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Air And Radiation
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EPA 402-R-93-009
March 1993
Pathway Model
Ground-Water Modeling
In Support Of Remedial
Decision-Making At Sites
Contaminated With Radioactive
Material
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ENVIRONMENTAL PATHWAY MODELS -
GROUND-WATER MODELING IN SUPPORT OF
REMEDIAL DECISION-MAKING AT SITES
CONTAMINATED WITH RADIOACTIVE MATERIAL
March 1993
A Cooperative Effort By
Office of Radiation and Indoor Air
Office of Solid Waste and Emergency Response
U.S. Environmental Protection Agency
Washington, DC 20460
Office of Environmental Restoration
U.S. Department of Energy
Washington, DC 20585
Office of Nuclear Material Safety and Safeguards
Nuclear Regulatory Commission
Washington, DC 20555
CVJ
WASHINGTON, D.C. 20400
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PREFACE
This report is the product of the Interagency Environmental Pathway Modeling Workgroup.
The Workgroup is composed of representatives of the Environmental Protection Agency
Office of Radiation and Indoor Air and Office of Solid Waste and Emergency Response, the
Department of Energy Office of Environmental Restoration, and the Nuclear Regulatory
Commission Office of Nuclear Material Safety and Safeguards. This report is one of several
consensus documents being developed cooperatively by the Workgroup. These documents
will help bring a uniform approach to solving environmental modeling problems common to
these three participating agencies in their site remediation and restoration efforts. The
conclusions and recommendations contained in this report represent a consensus among the
Workgroup members.
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ACKNOWLEDGEMENT
This project is coordinated by the Office of Radiation and Indoor Air, U.S. Environmental
Protection Agency, Washington, D.C. and jointly funded by the following organizations:
EPA Office of Radiation and Indoor Air (ORIA)
EPA Office of Solid Waste and Emergency Response (OSWER)
DOE Office of Environmental Restoration and Waste Management (EM)
NRC Office of Nuclear Material Safety and Safeguards (ONMSS)
The project Steering Committee for this effort includes:
EPA
Beverly Irla, EPA/ORIA - Project Officer
Ron Wilhelm, EPA/ORIA
Kung-Wei Yeh, EPA/ORIA
Loren Henning, EPA/OSWER
DOE
Paul Beam, DOE/EM
NRC
Harvey Spiro, NRC/ONMSS
Contractor Support
John Mauro, S. Cohen & Associates, Inc.
Paul D. Moskowitz, Richard R. Pardi, Brookhaven National Laboratory
Consultants
David Back, Hydrogeologic, Inc.
Jim Rumbaugh, III, Geraghty & Miller, Inc.
We acknowledge the technical support and cooperation provided by these organizations and
individuals. We also thank all reviewers for their valuable observations and comments.
u
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CONTENTS
Preface i
Acknowledgement ii
Acronyms v
1. Introduction 1-1
1.1 Purpose and Scope of the Joint EPA/DOE/NRC Program on Modeling . . 1-1
1.2 Purpose and Scope of This Report 1-5
1.3 Key Terms 1-6
1.4 Organization of the Report 1-7
2. The Need For and Role of Fate and Effects Modeling on a Remedial Project ... 2-1
2.1 Why Do We Need to Model? 2-1
2.2 What Determines Modeling Needs? 2-3
2.3 What Needs to Be Modeled? 2-5
2.4 What Scenarios Need to Be Modeled? 2-6
2.5 What Pathways Need to Be Modeled? 2-8
2.6 When Is Modeling Not Needed or Inappropriate? 2-9
3. Processes that Need to be Modeled: The Need for Complex Versus Simple
Ground-water Flow and Transport Models 3-1
3.1 Site Conditions and Processes that Need to Be Modeled 3-3
3.2 Reasons for Modeling 3-3
3.3 Characteristics of the Waste - Modeling the Source Term 3-9
3.3.1 Waste Form and Containment 3-10
3.3,2 Physical and Chemical Properties of the Radionuclides 3-11
3.3.3 Geochemical Setting 3-13
3.4 Environmental Characteristics - Modeling Flow and Transport 3-14
3.4.1 Sub-Regional Scale Characteristics 3-14
3.4.2 Detailed Hydrogeological Characteristics of the Site 3-17
3.5 The Phase of the Remedial Process 3-21
3.5.1 Phase 1 - Planning and Scoping 3-22
3.5.2 Phase 2 - Site Characterization 3-22
3.5.3 Phase 3 - Remediation 3-24
3.6 Land Use and Demography 3-29
4. Summary and Conclusions 4-1
References R-l
Appendix A Regulatory Requirements and Guidelines Pertaining to Fate and
Effects Modeling A-l
Appendix B Environmental Characteristics of NPL Sites Contaminated with
Radioactive Waste and NRC Sites in the SDMP B-l
111
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TABLES
2-1 Why is Modeling Needed? 2-2
3-1 Transport Processes 3-4
3-2 Matrix of Reasons for Modeling 3-7
3-3 Ground-water Flow and Transport Processes that May Need to be Modeled
and Site-specific Information Needed to Identify the Processes to be Modeled . . 3-23
FIGURES
1-1 Exposure Pathways 1-2
IV
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ACRONYMS
A/E Architect/Engineer
AEA Atomic Energy Act
ARARs Applicable or Relevant and Appropriate Regulations
CERCLA Comprehensive Environmental Response, Compensation, and Liability Act
CFR Code of Federal Regulations
CMA Corrective Measures Assessment
CMI Corrective Measures Investigation
D&D Decontamination and Decommissioning
DOD Department of Defense
DOE Department of Energy
EDE Effective Dose Equivalent
EM Office of Environmental Restoration and Waste Management
EPA Environmental Protection Agency
FFA Federal Facility Agreement
FERP Fernaid Environmental Remediation Project
FMPC Feed Materials Production Center
FS Feasibility Study
GSA General Services Administration
HHEM Human Health Evaluation Manual
IAG Interagency Agreement
INEL Idaho National Engineering Laboratory
MFDS Maxey Flats Disposal Site
NCP National Contingency Plan
NORM Naturally Occurring Radioactive Materials
NPL National Priorities List
NRC Nuclear Regulatory Commission
ONMSS Office of Nuclear Material Safety and Safeguards
ORP Office of Radiation Programs
OSC On-Scene Coordinator >
OSWER Office of Solid Waste and Emergency Response
RCRA Resource Conservation and Recovery Act
RFI RCRA Facility Investigation
RI Remedial Investigation
RPMs Remedial Project Managers
ROD Record of Decision
SARA Superfund Amendments and Reauthorization Act of 1986
SDMP Site Decommissioning Management Program
TEDE Total Effective Dose Equivalent
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1. Introduction
1.1 PURPOSE AND SCOPE OF THE JOINT EPA/DOE/NRC PROGRAM ON
MODELING
A joint program is underway between 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 NRC Office of Nuclear Material Safety
and Safeguards (ONMSS). The purpose of this program is to promote the appropriate and
consistent use of mathematical models in the remediation and restoration process at sites
containing, or contaminated with, radioactive materials. This report is one of a series of
reports designed to accomplish this objective. The report specifically addresses the role of,
and need for, modeling in support of remedial decision-making at sites contaminated with
radioactive material. Other reports in this series will address the selection and application of
models.
This report is intended to be used by the Remedial Project Manager (RPM) at National
Priorities List (NPL) sites or the equivalent at non-NPL sites containing radioactive
materials. It is also intended to be used by geologists and geoscientists responsible for
identifying and implementing ground-water flow and transport models at such sites.
The overall joint program is concerned with the selection and use of mathematical models
that simulate the environmental behavior and impacts of radionuclides via all potential
pathways of exposure, including the ah-, surface water, ground water, and terrestrial
pathways. Figure 1-1 presents an overview of the various exposure pathways.
Though the overall program is concerned with all pathways, it has been determined that, due
to the magnitude of the undertaking, it would be appropriate to divide the program into
smaller, more manageable phases, corresponding to each of the principal pathways of
exposure. It was also determined that, in this, the first phase of the project, greatest
attention would be given to the ground-water pathways.
Ground-water pathways were selected for consideration first for several reasons. At many
sites currently regulated by the Environmental Protection Agency (EPA) and the Nuclear
Regulatory Commission (NRC) or owned by the Department of Energy (DOE), the principal
concern is the existence of, or potential for, contamination of the underlying aquifers.
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In addition, compared to the air, surface water, and terrestrial pathways, ground-water
contamination is more difficult to sample and monitor, thereby necessitating greater
dependence on models to predict the locations and levels of contamination in the
environment. The types of models used to simulate the behavior of radionuclides in ground
water must be more complex than surface water and atmospheric pathway transport models
in order to address the more complex settings and the highly diverse types of settings
associated with different sites. As a result, the methods used to model ground water are not
as standardized as those used in surface water and air dispersion modeling, and, therefore,
there is considerably less regulatory guidance regarding appropriate methods for performing
ground-water modeling.
In planning this project, it was also necessary to make judgments regarding the categories of
sites that should be addressed. The categories of sites are relevant to the selection and use of
models because they define the range of environmental settings of concern, the types and
forms of the radioactive material requiring modeling, and, most importantly, the regulatory
structure within which models are used to support remedial decisions. Investigations
performed by the Agency have identified thousands of sites that contain, or are potentially
contaminated with, radioactive materials and that may require some remediation. These sites
include:
• Federal facilities under the authority of 18 federal agencies, predominantly
consisting of DOE and Department of Defense (DOD) sites and facilities, and
sites listed on the National Priorities List (NPL),
• Facilities licensed by the NRC and NRC Agreement States,
• State-licensed facilities,
• Facilities and sites under the authority of the states but not governed by
specific regulations. These include sites containing elevated levels of naturally
occurring radioactive materials (NORM).
All of these sites are of interest to this program. However, some categories of facilities and
sites are not considered because they are being designed and licensed specifically to receive
radioactive material for storage and disposal; i.e., licensed low-level and high-level waste
storage and disposal sites. These sites are being managed within a highly structured
regulatory context, and, though models are used to support the siting and design of such
facilities, they are not remedial sites.
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It was also necessary to limit the range of the categories of sites of interest to this program in
order to keep their number at a manageable size. Specifically, it was determined that this
phase of the program will be limited to (1) sites currently listed on the National Priorities
List (NPL) that contain radioactive materials, and (2) sites currently or formerly licensed by
the NRC that are part of the Site Decommissioning Management Program (SDMP). The
NRC established the SDMP to decontaminate 46 facilities that require special attention by the
NRC staff (NRC 91). As will be discussed, ground-water modeling needed to support
remedial decision-making at NPL sites containing radioactive materials is in many ways
similar to the ground-water modeling needs of the SDMP.
These categories of sites were selected because decisions are currently being made regarding
their decontamination and remediation. In many cases, models are being used to support
decision-making and demonstrate compliance with remediation goals. Though the program is
designed to address the modeling needs of these categories of sites, the information contained
in this report and the other reports prepared under this program should apply, to varying
degrees, to the full range of categories of sites concerned with the disposition of radioactive
contamination. In addition, much of the material may also apply to sites contaminated with
chemically hazardous substances.
In order to meet its mission of promoting the appropriate and consistent use of mathematical
models in the remediation and restoration process at sites containing, or contaminated with,
radioactive materials, the overall program is designed to achieve the following four
objectives:
1. Describe the roles of modeling and the modeling needs at each phase in the
remedial process;
2. Identify models in actual use at NPL sites and facilities permitted under
RCRA, at DOE sites, and at NRC sites undergoing decontamination and
decommissioning (D&D);
3. Produce detailed critical reviews of selected models in widespread use; and
4. Produce informal guidance for Remedial Project Managers (RPMs), On-Scene
Coordinators (OSCs), or their equivalents to use in selecting and reviewing
models used in the remediation and restoration process.
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1.2 PURPOSE AND SCOPE OF THIS REPORT
This report, which is the third in a series of reports planned for this program, identifies the
role of modeling and modeling needs in support of remedial decision-making at sites
contaminated with radioactive materials. For the reasons previously cited, the discussion of
modeling needs focuses primarily on ground-water modeling at NPL and SDMP sites.
The two previous reports in this series include a survey of model users and a summary of the
characteristics of selected sites contaminated with radioactive materials. The model survey
report is entitled, "Computer Models Used to Support Cleanup Decision-Making at
Hazardous and Radioactive Waste Sites." The site characterization report is entitled,
"Environmental Characteristics of EPA, NRC, and DOE Sites Contaminated with
Radioactive Substances."
The primary objective of this report is to describe when modeling is needed and the various
processes that need to be modeled. In order to accomplish this objective, modeling needs are
defined in terms of the various factors that determine the need. These include:
* The existing regulatory requirements that apply to the remediation of different
categories of sites.
The regulatory structure for sites on the NPL is different than that for sites in
the SDMP. As a result, the role of and need for modeling may be expected to
differ depending on specific regulatory requirements.
• The phase in the remedial process.
The role of modeling may be quite different during the early scoping and
planning phases of a remedial project, as compared to the later phases, when
remedial decisions are made.
• Site characteristics.
The need for modeling and the types of models that are needed depend, in
part, on the characteristics of the site. For example, for ground-water
modeling, waste form, the chemical and physical characteristics of the
radionuclides, and the hydrogeological and demographic setting will, in part,
determine modeling needs.
1-5
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In this report, modeling needs are described in terms of the applicable regulatory
requirements, the phase in the remedial process, and the site characteristics that apply to sites
on the NPL contaminated with radionuclides and sites in the SDMP.
This report is intended for use by Remedial Project Managers at NPL sites or their
equivalent at SDMP sites. It is assumed that the RPM or equivalent is not a modeler but is
responsible for deciding when modeling is needed, authorizing the selection and
implementation of the models, and determining how the results of the models will be used in
support of the remedial process.
1.3 KEY TERMS
The following are definitions of some key terms used throughout the report.
Conceptual Model. The conceptual model of a site is a flow diagram or sketch of a site and
its setting that depicts the types of waste and where they are located, how the waste is being
transported offsite by runoff, percolation into the ground and transport in ground water, or
suspension or volatization into the air and transport by the prevailing meteorological
conditions. The conceptual model also attempts to help visualize the direction and path
followed by the contaminants, the actual or potential locations of the receptors, and the ways
in which receptors may be exposed, such as direct contact with the source, ingestion of
contaminated food or water, or inhalation of airborne contaminants. As information
regarding a site accumulates, the conceptual model is continually revised and refined.
Mathematical Model. A mathematical model translates the conceptual model into a series of
equations which simulate the fate and effects of the contaminants as depicted in the
i
conceptual model at a level of accuracy that can support remedial decision-making.
Computer Code. A computer code is simply a tool that is used to solve the equations which
constitute the mathematical model of the site and display the results in a manner convenient
to support remedial decision-making.
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1.4 ORGANIZATION OF THE REPORT
Following this introduction, Section 2 presents a generic discussion of the role and purpose
of modeling in support of remedial decision-making. The section discusses why modeling is
needed and when it is needed. Beginning with Section 3, the report focusses on ground-
water modeling. Section 3 describes the various ground-water flow and transport processes
that may need to be modeled. A matrix is provided that describes ground-water modeling
needs as a function of site characteristics and phase in the remedial process. Other reports
being prepared under the joint program describe methods for selecting and applying models
that meet these needs.
The report also include two appendices. Appendix A is an extension of Section 2 in that it
addresses the role of modeling; however, it does so within the context of specific regulations
and programs. An overview is provided of the current and developing CERCLA/RCRA
requirements and guidelines that establish the role of, and need for, modeling in support of
remedial decision-making. In addition, the principal DOE Orders for meeting these
requirements are summarized. Appendix A also draws parallels between modeling required
to support CERCLA/RCRA decision-making and the NRC modeling requirements needed to
support remedial decision-making on the SDMP. The purpose of Appendix A is to provide
background information on modeling needs within the context of specific EPA, DOE, and
NRC requirements and programs pertaining to the remediation of sites contaminated with
radioactive material.
Appendix B summarizes the characteristics of the current NPL sites contaminated with
radioactive material and the sites currently being addressed by the NRC on the SDMP. The
purpose of Appendix B is to define the range of site conditions where modeling may be used
to support remedial decision-making. This information is used in Section 3, which addresses
the need to use simple versus complex models to simulate flow'and transport processes.
1-7
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2. The Need For and Role of Fate and Effects Modeling on a Remedial Project
Modeling needs on a remedial project can be discussed from a number of perspectives,
including: why do we need to use models, what determines modeling needs, what needs to be
modeled, when is modeling needed, and when is it not needed? These questions are different
but interrelated. This section provides generic responses to these questions.
2.1 WHY DO WE NEED TO MODEL?
Modeling is one of the techniques used to fulfill the regulatory requirements that apply to a
specific site or category of sites. Ultimately, decisions regarding the selection and
implementation of models on a remedial project are driven by the applicable regulations and
regulatory guidelines. As discussed in Appendix A, currently no regulations pertaining to
the early scoping, characterization, or remediation of NPL or SDMP sites explicitly require
modeling. However, in order to make informed decisions about remedial actions at a site
and in order to demonstrate compliance with remedial criteria, modeling is often required.
