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Fire Station No. 12 TL A68105.01 March 10, 1995 Page 2 3.0 BACKGROUND A description of the site, the geologic and hydrologic characteristics, and the project history are summarized in the following subsections. 3.1 Site Description The site is located at 4010 East Main Street in the southeastern portion of Stockton, California (Drawing 1). - r3.2 Geologic and Hydrologic Characteristics 3.2.1 Geology The site is located on the northern boundary of the San Joaquin Valley which comprises the southern segment of the larger Great Valley. The Great Valley is an asymmetrical synclinal trough interrupted by two major surface cross structures: the Stockton Fault in the Stockton Arch and the White Wolf Fault in and south of the Bakersfield Arch. The Stockton Fault is located approximately 1/8 mile to the north. The Great Valley has been filled with a sequence of older to younger alluvium of Pliocene to Holocene age which overlie sediment rocks of Cretaceous to Tertiary age. These g �' rY g 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). Based on field notes taken during the reifioval of product piping on April 22, 1994, soils encountered at the site consisted of silty sand and sandy silt to a depth of approximately five feet. 3.2.2 Hydrology Sediments containing fresh groundwater are largely unconsolidated silts and sands derived from 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 Public Health Services Environmental Health Division (PHS/EHD), the expected seasonal high depth to groundwater is approximately 50 feet below site grade (BSG). Local groundwater flow 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.