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F In July 1990, approximately 5,080 cubic yards of soil were excavated to a total depth of approximately 10 feet below W grade (depth of water table) , characterized, and disposed of at appropriate facilities. In October 1990, a total of four 4-inch-diameter ground water monitoring wells were installed onsite around the perimeter of the excavated area (Alton, . In April and May 19911 a total of nine soil borings 1490) were drilled onsite and downgradient of the site as part of a supplemental site in.'estigation to determine the extent of the dissolved-phase hydrocarbon constituent plume. Four of the downgradient borings were converted into 2-inch-diameter monitoring wells. Figure 2 shows the location of the monitoring wells and borings. A chronology of previous activities is presented in Appendix A. 2.1.1 Site Geology and Hydrogeology W Geologically, the scope of this investigation includes only the uppermost deposits within the Central Valley. Continental rocks and deposits of Tertiary and Quaternary age compose a number of formations; in total they constitute the major aquifer of the Central Valley .and yield large quantities of water to wells. btrlgisorted these deposits are heterogeneous mixtures of POOy clay, silt, sand, and gravel and, in ,places, beds of claystone, siltstone, sandstone, and conglomerate. The maximum thickness reported in these formations is 3,000 feet (United States Geological Survey, 1986) . Based on field observations during drilling and excavation, the predominant sediment type beneath the site is a mixture of sand, silt, and clay from the ground surface to a depth of greater than 11 feet below grade. Sandy silt may persist to nearly 15 feet below grade. Sand was observed in Monitoring Well AW-3 at a depth of 15 feet. This well was converted to Monitoring Well AW-3. A stiff to hard silty clay unit underlies the sand and sandy silt unit, persisting to the total depth of all borings (approximately 25 feet) . Ground water was first encountered during drilling at approximately 10 feet nelow grade in all soil borings, stabilizing at an average depth of about 10.5 feet below the top of casing. A ground water elevation contour mag, based on data collected on May. 7, .1991, , i.s shown in Figure 3• This map indicates an average hydraulic gradient of approximately 0.014 foot per fo,.r, generally to the northeast. 2.2 Extent Of H - - Sgil Based on the results of previous activities performed at the site, petroleum hydrocarbon constituents in the soil have been removed by excavation to the full-4st practical extent. 2