In general, models are used to evaluate existing data or assumptions and to make testable
predictions about relationships among parameters and/or the behavior and actual or potential
impacts of contaminants in the environment. Models are primarily hypothesis generators,
which must be tested and supported by empirical field data. When used in the proper
context of hypothesis generators, models can greatly assist in focusing expensive and time-
consuming field sampling and monitoring activities. Table 2-1 presents the principal reasons
why modeling may be needed on a project. These reasons for modeling can surface during
any phase of the remedial process. However, as will be discussed in Section 3, some of
these reasons for modeling are more likely to occur during specific phases of a remedial
project.
In general, a combination of field measurements and fate and effects models is used at sites
contaminated with radioactive material to determine the average and tune-varying
radionuclide concentrations in various media, the rate of transport of the radionuclides in the
media, radiation fields, and the radiation doses and risks to individuals and populations
exposed to the radioactive material. Models are used to screen sites to determine the need
for remedial activities and remedial priorities. They are also used to support the design of
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Table 2-1. Why is Modeling Needed?
1. When it is not feasible to perform field measurements; i.e.,
• Cannot get access to sampling locations
• Budget is limited
• Time is limited
2. When there is concern that downgradient locations may become contaminated at some time in the
future; i.e.,
• When transport times from the source of the contamination to potential receptor locations
are long relative to the period of time the source of the contaminant has been present.
• When planning to store or dispose of waste at a specific location and impacts can be
assessed only through the use of models.
3. When field data alone are not sufficient to characterize fully the nature and extent of the
contamination; i.e.,
• When field sampling is limited in space and time, and
• When field sampling results are ambiguous or suspect.
4. When there is concern that conditions at a site may change, thereby changing the fate and transport
of the contaminants; i.e.,
• seasonal changes in environmental condition
• severe weather (e.g., floods, tornadoes)
» accidents (e.g., fires)
5. When there is concern that institutional control at the site may be lost at some time in the future
resulting in new or unusual exposure scenarios, or a change in the fate and transport of the
contaminants; i.e.,
* trespassers
* inadvertent intruder (construction/agriculture)
• human intervention (drilling, excavations, mining)
6. When remedial actions are planned and there is a need to predict the effectiveness of alternative
remedies.
7. When there is a need to predict the time when the concentration of specific contaminants at specific
locations will decline to acceptable levels.
8. When there is concern that at some time in the past individuals were exposed to elevated levels of
contamination and it is desirable to reconstruct the doses.
9.
When there is concern that contaminants may be present but below the lower limits of detection.
10. When field measurements reveal the presence of some contaminants, and it is desirable to
determine if and when other contaminants associated with the source may arrive, and at what
levels.
11. When field measurements reveal the presence of contaminants and it is desirable to identify the
source or sources of the contamination.
12. When there is a need to determine the timing of the remedy; i.e., if the remedy is delayed, is there
a potential for environmental or public health impacts in the future?
13. When there is a need to determine remedial action priorities.
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Table 2-1 (Continued)
14.
When estimating the benefit in a cost-benefit analysis of alternative remedies.
15. When demonstrating compliance with regulatory requirements.
16. When performing a quantitative dose or risk assessment pertaining to the protection of remediation
workers, the public, and the environment prior to, during, and following remedial activities.
17. When designing the site characterization program (e.g., placement of monitor wells, determining
data needs).
18. When there is a need to compute or predict the concentration distribution in space and time of
daughter products from the original source of radionuclides.
19. When there is a need to quantify the degree of uncertainty in the anticipated behavior of the
radionuclides in the environment and the associated doses and risks.
20.
To facilitate communication with the public.
environmental measurement programs, identify the types and locations of samples, the types
of analyses, and the required sensitivities of analytical techniques. In addition, modeling is
used as a tool to help understand the processes that affect the behavior of the radionuclides at
a site and the effectiveness of alternative strategies and techniques for mitigating the impacts
of the contaminants.
Though models can be useful, remedial decision-making at a site can proceed effectively
without the use of models. For example, if the levels of contamination in the environment
are above predesignated criteria, and it is apparent that removal and proper permanent
disposal of the contaminants is the appropriate remedy, modeling may be unnecessary.
However, if a remedial decision is delayed and/or the remedy is other than removal, it is
difficult to judge the prudence of such decisions without the aid of fate and effects models.
2.2 WHAT DETERMINES MODELING NEEDS?
Once it is determined that modeling may be needed to fulfill, at least in part, the letter or
intent of the applicable regulatory requirements or guidelines, the need for modeling is
further defined by:
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• the reasons for modeling (as described in Table 2-1),
* the phase of the remedial process,
• the environmental setting and conditions,
• the characteristics of the waste, and
• the land use and demography in the vicinity of the site.
Throughout this report, the remedial process is divided into three phases. Phase 1 is referred
to as the scoping and planning phase, wherein regional, sub-regional, and site-specific data
are reviewed and analyzed in order to define the additional data and analyses needed to
support remedial decision-making. Phase 2 is referred to as the site characterization phase,
wherein the plans developed in Phase 1 are implemented. These data are used in Phase 2 to
characterize more fully the nature and extent of the contamination at the site, to define the
environmental and demographic characteristics of the site, and to support assessments of the
actual or potential impacts of the site. In Phase 2, the data are analyzed to determine
compliance with applicable regulations and to begin to define strategies for the remediation
of the site. Phase 3 is referred to as the site remediation phase, wherein alternative remedies
are identified, evaluated, selected, and implemented.
Conceptually, any remedial process can be described in terms of these three phases.
However, depending on the regulatory framework, each phase may be highly structured, with
clear boundaries between phases, as is the case for sites on the NPL. Conversely, each
phase may be relatively unstructured, as is the case for sites in the SDMP. In either case,
modeling needs will vary as a function of the phase of the remedial process.
During Phase 1, modeling can be used to identify the potentially significant radionuclides and
pathways of exposure, which, in turn, can be used to support the design of comprehensive
and cost-effective waste characterization, environmental measurements, and site
characterization programs. During Phase 2, modeling is used primarily in support of dose
and risk assessment of the site and to evaluate the adequacy of the site characterization
program. During Phase 3, modeling is used primarily to support the selection and
implementation of alternative remedies and, along with environmental measurements
programs, is used to determine the degree to which the remedy has achieved the remedial
goals.
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The environmental setting and conditions at the site will also define modeling needs. The
critical pathways of exposure and the complexity of the site will define the types of models
needed to support decision-making at each phase in the remedial process.
The types of radionuclides, their chemical and physical form, and the degree to which they
are contained in an engineered barrier will also define modeling needs. Finally, the land use
and demography in the vicinity of the site will partly determine the potentially important
exposure pathways, which, in turn, will define modeling needs.
2.3 WHAT NEEDS TO BE MODELED?
For a site contaminated with radioactive materials, the "end product" of the modeling process
is typically one or more of the following results for a broad range of exposure scenarios,
exposure pathways, and modeling assumptions:
• The time-varying and time-averaged radionuclide concentrations in air, surface
water, ground water, soil, and food items. These are usually expressed in
units of pCi/L of water or pCi/kg of soil or food item.
• The radiation field in the vicinity of radioactive material, expressed in units of
pR/hr.
• The radionuclide flux in units of pCi/m2-sec.
• The transit time or time of arrival of a radionuclide at a receptor location.
• The volume of water contained within or moving through a hydrogeological
setting.
• Radiation doses to individual members of the public under expected and
transient conditions and following accidents. The doses are evaluated for the
site in its current condition (i.e., the no action alternative) and during and
following a broad range of feasible alternative remedies. These doses are
usually expressed in units of mrem/yr effective dose equivalent (EDE) for
continuous exposures and mrem per event (EDE) for transients and postulated
accidents.
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• Radiation risks to individual members of the public under expected and
transient conditions and following accidents. The risks are evaluated for the
no action alternative and during and following a broad range of feasible
remedies. These are usually expressed in units of individual lifetime risk of
total and fatal cancers.
• Cumulative radiation doses to the population in the vicinity of the site under
expected and transient conditions and following accidents. The cumulative
doses are evaluated for the no action alternative and during and following a
broad range of feasible remedies. These are usually expressed in units of
person rem/yr (EDE) for continuous exposures and person rem per event
(EDE) for transients and accidents.
• Cumulative radiation risks to the population in the vicinity of the site under
expected and transient conditions and following accidents. The cumulative
population risks are evaluated for the no action alternative and during and
following a broad range of feasible remedies. These are usually expressed in
units of total and fatal cancers per year in the exposed population.
• Radiation doses and risks to remedial workers for a broad range of alternative
remedies. The units of dose and risk for individual and cumulative exposures
are the same as those for members of the public.
• Uncertainties hi the above impacts, expressed as a range of values or a
cumulative probability distribution of dose and risk.
The specific regulatory requirements that apply to the remedial program determine which of
these "end products" is needed. In general, these modeling results are used to assess impacts
or compliance with applicable regulations; however, information regarding flux, transport
tunes, and plume arrival times is also used to support a broad range of remedial decisions.
2.4 WHAT SCENARIOS NEED TO BE MODELED?
In order to model radionuclide concentrations, radiation fields, doses, and risks, as delineated
in Section 2.3, it is necessary to postulate a set of hypothetical exposure scenarios.
Depending on the regulatory requirements and the phase in the remedial process, the
exposure scenarios that will need to be modeled can include any one or combination of the
following:
i
2-6
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• The no action alternative
• Trespassers
• Inadvertent intruder (construction, agricultural, and well water)
• Routine and transient emissions
• Accidents
• Alternative remedies
The "no action" alternative is a term used primarily with sites on the NPL and has a very
specific meaning delineated in EPA 88. However, in its broadest interpretation, it simply
refers to the impacts of the site if no action is taken to clean up the site or protect the public
from the radioactive material at the site. As applied to NPL sites, the no action alternative
refers to site conditions and exposure scenarios prior to any remedial action, including
institutional control (EPA 88). In addition, the exposure scenarios must consider future use
of the site for either residential, commercial, or recreational use (EPA 91). For sites not on
the NPL, or portions of sites that are on the NPL but are in active use, such as several of the
federal facilities on the NPL, it may be more appropriate to assume that institutional controls
are in place to control access to the site, maintain the site, and ensure that emissions from
the site are monitored and kept within acceptable levels.
In between these two extremes are scenarios where credit for institutional controls is taken
for some reasonable period of time, such as 100 years, after which unrestricted access to the
site is postulated.
Within the context of the no action alternatives, a number of scenarios may be postulated.
For example, it may be assumed that a trespasser gams access to the site periodically. It
may also be postulated that an individual gains access to the site and establishes residence.
This scenario could include the construction, agriculture, and well water scenarios. The
construction scenario postulates that an individual builds a home on the site. The agriculture
scenario assumes that the resident maintains a farm or backyard garden at the site. The well
water scenario assumes that the individual establishes a well on site. These scenarios are
often referred to as the intruder scenarios because they postulate that an individual gains
access to, or intrudes onto the site, either deliberately or inadvertently. These scenarios are
2-7
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addressed at NPL sites in accordance with guidance provided in EPA 91, 91a, and 91b.
However, at sites in the SDMP, the concept of an intruder may not be appropriate since the
site is under die direct control of an NRC licensee and will not be released for unrestricted
use prior to decontamination to levels that will permit unrestricted access to the site.
In addition to the intruder scenarios, which are concerned with the potential impacts
associated with a person gaining access to a site, it is usually necessary to model the impacts
on the people and the environment in the vicinity of the site, outside the area where credit is
taken for institutional control. The scenarios that could be postulated are routine or chronic
releases to the atmosphere, surface water, or ground water, transient emissions, and
postulated accidents. Routine emissions of radioactive materials are associated with normal
erosion and transport processes. Transient releases typically include periodic releases
associated with severe weather or other such phenomena that are anticipated to occur during
the hazardous life of the contaminants at the site. Postulated accidents include unlikely
events, such as fires, severe flooding, or earthquakes, which have a low probability of
occurring but relatively large impacts as compared to routine or transient conditions.
Finally, it may also be necessary to model the effectiveness of a broad range of alternative
remedies. Each remedy can be considered a separate scenario. In addition, for each
remedy, it may be necessary to model the performance of the remedy under anticipated and
offnormal conditions. For example, if stabilization of the site using an engineered cap is a
feasible alternative, it will be necessary to model the impacts (i.e., doses and risks) of the
site with the cap performing as designed and also following its postulated failure.
Clearly, the number of scenarios that can be postulated is virtually unlimited. Accordingly,
it is necessary to determine which scenarios reasonably bound what may in fact occur at the
site. The types of scenarios selected for consideration influence modeling needs because they
define the receptor locations and exposure pathways that need to be modeled.
2.5 WHAT PATHWAYS NEED TO BE MODELED?
For each scenario, an individual or group of individuals may be exposed by a wide variety of
pathways. The principal pathways include:
2-8
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* External exposure to deposited radionuclides
• External exposures to airborne, suspended, and resuspended radionuclides
• Inhalation exposures to airborne, suspended, and resuspended radionuclides
• Ingestion of radionuclides in food items and drinking water
• Ingestion of contaminated soil and sediment
• External exposures from immersion in contaminated water
Each of these pathways, within the context of each postulated scenario, creates unique
modeling needs. Which of these pathways will need to be explicitly modeled is initially
determined during the planning phases and is based on judgments 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 is exhumed or the water
covering the material is drained or evaporates. The relative importance of each pathway may
also be evaluated by the use of scoping calculations, such as those described in Til 83, SCA
90, and EPA 88a.
2.6 WHEN IS MODELING NOT NEEDED OR INAPPROPRIATE?
The previous sections have identified the possible uses of models and the factors that
determine modeling needs. However, it is equally important to be able to recognize the
circumstances under which modeling would be ineffective and should probably not be
performed. There are three general scenarios in which modeling would be of limited value.
These are:
1. Presumptive remedies can be readily identified,
2. Decision-making is based on highly conservative assumptions, and/or
3. The site is too complex to model realistically.
The first case arises in situations where a presumptive remedy is apparent; that is, where the
remedy is obvious based on regulatory requirements or previous experience, and there is a
high level of assurance that the site is well understood and the presumptive remedy will be
2-9
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effective. An example would be conditions that obviously require excavation or removal of
the contaminant source.
The second case is based on the assumption that decision-making can proceed based on
conservative estimates of the behavior and impacts of contaminants at the site rather than
detailed modeling. This strategy could be used in the initial scoping, site characterization, or
remedial phase of the investigation. For example, a conservative approach to the risk
assessment would be to assume that the contaminant concentrations at the receptor(s) are
identical to the higher concentrations detected at the contaminant source, and that the
concentrations diminish in time only through radioactive decay. Thus, the need for modeling
to determine the effects of dilution and attenuation on contaminant concentrations is
subsequently removed.
Although the need for modeling may be eliminated through the adoption of a conservative
approach, a conservative approach should not be taken just to avoid ground-water modeling.
There are far more important aspects of the remedial program which will dictate an
acceptable remedial approach and which usually focus on the optimization between the
remedial activities and the accompanying reduction in risk, whereas, an overly conservative
approach may be contradictory to these objectives.
The third case involves sites where modeling would be helpful in supporting remedial
decision-making, but the complexity of the site precludes reliable modeling. These
complexities could be associated with the contaminant source, flow and transport processes,
or characteristics of the wastes. For example, the contaminant source may be so poorly
defined in terms of areal extent, release history, and composition that it cannot be reliably
defined and little would be gained from flow and transport modeling. Complex flow and
transport processes present another difficulty in that computer codes currently do not exist
that accommodate a number of these processes, which include: turbulent ground-water flow,
facilitated transport, and flow and transport through a fractured unsaturated zone.
The availability of computer codes is also an issue when characteristics of the wastes are
typified by complex geochemical reactions; such as phase transformations and non-linear
sorption processes. Currently, ground-water flow and contaminant transport codes have not
been developed which provide credible mathematical descriptions of the more complex
geochemical processes. If modeling is not possible because of the overall complexity of the
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site characteristics, it is common for a greater emphasis to be placed on empirical rather than
predicted data. This may involve establishing long-term monitoring programs, which in
effect, have similar objectives of the ground-water modeling.
In summary, models are data analysis tools which can be useful in supporting decisions, but
are not substitutes for data acquisition and expert judgement. No model is "correct," but
some are useful when used in the proper context. Models should not be used until the
specific objectives of the modeling exercise are defined and the limitations of the models
fully appreciated.
If a site is poorly characterized or poorly understood, any simulation of the transport and
impacts of contaminants using mathematical models is speculative at best and could be highly
misleading. Accordingly, the use of models under such circumstances is limited with regard
to the types of decisions they can help to support. For example, when site specific data are
limited or a site is poorly understood, models may be used to make conservative (i.e., upper
bound) estimates of the potential public health and environmental impacts of a site or to
identify those pathways and environmental parameters that must be better characterized in
order to make more realistic estimates of the potential impacts of a site. As such, models
may be helpful in planning and prioritizing activities. However, modeling alone generally
cannot be used to support reliable risk assessments or remedial decisions.
