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APPENDIX HELP MODEL AND LEACHATE GENERATION QUANTITIES 1. Description of HELP Model The Hydrologic Evaluation of Landfill Performance (HELP) computer program is a quasi-two- dimensional hydrogeologic model of water movement across, into, through, and out of landfills. It has become a standard method for the performance evaluation of landfill liner systems, leachate collection systems, and cover systems. The version used in this analysis is HELP 2.05. I The HELP model calculates water balances in daily increments and can provide daily values, monthly totals, annual totals, and annual averages of vertical percolation and lateral drainage as # a final output. The water balance calculation incorporates the following user selected variables: a ■ Precipitation ■ Evapotranspiration 1 ■ Surface run-off ■ Soil and refuse moisture storage capacity ■ Root zone depth ■ Surface slope ■ Subsurface lateral drainage ■ Average monthly temperature s Figure 1 shows a representative HELP model profile with final cover. 2. Data Assumptions for Austin Road Landfill HELP Analysis The following data assumptions were used for the HELP model analysis: a. Climatological Data HELP model default rainfall and other climatological data for Sacramento,California i were used in this analysis. The precipitation data covers the five year period from 1974 to 1978. As the largest storm defined in the HELP model default data set for this period was 1.5 inches, the HELP model data set was augmented to include one 100-year, 24-hour storm event of 3.2 inches. This value was obtained from NOAA Atlas 2, "Precipitation Frequency Atlas of the Western United States", Volume 11 [California]. It was thought better to be conservative in estimating to use the higher value. Average annual rainfall for the precipitation record period was 14.54 inches. Peak annual precipitation was 26.38 inches. Figure 2 shows the j - average and average plus one standard deviation monthly precipitation curves. i b. Initial Moisture Content In general, when refuse is placed, it has a moisture content below field capacity (where field capacity is the water content which will allow for free drainage from a soil, i.e., it cannot "soak up" any more moisture). When modeling the active landfill area without run-off, the initial moisture content was set less than field capacity in order to represent the initial placement condition of the refuse.