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L �41V �Ij ltw" city of Stockton A681OZO1B July 18, 1997 Page 2 L • This report was prepared to document the investigative procedures, summarize the findings, and present conclusions and recommendations. L 3.0D UI�T LA description of the site, the geologic and hydrologic characteristics, and the project history are summarized in the following subsections. L3.1 Site Description: Fire Station Number Seven is located at 1767 West Hammer Lane on the southeast corner of Hammer Lane and Amber Way in the city of Stockton, California (Drawing 1). A site plan of the site depicting existing features and the former UST location is shown on Drawing 2. 3.2 Geologic and H d Iogig; Characteristics: The following subsections summarize the geology and hydrology of the Stockton area. 3.2.1 Geology: 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 L.'e 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 (Haskel, 1966). 3.2.2 Mdrolo : First encountered groundwater in the site vicinity occurs under unconfined (water table) conditions at a depth of approximately 39 feet below site grade (BSG) (San Joaquin County, Public Health Services, 1996). During the performance of the Lwork herein documented, groundwater was encountered at approximately 40 feet BSG. Sediments containing fresh groundwater are largely unconsolidated silts and sands which Lwere 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 Lon groundwater beneath the site is beyond the scope of this investigation. 33 eject Histoiv: On March 9, 1994, one 500-gallon diesel fuel UST was removed Lfrom the site. An analysis of a soil sample collected at 13 feet BSG beneath the tank detected total petroleum hydrocarbons - diesel range constituents (TPH-D) at a L concentration of 4,140 milligrams per kilogram (mg/kg), ethylbenzene at 3.2 mg/kg and xylenes at 8.6 mg/kg. Analysis of a soil sample collected at 7 feet BSG on the north wall t L all, L