Inappropriate use of models can lead to costly mistakes. Not only are models often
expensive to implement, but, if used incorrectly, can lead to poorly designed site
characterization programs, the selection and implementation of ineffective remedies, and
erroneous conclusions regarding the actual or potential public health and environmental
impacts of a site.
Notwithstanding the limitations of models, it is difficult to support remedial decisions or the
assessment of risks at a site without the use of models. There are no easy answers or simple
instructions that can be used to ensure the intelligent and effective use of models in support
of remedial decision-making. However, as a general rule of thumb, it is prudent to ask
continually, under what circumstances could the results of a given modeling exercise be
wrong or misleading, what are the potential consequences of our decisions if the modeling
exercise is wrong, and what can we do to verify independently the reliability of our modeling
results?
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3. Processes that Need to be Modeled: The Need for Complex Versus
Simple Ground-water Flow and Transport Models
The previous sections of this report describe the need for, and role of, modeling on both a
generic basis (Section 2) and also within the context of the regulations and guidelines
governing site remedial activities (see Appendix A). This section describes the site
conditions and fate and effects processes that may need to be modeled, which, in turn,
determine whether simple or complex models are needed. The discussion emphasizes
ground-water flow and transport.
For the purpose of this discussion, the distinction between simple and complex ground-water
modeling is based upon broad classifications of ground-water models as either analytical or
numerical. Analytical models are usually approximate or exact solutions to simplified forms
of the differential equations for water movement and solute transport. Such models are
simpler to use than numerical models and can generally be solved with the aid of a
calculator, although computers are also used. Analytical models are limited to simplified
representations of the physical situations and generally require only limited site-specific input
data. They are useful for screening sites and scoping the problem to determine data needs or
the applicability of more detailed numerical models.
Numerical models generally provide solutions to the differential equations describing water
movement and solute transport using numerical methods, such as finite differences and finite
elements. These methods always require a digital computer, a large quantity of data, and an
experienced modeler-hydrologist. The validity of the results from numerical models depends
strongly on the quality and quantity of the input data.
In the sections that follow, reference to complex sites and complex models generally means
that the processes of interest at a site can be best simulated with numerical models.
Reference to simple sites, simple models, or scoping or screening calculations generally
means that the processes of interest can be modeled with analytical models. Notwithstanding
these definitions, it should be understood that there are also degrees of complexity among
both analytical and numerical models. For example, in Section 3.5, which addresses
remedial technologies, numerical modeling is generally required to simulate the performance
of alternative remedies. However, some remedies require more complex numerical models.
The purpose of referring to simple and- complex sites and models in this fashion is to alert
the RPM or equivalent to circumstances when relatively complex processes may need to be
3-1
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simulated so that the appropriate resources and expertise are included in the planning
process.
In general, the site conditions and the processes that need to be modeled, and therefore the
complexity of the models, are determined by a combination of the following five factors:
1. the reasons for or objectives of modeling (as discussed in the previous
sections, the regulatory requirements and guidelines ultimately establish the
reason for modeling),
2. the phase of the remedial process,
3. the chemical and physical form, distribution, and radionuclide composition of
the waste,
4. the environmental characteristics of the site, and
5. the site demography and land use.
In general, factors 1,3, and 4 have the greatest influence on determining the processes that
need to be modeled and therefore the required complexity of the models. However, as will
be discussed, all five factors hi combination influence whether complex or simple models
will be needed: Accordingly, the site conditions and the processes that need to be modeled
can be defined in terms of a five-dimensional matrix; that is, given the reason for modeling,
the phase of the remedial process, the characteristics of the waste, and the environmental and
demographic characteristics of the site, the site conditions and the processes that need to be
explicitly modeled can be defined.
In the following sections, site conditions and the processes that need to be modeled are
discussed in terms of each of the five controlling factors. In each section, the
interdependencies among the controlling factors are discussed briefly. Accordingly, the
discussion of each factor must include some discussion of each other factor, resulting in
some redundancy. Cross-referencing is used to minimize redundancies.
The discussion begins with a listing of the full range of site conditions and processes that
may need to be modeled. This is followed by a discussion of how modeling complexity
(i.e., modeling needs) is determined, at least hi part, by each of the five controlling factors.
Bear in mind that it is the combination of the five factors that determines modeling needs.
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3.1 SITE CONDITIONS AND PROCESSES THAT NEED TO BE MODELED
Table 3-1 presents an overview of the full range of site conditions, transport processes,
doses, and risks from all 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. That is, if it is determined that a given process needs
to be modeled at a given site, and for a given phase of the remedial process, a model or
group of models needs to be selected that addresses that process.
3.2 REASONS FOR MODELING
Table 3-2 presents a list of the reasons for modeling and the phases in the remedial process
when those reasons are likely to occur. In general, the ground-water flow and contaminant
transport processes that need to be modeled are dependent primarily on the modeling
objectives and site conditions. However, many of the reasons for modeling listed in Table
3-2 will also affect the processes requiring modeling, and therefore the complexity of the
models. For example, the assessment of the onsite intrusion scenarios (item 5) does not
require modeling complex flow and transport processes, while modeling the effectiveness of
alternative in-situ remedies (item 15) may require modeling complex processes to support the
design of the remedy.
As discussed later in Section 3.5, the modeling needs associated with the early phases of the
remedial process generally do not require complex modeling. In addition, the detailed site-
specific data required to perform complex modeling are usually not available at this early
stage in the process. As a result, modeling often consists of screening-level calculations that
tend to bound the potential impacts associated with the site and simulate flow and transport
using simplifying, conservative assumptions.
The two primary reasons for ground-water modeling in the site characterization phase of the
remedial process are to: (1) support the baseline risk assessment and (2) optimize the
effectiveness of the site characterization program. Each of these modeling objectives
presents distinct modeling needs.
The demands of the baseline risk assessment that are supported by ground-water modeling
generally range from determining peak concentrations of radionuclides arriving at the
3-3
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Table 3-1. Transport Processes
i. Source Term - Determine routine emissions/leach rate in terms of Ci/yr or accidental emissions in
terms of Ci/event as a function of time
• Routine Emissions
- Waste Form/Waste Container Performance
- Natural Barrier Performance
- Engineered Barrier Performance
• Transient or Accident Emissions
- Natural
Flood
High Winds
Tornado
Earthquake
- Anthropogenic
Construction
Agriculture
Drilling
2. Environmental Transport - Determine radionuclide concentrations in air (pCi/mJ), soil and sediment
(pCi/g), surface and ground water (pCi/L) as a function of time and receptor location
• Air Transport Processes
- Suspension
- Evaporation
- Volatilization
- Dispersion
- Deposition
- Radioactive Decay and Buildup
• Surface Water Transport in Streams, Rivers, Lakes, Estuary and Marine Environments
- Dispersion
- Deposition in Sediments
- Sediment Transport
- Radioactive Decay and Buildup
• Ground-Water Transport Processes
- Unsaturated Zone
Miscible
Immiscible
Vapor Transport
Mass Transport
Advection
Diffusion
Dispersion
Physical/Chemical Processes
Decay
Sorption
Dissolution/Precipitation
Acid/Base Reactions
Complexation
Hydrolysis/Substitution
Redox Reactions
Density Dependent Flow
Biologically Mediated Transport
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Table 3-1 (Continued)
Environmental Transport (Continued)
- Saturated Zone
Miscible
Mass Transport
Advection
Diffusion
Dispersion
Physical/Chemical Processes
Decay
Sorption
Dissolution/Precipitation
Acid/Base Reactions
Complexation
Hydrolysis/Substitution
Redox Reactions
Density Dependent Flow
Biologically Mediated Transport
Immiscible
- Fractured Zone
Nonpercolating
Percolating
Matrix diffusion effects
3. Exposure Scenarios
• Postulated scenarios causing radiation exposure via various pathways
* The no action alternative
• Alternative remedies
• Trespassers
• Inadvertent intruder (construction and agricultural)
* Routine and transient emissions
• Accidents
4. Exposure Pathways
• The Pathway or Medium to Which Individuals and Populations Are Exposed
• External Exposure to Deposited Radionuclides
• External Exposures to Airborne, Suspended, and Resuspended Radionuclides
• Inhalation Exposures to Airborne, Suspended, and Resuspended Radionuclides
• Ingestion of Radionuclides in Food Items and Drinking Water
• Ingestion of Contaminated Soil and Sediment
• External Exposures from Immersion in Contaminated Water
5. Doses
• mrem/yr EDE to individuals
• person rem/yr EDE to population
3-5
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Table 3-1 (Continued)
6. Public Health Impacts
• Individual Risk (risk per yr and per lifetime)
- Acute Effects
- Carcinogenic Effects
- Mutagenic Effects
- Teratogenic Effects
• Population Impacts (Effects/yr)
- Acute Effects
- Carcinogenic Effects
- Mutagenic Effects
- Teratogenic Effect
ground-water table, which have been derived from an immediately overlying source, to the
determination of radionuclide arrival times and concentrations at receptors that may be
located miles downgradient. The most acceptable method of predicting peak concentrations
of radionuclides emanating from a source and reaching the water table is to model the
movement of ground water and radionuclides through the unsaturated zone. A number of
ground-water models are available to model flow and transport processes in the unsaturated
zone, each with their own strengths and weaknesses (see EPA 88a).
In some instances, the risk assessment may require that radionuclide concentrations be
determined at a receptor located at some distance downgradient from the source. In this
case, a model that can simulate flow and transport in the saturated zone should be used.
Currently, very few ground-water computer codes are available that satisfactorily couple the
flow and transport processes occurring in the unsaturated zone with those of the saturated
zone. However, it is not essential to use a coupled code to obtain reliable results of ground-
water flow and radionuclide transport moving from the unsaturated zone into the saturated
zone. Decoupled codes can use the output from the unsaturated flow and transport model
(radionuclide concentrations reaching the water table) as input (boundary and/or initial
conditions) to the saturated zone flow and transport code. In essence, the codes may be
coupled in terms of input and output.
Ground-water modeling during the site characterization phase is also used to: (1) refine the
existing site conceptual model; (2) optimize the number and location of monitoring wells; and
(3) evaluate the sensitivity of ground-water flow and contaminant transport to various
parameters. To accomplish these goals, it is generally necessary to apply relatively complex
ground-water models that can simulate flow in the saturated zone as well as transport
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Table 3-2. Matrix of Reasons for Modeling1
1.
2.
3.
4.
5.
6.
7.
Opportunities for Modeling
When it is not feasible to perform field
measurements; i.e.,
• Cannot get access to sampling locations
• Budget is limited
• Time is limited
When there is concern that downgradient locations
may become contaminated at some time in the future.
When field data alone are not sufficient to fully
characterize the nature and extent of the
contamination; i.e.,
• when field sampling is limited in space and time
and needs to be supplemented with models
• when field sampling results are ambiguous or
suspect
When there is concern that conditions at a site may
change, thereby changing the fate and transport of
the contaminants; i.e.,
• seasonal changes in environmental conditions
• severe weather (floods, tornadoes)
• accidents (fire)
When there is concern that institutional control at the
site may be lost at some time in the future resulting
in unusual exposure scenarios.or a change in the fate
and transport of the contaminants; i.e.,
• trespassers
• inadvertent intruder (construction/agriculture)
• drilling, mineral exploration, mining
• human intervention (drilling, excavations, mining)
When remedial actions are planned and there is a
need to predict the effectiveness of alternative
remedies.
When there is a need to predict the time when the
concentration of specific contaminants at specific
locations will decline to acceptable levels.
Scopinig
•
•
•
0
o
o
o
Site;
Characterization
O
•
•
•
•
O
•
Remediation
O
•
•
•
•
•
•
• Denotes an important role
O Denotes a less important role
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Table 3-2 (Continued)
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
j«ft^Miwir -:;lh
When there is concern that at some time in the past
individuals were exposed to elevated levels of
contamination and it is desirable to reconstruct the
doses.
When there is concern that contaminants may be
present but below the lower limits of detection.
When field measurements reveal the presence of
some contaminants and it is desirable to determine if
and when other contaminants associated with the
source may arrive, and at what levels.
When field measurements reveal the presence of
contaminants and it is desirable to identify the source
or sources of the contamination.
When there is a need to determine the timing of the
remedy; i.e., if the remedy is delayed, is there a
potential for environmental or public health impacts
in the future.
When there is a need to determine remedial action
priorities.
When demonstrating compliance with regulatory
requirements.
When estimating the benefit in a cost-benefit analysis
of alternative remedies.
When performing a quantitative dose or risk
assessment.
When there is uncertainty regarding the proper
placement of monitor wells.
When developing a site conceptual model.
When developing a site characterization plan and
determining data needs.
When there is a need to anticipate the potential doses
to remediation workers.
Scoping
O
O
O
•
O
0
•
0
O
9
•
•
•
Site;.::' .... .
Characterization
*
*
*
*
O
O
•
O
•
O
O
O
O
Remediation
O
O
O
O
*
•
•
•
•
•
O
O
•
• Denotes an important role
O Denotes a less important role
3-8
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processes that will affect downgradient radionuclide concentrations. However, the lack of
available data throughout much of the site characterization phase will often limit a
meaningful analysis to two dimensions.
The objectives of modeling required to support the selection and implementation of
alternative remedies are generally more ambitious than those associated with the site
characterization phase of the remedial process. Therefore, it is often necessary to select a
computer code with more advanced capabilities in order to simulate the more complex
conditions inherent in the remedial design. For example, the following specific processes are
rarely essential to the baseline risk assessment and site characterization but are often very
important to the remedial design:
(1) three-dimensional flow and transport;
(2) matrix diffusion (pump and treat);
(3) resaturation of the nodes (pump and treat);
(4) heat-energy transfer (in-situ vitrification/freezing);
(5) sharp contrasts in hydraulic conductivity (barrier walls);
(6) multiple aquifers (barrier walls);
(7) the capability to move from confined to unconfined conditions (pump and
treat); and
(8) ability to simulate complex flow conditions (pumping wells, trenches, injection
wells).
If these types of remedies may be employed, complex models will likely be needed to
support the selection, design, and implementation of the remedy.
3.3 CHARACTERISTICS OF THE WASTE - MODELING THE SOURCE TERM
Within the context of ground-water modeling, the "source term" refers to the rate at which
radionuclides are mobilized from the waste and enter the unsaturated and saturated zones of a
site. The following characteristics of the waste will determine the complexity of the models
required to simulate realistically the source term:
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• Waste container
• Waste form
• Source geometry (e.g., volume, area, depth, homogeneity)
• Types and chemical composition of the radionuclides
• Geochemical environment in the vicinity of the waste
Analytical models for the geosphere can only simulate simple approximations of the source
term; therefore, if it is suspected that the source term is transient or that the waste container
has a significant impact on the release rates, models that simulate complex source terms, in
addition to the more traditional flow and transport computer codes, may be needed.
3.3.1 Waste Form and Containment
As indicated in Appendix B, radioactive contaminants are present hi a wide variety of waste
forms that may have some influence on their mobility. However, hi most cases, the
radionuciides of concern are long-lived and the integrity of the waste form or container
cannot be relied upon for long periods of tune. Therefore, the source term can often be
modeled as a uniform point or areal source and no credit taken for waste form or
containerization.
If it is desired to model explicitly the performance of the waste form (e.g., rate of
degradation of solidified waste or containerized waste) or transport in a complex geochemical
environment (changing acidity, presence of chelating agents or organics), more complex
geochemical models may be needed. Depending on the waste form and container, such
models would need to simulate the degradation rate of concrete, the corrosion rate of steel,
and the leaching rate of radionuclides associated with various waste forms (i.e, soil, plastic,
paper, wood, spent resin, concrete, glass, etc.). These processes would depend, in part, on
the local geochemical setting. However, it is generally acknowledged that the current state-
of-the-art does not permit the explicit modeling of geochemical processes responsible for the
degradation of the waste containers or the waste itself (NRC 90a). As a result, in order to
account for container and waste form performance, the model would need to include terms
that provide for a user-defined algorithm which accounts for the delay in release associated
with the performance of the barrier or waste form.
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3.3.2 Physical and Chemical Properties of the Radionuclides
Certain radionuclides have properties that are difficult to model and may not be adequately
simulated with analytical models. For instance, most of the NPL sites are contaminated with
thorium and uranium, both of which decay into multiple daughters which may differ from
their parents both physically and chemically. Some of the radionuclides (e.g., uranium)
exhibit complex geochemistry, and their mobility is dependent upon the redox conditions at
the site. The following discusses some of the chemical properties of common radionuclide
contaminants found at many NPL and SDMP sites and how these properties can influence
radionuclide transport.
