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City of Stockton A68107.07A May 29, 1996 Page 2 �.. 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). �. 2.2.2 Hydrology: First encountered groundwater in the site vicinity occurs under unconfined (water table) conditions at a depth of 39 feet below site grade (BSG) (San Joaquin County, Public Health Services, 1996). Sediments containing fresh groundwater are +r 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). L" 2.3 Pro'ect Histo : On March 9, 1994, one 500-gallon diesel fuel UST was removed from 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 concentration of 4,140 milligrams per kilogram (mg/kg), ethylbenzene at 32 mg/kg and xylenes at 8.6 mg/kg. Analysis of a soil sample collected at 7 feet BSG on the north wall of the UST excavation detected TPH-D at a concentration of 1,880 mg/kg, ethylbenzene at concentration of 0.03 mg/kg and xylenes at a concentration of 0.11 mg/kg. In a request for bid letter dated February 8,. 1996, the City of Stockton (City) requested a work plan for the project site to include one soil boring advanced to groundwater depth and three soil borings advanced to depths of 25 feet BSG using the direct push (soil probe) technique. In a letter dated March 21, 1996 the San Joaquin County Public Health Services (SJCPHS) suggested that soil borings be advanced to confirm the analytical results and determine the vertical and horizontal extent of soil contamination. In addition, if soil contamination appears to extend to groundwater, the SJCPHS suggested that a groundwater sample be collected. The following work pian provides an outline of actions to satisfy the request of the City and the suggestions of the SJCPHS. 3.0 PURPOSE AND SCOPE The purpose of this site characterization is to delimit the lateral and horizontal extent of soil impacted with PPCs. Furthermore, the presence or absence of PPCs in groundwater will be assessed. The following scope of work is proposed to achieve the stated objective: • Obtain permits from the SJCPHS; • Collect subsurface soil samples from selected locations at the project site; ti.. • Collect a grab groundwater sample; • Chemically analyze soil and groundwater samples in the laboratory for selected PPCs.