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1 <br /> City of Stockton A68104 01A <br /> October 25, 1995 Page 2 <br /> 3.0 BACKGROUND <br /> t The following subsections describe the site, the geologic and hydrologic characteristics, and <br /> the project history <br />' 3.1 Site Description Victory Park is located in the central western portion of the city <br /> of Stockton, California on the west side of Pershing Avenue between Argonne and Picardy <br /> Drives (Drawing 1) The maintenance building is located along the western site boundary <br /> near the intersection of Argonne and Picardy Drives (Drawing 2) <br /> 3.2 Geologic and Hydrologic Characteristics The following subsections summarize the <br /> geology and hydrology of the Stockton area <br /> 3.2.1 Geology The site is located in the northern portion of the San Joaquin Valley which <br />' comprises the southern segment of the larger Great Valley geomorphic province The Great <br /> Valley is a nearly flat asymmetrical synclinal trough trending northwest to southeast <br /> approximately 450 miles long by 50 miles wide The Great Valley has been filled with a <br />' sequence of older to younger alluvium of Pliocene to Holocene age which overlie <br /> sedimentary rocks of Cretaceous to Tertiary age These sedimentary units, in turn, overlie <br /> a crystalline basement of Paleozoic and Mesozoic metamorphic and igneous rocks The <br /> shallow subsurface geology in the site vicinity consists of a heterogeneous mix of gravel, <br /> sand, silt, and clay (Hackel, 1966) <br />' 3.2.2 Hydrology According to the 1986 San Joaquin County Flood Control and Water <br /> Conservation District map, seasonally high groundwater is approximately 15 feet below site <br /> grade (BSG), and the regional flow direction for the Stockton area is toward the east <br /> Sediments containing fresh groundwater are largely unconsolidated silts and sands which <br />' were derived from river channel, flood plain, and alluvial fan deposits of Pliocene to Recent <br /> age (Davis and others, 1959) Local groundwater flow may be influenced by pumping of <br /> water wells and recharge from various sources The effect of these influences on <br />' groundwater beneath the site is beyond the scope of this investigation <br /> 3.3 Prosect History Twining understands that a 500-gallon gasoline UST was removed <br />' from the site in 1993 An investigation was performed on July 7, 1994 by over-excavating <br /> the former UST location and collecting soil samples The analytical results of a soil sample <br /> collected at 19 5 feet BSG detected 89 milligrams per kilogram (mg/kg) total petroleum <br /> hydrocarbons - gasoline range constituents (TPH-G), 1 mg/kg toluene, 1 mg/kg <br /> ethylbenzene, and 8 5 mg/kg xylenes (McCampbell Analytical, Inc , 1994) <br /> In a letter date July 27, 1994 the San Joaquin County Public Health Services (SJCPHS) <br /> required a groundwater and soil investigation On August 30, 1994 The City of Stockton <br /> authorized Twining to perform a groundwater and soil investigation A work plan (Twining, <br /> 1994) was prepared by Twining and then approved by the SJCPHS on October 19, 1994 <br />