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41City of Stockton A68108.OIB July 18, 1997 Page 2 • This report was prepared to document the investigative procedures, summarize the findings, and present conclusions and recommendations. 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: Fire Station Number Three is located at 1116 East First Street at the northeast corner of First and Union Streets in the city of Stockton, California (Drawing 1). A site pian of the site depicting existing features and the former UST location is shown on Drawing 2. 3.2 Geologic and Hydrologic 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 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 `r 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 40 feet below site grade (BSG) (San Joaquin County, Public Health Services, 1996). During the performance of the work herein documented, groundwater was measured at approximately 45 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 History: On June 28, 1989, one 550-gallon gasoline fuel UST was removed from the site. Analysis of a soil sample collected at 8 feet BSG beneath the UST detected total petroleum hydrocarbons - gasoline range constituents (TPH-G) at a concentration of 1,200 milligrams per kilogram (mg/kg), benzene at less than 2.0 mg/kg, toluene at 3.7 mg/kg, ethylbenzene at 4.1 mg/kg and xylenes at 110 mg/kg.