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distance of travel between the leak and the aquifer, the hydraulic characteristics of the water bearing units, the natural ability of the vadose zone and water bearing units to attenuate the contaminants by adsorption, oxidation, reduction, and/or biological processes, and the ability to manually intercept the leak and prevent it from reaching the aquifer. These factors are largely controlled by the local site climate and the lithology and hydrogeology of the soils present along the antipated flow path. 4.1 Climate In the preamble to the federal Subtitle D municipal solid waste liner system regulations, it states that "little leachate is generated in areas where the precipitation does not exceed 25 inches annually, which suggests that precipitation is an indicator of the potential of a landfill to contaminate groundwater." Precipitation at the project site is typical of the Central Valley region with approximately 90 percent of the rainfall occurring between November and April. Snowfall is unusual at the site. The isohyetal precipitation map (San Joaquin County Department of Public Works, 1991) divides the whole of San Joaquin County into three rainfall intensity zones. The project site is located in Zone 2 of the three rainfall zones and a mean annual precipitation of approximately 13.5 inches is reported. The estimated annual evaporation for the site is 78.43 inches based on Pan A evaporation data recorded at Oakdale Woodward Dam, located approximately 13 miles southeast of the site (California Department of Water Resources, 1979). A minimum mean monthly evaporation of 1.14 inches is calculated for December and January, and a maximum mean monthly evaporation of 14.69 inches is calculated for July. On the basis of the preamble to Subtitle D, it is concluded that "little leachate is [likely to be] generated" at Forward. 4.2 Geology/Hydrogeology The geology and hydrogeology of the existing Forward and Austin Road Landfills have been detailed in several reports including Kleinfelder & Associates (1991, 1997, 1999), CH2MHill (1992) Beta Associates (1988), and EMCON Associates (1972). As determined in exploratory boring and well installation programs performed at the site, geologic materials beneath the site occur as lenticular units composed of clay, silt, and sand with minor amounts of gravel. The sediments observed in borings at the site appear to correspond with the Pleistocene Victor Formation which consists of over 100 feet of clay, silt, and fine to coarse sand. The near surface soils near the site are typically clayey, consisting of two soil types typical of the Victor Formation including the Clear Lake clay and Jacktone clay. As detailed in Appendix A, leachate transport and fate analyses were completed using the U.S. Environmental Protection Agency's MULTEVED computer program to simulate a leak through the proposed CCL and near surface clayey vadose zone soils beneath the site. The simulations anticipate that the vadose zone is only 17 -feet thick (approximating the historical minimum thickness), and conservatively assumed that one -foot of leachate head Ef CA2000.049\Fomard Composite 1.Iner Y3 .doe\09119102 Geologic Associates