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\TAT 4: A S S O C I A T E S I N C . consists of the more rugged foothills of the Sierra Nevada (maximum elevation due east is over ' 10,000 feet). The Great Central Valley contains a thick sequence of sediment, which, in places, reaches a depth of 10 miles. These sediments range in geologic age from Jurassic (205 million years before present) to Recent (present time) and include both marine and continental deposits. The site's immediate subsurface geology consists of Pleistocene and Recent alluvial deposits. These alluvial deposits consist of heterogeneous sequences of sand and gravel originating from active stream channels, and silt and clay originating from overbank and marsh depositional environments. ' The site is in the Eastern San Joaquin County Groundwater Basin (California Department of Water Resources, 1980). According to the San Joaquin Flood Control and Water Conservation District, the direction of regional groundwater flow is generally to the east. The groundwater basin has been developed for irrigation, domestic,and stock use. ' Lone Tree Creek is located approximately 1,500 feet west,northwest of site. 2.3 Site Geology and Hydro¢eology Based on information obtained from previous borings at the site, sediments underlying the site generally consist of silt from the ground surface to approximately 18 feet below ground surface (bgs) with an intermittent layer present at a depth of approximately 10 feet bgs. Sand was typically encountered between 18 and 47 feet bgs with a silt layer from 25 to 38 feet bgs ' m several borings. Silt with several thin layers of sand was encountered between 47 and 90 feet bgs. Beneath the eastern portion of the site, soil from depths between 18 and 80 feet bgs consists of sand with a silt layer present from 40 to 60 feet bgs. Soil from depths between 80 and 86 feet bgs consist of silt that is underlain by sand to a depth of 100 feet bgs. Soil boring logs are contained in Appendix A, cross section locations are shown on" Figure 3, and generalized, cross-sectional interpretation of subsurface conditions are shown on Figures 4 and 5. These cross-sections are based on the boring logs completed by WHF and BSK & Associates (BSK). ' Surface drainage at the site is controlled by the site topography, which slopes to the west. The site is located approximately 1,500 feet east, southeast of Lone Tree Creek, which flows west into French Camp Slough and eventually drains into the San Joaquin River, approximately 25 miles northwest of the site. A culvert oriented north south on the western portion of the property intermittently carries surface runoff. Water level measurements were collected from wells MWl, MW2, MW3, MW4, MW5, MW6, MW8, and MW9 on November 20, 2003. On November 20, 2003, water levels ranged from 70.92 to 75.01 feet below the tops of the well casing elevations. The data from MW6 was not ' used when considering the shallow groundwater flow direction because MW6 is screened in a deeper interval with respect to the remaining monitoring wells. The water level data were used to s:\environmental\25291\reports\irp.doe 2