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walls of the tankhold; however, samples were not collected to confirm that petroleum-affected soils had been adequately removed. Soil was also removed to within four feet of the building's wall (the north wall of the tankhold), but excavation activities were halted so as not to adversely impact the building's foundation. While the majority of the petroleum-affected soils appeared to have been removed, it was unclear to what extent the affected soils remained on the site, and to what extent groundwater was affected. The purpose of this Phase II assessment was to provide the SJCEHD enough information to determine if the subject site meets the criteria for closure. 3.4 REGIONAL GEOLOGY/HYDROGEOLOGY According to the U.S. Department of Agriculture Soil Survey of San Joaquin County, California, the subject site rests on Capay-Urban land complex. This unit consists of 50 percent Capay clay and 35 percent Urban land. The Capay soil is very deep and moderately well drained. It formed in alluvium derived from mixed rock sources. Permeability is slow in the Capay soil; available water capacity is high. The shrink-swell potential also is high. Urban land consists of areas covered by roads, driveways, sidewalks, parking lots, buildings, and other structures. The soil material under the impervious surface is similar to that of the ... Capay soil. Soils observed during the on-site drilling activities were generally inorganic clays and clayey silts. Occasionally, a silty sand layer was noted. The subject property is in the San Joaquin Valley groundwater basin, which is part of the San Joaquin Basin Hydrologic Study Area (HSA). Twenty-six groundwater basins and areas of potential groundwater storage have been identified in the San Joaquin Basin HSA. The HSA is bounded by the Sacramento-San Joaquin Delta to the north, the Sierra Nevada to the east, the Tehachapi Mountains to the south, and the Coast Ranges to the west (DWR Bulletin No. 118, 1975). The San Joaquin Valley groundwater basin occupies an area of 13,500 square miles and is drained by the San Joaquin River. The groundwater bearing material is younger and older alluvium. Based on regional geologic conditions, groundwater in the area is estimated to flow generally in a northerly direction. This was confirmed through groundwater elevation data collected from the on- site monitoring wells. v 5