Non-metals
The non-metallic elements (C, H, I, Rn, and Se) will, under normal geochemical conditions,
exist as either gases or as anions dissolved in water. As gases, these elements pose a
completely different set of problems from the other radioisotopes. Similarly, as anions, such
as carbonate or selenate, these radioisotopes will be much less affected by adsorption and ion
exchange than will the other, primarily cationic, radioisotopes.
The radon associated with radium-contaminated sites, as well as sites with tritium, has
special considerations in that these radionuclides can move in a gaseous phase. This gaseous
phase cannot be simulated with traditional flow and transport models. Furthermore, model
calibration of the radionuclide transport may be extremely difficult due to the radionuclide
gas or vapor phase moving independently of the ground water.
Transition. Noble Metals, and Lanthanides
These elements (Mn, Ni, Co, Ru, Tc, Eu, and Pm) exist as atoms with one, two, three, or
more valence electrons (except for the lanthanides, Eu and Ru, which exhibit only the +3
valence state and behave similarly). In solution, they exist as simple cations in most
common geochemical environments. Reactions that lead to the precipitation of oxides,
sulfides, carbonates and sulfates, etc., and ion-exchange will dominate the behavior of these
elements. In modeling the transport of these radionuclides, retardation factors are selected
based on knowledge of the geochemical environment and whether precipitation reactions are
anticipated.
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Alkaline Metals and Earths
Cesium, radium, and strontium occur in nature at only one valence state (+1 for Cs, and +2
for Ra and Sr). They tend to form very soluble cations in water. Radium and strontium will
behave similarly to calcium, which controls their behavior in nature, and cesium will tend to
follow K and Na in solution. Because of their relatively short half-lives, the retardation
factors for Cs-137 and Sr-90 can have a significant effect on the outcome of a modeling
exercise. In general, the retardation factors for these radionuclides depend on the
composition of the soil (i.e., clay vs. silt vs. sand).
Actinides and Transuranics
Unlike the lanthanide series, whose members have essentially identical chemistry, the
actinide series elements exhibit a more varied and complex array of chemical behaviors.
This complexity is the consequence of their potential for existing at more than one oxidation
state and their related tendency to form complexes with anions and/or organic substances
dissolved hi water.
The geochemical behavior of many of the actinides (and some of the transition metals) will,
therefore, be controlled not only by their concentration but also by the redox conditions
which prevail in the media through which the isotopes are transported. Uranium, for
example, can be found in any of five valence states (+2, +3, +4, +5, +6) with two (+4
and +6) of geochemical significance. In most geologic environments, the reduced uranous
ion (U4+) is insoluble, while the oxidized uranyl ion (UO2++) is considerably more soluble.
At virtually every site, the possibility exists for transitions within media from reducing to
oxidizing conditions on both a macro and micro scale.
While multiple valence states will generally suggest that redox conditions will be a
controlling factor in the behavior of radioactive materials, other properties may mask the
charge effect. For example, although plutonium can exist in any of five valence states (+3,
+4, +5, +6, +7), few of these are, in fact, of geochemical importance. In practice, the
property that controls the behavior of plutonium, for example, is the insolubility of Pu(IV)
hydrolysis products, which are, in turn, strongly adsorbed to particle surfaces. In general,
these processes cannot be reliably modeled. Instead, retardation factors are selected based on
an understanding of the site geochemistry and bench scale tests.
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Though the chemical form of the radionuclides and the geochemical setting can have a
profound effect on the transport of the radionuclides, it is generally acknowledged that the
various geochemical processes cannot be modeled reliably. Instead, based on knowledge of
the radionuclides, their chemical and physical form, and the local geochemistry, judgments,
along with bench scale tests, are used to identify the effective binding coefficients for the
radionuclides in that setting.
Decay Chains
Many of the radionuclides discussed in Appendix B have daughters or an entire decay chain
(such as uranium and thorium) that must be modeled if exposures over long periods of time
are of concern. This necessitates the use of models that explicitly address decay chains.
3.3.3 Geochemical Setting
In addition to the standard chemical properties of radionuclides, it is important to understand
the geochemical properties and processes of the radionuclides that are specific to the site.
These properties and processes include the following:
• Complexation of radionuclides with other constituents
• Phase transformations of the radionuclides
• Adsorption and desorption
• Radionuclide solubilities at ambient geochemical conditions
To model these processes explicitly, as opposed to using simplifying assumptions such as
default or aggregate retardation coefficients, more complex geochemical models may be
needed. However, as discussed above, it is generally acknowledged that explicit modeling of
complex geochemical processes in conjunction with ground water flow is currently not
feasible.
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3.4 ENVIRONMENTAL CHARACTERISTICS - MODELING FLOW AND
TRANSPORT
In general, the need for complex models increases with increasing complex lithology (i.e., a
thick unsaturated zone, and/or streams or other bodies of water on site (i.e., a complex site).
However, even at complex sites, complex models may not be needed. For example, if a
conservative approach is taken, where transport through the unsaturated zone is assumed to
be instantaneous, then the complex processes associated with flow and transport through the
unsaturated zone would not need to be modeled. Such an approach would be appropriate at
sites that are relatively small and where the extent of the contamination is well defined.
Under these conditions, the remedy is likely to be removal of the contaminated surface and
near-surface material. Many of the SDMP sites and several of the non-defense NPL sites
exhibit such conditions. In these cases, the use of conservative screening models may be
sufficient to support remedial decision-making throughout the remedial process.
3.4.1 Sub-Regional Scale Characteristics
At more complex sites, such as many of the defense facilities on the NPL, the remedial
process is generally structured so that, as the investigation proceeds, additional data become
available to support ground-water modeling. An understanding of the physical system, at
least at a sub-regional scale, may allow an early determination of the types of models that
may be appropriate for use at the site. Specifically, the following site characteristics may
have to be extrapolated from regional-scale information, and will, in part, determine the
types and complexity of models required:
• Approximate depth to ground water - A thick unsaturated zone
suggests the need for complex models.
• Lithology of the underlying rocks (e.g., limestone, basalt, shale)
- Layered, fractured, or heterogenous lithology suggests the
need for complex models.
• Presence of surface water bodies - The presence of water bodies
on or in the vicinity of the site suggests the need for complex
models.
• Land surface topography - Irregular topography suggests the
need for complex models.
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• Sub-regional recharge and discharge areas may indicate the need
for complex models.
• Processes or conditions that vary significantly in time may require
complex models.
The NPL sites are distributed more or less randomly across the 48 contiguous United States.
However, the SDMP sites are almost all concentrated in the Northeast. The geographic
location of the various sites provides some early clues as to the level of sophistication of any
required ground-water modeling.
Depth to Ground Water
Sites located in the arid west and southwest (e.g., Pantex, Hanford, and INEL) generally
have greater depths to ground water. The simulation of flow and transport through the
unsaturated zone may require more complex computer codes due to the non-linearity of the
governing equations. Modeling of the unsaturated zone is further hampered because the
necessary data are often difficult to obtain.
Sub-Regional Lithology
The lithology of the underlying rocks also provides insight into the expected level of
difficulty of modeling. A number of the NPL sites described in Appendix B overlie areas
where fractures are probably dominant mechanisms for flow and transport. These sites
include Hanford, the Idaho National Engineering Laboratory (INEL), Maxey Flats,
Jacksonville, and Pensacola Air Stations. In some cases, such as at Hanford, the fractured
zone is deep below the site, and concerns regarding ground-water contamination are limited
primarily to the near-surface sedimentary rock.
It is unlikely that analytical models could be used to describe adequately flow and transport
in the fractured systems because radionuclide transport and ground-water flow in fractured
media are much more complex than in unfractured, granular porous media. This is because
of the extreme heterogeneities, as well as anisotropies, in the fractured systems.
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Surface Water Bodies
Virtually all of the NPL sites and many of the SDMP sites have surface water bodies at or in
the immediate vicinity of the site. Bodies of water often have a significant impact on the
ground-water flow and cannot be neglected in the modeling analysis. In general, analytical
models are limited in their ability to simulate properly the effect that surface water bodies
have on contaminant flow and transport, particularly if the surface water body behaves
episodically, such as tidal or wetland areas. Several of the NPL sites are indurated with
wetlands, including Oak Ridge, Himco, and Shpack Landfill. At least two sites, Pensacola
and Jacksonville, are close to estuaries, which suggests that tidal as well as density-dependent
flow and transport may be significant.
Sub-Regional Topography
The land surface topography is often overlooked in the preliminary identification of potential
modeling needs but may be an important factor in evaluating the need for, and complexity
of, ground-water modeling. Topography may have a significant influence on ground-water
flow patterns. For instance, Maxey Flats is situated atop a relatively steep-sided plateau with
a stream situated at the bottom of the slope. The steep topography strongly controls the
direction of ground-water flow, making it much more predictable. Furthermore, estimating
the flux of ground water moving into the system from upgradient sources becomes much
simpler if the area of interest is a local recharge area, such as a hill or mountain. Steep
topography can also complicate the modeling by making it more difficult to simulate
hydraulic heads that are representative of the hydrologic units of interest. To solve this
problem, some computer codes have an option to use curvilinear elements.
Regional Recharge/Discharge
The ground-water flow paths will largely be controlled by regional and sub-regional ground-
water recharge and discharge areas. It is generally necessary to ensure that the flow and
transport simulated by the model on a local scale is consistent with the sub-regional and
regional scale. If the site is located in an aquifer recharge area, the potential for widespread
aquifer contamination is significantly increased, and reliable modeling is essential.
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3.4.2 Detailed Hvdrogeological Characteristics of the Site
Detailed knowledge pertaining to the hydrogeology of the site will generally become
available during site characterization. Ground-water systems can be grouped into several
broad categories. Each category has an associated set of data needed to support ground-
water modeling and to determine modeling complexity. These broad classifications are:
• Saturated versus unsaturated systems
• Porous media versus fractured systems
• Complex versus simple hydrologic boundary conditions
In general, sites with relatively simple hydrogeological characteristics pertaining to these
parameters may be reliably modeled with relatively simple analytical models.
Unsaturated Zone Properties
The following characteristics of the unsaturated zone determine, in part, whether the
unsaturated zone will need to be explicitly modeled. These are also the site-specific
parameters required to model transport through the unsaturated zone.
• Recharge through the unsaturated zone (infiltration)
• Thickness and geometry of the unsaturated zone
• Magnitude and distribution of the saturated hydraulic conductivity
• Matric potential and distribution at various soil moisture contents
• Delineation of discrete features
• Degree of isotropy
• Degree of homogeneity
• Distribution coefficients
• Bulk density
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• Boundary conditions
• Porosity
• Dispersivity
• Vapor phase transport effects
The sophistication of the unsaturated zone modeling approach will be based not only on the
complexity of the hydrogeology but also on the overall modeling objectives. For instance,
radionuclide flow and transport through a very thick unsaturated zone may be irrelevant if
credit is not taken for it in the baseline risk assessment. On the other hand, if the risk
assessment is based solely upon arrival times and peak concentrations of radionuclides
arriving at the ground-water table, then consideration of transport through even a thin
unsaturated zone is significant.
Situations may arise where reliable simulations of flow and transport of radionuclides through
the unsaturated zone may not be possible even with complex ground-water models. In
particular, if the unsaturated zone is indurated with fractures or macropores with high
permeability, the flow and transport processes become so involved that mathematical
formulations of porous media transport are poor representations of the physical phenomena.
Furthermore, localized zones of higher permeability may cause the wetting front to advance
at highly variable rates, which may introduce significant disparities between the actual and
predicted contaminant concentrations. It is also difficult to model saturated zones that are
"perched" above fine-grained sediments within the unsaturated zone. These perched-water
zones may have a significant impact on the flow and transport of radionuclides.
Saturated Zone Properties
The most frequently performed ground-water modeling is that of the saturated zone. The
parameter needs are well defined, and the field data collection activities are relatively
straightforward. The characteristics of the site that determine the complexity of saturated
zone modeling (and also the site-specific data required to perform saturated zone modeling)
include:
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• Geometry of the hydrogeologic units
• Specific storage
• Magnitude and distribution of hydraulic conductivity
• Vertical and horizontal hydraulic gradients
• Degree of isotropy
• Degree of homogeneity
• Dispersivity
• Distribution coefficients
• Bulk density
• Diffusion properties
• Effective porosity
• Boundary conditions
Three major hydrogeological factors that provide immediate insight into whether complex
ground-water modeling will be necessary are: the transient nature of the flow system; the
complexity of the dominant flow and transport processes; and the heterogeneity and
anisotropy of the hydrostratigraphic units.
A transient flow system simply means one that fluctuates with time. This fluctuation may be
induced by both natural (e.g., tides, rainfall) and manmade influences (e.g., wells, dams).
In many instances, transient systems, if observed over the long term, will approach relatively
steady-state conditions. If this is the case, it may be possible to undertake a simple modeling
approach even with a transient flow system.
Analytical models do not generally account for many of the more complex flow and transport
processes that may be occurring. For example, if it is necessary to model multi-phase fluid
conditions in order to accomplish the modeling objectives, a more complex modeling
approach will be needed.
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Hydrogeologic systems that are heterogeneous and/or anisotropic are often associated with
complicated flow patterns. Analytical models generally do not allow for the incorporation of
varying rock properties. Therefore, heterogeneous conditions are generally simulated with
numerical rather than analytical models.
Fracture Zone Properties
A number of the NPL sites described in Appendix B overlie areas where fractures and
solution channels are probably dominant mechanisms for flow and transport. These sites
include INEL, Maxey Flats, Jacksonville, and Pensacola Air Stations. The uncertainty
associated with fracture zone modeling is generally high, and much effort needs to go into
the field investigation. The data needs associated with fracture properties to support flow
and transport modeling include:
• Aperture
• Porosity
• Orientation
• Length
• Density
• Connectivity
• Roughness coefficient
• Matrix diffusion coefficient
• Effective surface area
• Fracture mineralization
It is unlikely that analytical models could adequately describe flow and transport in most
fractured systems because radionuclide transport and ground-water flow in fractured media
are much more complex than in unfractured granular porous media. This is due to the
extreme heterogeneities, as well as anisotropies, in the fractured systems.
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Boundary Conditions
Physical features within the modeled area will be translated into numerical terms as boundary
conditions for the ground-water modeling. The following features most commonly constitute
boundary conditions and need to be identified and characterized in order to determine the
types and level of complexity of ground-water flow and transport models required at a site:
• Surface water bodies
• Ground-water divides
• Fractures and faults
• Areal recharge
• Geologic contacts
• Freshwater-saltwater interface
• Waste source characteristics
• Wells (injection and withdrawal)
Some physical characteristics are more difficult than others to incorporate as boundary
conditions into a model and, therefore, will necessitate a more complex modeling approach.
In general, flow processes dominated by boundary conditions that are transient in time and/or
associated with discrete features (e.g., faults) will require a more complex modeling
approach.
3.5 THE PHASE OF THE REMEDIAL PROCESS
The reasons for modeling and the types and complexity of the models required to support
remedial decision-making will change as the project matures from scoping and planning
(Phase 1), to more detailed site characterization (Phase 2), to remedy selection and
implementation (Phase 3). In general, the complexity of modeling will increase as the
remedial process proceeds from Phase 1 to Phase 3.
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3.5.1 Phase 1 - Planning and Scoping
During Phase 1 (the scoping and planning phase), only limited site-specific data are generally
available. Accordingly, modeling during the scoping phase is generally limited
to simple one-dimensional or analytical models even if the characteristics of the waste and
the site indicate that more complex models may eventually be needed. As a result, modeling
in Phase 1 generally consists of screening-level calculations that tend to bound the potential
impacts associated with the site and simulate flow and transport using simplifying,
conservative assumptions.
3.5.2 Phase 2 - Site Characterization
Phase 2 of the remedial process (the site characterization phase) is designed to characterize
the nature and extent of the contamination and the potential risks posed by the site. As
indicated in Table 3-2, most of the reasons for modeling present themselves during Phase 2.
A properly designed and implemented site characterization program will generate the detailed
site-specific information necessary to perform the modeling required to meet the modeling
objectives.
Table 3-3 focuses on the various ground-water flow and transport processes at a site that
will, in part, determine whether simple or complex models will be needed during Phase 2 of
the remedial process. In general, simple models may be adequate under the following
combination of conditions: (1) credit is not taken for the waste form or engineered barriers
(i.e., it is assumed that the waste is being transported by simple leaching processes), (2) no
credit is taken for transport through the unsaturated zone (i.e., it is assumed that the leachate
percolates directly into the saturated zone), and (3) the saturated zone is treated as a
homogeneous, isotropic medium. Any other assumptions regarding the behavior of the waste
or site conditions will likely necessitate the use of more complex models. Additional
discussion of the various processes is provided in Sections 3.3 and 3.4, which address how
the characteristics of the waste and the characteristics of the site affect modeling needs.
