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3 2.1 TOPOGRAPHY,GEOLOGY AND HYDROGEOLOGY 2.1.1 Topography The Site is located in the northwest central portion of the San Joaquin Valley between the San Joaquin River (east), the Coastal Range (west), and the Old River, a former channel of the San Joaquin River (north), an area with very little topographic relief. The local topography slopes gently to the north-northeast at approximately a 0.4 percent (0.004 feet/foot) gradient (EDR, 2007). The Site drains to the west via concrete swales and asphalt that is graded toward the concrete swales. 2.1.2 Geology The Site is located within the southern portion of the Great Valley geomorphic province, which is a topographic and structural basin, bound on the east by the Sierra Nevada fault block and on the west by the Coastal Range. The basin is filled with a thick sequence of alluvial sediments and sedimentary rocks from Jurassic (approximately 200 million years ago) to recent Holocene deposits(PG&E, 1989). The alluvial materials were generated primarily from the Coastal Range with limited contributions from Sierra Nevada Mountains. The alluvial fan deposits are described as comprising a heterogeneous and discontinuous sequence of gravels, sands and clays. The alluvial fan deposits are approximately 100 feet thick in the vicinity of the Site (CDWR, 1967). The alluvial fan deposits are underlain by the Tulare Formation of Pliocene-Pleistocene age (approximately 700,000 to 5 million years ago). This Formation is comprised of semi- consolidated deposits of clays, sands, and gravels that are lenticular and highly discontinuous. However, the Corcoran Clay, which is present in the upper portions of the Tulare Formation in the vicinity of the Site, is a major aquitard within the San Joaquin Basin. This aquitard varies in thickness from zero to 150 feet in the vicinity of the Site and is encountered beginning at approximately 300 feet below ground surface(bgs) (Hotchkiss and Balding, 1971). Based on information provided in the SI Report (PG&E, 1989), soil beneath the Site consists of highly interbedded clay, silt, silty sand, sand and gravel, which is consistent with the description of regional alluvial sediments. Low permeability clay and silt are predominant in the unsaturated zone (0 to 7 feet bgs) and the matrix of the uppermost aquifer consists of interbedded silty sand, sand and gravel. This information is consistent with the geologic description in boring logs prepared as part of the RI. 2.1.3 Hydrogeology The Site is located within the Tracy Groundwater Subbasin of the San Joaquin Groundwater Basin. Locally, groundwater is recharged from the foothills of the Diablo Mountain Range to the west and through infiltration of precipitation. Groundwater production wells were the primary source of water in the Tracy area until approximately 1951 when the large scale importation of surface water from the Delta-Mendota Canal was initiated. Since 1951, the primary source of domestic water has been surface water (Hotchkiss and Balding, 1971). The Tracy Subbasin is drained by the San Joaquin River and one of its major west tributaries, Conal Hollow Creek (CDWR, 2003).