Abstract
ABSTRACT
This paper is one of a series of papers that reports the results of a benchmarking between three multimedia risk assessment models: RESRAD, MMSOILS, and MEPAS. Each multimedia model is comprised of a suite of modules (e.g., groundwater, air, surface water, exposure, and risk/hazard), all of which can impact the estimation of human health risk. As a component of the benchmarking exercise, the vadose-zone modules of each model were applied to an environmental release scenario, where uranium-238 was released from the waste site to a vadose zone with two layers. Time-varying emission rates, exiting the source and each of the layers, were compared. Different results are reported for RESRAD, MMSOILS, and MEPAS, which are solely due to the assumptions and mathematical constructs inherently built into each model, thereby impacting the potential risks predicted by each model. RESRAD transports its contaminants by pure translation (i.e., plug flow), and MMSOILS and MEPAS transport contaminants by solving the one-dimensional advective-dispersive equation. Because of the plug-flow assumption, RESRAD generally predicts higher emission rates (and, hence, concentrations). In addition, because of assumptions in the way each model calculates pore-water velocity, RESRAD travel times will always be shorter than MMSOILS, which will always be shorter than MEPAS. Shorter or longer travel times could have a profound impact for contaminants that exhibit significant degradation/decay.
Key words:
Multimedia; modeling; benchmarking; risk; groundwater; vadose zone
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