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With the exception of the iterative moisture contents described bed below, all soil and refuse parameters used in modeling the Forward Landfill LCRS are default values selected from the HELP program as typical for municipal solid waste landfills. Weather data was synthetically generated by the program for the Stockton area and corrected for the latitude of the Forward Landfill facility, yielding an average annual rainfall of approximately 15.8 inches. As a conservative measure, the HELP modeling that was completed for the project anticipated that the subgrade (liner) gradient beneath all development areas would be 2%, and that yearly thickness of refuse would be no greater than 10 feet. While wastes that are initially placed in landfills are typically dry, with a moisture content that is well below their field capacity (moisture holding capacity), for the Forward Landfill leachate generation analyses, as a conservative measure, it was assume that the upper 10 -foot refuse section in each yearly simulation would exist at its field capacity. The moisture content for the underlying refuse layers was manually specified for each yearly simulation based on the results that were calculated for the previous year. As additional conservative measures, runoff was assumed to be impeded by a "fair stand of grass," though, for simulating transpiration, a bare ground (daily cover) condition was assumed. Current estimates are that the expanded Forward Landfill may operate for approximately 20 years. The HELP simulations were run for a period of 30 years: 20 years of active refuse disposal and 10 years of post -closure period with final cover and no positive or negative flux through the landfill's geomembrane cover. The simulation conservatively ignored placement of intermediate cover soils, and assumed that a one -foot thick LCRS drainage layer would be placed at a 2% gradient on top of a geomembrane layer at the base of refuse. LCRS collection pipe was assumed to exist on 200 -foot centers. •a The average peak daily leachate that was calculated to be generated in a one -acre area with the greatest anticipated refuse thickness is approximately 397 cubic feet. As shown on the charts included in this section (Figures D-1 and D-2), this volume was generated after the first simulated year of refuse placement. Within the simulated one -acre landfill area, the average peak daily leachate generated is about 4000 gallons or .0045 cubic feet per second (cfs). The maximum peak daily head on the liner system was calculated to be approximately 1.6 inches. Accordingly, the 200 -foot spacing of the LCRS appears more than adequate to keep less than one foot of leachate head on the liner. LCRS Pipe Capacity Since the largest tributary area within the planned new liner system is approximately 23 acres, the peak daily flow through leachate collector pipes to the leachate sumps will be approximately 0. 10 cfs. Recognizing that 27 CCR regulations require that the LCRS pipes be designed to accommodate double the peak daily leachate generation rate, the LCRS pipe at the facility should be designed to accommodate at least 0.20 cfs. The