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Supplemental Investigation Report BP Oil Site No 11193 C A M B R I A Stockton, California October 13, 2000 Soil Handling Soil cuttings produced dunng dnlling activities were stored temporarily on visqueen and subsequently transported to Forward Landfill in Manteca, California Sampling Results Site Hydrogeology In general, the soil types and depth-to-water encountered dunng dnlling activities were consistent with the 1999 investigation at the site (see Appendix D) Soil types encountered below 30 feet bgs in well MW-8 were consistent with soil types encountered in the shallower bonngs The clay unit noted between 43 and 80 feet bgs in the bonng log for the Caltrans well (see NNW Appendix A)was not encountered to the total explored depth of 50 feet bgs Based on the water level data collected from the extended well network on July 14,2000, groundwater flows toward the east- northeast at a horizontal hydraulic gradient of 0 007 These measurements indicate that adjacent wells MW-2R and MW-8 both have a groundwater elevation of 7 33 feet above mean sea level suggesting little or no vertical hydraulic gradient Soil Analytical Data No TPHg was reported in the analyzed soil samples collected from wells MW-9 or MW-10 or deeper than 30 feet bgs in well MW-8 (see Table 2) TPHg concentrations in the remaining analyzed samples ranged from 0 16 mg/kg to 0 82 mg/kg No BTEX was reported in the analyzed soil samples No MTBE was reported in the analyzed soil samples collected from well MW-10 or from deeper than 30 feet bgs in well MW-8 MTBE concentrations in the remaining soil samples ranged from 0 014 mg/kg to 2 6 mg/kg Groundwater Analytical Data Groundwater samples from wells MW-1R, MW-2R, MW-3 and VW-1 contained more than 1,000µg/l TPHg, 10 µg11 benzene, and 10,000 gg/l MTBE(see Table 3) No TPHg was reported in wells MW-4,MW-6, MW-7, MW-9 or MW-10 No benzene was reported in wells MW-4,MW-5,MW-6,MW-7, MW-9 or MW-10 No MTBE was reported in well MW-10 I I Well MW-8, which screens from 33 to 48 feet bgs,contained 55 µg/1 TPHg, 0 86 µg11 benzene and 230 gg/1 MTBE Grab samples collected from adjacent boring CB-3 reported 27,000 µg/1 TPHg, 39 µg11 benzene and 34,000 µg/1 MTBE at 16 feet bgs, and no TPHg or benzene and 3,400 µg11 MTBE at 34 feet bgs The constituents reported in the initial sample from well MW-8 may be a result of minor mixing of shallow and deeper portions of the water-beanng zone dunng dnlling activities and are anticipated to attenuate with time 9