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Table 3-3. Ground-water Flow and Transport Processes that May Need to be Modeled
and Site-specific Information Needed to Identify the Processes to be Modeled
Leach rate of radionuclides
Structural integrity of the source of the waste (i.e.,
waste form, container, package)
Composition and thickness of the source of the waste
Geochemical environment surrounding the waste
Area! extent and geometry of the waste source
Inventories
Transport through the unsaturated zone
Recharge through the unsaturated zone
Thickness of the unsaturated zone
Moisture release curve parameters
Saturated hydraulic conductivity
Matric potential
Soil moisture content
Delineation of discrete features
Degree of isotropy
Degree of homogeneity
Distribution coefficients
Bulk density
Vapor phase transport effects
Porosity
Transport through the saturated zone
Geometry of the hydrogeologic units
Specific storage
Hydraulic conductivity
Vertical and horizontal hydraulic gradients
Degree of isotropy
Degree of homogeneity
Dispersivity
Distribution coefficients
Bulk density
Matrix diffusion properties
Effective porosity
Fractured media transport
Aperture
Porosity
Orientation
Length
Density
Connectivity
Roughness coefficient
Matrix diffusion coefficients
Simple hydrologic boundary conditions
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Table 3-3 (Continued)
Complex hydrologic boundary conditions
Surface water bodies
Ground-water divides
Fractures and faults
Non-uniform area! recharge
Geologic contacts
Freshwater-saltwater interface
Radioactive decay and ingrowth of daughters
Radioisotopes present
Half-life
3.5.3 Phase 3 - Remediation
As the site characterization process proceeds, data are acquired that will help in identifying
feasible remedial alternatives. These data, in combination with models, are used to simulate
the flow and transport of the radionuclides with and without the feasible alternative remedies.
The data and models are used to predict the behavior of the radionuclides and thereby aid in
the selection and design of the remedy. The models also help to demonstrate that the
selected remedy will achieve the remedial goals.
Each remedial alternative has costs and benefits that must be carefully weighed before a
specific remedy or set of remedies is selected and implemented. The benefit of a given
remedy is the reduction or elimination of the risks estimated in the risk assessments
performed during the site characterization phase. The costs of a given remedy include:
(1) the short- and long-term economic costs, (2) the public health impacts on the remediation
workers and general public associated with implementing the remedy, and (3) the public
health impacts associated with any residual contamination and modifications to the
environment associated with the remedy. The various remedial alternatives can be
conveniently grouped into the following three categories:
• Immobilization
• Isolation (Containment)
• Removal/Destruction
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This section briefly describes each category and the modeling needs that may be associated
with each category. In general, most remedies require complex models to support their
design and implementation and predict their performance.
Immobilization
Immobilization of the radioactive wastes refers to processes whereby physical or chemical
means are used to stabilize the radionuclides and preclude their transport. Biological
remedies are not addressed since they are employed primarily to degrade organic
contaminants and have limited applicability to sites contaminated with radioactive materials.1
The various physical and chemical treatment processes available for relatively long-term
immobilization can be grouped as follows:
• Physical
• water vapor extraction
* in-situ coating
» grouting of fissures and pores
* in-situ vitrification
• Chemical
* induce secondary mineralization
* induce complexation
• alter oxidation-reduction potential
• Physical/Chemical
• alter surface tension relationships
• alter surface charges
• in-situ binding
• adsorbent injection
• radionuclide particle size augmentation through
clay flocculation
The following are the types of physical and chemical processes that may need to be modeled
to support the selection and design of alternative immobilization remedies:
1 The use of microbes to help stabilize Sr-90 and uranium in waste ponds at Oak Ridge is one exception to this.
Another is the use of jimson weed at Los Alamos National Laboratory for the uptake of plutonium in soil.
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Physical Properties and Processes
* unsaturated zone flow and transport
• heat energy transfer
• multiple layers
• vapor transport
• density-dependent flow and transport
• extreme heterogeneity
• temperature-dependent flow and transport
Chemical Properties and Processes
oxidation-reduction reactions
system thermodynamics
chemical speciation
ion exchange phenomena
precipitation
natural colloidal formation
radiolysis
organic cornplexation
anion exclusion
It would be ideal if conventional and available models could reliably describe and model
these processes and properties. However, many of these properties and processes are not
well understood, and models do not exist that reliably simulate them. Accordingly,
treatability studies, as discussed in OSWER Directive 9355.3-01, may be the only reliable
method for assessing the performance of a remedy based on immobilization.
The results of the treatability studies can be used to determine aggregate modeling
parameters, such as the leach rates of waste stabilized in-situ. These empirically determined
parameters can then be used as input to simple or complex ground-water flow and transport
models, depending on the other factors discussed above.
Isolation
A common remedial alternative is to emplace protective barriers either to prevent
contaminated ground water from migrating away from a contaminated site or to divert
incoming (i.e., clean) ground water from the source of contaminants. Several types of
materials are being used to construct such barriers, including soil and bentonite, cement and
bentonite, concrete, and sheet piling. Examples of potential barriers include the following:
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• Physical
• cutoff curtains, sheet piling, slurry walls
• covers
• hydraulic control wells or trenches
(injection and/or extraction)
• Chemical
• ion-exchange barriers
If properly designed and emplaced, such barriers can last for several decades, barring
geological disturbances such as tremors, ground settling, significant changes in hydraulic
gradients, etc. Accordingly, such barriers can be useful in mitigating the impacts of
relatively short-lived radionuclides, or in controlling the migration of long-lived radionuclides
until a more permanent remedy can be implemented.
Several mechanisms or processes can affect the long-term integrity of such barriers. Once
the installation is complete, cracking, hydrofracturing, tunnelling and piping, and chemical
disruption can cause failures. Changes in the site's geological or hydrological characteristics
can also lead to catastrophic failures, such as partial collapse, settling, and breaking. If a
barrier should fail following installation, water may infiltrate or exfiltrate the site, and
contaminated leachate may move beyond the site. This type of failure could result in the
dispersion of contaminants in the environment. The risk assessment performed in support of
this category of remedial alternative should include an evaluation of the range of radionuclide
concentrations in down-gradient wells following such an accident and the associated doses
and risks to well users.
The following are the types of physical, chemical, and biological processes that may need to
be modeled to support the isolation alternative.
• Physical Properties and Processes
unsaturated zone flow and transport
runoff
transport through multiple layers
vegetative cover
transient source term
extreme heterogeneity
areal recharge and zero flux capability
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• Chemical Properties and Processes
* localized ion exchange phenomena
• oxidation-reduction
Attempts at modeling these processes will have varying degrees of success. Like
immobilization techniques, many of these properties and processes are not well understood,
especially over the long term, and models do not exist that reliably simulate these processes
and properties. Accordingly, prior experience with each remedy, engineering judgment, and
conservative design are the principal means for assessing and assuring the performance of an
isolation remedy.
The design criteria, such as the hold-up time of barriers and the life expectancy of a barrier,
can be used to determine, in part, the transit time to ground-water user locations. These
design criteria can then be used as input to simple or complex models appropriate for the
site.
Removal
The technologies most commonly used to remove solid, liquid, and vapor (e.g., tritium)
radionuclides include:
• soil excavation
• in-situ vaporization
• pump and treat
• soil washing
The following are the types of physical and chemical processes that may need to be modeled
to support alternative removal remedies. Most of these processes and properties are readily
described in mathematical terms and can be modeled relatively reliably.
• transient source term
• unsaturated and saturated zone flow and transport
• matrix diffusion
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• desaturation and resaturation of the aquifer
• vapor transport
• sorption
• mixing
• dilution
The removal alternative is probably the only truly permanent alternative for long-lived
radioactive contaminants. However, it is also generally the most expensive. Accordingly,
modeling, though expensive and time consuming, can be highly cost-effective if it can
convincingly demonstrate that remedies other than removal can be protective of human health
and the environment.
3.6 LAND USE AND DEMOGRAPHY
At sites where the ground water is currently being used or may be used in the future as a
municipal water supply, complex ground-water models may be needed to gain insight into the
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 the wells located at different directions, distances,
and depths relative to the sources of contamination.
Simple models are typically limited to estimating the radionuclide concentration in the plume
centerline. Accordingly, if it is assumed that the receptors are located at the plume
centerline, a simple model may be appropriate. Such an assumption is often made even if a
receptor isn't currently present at the centerline location because the results are generally
conservative. In addition, risk assessments often postulate that a receptor could be located
directly down-gradient of the source at some time in the future.
The need for complex models increases if there are a number of municipal water supplies in
the vicinity of the source. Under these circumstances, it may be necessary to calculate the
cumulative population doses and risks, which require modeling the radionuclide
concentrations at a number of specific receptor locations. Accordingly, off-centerline
dispersion modeling may be needed.
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4. Summary and Conclusions
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 NRC Office of Nuclear Material Safety and Safeguards (ONMSS) have
established an interagency agreement for promoting the appropriate and consistent use of
mathematical models in the remediation and restoration process at sites containing, or
contaminated with, radioactive materials. This report, which is one of a series of reports
prepared under the agreement, specifically addresses the role of, and need for, modeling in
support of remedial decision-making. Though the report addresses all pathways, the
emphasis is placed on ground-water flow and transport modeling. Other reports in this series
will address the selection and application of ground-water models and the evaluation of
multimedia models.
This report and the other reports in the series are intended to be used by the Remedial
Project Manager (RPM) at National Priorities List (NPL) sites or the equivalent at non-NPL
sites containing radioactive materials. They are also intended to be used by geologists and
geoscientists responsible for identifying and implementing ground-water flow and transport
models at such sites.
This report describes modeling in each of the phases of the remedial process; from scoping
and planning, to site characterization and risk assessment, to the selection and
implementation of remedial alternatives. The report attempts to address questions regarding
why modeling is needed, when it is needed, and when it is not needed. In addition, the
report describes when simple versus more complex models may be needed to support
remedial decision-making.
Modeling needs are described in terms of the following five interrelated factors:
1. reasons for modeling,
2. waste characteristics,
3. site environmental characteristics,
4. site land use and demography, and
5. phase of the remedial process.
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The principal reasons for modeling include: (1) the performance of risk assessments and the
evaluation of compliance with applicable health and safety regulations, (2) the design of
environmental measurements programs, and (3) the identification, selection, and design of
remedial alternatives. Each of these reasons for modeling influences modeling needs and
model selection differently.
Risk assessments can be performed with relatively simple models for site screening or may
require more advanced models when attempting to quantify risks at particular locations and
points in tune. The selection of the optimum sampling locations and the identification and
design of remedies often require the use of complex 2- and 3-dimensional models to achieve
the above objectives. Current regulations and guidelines, which are summarized in Appendix
A, do not explicitly require modeling. However, if used appropriately, modeling can be
useful in support of remedial decision-making in all phases of the remedial process.
A review of the physical, chemical, and radiological properties of the waste at a number of
remedial sites, which are described in Appendix B, reveals that the waste characteristics can
be diverse. At sites currently undergoing or scheduled for remediation, over 30 different
types of radionuclides have been identified, each with its own radiological and chemical
properties. The waste is found in a variety of settings, including contaminated soil, in
ponds, in storage piles and landfills, buried in trenches, and in tanks and drums. Each of
these settings influences the areal distribution of the contaminants and rate at which they may
leach into the underlying aquifer, which, in turn, influences modeling needs.
In a similar manner, the environmental characteristics of remedial sites are highly diverse.
The sites containing radioactive materials that are currently undergoing remediation include
both humid and dry sites, sites with and without an extensive unsaturated zone, and sites with
simple and complex hydrogeological characteristics. These different environmental settings
determine the processes that need to be modeled, which, in turn, influence modeling needs.
The land use and demographic patterns at a site, especially the location and extent of ground-
water use, affects the types and complexity of the models required to assess the potential
impacts of the site on public health. At many of the sites contaminated with radioactive
materials, the principal concern is the use of the ground water by present or future residents
located close to, and downgradient from, the source of contamination. At other sites, the
concern is the use of municipal welts located at some distance and in a variety of directions
from the source. Each of these usage patterns influences modeling needs.
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Superimposed on these waste and site related issues is the different modeling needs
associated with the various phases of the remedial process. The phase of the remedial
process from scoping and planning, to site characterization, to remediation, creates widely
different opportunities for modeling, which, together with the other factors, influences
modeling needs.
Finally, the report also emphasizes that modeling is not always needed or appropriate to
support remedial decision-making. If a site is poorly characterized or poorly understood, any
simulation of the transport and impacts of contaminants using mathematical models is
speculative at best and could be highly misleading. Accordingly, the use of models under
such circumstances is limited with regard to the types of decisions they can help to support.
In summary, models are data analysis tools which can be useful in supporting decisions, but
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 fully appreciated. Notwithstanding the limitations of models, it can be concluded that
it is difficult to support remedial decisions or the assessment of risks at a site without the use
of models.
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References
All 85 Aller, L., T. Bennet, J.H. Laher and RJ. Betty, "DRASTIC: A
Standardized System for Evaluating Ground Water Pollution Potential Using
Hydrologic Settings," EPA 600-285-018, National Water Well Association for
U.S. Environmental Protection Agency, Office of Research and Development,
Ada, Oklahoma, 1985.
CRC 90 CRC Press, Handbook of Chemistry and Phvsics. Robert C. Weast, ed.,
Chemical Rubber Company, Cleveland, Ohio, 1990.
Die 75 Diem, K., and C. Lentner, eds., Documenta GEIGY: Scientific Tables. 7th
ed.. Geigy Pharmaceuticals, Ardsley, New York, 810 pp., 1975.
DOE 89 Department of Energy, "A Manual for Implementing Residual Radioactive
Material Guidelines," DOE/CH/8901, June 1989.
DOE 89a Department of Energy, "CERCLA Requirements," DOE 5400.4, 10/6/89.
DOE 90 Department of Energy, "General Environmental Protection Program," DOE
5400.1, Change 1, June 29, 1990.
Eis 63 Eisenbud, M.,"Environmental Radioactivity," McGraw Hill, New York, 1963.
Eis 90 Eisenbud, M,, "An Overview of Sites Contaminated with Radioactivity," in:
Health and Ecological Implications of Radioactivelv Contaminated
Environments: Proceedings of the 26th Annual Meeting of the NCRP. No. 12,
Washington, D.C., pp. 5-19, 1990.
EPA 87 Environmental Protection Agency, "Guidance on Preparing Superfund
Decision Documents" (ROD guidance), EPA/624/1-87/001, 1987.
EPA 88 Environmental Protection Agency, "Guidance for Conducting Remedial
Investigations and Feasibility Studies Under CERCLA," EPA/540/G-89/004,
OSWER Directive 9355.3-01, October 1988.
EPA 88a Environmental Protection Agency, "Superfund Exposure Assessment Manual,"
EPA/540/1-88/001, OSWER Directive 9285.5-1, April 1988.
EPA 88b Environmental Protection Agency, "CERCLA Compliance with Other Laws
Manual," EPA/540/G-89/006,.August 1988.
EPA 89 Environmental Protection Agency, "Risk Assessment Guidance for Superfund,
Volume II, Environmental Evaluation Manual" (EPA/540/1-89/001), 1989.
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EPA 89a Environmental Protection Agency, "Ecological Assessment of Hazardous
Waste Sites: A Field and Laboratory Reference" (EPA/600/3-89/013), 1989.
EPA 89b Environmental Protection Agency, "Risk Assessment Guidance for Superfund:
Volume I, Human Health Evaluation Manual - Part A" (HHEM), EPA/540/
1-89/002, December 1989.
EPA 90 Environmental Protection Agency, "Scoping Study - Clean-up Criteria for Sites
Contaminated with Radioactivity - Site Contamination Assessment," Contractor
Report No. 1-42, Contract No 68D90107, Prepared by S. Cohen &
Associates, Inc. for the EPA Office of Radiation Programs, Work Assignment
Manager Jack Russell, August 1990.
EPA 91 Environmental Protection Agency, "Role of the Baseline Risk Assessment in
Superrund Remedy Selection Decisions," OSWER Directive 9355.0-30, April
1991.
EPA 91a Environmental Protection Agency, "Human Health Evaluation Manual,
Supplemental Guidance: Standard Default Exposure Factors," OSWER
Directive 9285.6-03, March 25, 1991.
EPA 91b Environmental Protection Agency, "Risk Assessment Guidance for Superfund:
Volume I, Human Health Evaluation Manual - Part B, Development of Risk
Based Preliminary Remediation Goals" (HHEM), PB92-963333, December
1991.
EPA 91c Environmental Protection Agency, "Risk Assessment Guidance for Superrund:
Volume I, Human Health Evaluation Manual - Part C, Risk Evaluation of
Remedial Alternatives" (HHEM), PB92-963334, December 1991.
EPA 91d Environmental Protection Agency, "Superfund Record of Decision - Maxey
Flats Nuclear Disposal, KY," EPA/ROD/RO4-91/097, September 1991.
EPA 91e Environmental Protection Agency, Health Effects Assessment Summary
Tables: FY-1991 Annual. OERR 9200.6-303(91-1), Office of Research and
Development and Office of Emergency and Remedial Response, Washington,
D.C., 1991.
