may need to be mod-
eled. A matrix is provided that describes ground-
water modeling needs as a function of site charac-
teristics and phase in the remedial process.
The report also includes two appendices. Appen-
dix A addresses the role of modeling within the
context of specific EPA, DOE, and NRC programs
pertaining to the remediation of sites contami-
nated with radioactive material. Appendix B
summarizes the characteristics of NPL and SDMP
sites contaminated with radioactive material and
defines the range of site conditions where model-
ing may be used to support remedial decision
making.
Role of Modeling
Modeling often is required to make informed deci-
sions about remedial actions at a site and to demon-
strate compliance with remedial criteria. In combi-
nation with field measurements, fate and effects
models are used to screen sites that may need
remedial action, support the design of environmen-
tal measurement /sampling programs, help under-
stand the processes that affect radionuclide behav-
ior at a site, and predict the effectiveness of alterna-
tive strategies for mitigating impacts.
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Models are not substitutes for data acquisition
and expert judgement. Models should not be
used until the specific objectives of the modeling
exercise are defined and the limitations of the
models are fully appreciated.
Why Modeling is Needed
The table below presents a list of the reasons for
modeling and the phases in the remedial process
when modeling likely will be needed. Many of
the reasons for modeling will affect the processes
that require modeling and the complexity of the
models.
What To Model
The table below presents an overview of the range
of site conditions, transport processes, doses, and
risks from all exposure scenarios and pathways
that may need to be modeled during the various
phases of the remedial process. These conditions
and processes also represent attributes of fate
and effects models.
Opportunities for Modeling
1
6
8.
9.
It is not feasible to perform field measurements due
to limited access, budget, or time.
There is concern that downgradient locations may
become contaminated in the future.
Field data atone are not sufficient to fully characterize
the nature and extent of contamination.
There is concern that conditions at a site may change,
affecting the fate and transport of contaminants.
There is concern that institutional control at the
site may be lost in the future.
Remedial actions are planned and there is a need to
predict the effectiveness of alternative remedies.
There is a need to predict when the concentration of con-
taminants at a location will decline to acceptable levels.
There is concern that individuals have been exposed to
contamination and it is desirable to reconstruct the doses.
There is concern that contaminants may be present but
below the tower limits of detection.
10. Field measurements reveal the presence of contaminants
and it is desirable to determine if and when other con-
taminants associated with the source may arrive and
at what levels.
Field measurements reveal the presence of contaminants
and it is desirable to identify their source.
There is a need to determine future environmental and
hearth impacts if the remedy is delayed.
13. There is a need to determine remedial action priorities.
14. Demonstrating compliance with regulatory requirements.
15. Estimating the benefit in a cost-benefit analysis of
alternative remedies.
16. Performing a quantitative dose or risk assessment.
17. There is uncertainty regarding the proper placement of
monitoring wells.
18. Developing a site conceptual model.
19. Developing a site characterization plan and
determining data needs.
20. There is a need to anticipate the potential doses to
remediation workers.
1 1
12
Scoping Characterization
O
O
O
o
o
o
o
o
o
o
o
o
o
o
o
o
Remediation
o
o
o
3
o
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Processes that Can Be Modeled
Source Term
Routing Emissions Transient or Accidental Emissions
Waste form/waste container performance Natural (flood, high winds, tornado, earthquake)
Natural barrier performance Antrtorpogenic (construction, agriculture, drilling)
Engineered barrier performance
Environmental Transport
Air Transport Processes Ground-Water TransportSaturated Zone
Suspension Miscible (mass transport, advectton, diffusion,
Evaporation dispersion)
Volatilization Immiscible
Dispersion Physical/chemical processes (decay, sorption,
Deposition dissolution/precipitation, acid/base
Radioactive decay and buildup reactions, complexation, hydrolysis/substitution,
redox reactions, density dependent flow)
Ground-Water Transport * Biologically mediated transport
Unsaturated Zone
Miscible Ground-Water TransportFractured Zone
Immiscible Nonpercolating
Vapor transport Percolating
Mass transport (advection, diffusion, Matrix diffusion effects
dispersion)
Physical/chemical processes (decay, Surface Water Transport
sorption, dissolution/precipitation, Dispersion
acid/base reactions, complexation, Deposition
hydrolysis/substitution, redox reactions, Sediment transport
density dependent flow) Radioactive decay and buildup
Biologically mediated transport
Exposure Scenario*
Postulated scenarios causing radiation Trespassers
exposure via various pathways Inadvertent intruder (construction, agriculture)
The no action alternative Routine and transient emissions
Alternative remedies Accidents
Exposure Pathways
Pathway or medium to which individuals Inhalation exposure to airborne, suspended,
and populations are exposed and resuspended radionuclides
External exposure to deposited radio- Ingestion of radionuclides in food and
nuclides drinking water
External exposure to airborne, suspended, Ingestion of contaminated soil and sediment
and resuspended radionuclides External exposure from immersion in contaminated
water
Doses
mrem/yr EDE to individuals Person rem/yr EDE to population
Public Health Impacts
Individual risk (acute, carcinogenic, mutagenic, Population impacts (acute, carcinogenic,
teratogenic risk per year and per lifetime) mutagenic, teratogenic effects per year)
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The products of the modeling process typically are
one or more of the following results for a broad
range of exposure scenarios:
Time-varying and time-averaged radionuclide
concentrations;
Radiation field in the vicinity of the radioactive
material;
Radionuclide flux;
Transit or arrival time of a radionuclide at a
receptor;
Volume of water contained within or moving
through a hydrogeological setting;
Radiation doses to individuals;
Radiation risks to individuals;
Cumulative radiation doses to the population
in the vicinity of the site;
Radiation doses and risks to remedial workers;
and
Uncertainties in the above impacts.
