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0 3.2.1 MULTIMED Simulations <br />The U.S. Environmental Protection Agency's Multimedia Exposure Assessment Model <br />(MULTIMED) was employed to evaluate potential impacts to groundwater associated <br />with leachate migration through the 17 -foot -thick vadose zone beneath the site. As <br />described by Sharp -Hansen et al. (1990) and Salhotra et al. (1990), MULTIMED <br />simulates the movement of liquids in both the unsaturated (vadose) and saturated <br />(groundwater) zones. A steady-state, one-dimensional, semi -analytical module simulates <br />flow in the unsaturated zone. The output from this module (water saturation as a function <br />of depth) is used as input to the unsaturated zone transport module. The latter simulates <br />transient, one-dimensional (vertical) transport in the unsaturated zone and includes the <br />effects of longitudinal dispersion, linear adsorption, and first -order decay. For the <br />Forward Landfill analyses, the decay coefficients were conservatively ignored (i.e., set to <br />0.0). The unsaturated zone transport module calculates contaminant concentrations at the <br />base of the vadose zone as a function of time. <br />Output from both unsaturated zone modules is coupled with the saturated zone transport <br />module to estimate contaminant concentrations in groundwater as a function of time at a <br />specified distance downgradient from the edge of the landfill. The saturated zone <br />transport module includes one-dimensional uniform flow, three-dimensional dispersion, <br />linear adsorption, and first -order decay. Again, for the saturated zone simulations, the <br />decay coefficients were conservatively set to 0.0. <br />0 Model Input <br />v <br />As indicated on Table 1 in Attachment A, the MULTIMED analyses assumed that <br />leachate leakage would occur at a rate of approximately 0.0 17 cubic feet per day per acre <br />for the approximately 127 acres where the new single composite liner would be installed. <br />This leakage rate is based on the geomembrane defects anticipated in the HELP3 model <br />and conservatively assumes that 1 -foot of leachate will be applied on a prescriptive liner <br />whose minimum hydraulic conductivity is 1 x 10"7 cm/sec. The MULTIMED <br />simulations also conservatively assumed that the 1 -foot of leachate head would be <br />applied for 200 years. <br />The MULTIMED simulations assumed that the hydraulic conductivity of the 17 -foot <br />thick clayey vadose zone soils would be about 1 x 10"5 cm/sec. Conservative <br />assumptions were also integrated with respect to soil properties such as suction, porosity, <br />and density. The potential for biological degradation within the vadose zone was <br />conservatively ignored, and conservative estimates of potential dispersivity were <br />integrated. <br />The groundwater properties integrated in the model are consistent with those identified by <br />HA/AEE (2001) which indicate that the hydraulic conductivity of aquifer materials in the <br />area is relatively high (250 ft/d). Using the hydraulic conductivity value selected by <br />HA/AEE (250 R/d), an average groundwater gradient of 0.003 percent, and an estimated <br />effective porosity of 0.25, the calculated average linear groundwater velocity at the site is: <br />CA2002-002\Revised Appx A - Leakageldoc\09/19/02 <br />Geologic Associates <br />