MULTIMEDIA BENCHMARKING ANALYSIS FOR 3 RISK ASSESSMENT MODELS - RESRAD, MMSOILS, AND MEPAS

This paper is one in a series that describes results of a benchmarking analysis initiated by the Department of Energy (DOE) and the United S tates Environmental Protection Agency (EPA). An overview of the study is provided in a companion paper by Laniak et al. presented in this jo urnal issue. The three models used in the study-RESRAD (DOE), MMSOILS (EPA), and MEPAS (DOE)-represent analytically-based tools that are use d by the respective agencies for performing human exposure and health risk assessments. Both single media and multimedia benchmarking scenar ios were developed and executed. in this paper, the multimedia scenari o is examined. That scenario consists of a hypothetical landfill that initially contained uranium-238 and methylene chloride. The multimedia models predict the fate of these contaminants, plus the progeny of ma nium-238, through the unsaturated zone, saturated zone, surface water, and atmosphere. Carcinogenic risks are calculated from exposure to th e contaminants via multiple pathways. Results of the tests show that d ifferences in model endpoint estimates arise from both differences in the models' mathematical formulations and assumptions related to the i mplementation of the scenarios.