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Monitoring Well Installation Report-Tosco(Former BP)Service Station No 11192 Septe iber 26,2001 2.3 Previous Environmental Work Based on information supplied by BP and Tosco the previous USTs (Three at 10,000 gallons and four at 2,000 gallons) were installed in 1976, and were removed in 1991 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 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 During eight weekly events between November 16, 2000 and January 2, 2001 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 1\4W-7 on November 16, 2000 The groundwater was purged directly from the wells into a Onyx vacuum truck and transported to the Tosco Refinery in Rodeo, California for disposal Approximately 27,200 gallons of groundwater were removed from the site during the eight weekly purging events Groundwater samples were collected from MW-3 prior to and following each of the eight purging events A water sample was also collected from the purged groundwater on each of the eight events, prior to its removal from site The estimated mass of MTBE removed 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 approved by the PHS-EHD In April 2001 GR conducted a subsurface investigation consisting of 8 GeoprobesO (2 on-site and 6 off-site) and one Cone Penetrometer Test (CPT) at the subject site The locations are shown on Figure 2 The purpose of the investigation was to delineate the lateral and vertical extent of Methyl tertiary Butyl Ether (MtBE) impacted groundwater at and in the vicinity of the subject site Selected soil and grab groundwater samples were collected and analyzed for MtBE and other petroleum hydrocarbons The soil samples submitted for analysis reportedly contained no detectable concentrations of MtBE or other petroleum hydrocarbon constituents The grab groundwater samples collected contained no detectable concentrations of total petroleum hydrocarbons as gasoline (TPHg) or benzene, but did contained MtBE in concentrations ranging from ND to 120 parts per billion (ppb) The highest concentration of MtBE was 'detected in the groundwater sample collected at 75 feet bgs in a groundwater sampling probe advanced adjacent to the CPT boring CPT-1 in northeast corner of the . site(see Figure 2) 14024505-2 3