Hea 68 Heath, R.C., and F.W. Trainer, Introduction to Groundwater Hydrology. John
Wiley and Sons, Inc., New York, 284 pp., 1968.
HEW 70 Department of Health, Education, and Welfare, Radiological Health
Handbook. Public Health Service, Food and Drug Administration, Bureau of
Radiological Health, Public Health Service Publication No. 2016, 458 pp.,
1970.
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Ken 92 Kennedy, W.E., Jr. and D.L. Strenge, "Residual Radioactive Contamination
from Decommissioning - Technical Basis for Translating Contamination Levels
to Annual Total Effective Dose Equivalent," Prepared by Pacific Northwest
Laboratory for the U.S. Nuclear Regulatory Commission, NUREG/CR-5512,
PNL-7994, Volume 1, October 1992.
Mur 66 Murphy, R.E., The American City: An Urban Geography. McGraw-Hill Book
Company, New York, 1966.
Naw 89 Nawar, M. (Work Assignment Manager), "Final Review of the Maxey Flats
Assessment Provided in Support of the December 1989 Feasibility Study
Report." Prepared by S. Cohen & Associates, Inc. for the EPA Office of
Radiation Programs, Contract No. 68D90170, Work Assignment 1-6, June
1991.
NRC 90 Nuclear Regulatory Commission, Policy Issue from James M. Taylor,
Executive Director for Operations, to The Commissioners, Site
Decontamination Management Program, SECY-90-121, March 29, 1990.
NRC 90a "Background Information for the Development of a Low-Level Waste
Performance Assessment Methodology," Volumes 1 through 5. Prepared by
Sandia National Laboratories for the U.S. Nuclear Regulatory Commission,
NUREG/CR-5453, August 1990.
NRC 91 Nuclear Regulatory Commission, Policy Issue from James M. Taylor,
Executive Director for Operations, to The Commissioners, Updated Report on
Site Decommissioning Management Plan, SECY-91-096, April 12, 1991.
Par 91 Pardi, R., P.D. Moskowitz, and M. Daum, "Environmental Characteristics of
Superfund Sites Contaminated with Radioactive Substances," Biomedical and
Environmental Assessment Group, Brookhaven National Laboratory,
Prepared for the Office of Radiation Programs, U.S. EPA, September 26,
1991.
SCA 90 Sanford Cohen & Associates, Inc. and Roy F. Weston, Inc., "Draft Report -
Radiological Risk Assessment Requirements Definition," Prepared for the EPA
Office of Radiation Programs, Contract No. 68D90170, Work Assignment 1-6,
Work Assignment Manager Robert S. Dyer, September 25, 1990.
Til 83 Till, John E. and H. Robert Meyer eds., "Radiological Assessment - A
Textbook on Environmental Dose Analysis," Prepared for the U.S. Nuclear
Regulatory Commission, NUREG/CR-3332, September 1983.
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APPENDIX A
REGULATORY REQUIREMENTS AND GUIDELINES
PERTAINING TO FATE AND EFFECTS MODELING
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REGULATORY REQUIREMENTS AND GUIDELINES
PERTAINING TO FATE AND EFFECTS MODELING
This appendix summarizes the RCRA/CERCLA requirements and guidelines, DOE Orders,
and NRC programs and guidelines that establish the regulatory framework pertinent to fate
and effects modeling. Like Section 2, this appendix describes the role of, and need for,
modeling; however, the discussion is presented within the context of specific regulations and
programs concerned with the remediation of sites contaminated with radioactive material.
The summary reveals that though current regulations and guidelines pertaining to site
remediation do not explicitly require fate and effects modeling, modeling is needed to
support informed decision-making pertinent to site remediation.
A.1 RCRA/CERCLA
Table A-l presents the overall structure of the RCRA and CERCLA process. This section
describes the role of fate and effects modeling in fulfilling the RCRA/CERCLA regulatory
requirements.
Fate and effects modeling is not explicitly required by CERCLA or RCRA. However, in
many cases, in order to meet CERCLA and RCRA requirements for risk assessments and in
support of remedial decision-making, fate and effects modeling is useful. It can be valuable
in each of the following phases of die remedial process:
• The scoping and planning phase of the Remedial Investigation/RCRA Facility
Investigation (RI/RFI). This early phase of the RI/RFI process is equivalent to
the generic Phase 1 defined in Section 2.
• The site characterization phase, including the performance of the baseline risk
assessment, of the RI/RFI. This phase of the RI/RFI process is equivalent to
the generic Phase 2 defined in Section 2.
• The Feasibility Study/Corrective Measures Assessment (FS/CMA) and
remedial implementation phases of the remedial process. These phases of the
RCRA/CERCLA processes are equivalent to the generic Phase 3 defined in
Section 2.
The following sections describe the RCRA/CERCLA regulations and existing and developing
guidelines that effectively require or create opportunities for modeling during each phase of
the remedial process.
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TABLE A-l*
COMPARISON OF THE RCRA CORRECTIVE ACTION PROCESS
AND THE CERCLA RESPONSE PROGRAM
RCRA
CERCLA
RCRA Facility Assessment
Performed to identify and gather information on
releases at RCRA facilities, make preliminary
determinations regarding releases of concern and
identify the need for further actions and interim
measures at the facility.
Performed in three phases: 1) Preliminary
Review. 2) Visual Site Inspection, and 3)
Sampling Visit (if necessary).
CERCLA Preliminary
Assessment/Site Inspection
(Site Assessment Process)
Performed to gather initial information on
identified sites in order to complete the
Hazard Ranking System to determine
whether removal (emergency) actions are
required.
The Preliminary Assessment and the Site
Inspection are performed in two phases and
may or may not involve sampling.
RCRA Facility Investigation
Defines the presence, magnitude, extent.
direction and rate of movement of any
hazardous wastes and hazardous constituents
within and beyond the facility boundary. The
scope is to:
a) characterize the potential pathways of
contaminant migration.
b) characterize the source(s) of
contamination.
c) define the degree and extent of
contamination,
d) identify actual or potential receptors, and
e) support the development of alternatives
from which a corrective measure will be
selected by the EPA.
The RFI is performed in seven tasks:
1) Description of current conditions
2) Identification of preliminary remedial
measures technologies
3) RFI workplan requirements
- project management plan
- data collection quality assurance plan
- data management plan
- health and safety plan
- community relations plan
4) Facility investigation
5) Investigation analysis
6) Laboratory and bench-scale studies
7) Reports
CERCLA Remedial Investigation
Performed to characterize the extent and
character of release of contaminants. The
RI is the mechanism for collecting data to
characterize site conditions; determine the
nature of the waste; assess risk to human
health and the environment: and conduct
treatability testing as necessary to
evaluate the potential performance
and cost of the treatment technologies
that are being considered.
Although EPA guidance presents a
combined RI/FS Model Statement of Work.
the RI is generally considered to be
performed in seven tasks:
1) Project planning (scoping)
- summary of site location
- history and nature of problem
- history of regulatory and response
actions
- preliminary site boundary
- development of site operations
plans
2) Field investigations
3) Sample/analysis validation
4) Data evaluation
5) Assessment of risks
6) Treatability study/pilot testing
7) RI reporting
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TABLE A-l CONTINUED
COMPARISON OF THE RCRA CORRECTIVE ACTION PROCESS
AND THE CERCLA RESPONSE PROGRAM
RCRA
CERCLA
Correctlve Measures Study
The purpose of the CMS is to identify, develop.
and evaluate potentially applicable corrective
measure(s) and to recommend the corrective
measure(s) to be taken. The CMS is performed
following an RFI and consists of the following
four tasks:
1) Identification and development of the
corrective measures alternative(s)
2) Evaluation of the corrective measure
alternative(s)
3) Justification and recommendations of the
corrective measures alternative(s)
4) Reports
CERCLA Feasibility Study
The FS serves as the mechanism for the
development, screening, and detailed
evaluation of alternative remedial actions.
As noted above, the RI and the FS are
intended to be performed concurrently:
however, the FS is generally considered to
be composed of four general tasks:
1) Remedial alternatives development
and screening
2) Detailed analysis of alternatives
3) Community relations
4) FS reporting
Corrective Measures Implementation
CERCLA Remedial Design/Remedial
Acti on
This activity includes the development of
the actual design of the selected remedy
and implementation of the remedy through
construction. A period of operation and
maintenance may follow the RA activities.
Generally, this aspect of CERCLA response
includes:
1) Plans and specifications, including:
- preliminary design
- intermediate design
- prefinal/final design
- estimated cost
- correlation of Plans and
Specifications
- selection of appropriate RCRA
facilities
- compliance with requirements of
other environmental laws
- equipment startup and operator
training
2) Additional studies
3) Operation and maintenance plan
4) Quality assurance project plan
5) Site safety plan
6) A/E services during construction
The authors would like to thank Roy F, Weston. Inc. for providing this comparison overview
of RCRA and CERCLA requirements.
The purpose of the CHI is to design.construct.
operate, maintain and monitor the performance
of the corrective measures selected. The CMI
consists of four activities:
1) Corrective Measure Implementation
Program Plan
2) Corrective Measure Design, including
- design plans and specifications
- operation and maintenance plan
- cost estimate
- schedule
- construction Quality/Assurance
Objectives
- health and safety plan
- design phases
3) Corrective Measures Construction
(including the preparation of a
Construction Quality Assurance Program)
4) Reporting
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A. 1.1 The National Contingency Plan
The 1990 National Contingency Plan (NCP) (55 Fed. Reg. 8665 - 8865, March 8, 1990)
calls for a site-specific baseline risk assessment to be conducted, as appropriate, as part of
the remedial investigation [Section 300.430(d)(l)]. Specifically, the NCP states that the
baseline risk assessment should "characterize the current and potential threats to human
health and the environment that may be posed by contaminants migrating to ground water or
surface water, releasing to air, leaching through soil, remaining in the soil, and
bioaccumulating in the food chain" [Section 300.430(d)(4)]. The primary purpose of the
baseline risk assessment is to provide risk managers with an understanding of the actual and
potential risks to human health and the environment posed by the site and any uncertainties
associated with the assessment. This information may be useful in determining whether a
current or potential threat to human health or the environment exists that warrants remedial
action.
In order to satisfy CERCLA requirements regarding the performance of a baseline risk
assessment, environmental measurements programs are performed to determine the nature
and extent of the contamination in the vicinity of the site. Based on these measurements and
other demographic, land use, and environmental data, the actual or potential human health
and environmental impacts attributable to the contaminants are assessed. However,
environmental measurements alone are not always sufficient to support the performance of a
baseline risk assessment because the number and types of samples can be representative only
of a limited area in the vicinity of the site. In addition, environmental measurements are
sometimes of little use in characterizing past and future conditions at the site and how the
conditions at the site may change due to changing environmental conditions, such as severe
weather and seasonal weather changes. There may also be questions about data quality and
representativeness. Because of these inherent limitations in environmental measurements
data, fate and effects modeling is used to supplement field measurements hi the performance
of baseline risk assessments.
A.1.2 Remedial Investigation (RI)/Feasibility Study (FS) Guidance
As applied to fate and effects modeling, RI/FS guidance can be conveniently discussed from
two separate perspectives. The first is RI/FS guidance as it applies to baseline risk
assessments and evaluating compliance with ARARs. The second is RI/FS guidance that,
though not directly related to modeling, creates opportunities for modeling.
Guidance Pertaining to the Performance of Baseline Risk Assessments and the Assessment of
Compliance with Site Specific ARARs
"Guidance for Conducting Remedial Investigations and Feasibility Studies Under CERCLA"
(EPA/540/G-89/004, October 1988) describes how the baseline risk assessment fits into the
overall RI/FS process. As indicated in the guideline "...the baseline risk assessments provide
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an evaluation of the potential threat to human health and the environment in the absence of
any remedial action. They provide the basis for determining whether or not remedial action
is necessary and the justification for performing remedial actions." The guideline also
addresses the role of, and need for, fate and effects modeling in evaluating risks and refers to
other guidelines describing methods acceptable to the Agency for performing risk
assessments.
"Superfund Exposure Assessment Manual" (EPA/540/1-88/001, April 1988) provides the
RPMs with the guidance necessary to conduct exposure assessments that meet the needs of
the Superfund human health evaluation process. Specifically, the manual "(1) provides an
overall description of the integrated exposure assessment process as it applies to uncontrolled
hazardous waste sites; and (2) serves as a source of reference concerning the use of
estimation procedures and computer modeling techniques for the analysis of uncontrolled
sites." The manual includes an overview of air, surface water, and ground-water modeling.
"CERCLA Compliance with Other Laws Manual" (EPA/540/G-89/006, August 1988,
referred to as the Other Laws Manual) presents guidance on identifying Applicable or
Relevant and Appropriate Regulations (ARARs) that may be used to determine if remedial
activities are warranted and for establishing remedial goals. Section 5 of the Other Laws
Manual presents a list of potential ARARs for sites contaminated with radioactive materials.
Inspection of the Other Laws Manual and other more recent EPA guidelines pertaining to
ARARs (EPA 91, EPA 91a,b) reveals that the ARARs for a site contaminated with
radioactive material can be expressed in terms of radionuclide concentrations in water
supplies (i.e., pCi/L), radionuclide concentrations in soil (pCi/g), and radiation dose
(mrem/yr). Accordingly, field measurements programs and fate and effects models must
provide results that have meaning in terms of the media, pathways, and units delineated in
the ARARs.
As part of the RI/FS process for the Maxey Flats Disposal Site (MFDS), the EPA defined
specific ARARs for radioactivity and required that the baseline risk assessment be performed
without taking credit for institutional controls. This requirement effectively defined the types
of exposure scenarios, and therefore the types of pathway models, required to evaluate
compliance with the ARARs. Specifically, the EPA required that the radiation doses and
risks be determined for a broad range of exposure scenarios, including the various intruder
and offsite exposure scenarios described in Section 2 (EPA 91d). Although these guidelines
apply specifically to the MFDS, they help to establish a regulatory precedent that may apply
to other sites contaminated with radioactive material and thereby establish modeling needs.
The "Risk Assessment Guidance for Superfund: Volume I, Human Health Evaluation
Manual - Part A" (HHEM) (EPA/540/1-89/002) provides guidance on how to conduct the
human health portion of the baseline risk assessment. A separate chapter specifically
addresses radiological risk assessment. Part B of this guideline presents methods acceptable
to the Agency for establishing remediation goals (EPA 91b). Specific calculational
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methodologies (i.e., models), along with default assumptions and calculational parameters,
are provided for determining remediation goals in terms of contaminant concentrations in
specific media.
Volume II of the "Risk Assessment Guidance for Superfund," the "Environmental Evaluation
Manual" (EPA/540/1-89/001), and the companion manual "Ecological Assessment of
Hazardous Waste Sites: A Field and Laboratory Reference" (EPA/600/3-89/013) provide
guidance on conducting the environmental portion of the baseline risk assessment. Other
pertinent guidance includes the "Guidance on Preparing Superfund Decision Documents"
(ROD guidance) (EPA/624/1-87/001), which provides information on how to document the
results of the baseline risk assessment in the ROD.
These documents establish the basic regulatory structure pertaining to the performance of risk
assessment in support of remedial decision-making. However, the guidelines are continually
being supplemented and refined. For example, additional guidance has been established
which further defines cleanup criteria and the assumptions that should be used to perform
risk assessments. OSWER Directive 9355.0-03, "Role of the Baseline Risk Assessment in
Superfund Remedy Selection Decisions," April 1991, presents additional numerical criteria
for identifying the risks that warrant remedial action and the criteria to be used in
establishing remediation goals. This document effectively supplements the "Other Laws
Manual." OSWER Directive 9285.6-03, "Risk Assessment Guidance for Superfund:
Volume I, Human Health Evaluation Manual Supplemental Guidance Standard Default
Exposure Factors," March 25, 1991, defines the current and future exposure scenarios that
should be used to perform risk assessments.
These documents require the performance of risk assessment throughout the remedial
process, and define methods generally acceptable to the Agency for performing risk
assessments.
Other Opportunities for Modeling
A review of the EPA guidance reveals that, in addition to the obvious need for fate and
effects modeling in support of the baseline risk assessment, the RI/FS process offers
numerous other opportunities for modeling. Ultimately, all of these opportunities for
modeling pertain to the assessment and management of risk. However, throughout the
remedial process, specific decisions need to be made in accordance with RI/FS guidance.
This section provides an overview of the role of modeling in support of these decisions.
During the early stages of the remedial process, fate and effects modeling may be used to
support decisions to implement a tune-critical removal action (53 FR 51109, December 21,
1988). Such a decision is appropriately based on environmental measurements or
observations. However, if the hazardous material has not yet reached receptor locations, but
fate and effects models reveal that the contaminants can reach receptors in the near future, a
tune-critical response may be appropriate.
