Human and Ecological Risk Assessment - Earthjustice

Section 3.0Analysis• The receptor well was located based on R rw and rw as shown in Figure 3-7.• The maximum lateral extent of a groundwater plume, based on lateral dispersion, wascalculated using the dimensions of the WMU sampled for that simulation, a sampledvalue for lateral dispersivity in the groundwater, and the downgradient distance to thereceptor well.• If the receptor well was located inside the idealized maximum plume extent, the shadedportion in Figure 3-7 (the distance from the well to the centerline was less than the lateralextent of the calculated in the previous step), the well location was used for thatsimulation. Otherwise, new values for R rw and rw were sampled and the process repeatedfor the same WMU. The depth of the well intake point was based on a uniformdistribution with limits of 0 (i.e., well at the water table) to 10 meters (or the totalsaturated aquifer thickness if the aquifer is less than 10 meters thick).The location of the surface waterbody intercepting groundwater flow was specified foreach flow and transport simulation. The waterbody was constrained to lie across the contaminantplume centerline and perpendicular to the groundwater flow direction. The waterbody isassumed to fully penetrate the aquifer thickness. Downgradient distance to the surface waterbodywas determined from an empirical distribution of distances measured for CCW landfills andsurface impoundments (see Appendix C), which was randomly sampled to develop the distancesused in EPACMTP to calculate groundwater concentrations at those distances in the MonteCarlo analysis.3.6.4 Groundwater Model OutputsThe output of EPACMTP is a prediction of the contaminant concentration arriving at adowngradient groundwater receptor location (either a well or a surface water body). Because afinite-source scenario was used, the concentration is time-dependent. A maximum time-averagedconcentration was calculated for each constituent across the exposure duration selected in eachMonte Carlo iteration.3.7 Surface Water ModelsFor the groundwater-to-surface-water pathway, chemical contaminants leach out ofWMUs and into groundwater, and this contaminated groundwater then discharges into a surfacewaterbody through groundwater discharge. Once in the waterbody, the continued fate andtransport of the contaminants is modeled with a surface water model, which uniformly mixes thecontaminants in a single stream segment. Surface water flows in and out of the stream segment.Surface water flowing into the stream segment is assumed to have zero constituentconcentration, and surface water flowing out has nonzero constituent concentrations due to thegroundwater contamination. The primary simplifying assumptions in EPACMTP are as follows:(1) the groundwater–surface water interface is assumed to be perpendicular to the regionalgroundwater flow direction; (2) the interface is infinite in its lateral extent so as to intercept theentire width of the dissolved contaminant plume; and (3) the intercepting surface water bodyfully penetrates the saturated region of the subsurface. Therefore, all of the mass in thecontaminated groundwater is available to be transferred to the surface water model. If streamApril 2010–Draft EPA document. 3-36