When Modeling May Not Be Needed
There are three general scenarios in which mod-
eling may be of limited value:
Presumptive remedies can be readily identi-
fied.
Available data indicate no problem.
The site is too complex to model realistically.
If a site is poorly characterized or poorly under-
stood, any simulation of the transport and im-
pacts of contaminants using mathematical mod-
els could be highly misleading. The use of mod-
els under such circumstances can help to support
only limited types of decisions, such as planning
and prioritizing activities. As a general rule of
thumb, it is prudent to continually question the
results of modeling and the potential conse-
quences of site decisions based on misleading
results, and consider what can be done to verify
modeling results.
Factors Affecting Model Complexity
The purpose of referring to simple and complex
sites and models is to alert the project manager to
circumstances when relatively complex processes
may need to be simulated so that the appropriate
resources and expertise are included in the
planning process. In general, analytical models
are considered simple models and numerical
models are considered complex, though there are
gradations of complexity within each category.
Analytical models are limited to simplified
representations of physical situations and
generally require only limited site-specific input
data. They are useful for screening sites to determine
data needs and the applicability of more detailed
numerical models. Numerical models generally
require a large quantity of data and an experienced
modeler-hydrologist.
The required complexity of the model is deter-
mined by a combination of five factors, the first
three of which generally have the greatest influ-
ence:
Objectives of the modeling;
Form, distribution, and composition of waste;
Environmental characteristics of the site;
Phase of the remedial process; and
Site demography and land use.
Modeling Objectives
Modeling objectives are often determined based
largely on existing regulatory requirements and
potential exposure scenarios at the site. Exposure
pathways that will need to be modeled initially are
determined during the planning phase, based on
judgement regarding the likelihood that a given
pathway may be an important contributor to risk.
For example, if available data indicate that the
contamination is buried or covered with water, the
suspension pathway need not be addressed unless
it is postulated that the buried material will be
removed or the water drains or evaporates.
Site Conditions
The environmental characteristics of remedial sites
are highly diverse. The sites containing radioactive
materials that are currently undergoing remedia-
tion include both humid and dry sites, sites with
and without an extensive unsarurated zone, and
sites with simple and complex hydrogeological
characteristics. These different environmental set-
tings determine the processes that need to be mod-
eled and required complexity of modeling.
In general, the need for complex models increases
with increasingly complex hydrogeology. How-
ever, if a conservative approach is taken at complex
sites, where transport through the unsarurated zone
is assumed to be instantaneous, then flow and
transport through the unsaturated zone may not
need to be modeled. Such an approach would be
appropriate at sites that are relatively small and
contamination is shallow and well defined. Under
these conditions, the remedy is likely to be removal of
the contaminated material, and use of conservative
screening models may be sufficient to support re-
medial decision making.
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At more complex sites, an understanding of the
physical system may allow an early determination
of die types of models appropriate for use at the site.
In general, relatively complex models may be re-
quired for complex hydrogeological characteristics
such as:
Thick unsarurated zone;
Layered, fractured, or heterogenous underlying
rocks;
Presence of surface water bodies on, or in the
vicinity of, the site;
Irregular land surface topography;
Sub-regional recharge and discharge areas; and
Processes or conditions that vary significantly
over time.
