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C 11 11 11 C I 1 The primary environmental concern for sanitary landfills is protecting usable supplies of ground water and surface water from leachate. Leachate is produced when the net infiltration of water into the land- fill exceeds the refuse's absorptive capacity. Environmental problems may result if wastes contact water, or if ground water, precipitation, or surface water is allowed to drain through wastes and exit as leach- ate. To protect ground water at the Corral Hollow Sanitary Landfill, a 1 -foot -thick (minimum), compacted low -permeability soil layer (permea- bility of 1 x 10-6 cm/sec or less) will be installed in the final cover profile. This soil layer should prevent moisture infiltration into the landfill, decreasing the leachate generation potential (Drawing 3). To evaluate leachate produced when precipitation infiltrates into landfills, the U.S. Environmental Protection Agency (EPA) utilizes the water balance method developed by C. W. Thornthwaite.l This method is based on the relationship between precipitation, surface runoff, evapotranspiration (evaporative loss through plants), and soil mois- ture storage. Using this method, the soil moisture remaining within the vegetated cover soil profile can be estimated monthly by account- ing for gains from precipitation and losses from evapotranspiration. To determine the water balance for the completed landfill, EMCON assumed that the final cover soil would be vegetated with shallow - rooted grasses and not irrigated. The water balance shows that precipitation will not infiltrate into the refuse if the cover soil 1. Thornthwaite, C.W. and J.R. Mather. "Instructions and Tables for Computing Potential Evapotranspiration and the Water Balance." Centerton, N.J., 1957, pp. 185-311 (Drexel Institute of Technology. Laboratory of Climatology. Publications in Clima olnov. V. 10, No. 3). PJ9 9390310.00D 10 cmirnn