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relationship. After calculation of soil retentivity and unsaturated hydraulic conductivity, LEACHM simulates unsaturated flow through the modeled profile using Richards' equation. The following sections describe the critical assumptions, variables, and input requirements incorporated into the LEACHM computer analysis. Variables specified in the model include the iteratively calculated transient soil water status factors, plant growth, plant maturity, plant harvest variables, soil matric potential and saturated hydraulic conductivity. Sample input/output files for the LEACHM program are provided in Appendix B. 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. IS C:\2003-033\EDOM HILL ALTERNATIVE COVER.DOC\6/16/2005 Geologic Associates