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City of Stockton A68101.02F July 24, 1997 Page 2 3.4 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 Van Buskirk Park Golf Course is located in the southwest portion of the city of Stockton, California, at 1740 Houston Avenue. The maintenance facility is located in the south central portion of the golf course on the north side of Walker i Slough (Drawing 1). A site plan of the maintenance facility is shown on Drawing 2. ■ 3.2 Geologic and Hydrologic Characteristics: The following subsections summarize the geology and hydrology of the Stockton area. 3.21 Geoio�v: The site is in the northern portion of the San Joaquin Valley within the southern portion of the Great Valley geomorphic province of California. The Great Valley is a nearly flat northwest to southeast trending structural basin approximately 450 miles long -by 50 miles wide. 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 is a heterogeneous mix of gravel, sand, silt, and clay (Hackel, 1966). 3.2.2 Hydrology: First encountered groundwater in the site vicinity occurs under unconfined (water table) conditions at a depth of approximately 14 feet below site grade (BSG). Groundwater flow direction was calculated to be towards the north during the monitoring events conducted in June and December, 1994 (Twining, 1995). During the t performance of the work herein documented, groundwater was measured at approximately 10.2 feet BSG. Sediments containing fresh groundwater are largely unconsolidated silts and sands which were derived from river channel, flood plain, and alluvial fan deposits of Pliocene to Recent age (Davis and others, 1959). Local groundwater flow may be influenced by pumping of water wells and recharge from various sources. Evaluation of the effects of these conditions on groundwater beneath the site is beyond the scope of this investigation. 3.3 Project HistM: Twining understands that a 550-gallon gasoline UST was removed from the site in April 1991 and that analysis of a soil sample collected from four feet BSG in the vicinity of the former gasoline fuel dispenser detected 350 milligrams per kilogram (rng/kg) total petroleum hydrocarbons - gasoline range constituents (TPH-G) and 7,200 mg/kg total petroleum hydrocarbons - diesel ranged constituents (TPH-D). Twining also understands that a 1,000-gallon diesel UST was removed from the site in January 1994. Analysis of a soil sample collected from 11 feet BSG at the west end of the UST removal excavation detected 135 mg/kg TPH-D,435 mg/kg TPH-G, 5.8 mg/kg benzene, 29 mg/kg f