Investigating uncertainty and sensitivity in integrated, multimedia environmental models: tools for FRAMES-3MRA

摘要

Elucidating uncertainty and sensitivity structures in environmental models can be a difficult task, even for low-order, single-medium constructs driven by a unique set of site-specific data. Quantitative assessment of integrated, multimedia models that simulate hundreds of sites, spanning multiple geographical and ecological regions, will ultimately require a comparative approach using several techniques, coupled with sufficient computational power. The Framework for Risk Analysis in Multimedia Environmental Systems - Multimedia, Multipathway, and Multireceptor Risk Assessment (FRAMES-3MRA) is an important software model being developed by the United States Environmental Protection Agency for use in risk assessment of hazardous waste management facilities. The 3MRA modeling system includes a set of 17 science modules that collectively simulate release, fate and transport, exposure, and risk associated with hazardous contaminants disposed of in land-based waste management units (WMU). The 3MRA model encompasses 966 multi-dimensional input variables, over 185 of which are explicitly stochastic. Design of SuperMUSE, a 215 GHz PC-based, Windows-based Supercomputer for Model Uncertainty and Sensitivity Evaluation is described. Developed for 3MRA and extendable to other computer models, an accompanying platform-independent, Java-based parallel processing software toolset is also discussed. For 3MRA, comparison of stand-alone PC versus SuperMUSE simulation executions showed a parallel computing overhead of only 0.57 seconds/simulation, a relative cost increase of 0.7% over average model runtime. Parallel computing software tools represent a critical aspect of exploiting the capabilities of such modeling systems. The Java toolset developed here readily handled machine and job management tasks over the Windows cluster, and is currently capable of completing over 3 million 3MRA model simulations per month on SuperMUSE. Preliminary work is reported for an example uncertainty analysis of Benzene disposal that describes the relative importance of various exposure pathways in driving risk levels for ecological receptors and human health. Incorporating landfills, waste piles, aerated tanks, surface impoundments, and land application units, the site-based data used in the analysis included 201 facilities across the United States representing 419 site-WMU combinations.