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During the scoping phase of the RI/FS process, fate and effects modeling may be appropriate
as part of the development of the conceptual model of the site and the development of the
site characterization plan, both of which are required by EPA/540/1-89/002 (EPA 89b).
Preliminary fate and effects modeling will help to identify the critical pathways of exposure
and thereby help to define sample types, sample locations, and analytical techniques; i.e.,
data needs. Data needed to perform more reliable modeling in subsequent phases of the
remedial process will also be identified.
At sites with multiple operable units, modeling can be used during planning to establish
priorities and determine the potential significance of hydrogeologic connections between
adjacent operable units.
Near-field modeling can be used in the preparation of the site health and safety plan by
predicting worker exposures associated with intrusive sampling. Models can be used to
support health and safety decisions regarding the use of protective clothing, personnel
dosimetry, bioassay, and field instrumentation and monitoring for the protection of workers.
During the scoping and site characterization phases of the remedial process, ground-water
flow and transport modeling can be used to identify the sources of the contamination
observed in an aquifer. Such a modeling need could arise at a site where contamination of
an aquifer is observed, but the source of the contamination is unknown or uncertain.
During site characterization, as data are acquired, modeling can be used to refine, in an
iterative manner, the design of the field investigation programs. For example, modeling can
be used to determine the optimum location of monitor wells for intercepting the plume.
Conversely, modeling can be used to justify the elimination of unnecessary monitoring
locations and thereby help to control the costs of site characterization.
During the later stages of the remedial process, fate and effects models can be used to
identify feasible remedial alternatives and define the performance criteria for alternative
remedies required to ensure that the remedial goals are achieved. For example, in-situ
remedies based on engineered barriers may be appropriate at sites with relatively short-lived
radionuclides. At such sites, models can be used to predict radionuclide concentrations at
receptor locations using alternative barrier designs. In a related manner, acceptance criteria
for treatability studies can be determined by using fate and effects models. For example,
models can be used to define the pump and treat decontamination factors required to ensure
that remediation goals at receptor locations are achieved.
Models are also used to predict the impacts on workers, the public, and the environment
associated with alternative remedies. For example, excavation can result in the airborne
dispersion and transport of radionuclides or the contamination of surface water due to runoff.
Fate and effects models can be used to quantify the impacts and evaluate the cost-
effectiveness of measures for mitigating these impacts. The use of models in support of the
identification and implementation of remedial alternatives is discussed in more detail in
Section 3.5.3.
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A.2 DOE ORDERS PERTAINING TO THE REMEDIATION OF NPL SITES
DOE Order 5400.4, "Comprehensive Environmental Response, Compensation, and Liability
Act Requirements," establishes and implements DOE CERCLA policies and procedures as
prescribed by the NCP and under the overall framework established by DOE Order 5400.1,
entitled "General Environmental Protection Program Requirements." DOE Order 5400.4
refers to a broad range of laws, DOE orders, and Executive Orders that, in combination,
establish the overall DOE regulatory framework for the remediation of sites under
CERCLA/SARA. Within this regulatory framework, DOE enters into Interagency
Agreements (lAGs) and/or Federal Facility Agreements (FFAs) at both NPL and non-NPL
sites, as appropriate, with federal, state, and local entities for the execution of remedial
investigation/feasibility studies and remedial actions.
The role of modeling in support of remedial decision-making within this framework is
identical to that described above in Section A. 1, since the DOE is subject to CERCLA and
RCRA. To facilitate the decision-making process for the design and implementation of
remedial activities, the DOE has published "A Manual for Implementing Residual
Radioactivity Material Guidelines" (DOE 89). The manual adopts the RESRAD computer
code as the means for assessing and demonstrating compliance with the applicable regulations
and DOE orders. The model employs multi-pathway fate and effects algorithms which are
acceptable to the DOE for demonstrating compliance with applicable cleanup criteria.
A.3 NRC PROGRAMS
Under the Atomic Energy Act (AEA), as amended, the NRC has commercial regulatory
authority for source, special nuclear, and byproduct material. Though specifically exempt
under CERCLA and RCRA (unless mixed waste is present1), the technical issues associated
with the cleanup of sites licensed by the NRC are in many respects similar to the technical
issues associated with the remediation of sites under CERCLA and RCRA. Accordingly, the
types of remedial decisions and the role of models used to support these decisions are similar
for CERCLA/RCRA and for AEA sites.
This section briefly summarizes the NRC SDMP and the use of fate and effects models in
support of remedial decision-making. The technical approaches being used or developed by
the NRC have applicability to the modeling needs under CERCLA and RCRA. Conversely,
the technical approaches identified and developed in support of CERCLA and RCRA
program may be found to be useful to the NRC in meeting its modeling needs.
This section does not address other NRC programs dealing with (1) low-level radioactive
waste management under 10 CFR 61, which are the NRC regulations for licensing near
1 Though materials regulated under the Atomic Energy Act, as amended, are not subject to RCRA,
hazardous material, as defined under RCRA, at NRC-regulated sites is subject to RCRA.
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surface low-level waste disposal facilities, and (2) high-level radioactive waste management
under 10 CFR 60, which are the NRC regulations for licensing a high-level radioactive waste
repository. Though these NRC waste programs are concerned primarily with the licensing of
radioactive waste treatment and disposal technologies, and not with the remediation of
environmental contaminants, they are concerned with technological issues that have a bearing
on the characterization and remediation of environmental contaminants.
The Site Decommissioning Management Program (SDMP) (see SECY-90-121, March 29,
1990, and SECY-91-096, April 12, 1991) has identified 46 facilities under the authority of
the NRC that have a sufficient level of soil or water contamination to require special
attention from the staff. The Office of Nuclear Material Safety and Safeguards (ONMSS)
has regulatory responsibility for these sites and cannot terminate the licenses or release these
sites for unrestricted use until the sites are decontaminated. Toward this end, the SDMP has
the following elements:
Definition of a project management plan;
Identification of the sites requiring decontamination;
Prioritization of efforts;
Definition of decontamination schedule and resources;
Resolution of policy issues.
The last item is of particular relevance to this project, since it includes the need to develop
residual radioactivity criteria and methods for demonstrating compliance with the criteria.
The methods include the application of fate and effects models.
The SDMP makes specific reference to NUREG/CR-5512, entitled "Residual Radioactive
Contamination from Decommissioning - Technical Basis for Translating Contamination
Levels to Annual Total Effective Dose Equivalent" (Ken 92). NUREG/CR-5512 is adopting
standard, conservative, multi-pathway exposure scenarios for demonstrating compliance with
cleanup criteria. When completed, the document and the accompanying computer code will
provide a screening method for determining acceptable levels of residual radioactivity. It is
expected that many SDMP sites will fail that screening and require reanalysis using the same
models but with site-specific input data and assumptions. Should the site still fail the
screening process, more sophisticated modeling may be required.
NUREG/CR-5512 will be used to derive unit concentration total effective dose equivalent
(TEDE) factors, which relate a unit concentration of individual radionuclides in the
environment, including soil, to a dose. The TEDE factors for soil are being derived using a
set of conservative assumptions regarding the transport of residual contamination in multiple
pathways, including external exposure, inhalation of suspended dust, ingestion of garden
crops, and drinking water. The TEDE factors place an upper bound on the possible doses
associated with a unit concentration of residual radioactivity in soil. Once the TEDE factors
are adopted, the need to perform site-specific fate and effects modeling at SDMP sites may
be reduced.
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APPENDIX B
ENVIRONMENTAL CHARACTERISTICS OF NPL SITES
CONTAMINATED WITH RADIOACTIVE WASTE
AND NRC SITES IN THE SDMP
B-l
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ENVIRONMENTAL CHARACTERISTICS OF NPL SITES
CONTAMINATED WITH RADIOACTIVE WASTE
AND NRC SITES IN THE SDMP
This appendix briefly summarizes the overall waste types, waste forms, and site
characteristics of the National Priorities List (NPL) sites that are contaminated with
radioactive materials and the sites in the NRC's Site Decommissioning Management Program
(SDMP). The purpose of this appendix is to describe the range of site conditions and
processes that may need to be modeled to support remedial decision-making at sites
contaminated with radioactive material.
For each set of sites (i.e., NPL and SDMP sites), the following information is provided:
• a listing of the sites containing radioactive waste materials;
• the types of radionuclides found at each site;
• a description of the physical forms of the waste;
• a description of the physical characteristics of the site itself; and
• demographic characteristics of the region surrounding the site.
B.I NPL SITES CONTAINING OR CONTAMINATED WITH RADIOACTIVE
MATERIALS
Data characterizing the NPL sites were obtained from "Environmental Characteristics of
EPA, NRC, and DOE Sites Contaminated with Radioactive Substances," EPA 402-R-93-011,
March 1993. The information contained in this report was obtained from readily available
EPA, DOE, and NRC publications. No attempt was made to compile and review the large
amount of information being gathered as part of the RI/FS process. The intent of the report
is to provide survey-level information describing the general types and characteristics of the
sites on the NPL and the SDMP contaminated with radioactive materials.
B.I.I Categories of Sites
The NPL currently includes 45 sites contaminated with radioactive materials (FR
54(134):29820-29825, July 14, 1989)1. While each of the sites has specific physical and
1 At the time of the preparation of this report, there were 45 sites on the NPL containing substantial
quantities of radioactive material. The number of sites on the NPL with radioactive contamination has since
increased to 48. In addition, DOD sites containing radioactive material have also been identified. It was not
possible to revise this report to reflect these recent developments. In addition, since the primary purpose of this
appendix is simply to disclose the range of waste types and site characteristics, a detailed description of the most
current list of sites is not essential.
B-2
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environmental characteristics which will be discussed later in this section, and while some
sites could be placed in more than one group, it is possible to characterize these sites broadly
based on their historical usage. Seven general site types can be identified:
Defense Plants (all DOE facilities)
Mill Tailings, Processing, and Disposal Sites
Radium Sites
Commercial Landfills
Research Facilities
Commercial Manufacturing
Low-level Waste Disposal
Defense Plants
Of the 45 sites, 15 were involved in operations related to weapons manufacture. Included in
this group (all of which are under DOE supervision) are:
Fernald Environmental Remediation Project (FERP)
Hanford (4 sites)
Lawrence Livermore Laboratory (2 sites)
Idaho National Engineering Laboratory
Mound Plant
Oak Ridge Reservation (includes Oak Ridge National Lab)
Pantex Plant
Rocky Flats Plants
Savannah River Site
Weldon Springs (2 sites)
Many of these sites have been in operation since World War II. They were involved in the
handling of high levels of actinides, transuranics, and fission products. Each site's
involvement with these radioactive materials spans a time during which there was an
evolving concern for the environmental consequences of such disposal (see, for example,
Eis 63 and Eis 90). The potential risks to health and environment from these sites are high
relative to the other categories of sites due to the high level and huge quantity of materials at
each of these sites. In addition, the distribution of the contamination tends to be complex.
For security reasons, many of these sites are located in sparsely populated regions. They are
generally located in arid or semi-arid regions not particularly suitable for agriculture or
residential use.
Mill Tailings. Processing, and Disposal
Twelve sites were or still are involved with the processing and disposal of uranium ore for
military and commercial operations. These operations easily rank first in terms of the shear
volume of contaminated materials involved. Included in this list are:
B-3
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Homestake Mining Co.
Kerr-McGee (Kress Creek, Reed-Kepler Park, Residential Areas, Sewage Treatment
Plant, Gushing) (4 sites)
Lincoln Park
United Nuclear - Church Rock
Monticello Mill Tailings & Radioactively Contaminated Properties (2 sites)
St. Louis Airport/Hazelwood Interim Storage/Futura Coatings
Uravan Uranium (Union Carbide)
WR Grace/Wayne Internment (DOE)
Common to these sites are very large volumes of wastes containing primarily uranium and
thorium along with their daughters. The activity levels, however, are generally low. In
some cases, contaminated materials have been widely distributed long after their initial
disposal.
Radium Sites
Eleven of the sites fall in this radium processing category:
Denver Radium Site
Glen Ridge Radium Site
Jacksonville Naval Air Station
Lansdowne Radiation Site
Lodi Municipal Well
May wood Chemical Co.
Montclair/West Orange Radium Site
Ottawa Radiation Sites
Pensacola Naval Air Station
Radium Chemical Corporation
U.S. Radium Corp.
Many of these sites were in operation long before any harmful effects of radiation were
recognized and before any regulatory mechanisms were in place to control the use of
radioactive materials. The operations were, in general, relatively small, primarily limited to
radium and its daughters (especially radon), and often located in urban areas. Because of the
relatively long history of these sites, contaminated materials have been widely distributed,
including incorporation into building materials.
Commercial Landfills
Four of the sites were operated as general-purpose waste landfills, which were, at some time
during their operation, contaminated by radioactive wastes:
B-4
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Forest Glen Mobile Home
Himco Inc., Dump
Shpack Landfill (DOE)
Westlake Landfill
There is no indication that any special plans were made to isolate or contain radioactive
materials, other than routine practices at landfills. The isotopes present at these sites vary
widely, as they originated from various medical, research, and defense operations. For these
sites, the precise form or original concentration of the radioactive materials present are not
known.
Research Facilities
One of the sites, Brookhaven National Laboratory, is a dedicated research facility operated
for the Department of Energy. Radioactive materials are employed or produced in various
research activities not necessarily related to defense. A wide range of isotopes were disposed
of at very low levels in landfills or into ground water, trenches, and other disposal facilities,
which has resulted in ground-water contamination.
Commercial Manufacturing
One site, Teledyne Wah Chang, is involved in the commercial manufacture of products
related to the nuclear industry. In that capacity, sludge materials were contaminated with
actinide elements. The nature and distribution of contaminated materials is fairly well
defined at this site.
Low-Level Waste Disposal
One site, the Maxey Flats Disposal Site (MFDS) in Morehead, Kentucky, operated as a
licensed low-level radioactive waste disposal site from 1963 to 1977, when operations ceased
due to the determination that waste was migrating through the subsurface medium. As a
low-level waste disposal site, the MFDS received a variety of radioactive waste types.
However, the risk assessments performed in support of the RI/FS Report for the site reveal
that tritium in the leachate is of primary concern due to its relatively large inventory and
mobility (Naw 89).
B.1.2 Chemical and Radiological Properties of the Waste
Table B-l presents the number of sites containing specific radionuclides. The data are based
on Par 91. A more exhaustive review of the literature and ongoing RI/FS activities will
certainly change this distribution. However, the table provides insight into the relative
distribution of the various radionuclides. It is clear that the more prevalent radionuclides are
Co-60, Cs-137, H-3, Pu-238/239, Ra-226/228, radon, and the isotopes of thorium and
uranium.
B-5
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Table B-l. Number of Sites Containing Specific Radionuclides
Radioauclide
Actinium-227
Americium-241
Antimony- 125
Carbon- 14
Cobalt-60
Cerium-144
Cesium- 134
Cesium-135
Cesium-137
Curium-244
Europium- 152
Europium-154
Europium- 155
Hydrogen-3
Iodine- 129
Iodine- 131
Kiypton-85
Manganese-54
Nickel-63
No. pf Sites
2
3
1
2
7
1
2
1
10
1
1
1
1
11
3
1
1
1
1
Radionudide
Protactinium-231
Plutonium-238
Plutonium-239
Plutonium-240
Radium-226 (+ progeny)
Radium-228
Radon-220
Radon-222
Ruthenium-106
Selenium-79
Strontium-90
Technetium-99
Thorium-228
Thorium-230
Thorium-232
Uranium-234
Uranium-235
Uranium-238
No. of Sites
I
10
10
5
28
9
5
23
2
1
11
7
3
14
14
32
10
30
Chemical Properties of the Radionuclides
The preceding table shows that about 30 isotopes are present, which, for the purposes of this
report, can be assigned to the following four chemical groups based on then* environmental
behavior:
1. Non-metals (C, H, I, Rn, Se)
2. Transition, platinum-group metals, and lanthanides (Mn, Ni, Co, Ru, Tc, Eu,
Pm)
3. Alkaline metals and earths (Cs, Ra, Sr)
4. Actinides and transuranics (U, Th, Pu, Am, Ac, Pa)
Section 3.3 describes how the chemistry of these different radionuclides can affect transport
and, therefore, influence modeling needs.
Radioactive Properties
Many of the radionuclides have very long half-lives. As a result, they can be assumed to
persist indefinitely and, depending on the time period of interest, radioactive decay need not
be explicitly modeled. Examples of such long-lived radionuclides are U-238 and Th-232;
however, the ingrowth of progeny could be significant.
B-6
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r
Among the various radionuclides present at these sites, there are isotopes that decay to stable
daughters (e.g., C-14 to N-14) and others that decay to unstable daughters (e.g., 1-131 to
Xe-131). Some radionuclides present are associated with radioactive decay chains (e.g., Th
and U) and a mix of alpha (e.g., U-238), beta (e.g., Ra-228), and gamma (e.g., Mn-54)
emitters. Isotopes that decay to stable daughters through long chains of radioactive daughters
are, in fact, the most common materials at the NPL sites contaminated with radioactive
material.
