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4.2.1 COVER CONFIGURATION The first step in using the LEACHM model involves definition of the soil profile. This is accomplished by defining the total thickness of the profile and a nodal or profile segment thickness. Modeling of the proposed alternative final cover system at the FSL was completed assuming a total final cover thickness of 4.0 feet. As a result, a total profile thickness of 1220 -mm (4.0 feet) was stipulated, as was a nodal frequency of 152.5 -mm (6 -inches). This yielded a total of 8 profile segments or nodes and the analysis assessed water flux at each of these nodes ten times a day throughout the modeling period. 4.2.2 BOTTOM BOUNDARY CONDITIONS One of the most essential parameters included in the analysis a final cover proposed over municipal solid waste is definition of the bottom boundary condition of the final cover soil section. Based on recent large-scale demonstration project data, it is clear that the most realistic characterization of the bottom boundary is represented by the constant potential condition (GLA, 1999, GLA 2001, Lass et. al., 2000, Lass et. al., 2001). In this case, it is assumed that there is a source of moisture available within the landfill (e.g. high humidity landfill environment) that would represent a continuous source of moisture (and therefore constant potential) within the waste and near the base of the cover section. In addition to being the most realistic, this bottom boundary condition also allows for more effective characterization of two directional moisture movement and allows for the extraction of moisture through the final cover if drying conditions predominate (i.e., downward migration is calculated when the final cover moisture content is high and allows for extraction of water from the landfill [i.e., drying of the waste] when the moisture in the final cover is low). As a result, in estimating the nature of flow through the bottom boundary of the final cover section at the FSL, the constant potential boundary condition was utilized. 4.2.3 SOIL COVER CHARACTERISTICS For each node or interval in the defined profile, LEACHM requires input of specific soil properties including: remolded dry bulk density; initial matric potential and soil moisture content; Campbell's "a" and "b" coefficients (Campbell, G.S., 1974), and saturated hydraulic conductivity. Porosity is then calculated as a function of the stipulated specific gravity of the soils and their remolded dry bulk density. Prior to initiation of modeling, the existing cover and the proposed borrow source soils were tested for grain size (ASTM D-422), maximum density/optimum moisture (ASTM D-1557), hydraulic conductivity (ASTM D 5084), and matric potential testing (ASTM D- 6836) (Table 1). Table 2 lists the soil characteristics actually incorporated in modeling. It should be noted that for modeling purposes, the soil characteristics of a long-term weathered soil profile similar to the existing interim cover were utilized. Campbell's "a" and "b" coefficients were calculated using the LEACHM RETFIT program and are based on soil grain size distribution, bulk density, matric potentials and hydraulic conductivity. -4- C:00050082WOR WARD/ForwerdCOVER.DOa7/t3nUU$ GeoLogic Associates