J.L. Regens and K. Watanabe
Tulane University Medical
Center
C. Whipple
ICF Kaiser
C. Travis
Informatics International
D. Hoel
Medical University of South Carolina
G. Chieruzzi
ManTech Environmental
ABSTRACT
Multimedia risk assessment models are key tools for organizing and summarizing knowledge about environmental settings, contaminant sources, pollutant transport, and exposures. The model comparison effort conducted by the Consortium for Environmental Risk Evaluation is designed to compare model attributes and determine situations for which the models are best suited for applications to subsurface contamination.
INTRODUCTION
Designing and implementing effective strategies for environmental restoration at U.S. Department of Energy (DOE) sites requires valid, reliable estimates of the potential human health risks posed by the widespread radioactive and hazardous waste contamination of groundwater, soils, sediments, and surface water at over 100 DOE or its predecessor agency installations and facilities covering more than 3,300 square miles in 34 states and territories (1,2,3). This challenge underscores the importance of delineating health risks so remediation priorities and alternatives can be appropriately identified. Multimedia risk assessment models are a powerful decision support tool that provide a broad understanding of complex environmental processes (4). The models integrate information about contaminant concentrations and pathways as a basis for assessing human exposures in order to develop estimates of health risks associated with cleaning up the post-cold war legacy of the Manhattan Project.
The Consortium for Environmental Risk Evaluation (CERE), an integrated team of experts from universities, national laboratories, and consulting firms, is helping DOE identify the most urgent risks posed by weapons complex cleanup. CERE is designing, applying, and evaluating decision support tools for environmental management that can help transform DOE's tremendous information and knowledge base into the practical wisdom needed to perform successfully the cleanup mission by providing useful mechanisms to compare estimates of relative risks.
OVERVIEW OF THE CERE PROGRAM
CERE is a Tulane University/Xavier University partnership established in 1993 under a DOE cooperative agreement. CERE develops and applies credible scientific approaches, with meaningful stakeholder involvement, to assess technology performance, evaluate risks, estimate life cycle costs, and analyze public concerns linked to the cleanup of the weapons complex. Managed by Tulane University, the CERE program is funded by EM.
A guiding principle for CERE's efforts is to concentrate on significant EM problem areas. As a result, CERE is committed to maintaining a high-quality research effort in support of the Nation's need for credible scientific approaches addressing problems characterized by:
- Real DOE need for risk-based environmental management decisions
- Availability of viable alternative technologies
- Consistency with risk analysis applicability
- Amenable to solution in light of institutional issues
CERE's multimedia model comparison efforts focusing on subsurface contaminants are responsive to that need. The insights about baseline risk derived from the model comparison exercise illustrate how decision support tools can aid internal DOE decision makers, both at headquarters and the field, in appraising strategies for environmental management at the weapons complex. In addition to this primary audience, there are several secondary audiences which are likely to be interested in our results: 1) an expert audience of modelers with an interest in the model comparison's findings; and 2) a potential non-expert/expert audience of stakeholders (i.e., regulators, local publics) with a general interest in how DOE makes and communicates cleanup decisions.
EXTENT OF THE SUBSURFACE CONTAMINATION PROBLEM AT DOE SITES
The model comparison activities being conducted by CERE focus on aspects of the subsurface contamination problem confronting DOE as pursues cleanup of the weapons complex. Contamination of groundwater and soils is a pervasive problem across the weapons complex (5). The contaminants of concern include radionuclides such as plutonium, tritium, and strontium-90; heavy metals; and dense, nonaqueous phase liquids (DNAPLs) formed by chlorinated solvents such as trichloroethelene and carbon tetrachlorides. For example, DOE estimates reveal that more "than 5,700 known groundwater plumes have contaminated over 600 billion gallons of water and 50 million cubic meters of soil" (6). Migration of these plumes off-site as well as considerations about future land use on-site are significant considerations for EM managers.