B.I.3 Source Types, Containment, and Waste Forms
Hazardous wastes are most often stored or disposed of in a form designed to limit their
release to the environment. Even if containment was not deliberate, such as is the case for
many of the urban, radium-contaminated sites, soils and manmade materials themselves
provide some containment. The form of containment varies widely, but some broad classes
can be defined. Table B-2 presents the number of sites associated with specific source types
and waste forms, along with a brief description of the general characteristics of the sources
and forms of the waste.
Table B-2. Number of Sites Associated with Specific Source Types and Waste Forms
Settiag
ponds
surface water
wells
drums
tanks
landfills
piles
burial
asphalt & aggregate
Number
16
7
2
3
5
14
12
27
3
; Description
unlined or lined, excavated into the land surface into
which hazardous wastes were originally deposited in
fluids, usually water.
deliberate or accidental discharge to streams.
deliberate or accidental subsurface injection of waste.
usually SS-gallon steel drums.
large surface or buried structures designed to contain
waste for long periods.
deliberate above-ground facilities designed to limit the
escape of materials more or less indefinitely - usually
excavated into the landscape somewhat and surrounded by
some form of dike or embankment.
above-ground heaps of material without controls on
leaching or erosion.
accidental or indiscriminate burial of wastes above or
below ground level.
the use of materials contaminated with radioactivity for
construction purposes, generally by those unaware that
the materials were contaminated.
B-7
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Only about a quarter of the containment of radioactive materials at these sites is in a form
that might be described as a "point" source. A point source is one in which the area of the
source is small compared to the distance to the nearest potential receptor. Section 3.3
discusses how the different waste forms and types of containment may affect modeling needs.
B.I.4 Site Environmental Characteristics
The sites are distributed more or less randomly across the 48 contiguous states (see Figure
B-l). They span most of the large-scale physiographic and climatic regions of the North
American continent. The terrain where these sites are located is underlain by a wide range
of soils and geologic formations. Therefore, no general assumption can be made about the
climatic or hydrogeologic characteristics of these sites. Each site must be evaluated for the
specific conditions under which contaminants may be mobilized and the atmospheric and
subsurface properties that will control the direction and velocity of contaminant transport.
Surface Characteristics
Surface water transport is relevant to the ground-water pathway because surface water can
serve as a discharge location for ground-water flow or as a source of ground-water recharge.
Precipitation. The precipitation rate at the 45 NPL sites is as follows:
Category Precipitation (cm/vr) No. of Sites
Arid
Semiarid
Semihumid
Humid
Very Wet
0-25
25-50
50- 100
100-200
> 200
8
8
14
14
1
Gross precipitation, total average rainfall for a site, is a rough measure of the relative
importance of runoff and infiltration to the overall transport processes. As stated earlier,
these sites span a considerable geographic area. The same amount of rain falling in
Richland, Washington, as in Pensacola, Florida, will not have the same consequences for
contaminant transport. Similarly, a simple measure of net precipitation (average precipitation
less mean evaporation) does not adequately describe that fraction of precipitation which
ultimately enters into surface and ground-water systems even in arid regions. On the other
hand, an exhaustive analysis of the actual rate of runoff, soil percolation, and ground-water
recharge is beyond the scope of this report. For the present purpose, precipitation is used as
a general index of runoff and infiltration.
Surface Water Transport. Virtually all of the sites include perennial streams that can or do
act as vectors for contaminant transport. In addition, several of the sites contain or are
contained within freshwater wetlands. The presence of continuous standing bodies of water
B-8
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i
5
o o
r M
O
U
CO
J
(X
B-9
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surrounding contaminated materials or of bodies of water that may act as transient sinks for
contaminants is an important site characteristic. Only two of the sites (Pensacola and
Jacksonville in Florida) are sufficiently close to estuaries to suggest that tidal, as well as
density dependent, flow and transport mechanisms need to be considered. In addition, one
site, Idaho National Engineering Laboratory, is flooded with sufficient frequency to suggest
that transient flow and transport mechanisms should be given particular attention.
Subsurface Transport
Various attempts have been made to categorize hydrogeologic regions throughout the United
States (for a review, see All 85). Figure B-2 presents Heath's 1968 categorization system,
which divides the continental United States into 11 hydrogeologic regions (Hea 68). At least
one of the 45 NPL radioactively contaminated sites is in each of these regions.
For any given hydrogeologic setting, water movement below the ground surface can be
divided into two relatively distinct phases - transport through the vadose zone (subsurface
materials not saturated with water) and transport through the phreatic zone (water-saturated).
Depth-to-ground water is a measure of the thickness of the vadose zone. Of the 28 NPL
sites for which information regarding depth to ground water was readily available, 16 report
a relatively thin vadose zone (i.e., < 10 meters), 7 report a 10 to 20 meter vadose zone, and
5 report a vadose zone greater than 20 meters. As discussed in greater detail in Section 3.4,
depth-to-ground water is a key factor in determining modeling needs.
B.1.5 Receptor Characteristics
Contamination at a site is important because of the potential for causing harm to people or to
the natural environment. Most critical is the risk to the health of the human population in
the immediate area surrounding a contaminated site. This section provides data regarding the
population distribution and ground water use in the vicinity of the 45 NPL sites containing
radioactive material.
Population Data
County population or population density is frequently chosen as a gross measure of the
population potentially at risk because it is an easily available statistic and provides a rough
"region" around the site. However, county population is not necessarily characteristic of the
area immediately surrounding the site. For this reason, population in the immediate area
must also be known.
The county population densities at the 45 NPL sites tend to fall into four logarithmic groups,
as follows:
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en
PQ
o
o
I
a
u
S
I
Ou
I
I
U
«
K
It
B-ll
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Population Density
(persons/km2)
0- 10
10- 100
100 - 1,000
1,000-10,000
Number of NPL Sites
(radiological)
7
14
17
7
Where the population is less than 10/km2, an area may be considered unpopulated. Rural
regions are those with a population density of 10-100/km2. From 100-1,000/km2 an area is
classed as suburban, and population densities of 1,000/km2 or greater are labeled urban.
Defense plants are generally located in sparsely populated areas, frequently in the western
United States, largely for security reasons. For large facilities, like Hanford and Savannah
River, the presence of the facility itself may have significantly affected the population density
hi the area. Even sites in densely populated counties (e.g., FMPC) are located in
less-populated rural areas. Similarly, mining and processing of ore is a spatially extensive
industrial activity necessarily found in areas of low population density, though disposal of
wastes from the final processing stages may occur in more populated settings (e.g.,
Kerr-McGee, W.R. Grace). In contrast, the radium sites are more characteristically located
in higher-density urban areas. Other types of sites are less easily categorized.
It is necessary to examine population at several impact distances, since neither small-scale
nor large-scale regional densities can necessarily be extrapolated to each other and since
contaminant concentrations may vary considerably over distance. For example, the
Monticello site is located in a remote rural county hi southeastern Utah with a very low
population density. However, the commercial center of the town of Monticello, as well as
several residences, lies adjacent to the mill site; the population density of the immediate area
is nearly 200 tunes that of the county as a whole. Conversely, the Maywood site is located
hi a heavily urbanized county in New Jersey with a high population density. The area
immediately surrounding the site is mainly industrial, with a residential population density
less than a third that of the surrounding county. The appropriate target distance for
population estimates/measurements depends on the contaminants) in question and the various
factors affecting contaminant transport to the potential receptors.
The principal transport media for radioactive contaminants are water and air. Unlike air,
water use may vary from site to site. Ground water or surface water in the immediate site
area may be used for drinking, irrigation, watering of livestock, or recreational purposes.
Water for drinking is obtained from local supplies at 30 of the 45 sites, though this does not
necessarily mean that the water supplies are obtained from aquifers interconnected with the
sites. Two sites, Brookhaven National Laboratory and the Himco, Inc., Dump, are located
above Safe Drinking Water Act "Sole-Source Aquifers." Local surface waters are used for
recreation at seven sites and for agricultural purposes at five sites.
B-12
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Another aspect of receptor characterization is the land use associated with the area
surrounding a site. Broadly speaking, land use and population density are related. The four
population density groupings previously mentioned can be translated into categories of land
use.
Urban land use includes commercial, industrial, and medium- to high-density residential
uses. Commercial zones have a large but transient (non-resident) population. Population
density is far higher during the day than in residential areas, but may drop to nearly nothing
at night. Of the primary urban land use groups, industrial areas have the lowest potential
population at risk (Mur 66).
Suburban land use is usually thought of as medium- to low-density residential, but it also
includes large areas of commercial land (shopping malls) and industrial parks. The transient
populations of suburban commercial and industrial areas tend to be lower than those of then*
urban counterparts.
Rural areas include both agricultural and non-agricultural land. Agricultural land uses can
expose non-resident populations to risks from contamination in several ways. Cattle or other
animals may graze on contaminated ground or drink contaminated water. Contaminated
water may also be used to irrigate crops. The primary non-agricultural rural land use which
may result hi population exposures is recreation. Potential risks arise from swimming in
contaminated surface waters, soil ingestion by children at picnic sites, or eating contaminated
fish or game.
Unpopulated regions have essentially no regular use by human populations. Large areas of
desert which were used for nuclear weapons testing are an example of this land "use."
The majority, nearly 70 percent, of the sites are located in rural or suburban areas. Only
seven sites are located in urban areas. All are radium sites. Six of the seven are hi the
greater New York City area; the other (Lansdowne) is near Philadelphia. There are also
seven sites in counties classed as unpopulated. These sites are all located in the western
United States; all are mill tailings/processing/disposal sites.
B.2 DESCRIPTION OF SITES IN THE NRC SITE DECOMMISSIONING
MANAGEMENT PROGRAM
The data in this section were summarized from "Updated Report on the Site
Decommissioning Management Plan," SECY-91-096, April 12, 1991, and the database
provided hi Par 91.
Of the 41 SDMP sites, 30 are contaminated with large volumes of relatively low
concentrations of naturally occurring radionuclides associated with the ore and metal
processing industries. These residues are typically located in storage piles or ponds and
represent an actual or potential source of ground-water contamination. Most of the sites also
B-13
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contain contaminated buildings and structures, which represent a potential risk from direct
radiation and airborne participates in an occupational setting. Several of the sites report the
presence of fission products, including Sr-90 and Co-60.
Many of the sites are located in urban or industrialized areas, and the contaminated
properties are relatively small compared to many of the defense plants listed on the NPL (see
Section B. 1.1). The sites have a lot in common with many of the non-defense facilities on
the NPL. In fact, some of the SDMP sites are also Superfund sites (i.e., Kerr McGee
(Gushing), Westlake, Shpack). Figure B-3 presents the geographical distribution of the sites.
The distribution of geohydrological settings, according to Hea 68 (see Figure B-2), is as
follows.
Nonglaciated Central Region
Glaciated Central Region
Northeast and Superior Uplands
Piedmont and Blue Ridge
Number
15
10
12
4
Most of the NRC SDMP sites are located in the northeastern United States, with the
remainder in the middle west. This distribution reflects the fact that most of these sites are,
or were, commercial enterprises, engaged in manufacturing, uranium processing, or other
industrial activity. The NRC sites have been grouped according to the same classification
used for the NPL sites (Section B. 1), with the addition of two categories, Fuel Fabrication
and Processing and Scrap Metal Recovery. Though there are no low-level waste disposal
sites on the list, many of these and other NRC-licensed sites have waste buried on site in
accordance with Part 20.302 of Title 10 of the Code of Federal Regulations. These is also
one radium site, the Safety Light Corporation.
Defense Plants
Three NRC sites were involved in weapons manufacture:
Aberdeen Proving Ground
GSA Watertown Arsenal Site
Remington Arms Co., Inc.
Of these, the Watertown Arsenal (GSA) is currently operated by DOE; the other two are
under the control of the Department of Defense (DOD).
Whereas the NPL defense sites are generally located in the western United States, these sites
are located in the east or midwest. The NPL (DOE) sites are large, having been developed
primarily for the manufacture and testing of nuclear weapons under the Manhattan
Engineering District during World War II. The NRC sites, on the other hand, were involved
in the development and testing of ammunition for the U.S. Army and are comparatively
smaller.
B-14
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r
DSQQQ
B-15
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Mill Tailings. Processing, and Disposal
Eleven NRC sites fall into this category:
Cabot Corporation (3 sites)
Fansteel, Inc.
Heritage Minerals
Kerr-McGee Rare Earths Facility (West Chicago)2
Magnesium Elektron, Inc.
Molycorp., Inc. (2 sites)
Shieldalloy Metallurgical Corporation (2 sites)
Unlike the NPL sites in this group, not all of these sites deal with uranium ore. Some sites
process other ores (tantalum, columbium, zircon, leucoxene) which contain uranium or
thorium as a byproduct.
Commercial Landfills
The two commercial landfills in the NRC Site Decommissioning Management Plan are:
Kawkawlin Landfill
Westlake Landfill
The Westlake Landfill is also on the NPL list.
Research Facilities
The NRC research facilities are all private (commercial) operations:
Gulf United Nuclear Fuels Corporation
Permagrain Products
Westinghouse Electric (Waltz Mill)
The Gulf site carried out nuclear fuels research and development and included both
laboratories and reactors. The site was previously decontaminated, but additional
contamination was discovered, indicating a need for additional cleanup. The Permagrain site
is now owned by the Pennsylvania Forest Service. In addition to engineering design,
research, and development, the Westinghouse facility provides decontamination services to
nuclear power plants. Contamination from this site originated from a reactor accident in the
1960s.
2 Responsibility for this site was recently transferred to Superfund.
B-16
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Commercial Manufacturing
Twelve NRC/SDMP sites are or were involved in manufacturing processes:
Advanced Medical
Allied Signal
BP Chemicals
The Budd Company
Dow Chemical Company
Mallinckrodt Specialty Chemicals
Nuclear Metals, Inc.
Process Technology of New Jersey, Inc.
Safety Light Corporation
Schott Glass Technologies
Whittaker Corporation
Wyman-Gordon Company
The Dow Chemical site is actually three sites, though they are combined under the SDMP.
Two of these, at Midland and Bay City, Michigan, are manufacturing operations; the third,
at Salzburg, Michigan, is a landfill owned and operated by Dow, which Dow proposes to use
for disposal of low-level radioactive materials from the other two sites.
Fuel Fabrication and Processing
Eight NRC sites are, or have been, involved in uranium fuel fabrication and processing:
Amax
Babcock & Wilcox (Apollo)
Babcock & Wilcox (Parks Township)
Chemetron (Best Ave.)
Chemetron (Harvard Ave.)
Kerr-McGee (Cimmaron)
Kerr-McGee (Gushing)
Texas Instruments
One, Babcock & Wilcox (Parks Township), was also used for plutonium fuel fabrication.
Contamination at these sites is usually in the form of enriched and depleted uranium in the
soils in the vicinity of burial trenches, occasionally in surface soil around buildings in former
processing areas. Thorium, plutonium, and radium are also present.
B-17
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Scrap Metal Recovery
Two sites are industrial but cannot properly be classified as manufacturing operations:
Pesses Company (METCOA)
UNC Recovery Systems, Wood River
Both were involved in scrap metal recovery from contaminated materials (although
contamination may have resulted from other activities), and both have been closed for about
a decade. The UNC site is no longer functioning and has been remediated, though some
traces of uranium, strontium, and nitrates exist hi the ground water. Sources of thorium
contamination at the Pesses site include leaking containers and several slag piles.
B-18
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TECHNICAL REPORT DATA
(Please read instructions on the reverse before completing)
1. REPORT NO.
EPA 402-1^3-009
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Environmental Pathway Models-Ground-Water Modeling in
Support of Remedial Decision-Making.at Sites
Contaminated with Radioactive Material
5. REPORT DATE
"arch 1993
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
P.D. Moskowitz, Brookhaven National Laboratory
John Mauro, S. Cohen and Associates, Inc.
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
US EPA
0 ffice of Radiation and Indoor Air (6603J)
401 M St., SW
l-l ashington.DC 20460
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO
68P90170
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This report describes when modeling is neededand the various processes that need to
be modeled. Modeling needs are defined in the following terms:
1. Exisitng apllicable regulatory requirements
.2. The phase of the remedial process.
3. Site characteristics.
The report presents a generic discussion of the role and purpose of modeling in
support of remedial decision-making, with a particular emphasis on ground-water
modeling. Descritptions are provided for for various ground-water flow and
transport processes that may need to be modeled. A matrix describes ground-water
modeling needs as a function of site characteristics and phase in the remedial
process.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
ground-water, radionuclide, remediation,
computer models, decision-making,environmenta
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI field,Gtoup
18. DISTRIBUTION STATEMENT
19. SECURITY CLASS t This Rtporti
NO OF PAGES
20. SECURITY CLASS iTllis patv
11. "91CE
EPA Form 2220-1 (R»v. 4-77) PREVIOUS EDITION is OBSOLETE
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