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City of Stockton A68107.09A 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 43 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 Pro'ect Histola: On November 7, 1990, one 500-gallon diesel fuel UST was removed from the site. An analysis of a soil sample collected at 5 feet below site grade (BSG) beneath the product delivery line detected toluene at a concentration of 0.0087 milligrams ' per kilogram (mg/kg), ethylbenzene at 0.009 mg/kg and xylenes at 0.21 mg/kg. Analysis of a soil sample collected at 12 feet beneath the UST did not detect petroleum product constituents (PPCs) at concentrations above the detection limit. V In a request for bid letter dated February S, 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 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 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 plan provides an outline of actions to satisfy the request of the City and the suggestions of the SJCPHS. La 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; • Collect a grab groundwater sample; • Chemically analyze soil and groundwater samples in the laboratory for selected PPCs. • Evaluate the field and analytical data and assess the lateral extent of PPC migration La in soil;