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L.i City of Stockton A68106.03A 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 40 feet below site grade (BSG) (San Joaquin County, Public Health Services, 1996). 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). 2.3 Project History: On June 28, 1989, one 550-gallon gasoline fuel UST was removed from the site. An analysis of a soil sample collected at 8 feet below site grade (BSG) beneath the UST detected total petroleum hydrocarbons -gasoline range constituents (TPH- G) at a concentration of 1,200 milligrams per kilogram (mg/kg), toluene at 3.7 mg/kg, I ethylbenzene at 4.1 mg/kg and xylenes at 110 mg/kg. I 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 four soil borings advanced by the direct push technique (soil probe) to depths of 30 feet BSG. In a letter dated March 21, 1996 the San Joaquin County Public Health Services (SJCPHS) suggested that soil borings be advanced to determine the vertical and horizontal extent of soil contamination. In addition, if soil i contamination appears to extend to groundwater, the SJCPHS suggested that a groundwater j sample be collected. The following work plar_• provides an outline of actions to satisfy the I - ik request of the City and the suggestions of the SJCPHS. 3.0 PURPOSE AND SCOPE j The purpose of this site characterization is to delimit the lateral and horizontal extent of soil 1 impacted with PPCs. Furthermore, the presence or absence of PPCs in groundwater will I L be assessed. The following scope of work is proposed to achieve the stated objective: • Obtain permits from the SJCPHS; i • Collect subsurface soil samples from selected locations at the project site; L • Collect a grab groundwater sample; • Chemically analyze soil and groundwater samples in the laboratory for selected PPCs. i • Evaluate the field and analytical data and assess the lateral extent of PPC migration in soil; t s