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Supplemental Investigation Work Plan BP Site No 11191 C A M B R I A Stockton, California June 21, 2002 Surface Water Bodies and Water Supply Wells Mosher Creek is located approximately %-mile north of the site Based on a California Department of Water Resources file review and information acquired from the City of Stockton, four water supply wells and one test boring are located within !/2-mile of the site The nearest water supply well is City of Stockton well 3-2, located approximately 2,000 feet to the southeast of the site at 7400 West Lane Frontage (see Figure 2) Well 3-2 was drilled to a total depth of approximately 625 feet below ground surface (bgs) and is sealed with cement grout from the surface to 144 feet bgs and with bentonite from 376 to 386 feet bgs The other wells and the test boring are located approximately %-mile from the site, were drilled to 440 to 600 feet bgs, and are sealed with cement grout to at least 60 feet bgs Previous Activities. Eleven soil borings were advanced to depths of up to 80 feet bgs during site investigations conducted at the site between 1992 and 1995 In 1998, TOSCO replaced the underground storage tanks, dispensers, and associated piping In December 2001, four monitoring wells were installed at the site Two-inch diameter wells MW-1, MW-2, and MW4 were drilled to a total depth of 60 feet bgs and screen from 45-60 feet bgs Four-inch diameter well MW-3 was drilled to a total depth of 60 feet bgs and screens from 40-60 feet bgs Site Hydrogeology. The soil types at the site consist primarily of silty clays and clayey and sandy silts to approximately 25-35 feet bgs, which are underlain by sandy gravels, silty and gravelly sands, and sands to the total depth explored of 80 feet bgs Groundwater was not reported during site investigations between 1992 and 1995 In December 2001, groundwater was first observed at depths ranging from 40 to 45 feet bgs and static water levels range from 44 to 49 feet bgs Based on first and second quarter 2002 monitoring data, groundwater beneath the site flows westward at a horizontal hydraulic gradient of 0 001 to 0 003 Hydrocarbon Distribution• Soil analytical results for samples collected from the eleven soil borings from 1992 to 1995 indicated up to 0 15 milligrams per kilogram (mg/kg) of benzene and 0 37 mg/kg of (suspected) MTBE No hydrocarbons were reported in the deepest analyzed samples or at depth in adjacent borings (e g , borings B-4 and B-5) Based on these results, the extent of hydrocarbons in soil was considered defined 2