In addition to groundwater plumes, landfills at DOE sites also "are estimated to contain over 3 million cubic meters of buried waste" (6). As the containers in which that waste is stored has degraded over time, the surrounding soils and groundwater have been contaminated by transuranic (TRU), low-level, and hazardous waste.
OVERVIEW OF THE MODEL COMPARISON EFFORT
Multimedia risk assessment models are key tools for organizing and summarizing knowledge regarding environmental settings, contamination sources, pollutant transport, and exposures for application in the environmental management decision-making process. They offer a consistent, analytically plausible means of describing relative risks. The multimedia approach involves tracking contaminants-of-concern from sources through multiple media (i.e., air, water, soil, and biota) to points of human and ecological exposure. The available models typically combine estimated human exposure via ingestion, inhalation, external radiation, and dermal a absorption with measures of potency for cancer and non-cancer endpoints to arrive at estimates of risk.
Risk assessments do not provide absolute estimates of risk. Instead, they offer conditional estimates that are contingent on a variety of assumptions about source terms, environmental settings, pollutant transport and fate, exposure scenarios as well as data quality and availability. As a result, the output of multimedia risk models is subject to model, scenario, and parameter uncertainties. Model uncertainty stems from the inability of mathematical models to simulate accurately complex environmental processes, especially when the simplifying assumptions inherent in model construction do not address technical issues such as fractured and karst media, complex terrain, multiple and perched aquifers, mass balance between pathways, nonaqueous phase liquids, and radionuclide progeny frequently encountered in the context of DOE sites. Scenario uncertainty results from incorrect conceptualization of a release and/or exposure scenario because of incomplete knowledge of historical events or erroneous assumptions about future land use. Parameter uncertainty occurs when site-specific values for input parameters are unavailable and generic data are substituted which fail to represent adequately site-specific variability.
The CERE model comparison effort is designed to determine whether and to what extent multimedia models are useful for risk assessments at DOE sites. It also is designed to compare model attributes and determine situations for which the models are best suited, using actual release site data with a common set of assumptions. CERE's model comparison effort will identify and evaluate:
- Inconsistencies among model parameters
- Data quality issues
- Barriers to effective use of models
- Potential for models to contribute to technology decisions for cleanup
Four multimedia models -- MEPAS from Pacific Northwest National Laboratory, MMSOILS from the U.S. Environmental Protection Agency (EPA), RESRAD from Argonne National Laboratory, and TRUE from the Electric Power Research Institute (EPRI) -- will be compared. These models were selected because of their ability to consider multiple exposure pathways and capability of computing environmental transport over both long and short time periods.
In making the model comparison, CERE will focus on significant subsurface contamination problem, especially groundwater pathways, to derive risk estimates. Criteria used for comparison of the models include:
- Mathematical models used to simulate contaminant transport and fate
- Internal chemistry and physics
- Resource requirements
- Treatment of source terms
- Treatment of exposure scenarios
- Similarity of results
Application of these criteria will allow CERE to delineate the performance characteristics of the four multimedia models for informing judgments about relative risk as a basis for making choices about DOE's cleanup program.
CONCLUSIONS
Model comparison activities offer a means to develop a better scientific understanding of differences in the formulation and performance characteristics of multimedia risk assessment models, including their relative sensitivity to uncertainty. Because multimedia models provide approximations of the movement, degradation, and accumulation of contaminants in the environment and the resulting health risk posed by exposures to those contaminants, insights gained from the use of such models -- especially for screening-level analyses and analyses of situations over large spatial or temporal domains -- can aid DOE in planning, making, and communicating the underlying basis for its environmental restoration choices. For example, knowing what contaminants drive risk at a given site allows decision makers to better understand the remediation problem and choose technologies that maximize risk reduction. Similarly, once multimedia models delineate the relative importance of exposure pathways, decision makers can weigh options to "break" key pathways, even if contamination remains at the site.
REFERENCES