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r✓ r ,r City of Stockton, Fire Station No. 12 A68105.01 August 7, 1995 Page 2 2.2 Geologic and Hydrologic Characteristics: The following subsections summarize the geology and hydrology of the Stockton area. 2.2.1 Geology: The site is located on the boundary of the Sacramento Valley and San Joaquin Valley which comprises the larger Great Valley geomorphic province of California. The Great Valley is an asymmetrical synclinal trough interrupted by two major surface cross structures: the Stockton Fault in the Stockton Arch and the Bakersfield Arch. The Stockton Fault is located approximately 1/8 mile north of the site. The Great Valley has been filled with a sequence of older to younger alluvium of Pliocene to Holocene age which overlie sedimentary rocks of Cretaceous to Tertiary age. These sedimentary units, in turn, overlie a crystalline basement of Paleozoic and Mesozoic metamorphic and igneous rocks. The shallow subsurface geology in the site vicinity consists of a heterogeneous mix of gravel, sand, silt, and clay (Hackel, 1966). 2.2.2 Hydrology: Sediments containing fresh groundwater are largely unconsolidated silts and sands deposited as river channel, flood plain, and alluvial fan deposits of Pliocene to Recent age (Davis and others, 1959). First encountered groundwater in the site vicinity occurs under unconfined (water table) conditions. According to the San Joaquin County Flood Control District (SJCFCD), the expected seasonal high groundwater corresponds to a depth of approximately 50 feet below site grade (BSG). Local groundwater elevation and flow direction may be influenced by pumping of water wells and recharge from various sources. The effect of these influences on groundwater beneath the site is beyond the scope of this investigation. 2.3 Project History: According to information provided by the City of Stockton, a 550- "' gallon diesel UST and a 550-gallon unleaded gasoline UST were removed from a single excavation pit at the site in September 1994. Initial soil analyses conducted following removal of the USTs detected PPCs. Additional excavation of the UST pit was conducted and additional soil samples were collected (City of Stockton, 1995). According to the City of Stockton, approximately 37 cubic yards of soil was removed and disposed of at REMCO in Richmond, California. The dimensions of the excavations were reported as 8 feet by 10 feet by 14 feet deep. Analytical results for the soil samples collected following excavation activities are summarized in Table 1 (McCampbell Analytical, Inc., November 1994). Soil sample SP-1-2 was a composite sample collected from the excavated soils. r r r