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• Corrective Action Plan—Tosco (76)Service Station No 11192, Stockton, California March 13 2002 22 Previous Environmental Work Based on information supplied by BP and Tosco, the previous USTs (three 10,000-gallon and four 2,000-gallon) were installed in 1976, and were removed in 1991 Also in 1991, the three current 10,000-gallon gasoline USTs were installed A soil gas survey was performed in 1989 A well survey was performed in 1989 that indicated a water supply well is located 1,100 feet northwest from the site, and a private well is located 900 feet from the site Seven groundwater monitoring wells and two vapor extraction wells were installed at the site between 1990 and 1991 A hydropunch study, including a soil vapor extraction test, was conducted in 1992 Four soil borings were advanced in 1994 Eight weekly groundwater extraction events occurred between November 16, 2000 and January 2, 2001 During these events, approximately 2,400 to 3,800 gallons of groundwater per event were removed from monitoring well MW-3 by Onyx Industrial Services (Onyx)of Benicia, California In addition, approximately 600 gallons of groundwater were removed from well MW-7 on November 16, 2000 The groundwater was extracted directly from the wells into an Onyx vacuum truck and transported to the Tosco Refinery in Rodeo, California, for disposal A total of approximately 27,200 gallons of groundwater were removed from the site during the eight weekly extraction events Groundwater samples were collected from well MW-3 prior to and following each of the eight • extraction events A water sample was also collected from the purged groundwater during each of the eight events, prior to removal of the water from the site The estimated mass of methyl tertiary butyl ether(MtBE) removed with the extracted groundwater was calculated using the average MtBE concentration from the pre-purging(MW-3-PRE) and post-purging(MW-3-POST) grab groundwater samples for each purging event Based on these average concentrations, an estimated total of 26 63 pounds of MtBE was removed from the site during the eight purging events between November 16, 2000 and January 2, 2001 The weekly purging was discontinued after the eight week period by approval from the PHS-EHD In April of 2001, GR conducted a subsurface investigation at the site involving eight geoprobe soil borings (six offsite and two onsite) and two onsite Cone Penetrometer Tests (CPT) Selected soil samples collected during probing and CPT were reported by the laboratory as not detected (ND) for Total Petroleum Hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, xylenes (BTEX), MTBE, ethanol, tent-butyl alcohol (TBA), di-isopropyl ether (DIPE), ethyl tert-butyl ether (ETBE), 1,2-dichloroethane (1,2 DCA) tert-amyl methyl ether (TAME) and ethylene dibromide (EDB) Groundwater samples from the Geoprobes and CPT were reported as ND for all analytes in samples B-3, B-5, B-6, B-7 and B-8 Sample B-1 contained MtBE at 90 ppb and DIPS at 2 8 ppb Sample B-2 contained only MtBE at 15 ppb Sample B-4 contained MtBE at 11 ppb Sample CPT- 1 contained MTBE at 120 ppb and 1,2 DCA at 23 ppb • 140245 08 2