Human and Ecological Risk Assessment - Earthjustice

Section 3.0AnalysisFigure 3-6. Conceptual model of the groundwater modeling scenario.3.6.2 Modeling Approach and AssumptionsThe transport of leachate from the WMU through the unsaturated and saturated zoneswas modeled using EPACMTP (U.S. EPA, 1996a, 1997a, 2003a,c,d). EPACMTP is a compositemodel consisting of two coupled modules: (1) a one-dimensional module that simulatesinfiltration and dissolved contaminant transport through unsaturated soils, and (2) a3-dimensional saturated zone flow and transport module to model groundwater fate andtransport. EPACMTP has been used by EPA to make regulatory decisions for wastes managed inland disposal units (including landfills and surface impoundments) for a number of solid wasteand hazardous waste regulatory efforts, and as noted earlier, has undergone extensive peerreview. EPACMTP simulates the concentration arriving at a specified receptor location (such asa well or stream).The primary subsurface transport mechanisms modeled by EPACMTP are (1) downward(1-dimensional) movement along with infiltrating water flow in the unsaturated zone soils and(2) movement and dispersion along with ambient groundwater flow in the saturated zone.EPACMTP models soils and aquifer as uniform porous media and does not account forpreferential pathways such as fractures and macropores or for facilitated transport, which mayaffect migration of strongly sorbing constituents such as metals.In the unsaturated zone, flow is gravity driven and prevails in the downward direction.Therefore, the flow is modeled in the unsaturated zone as one-dimensional in the verticaldirection. The model also assumes that transverse (sideways) dispersion (from both mechanicaland molecular diffusion processes) is negligible in the unsaturated zone because the scale oflateral migration due to transverse dispersion is negligible compared with the size of the WMUs.This assumption is also environmentally protective because it allows the leading front of thecontaminant plume to arrive at the water table with greater peak concentration in the case of afinite source.April 2010–Draft EPA document. 3-33