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• Thrifty Oil Co Station # 172 and at the south end of the valley between Arvin and Maricopa, most of the subsidence has been caused by lowering of the artesian head in confined aquifer systems, due to the intensive pumping of groundwater Locally, on the west and south flanks of the valley, a third type of subsidence has been caused by near-surface compaction of moisture-deficient has been caused by near-surface compaction of moisture-deficient alluvial fan deposits above the water table, after initial wetting by percolating irrigation water (Poland and Evenson, 1996) SSB conducted a well survey within a half-mile radius during their preliminary investigation Their findings indicated a City of Stockton well (No 11) is located at the corner of Inglewood and Leslie Avenues (approximately 2,100 feet southeast of the site) A number of private wells are located within 1,000 feet to the west of the site in a county residential area and one to the northwest in the Carden School In addition, a number of wells are located in a small commercial center south of the site, however, they are no longer used, as the occupants currently utilize the City of Stockton water system (Thrifty, 1995) In May, 1991, SSB encountered groundwater at a depth of approximately 58 feet bgs during the installation of groundwater monitoring wells The hydraulic gradient was measured to be 0 0017 feet per foot towards the east-southeast Subsequent measurements have confirmed this flow direction although the static groundwater level has gradually risen since this time (Thrifty, 1995) In July 1995, groundwater (the first water-bearing zone) was encountered at a depth of approximately 48 feet bgs The hydraulic gradient was measured to be 0 0013 feet per foot to the east-southeast on August 16, 1995 (Thrifty, 1995) 4.0 HYDROCARBON VOLUME ESTIMATION A summary of historical soil laboratory analytical results are included in Table 2. Figure 3 presents the line of cross-section A-A', and Figure 4 is the geological cross-section showing the extent of hydrocarbon concentrations in soil as depicted in the cross-section line A-A' Figures 5 and 6 present the TPH soil isoconcentration plumes at depths of 25 feet and 40 feet below surface grade (bsg), respectively Based on this data,the estimated volume of soil affected by hydrocarbons with concentrations greater than 100 mg/kg is approximately 25,600 cubic feet Assuming a soil density of 110 pounds/cubic foot and an average hydrocarbon concentration of 1300 mg/kg, Thrifty Oil Co estimates approximately 3,660 pounds of hydrocarbons are present in the subsurface soil beneath the site 5.0 GROUNDWATER SITE CONDITIONS Groundwater beneath this site is present under water table conditions and has been steadily rising 7