Benchmarking of the saturated-zone module associated with three risk assessment models: RESRAD, MMSOILS, and MEPAS

A comprehensive benchmarking is being performed between three multimedia ri sk assessment models: RESRAD, MMSOILS, and MEPAS. Each multimedia model is comprised of a suite of modules (e.g., groundwater, air, surface water, exp osure, and risk/hazard), all of which can impact the estimation of the huma n health risk. As a component of the comprehensive benchmarking exercise, t he saturated-zone modules of each model were applied to an environmental re lease scenario, where uranium-234 was released from the waste site to a sat urated zone. Uranium-234 time-varying emission rates exiting from the sourc e and concentrations at three downgradient locations (0, 150, and 1500 m) a re compared for each multimedia model. Time-varying concentrations for uran ium-234 decay products (e.g., thorium-230, radium-226, and lead-210) at the 1500-m location are also presented. Different results reported for RESRAD, MMSOILS, and MEPAS are solely due to the assumptions and mathematical cons tructs inherently built into each model, thereby impacting the potential ri sks predicted by each model. Although many differences were identified betw een the models, differences that impacted these benchmarking results the mo st are as follows: (1) RESRAD transports its contaminants by pure translati on, and MMSOILS and MEPAS solve the one-dimensional advective, three-dimens ional dispersive equation. (2) Due to the manner in which the retardation f actor is defined, RESRAD contaminant velocities will always be faster than MMSOILS or MEPAS for long-lived contaminants. (3) RESRAD uses a dilution fa ctor to account for a withdrawal well; MMSOILS and MEPAS were designed to c alculate in situ concentrations at a receptor location. (4) RESRAD allows f or decay products to travel at different velocities, while MEPAS assumes th e decay products travel at the same speed as their parents. MMSOILS does no t account for decay products and assumes degradation/decay only in the aque ous phase.