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LEACHATE GENERATION <br />Leachate generation analyses were completed to evaluate the volume of leachate that may <br />collect at the base of the planned Forward Landfill expansion area and to develop design <br />criteria for sizing leachate collection and recovery system (LCRS) appurtenances. The <br />LCRS calculations for the Forward Landfill development include calculations for LCRS <br />lateral spacing based on infiltration modeling, pipe capacity based on anticipated <br />tributary flows, and LCRS pipe wall thickness based on deflection analysis. The sections <br />that follow present the background and results for the LCRS calculations. <br />Leachate Generation Analyses <br />Modeling of potential leachate generation within the planned vertical and horizontal <br />landfill expansion was performed using the United States Army Corps of Engineers <br />Waterways Experiment Station's Hydrologic Evaluation of Landfill Performance model <br />(HELP v. 3.07, November 1997). This modeling simulated a refuse fill placed over the <br />entire liner development system. Sample input and output files are included. The <br />calculated rate of leachate generation and accumulation were the primary parameters <br />used in designing the LCRS. <br />Model Description and Setup <br />HELP is a quasi -two-dimensional model that simulates moisture flux into and out of <br />landfills. It has become the standard method for evaluating the potential performance of <br />qW landfill liner and cover systems. The model calculates water balances in daily <br />increments, and can provide output for daily, monthly, and annual values of precipitation, <br />runoff, evapotranspiration, percolation, and lateral drainage. The calculations integrated <br />the following user -selected variables: <br />Input Parameters <br />Parameter <br />Selection Criterion <br />Precipitation <br />Daily values synthetically developed by <br />HELP using average monthly rainfall data <br />from weather station in Stockton, <br />California. <br />Evapotranspiration <br />Default values for Sacramento, California. <br />Runoff <br />Runoff coefficient modified for "poor stand <br />of grass." <br />Soil and refuse moisture storage capacity <br />Upper zone conservatively assumed to <br />always be at field capacity. <br />Root zone depth <br />No vegetation included. <br />Surface slope <br />3% surface gradient. <br />Subsurface lateral drainage <br />2% gradient with LCRS pipe at 200 -foot <br />centers. <br />Solar radiation <br />Data for Fresno, California, synthetically <br />modified by HELP for Stockton's latitude. <br />