An Evaluation of Three Representative Multimedia Models Used to Support Cleanup Decision-Making at Hazardous, Mixed, and Radioactive Waste Sites

The decision process involved in cleaning sites contaminated with hazardous, mixed, and radioactive materials is supported often by results obtained from computer models. These results provide limits within which a decision-maker can judge the importance of individual transport and fate processes, and the likely outcome of alternative cleanup strategies. The transport of hazardous materials may occur predominately through one particular pathway but, more often, actual or potential transport must be evaluated across several pathways and media. Multimedia models are designed to simulate the transport of contaminants from a source to a receptor through more than one environmental pathway. Three such multimedia models are reviewed here: MEPAS, MMSOILS, and PRESTO-EPA-CPG. The reviews are based on documentation provided with the software, on published reviews, on personal interviews with the model developers, and on model summaries extracted from computer databases and expert systems. The three models are reviewed within the context of specific media components: air, surface water, ground water, and food chain. Additional sections evaluate the way that these three models calculate human exposure and dose and how they report uncertainty. Special emphasis is placed on how each model handles radio-nuclide transport within specific media. For the purpose of simulating the transport, fate and effects of radioactive contaminants through more than one pathway, both MEPAS and PRESTO-EPA-CPG are adequate for screening studies; MMSOILS only handles nonradioactive substances and must be modified before it can be used in these same applications. Of the three models, MEPAS is the most versatile, especially if the user needs to model the transport, fate, and effects of hazardous and radioactive contaminants.     

Hazardous Waste Incineration at Sea: EPA Decision Making on Risk

This paper critiques the Environmental Protection Agency's assessment of risk for hazardous waste incineration at sea. It reviews operational and transportation risks and considers alternative approaches for managing chlorinated organic hazardous wastes. It concludes that depending on the scale of the program, ocean incineration will either contribute little to the overall management of this waste stream or else it will engender significant risks, especially in the coastal environment. Furthermore, past assessments on the part of EPA have tended to understate the risks of incineration at sea while simultaneously holding out the promise of the technology as a commercial-scale management option. Finally, this paper observes that the Western European countries that pioneered incineration in the North Sea are now finding practical alternatives. It is recommended that waste reuse, on-site treatment, and techniques of waste reduction provide viable alternatives and obviate the need for incineration at sea.     

Practical Methods for Meeting Remediation Goals at Hazardous Waste Sites

Risk-based cleanup goals or preliminary remediation goals (PRGs) are established at hazardous waste sites when contaminant concentrations in air, soil, surface water, or groundwater exceed specified acceptable risk levels. When derived in accordance with the Environmental Protection Agency's risk assessment guidance, the PRG is intended to represent the average contaminant concentration within an exposure unit area that is left on the site following remediation. The PRG, however, frequently has been used inconsistently at Superfund sites with a number of remediation decisions using the PRG as a not-to-exceed concentration (NTEC). Such misapplications could result in overly conservative and unnecessarily costly remedial actions. The PRG should be applied in remedial actions in the same manner in which it was generated. Statistical methods, such as Bower's Confidence Response Goal, and mathematical methods such as "iterative removal of hot spots," are available to assist in the development of NTECs that ensure the average postremediation contaminant concentration is at or below the PRG. These NTECs can provide the risk manager with a more practical cleanup goal. In addition, an acute PRG can be developed to ensure that contaminant concentrations left on-site following remediation are not so high as to pose an acute or short-term health risk if excessive exposure to small areas of the site should occur. A case study demonstrates cost savings of five to ten times associated with the more scientifically sound use of the PRG as a postremediation site average, and development of a separate NTEC and acute PRG based on the methods referenced in this article.     

An Approach for Balancing Health and Ecological Risks at Hazardous Waste Sites

Human health and ecological risks must be balanced at hazardous waste sites in order to ensure that remedial actions prevent unacceptable risks of either type. Actions that are designed to protect humans may fail to protect nonhuman populations and ecosystems or may damage ecosystems. However, there is no common scale of health and ecological risk that would allow comparisons to be performed. This paper presents an approach to addressing this problem based on classifying all risks (i.e., health and ecological risks due contaminants and remediation) as insignificant ( de minimis ), highly significant ( de manifestis ), or intermediate. For health risks the classification is based on standard criteria. However, in the absence of national guidance concerning the acceptability of ecological risks, new ecological criteria are proposed based on an analysis of regulatory precedents. Matrices and flow charts are presented to guide the use of these risk categories in remedial decision making. The assessment of mercury contamination of the East Fork Poplar Creek is presented as an example of the implementation of the approach.     

Life Years Lost at Hazardous Waste Sites: Remediation Worker Fatalities vs. Cancer Deaths to Nearby Residents

We present a hypothetical case study using the Years of Potential Life Lost (YPLL) metric to compare cancer risks incurred by residents living near a Superfund site to occupational fatality risks incurred by workers employed in that site's remediation. Since cancer occurs late in life, and because we assume its mortality rate is 60%, each case results in 8.8 YPLL. Each occupational fatality, which typically occurs earlier in life, results in 38.1 YPLL. In our case study, the residential population of 5000 incurred 1.3 YPLL, compared to 5.7 YPLL incurred by the 500 workers. Several uncertain assumptions may influence our calculations; moreover, occupational risks may be viewed as more "voluntary" than risks incurred by residents. However, because the magnitude of the YPLL incurred by workers and residents may be comparable, risk managers should consider occupational risks when evaluating remedial alternatives.     

A Comparison Between Probabilistic and Dempster‐Shafer Theory Approaches to Model Uncertainty Analysis in the Performance Assessment of Radioactive Waste Repositories

Model uncertainty is a primary source of uncertainty in the assessment of the performance of repositories for the disposal of nuclear wastes, due to the complexity of the system and the large spatial and temporal scales involved. This work considers multiple assumptions on the system behavior and corresponding alternative plausible modeling hypotheses. To characterize the uncertainty in the correctness of the different hypotheses, the opinions of different experts are treated probabilistically or, in alternative, by the belief and plausibility functions of the Dempster‐Shafer theory. A comparison is made with reference to a flow model for the evaluation of the hydraulic head distributions present at a radioactive waste repository site. Three experts are assumed available for the evaluation of the uncertainties associated with the hydrogeological properties of the repository and the groundwater flow mechanisms.