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I ! Supplemental Investigation Report BP Oil Site No 11193 C A M B R ! A Stockton, California October 13, 2000 CONCLUSIONS AND RECOMMENDATIONS Soil beneath the site consists of shallow fill underlain by silt and sand mixtures A clay interval encountered at 43 feet bgs in the Caltrans irrigation well located 315 feet west of the property boundary was not encountered during drilling to 50 feet bgs at the site Groundwater was encountered at approximately 11 to 14 feet bgs during well installation activities No BTEX was reported in the analyzed soil samples TPHg and MTBE concentrations were reported in soil collected from well MW-1R, located south of the USTs, and in shallow soil collected from well MW-8, located northeast of the USTs No TPHg or MTBE was reported in soil samples collected deeper than 30 feet bgs in well MW-8 MTBE was reported in the shallow soil sample collected from well MW-9, no TPHg was reported in soil samples collected from well MW-9 No TPHg or MTBE was reported in soil collected from well MW-10 MTBE concentrations of more than 10,000µg/1 were reported in groundwater samples collected from wells MW-1R, MW-2R and VW-1, located near the USTs, and from well MW-3, located near the northern dispenser at the site The MTBE concentration in groundwater in well MW-8, which screens from 33 to 48 feet bgs, was two orders of magnitude lower than concentrations in nearby wells VW-I (which screens from 7 to 17 feet bgs), MW-IR and MW-2R (which screen from 10 to 30 feet bgs) More than 10,000 gg/l TBA was reported in groundwater collected from well MW-2R,and more than 1,000 gg/l TBA was reported in the groundwater sample collected from well MW-3 DIPE,ETBE, and TAME concentrations were at least three orders of magnitude lower than MTBE and TBA concentrations in groundwater collected from well MW-2R Light oil, possibly from turbine pump operation, was encountered on the water surface in the Caltrans well Initial sampling of the Caltrans well reported no hydrocarbons, oxygenates or additives except for 11 gg/l TBA A second sample of the Caltrans well reported no hydrocarbons, oxygenates or additives except for 270 ltg/l TPHg (perhaps due to the light oil) The Caltrans well has been destroyed Fate and transport modeling results suggest that under the natural northeastward groundwater flow conditions,BTEX and MTBE at the site will not migrate to the former Caltrans well location To confirm groundwater flow direction and concentration trends,and to monitor the effectiveness of TOSCO'S interim groundwater extraction activities, continued quarterly groundwater monitoring is recommended Groundwater samples will be analyzed for TPHg, BTEX, MTBE and TBA Additional site assessment will be evaluated based on future monitoring results 14