The Multimedia Environmental Pollutant Assessment System (MEPAS) is an objective, scientific methodology to assess and prioritize hazardous chemical and radionuclide waste disposal sites based on a limited amount of site information. The vadose zone/groundwater transport module (RADCON) of MEPAS was enhanced to simulate scenarios where water-infiltration barriers (caps) have been emplaced over contaminated sites. The computer code was modified to accommodate three periods of constant water flux through the vadose zone (i.e., flux with no cap, fully functioning cap, and partially failed cap). The simulation results from test problems where the cap should have no effect essentially duplicated the simulation results from the same test problems evaluated with the earlier (baseline) version of RADCON (which requires a single constant value of water flux). Therefore, MEPAS assessments of baseline (no-cap) scenarios should be the same as they were when the baseline RADCON code was used as the vadose zone/groundwater transport module. As expected, simulations of preliminary test problems where the cap should have an effect showed that peak concentrations arrived at the receptor later and were reduced in magnitude compared to the no-cap case. Simple criteria were derived to indicate the degree to which results could be affected by violation ofmore » the assumptions of the conceptual model. However, even when assumptions are violated, errors on the nonconservative side that could occur as the cap fails should be offset by errors on the conservative side that would have already occurred as the cap was emplaced, resulting in simulations that are conservative overall.« less