Waste Characteristics
Radioactive contaminants are present in a wide
variety of waste forms that may influence their
mobility. In most cases, the radionuclides of concern
are long-lived and the integrity of the waste form or
container cannot be relied upon for long periods of
time. Therefore, the source term often can be
modeled as a uniform point or areal source and the
waste form does not need to be accounted for,
allowing the use of relatively simpler models.
More complex geochemical models may be needed
to predict the performance of the waste container or
transport in a complex geochemical environment.
Such models would need to simulate the degrada-
tion rate of concrete, corrosion rate of steel, and
leaching rate of radionuclides associated with vari-
ous waste forms. To account for container and
waste form degradation, the model would need to
include a user-defined algorithm that estimates the
delay in contaminant release.
Certain radionuclides have properties that are dif-
ficult to model and may not be adequately simu-
lated with analytical models. For example, thorium
and uranium decay into multiple daughters whose
mobility may differ from their parents. Geochemi-
cal processes that affect radionuclide transport in-
clude: complexation of radionuclides with other
constituents; phase transformations of the radionu-
clides; adsorption and desorption; and radionu-
clide solubilities at ambient geochemical condi-
tions. To model these processes, complex geochemi-
cal models may be needed.
Phase of Remedial Process
The remedial process is divided into three phases: the
scoping and planning phase; the site characterization
phase; and the site remediation phase. In general, the
complexity of modeling will increase as the reme-
dial process proceeds.
During planning and scoping phase, only limited
site-specific data generally are available. There-
fore, modeling is limited to simple analytical mod-
els even if the characteristics of the waste and the
site indicate that more complex models eventually
may be needed. As a result, modeling during
scoping generally consists of screening-level calcu-
lations to identify potentially significant radionu-
clides and pathways of exposure using simplified,
conservative assumptions.
Site characterization is designed to determine the
nature and extent of contamination and the poten-
tial risks posed by the site. In general, simple
models may be adequate where: 1) the waste form
or engineered barriers are not accounted for; 2)
transport through the unsarurated zone is not ac-
counted for; and 3) the saturated zone is treated as
a homogeneous, isotropic medium. Any other as-
sumptions regarding the behavior of the waste or
site conditions will likely necessitate the use of
more complex models.
A method of predicting peak concentrations of
radionuclides emanating from a source and reach-
ing the water table is to model the movement of
ground water and radionuclides through the un-
sarurated zone. In some instances, the risk assess-
ment may require that radionuclide concentrations
be determined at a receptor located at some dis-
tance downgradient from the source. In this case a
model that can simulate flow and transport in the
saturated zone should be used.
During the site remediation phase, modeling pri-
marily is used to support the selection and imple-
mentation of alternative remedies and determine
the degree to which the remedy has achieved reme-
dial goals. Remedial alternatives can be grouped
into three categories: immobilization, containment,
and removal /destruction. Treatability studies, prior
experience, engineering judgement, and conserva-
tive design may be the only reliable methods for
ensuring the performance of a containment or im-
mobilization remedy.
The removal alternative is generally the most ex-
pensive remedy for long-lived radioactive contami-
nants. Though modeling is expensive and time con-
suming, it can be cost-effective if it convincingly
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demonstrates that remedies other than removal
will protect human health and the environment.
Physical and chemical processes that may need to
be modeled to support removal remedies include:
three dimensional flow and transport; matrix dif-
fusion; resaturation of the nodes; heat energy
transfer; sharp contrasts in hydraulic conductiv-
ity; multiple aquifers; and complex flow condi-
tions.
Land Use
The land use and demographic patterns at a site,
especially the location and extent of ground-water
use, affects potential exposure pathways and
modeling needs. At sites where the ground water
currently is being used, or may be used in the
future, complex ground-water models may be
needed to gain insight into plume arrival times
and geometries. At sites with multiple user
locations, two- and three-dimensional models
may be needed to realistically estimate the
likelihood that the contaminated plume will be
captured by wells located at different directions,
distances, and depths relative to the sources of
contamination.
Simple models typically are limited to estimating
the radionuclide concentration in the plume
centerline. If it is assumed that the receptors are
located at the plume centerline, a simple model
may be appropriate. Such an assumption often is
made even if a receptor is not currently present at
the centerline location because the results gener-
ally are conservative.
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 (6603))
U.S. Environmental Protection Agency
401 M St., S.W.
Washington, DC 20460
(202) 233-9396
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 Sub-
stances, 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.
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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