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L. <br /> City of Stockton A68109.01B <br /> July 18, 1997 Page 2 <br /> �. • This report was prepared to document the investigative procedures, summarize the <br /> findings, and present conclusions and recommendations. <br /> 4 <br /> 3.0 BACKGROUND <br /> L.. <br /> A description of the site, the geologic and hydrologic characteristics, and the project history <br /> are summarized in the following subsections. <br /> 3.1 Site DescriQtiQn: Fire Station Number Nine is located at 550 East Harding Way <br /> ` between California Avenue and Cemetery Lane in the city of Stockton, California (Drawing <br /> �. 1). A site plan of the site depicting existing features and the former UST location is shown <br /> on 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 in the northern portion of the San Joaquin Valley within the <br /> southern portion of the Great Valley geomorphic province of California. The Great Valley <br /> �- is a nearly flat northwest to southeast trending structural basin approximately 450 miles long <br /> by 50 miles wide. The Great Valley has been filled with a sequence of older to younger <br /> alluvium of Pliocene to Holocene age which overlie sedimentary rocks of Cretaceous to <br /> '-' Tertiary age. These sedimentary units, in turn, overlie a crystalline basement of Paleozoic <br /> and Mesozoic metamorphic and igneous rocks. The shallow subsurface geology in the site <br /> vicinity is a heterogeneous mix of gravel, sand, silt, and clay (Hackel, 1966). <br /> 3.2.2 Hydrology: First encountered groundwater in the site vicinity occurs under <br /> unconfined (water table) conditions at a depth of 43 feet below site grade (BSG) (San <br /> Joaquin County, Public Health Services, 1996). During the performance of the work herein <br /> documented, groundwater was encountered at approximately 48 feet BSG. <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. Evaluation of the effects of these conditions <br /> on groundwater beneath the site is beyond the scope of this investigation. <br /> 3.3 Project History: On November 7, 1990, one 500-gallon diesel fuel UST was removed <br /> from the site. An analysis of a soil sample collected at 5 feet below site grade (BSG) <br /> beneath the product delivery line detected toluene at a concentration of 0.0087 milligrams <br /> per kilogram (mg/kg), ethylbenzene at 0.009 mg/kg and xylenes at 0.21 mg/kg. Analysis <br />