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{� Subsurface investigation Report,Tosco(Former BP) Service StationNo 11192 June 8,2001 Six groundwater monitoring wells have been installed by others at the former Shell site Pertinent site features are shown on Figure 2 2.2 Geology and Hydrogeology The site is situated on generally flat topography at an elevation of approximately six feet above sea level Smith Canal is located approximately 1,300 feet south of the site and the Calaveras River is approximately 3,500 feet north of the site Boring logs from the existing wells indicate that the site is predominantly underlain by clay and clayey sand with lesser amounts of silt, except in monitoring will MW-6, in which poorly graded sand with gravel was encountered from 25 to 40 feet north of the site The static depth to groundwater in the six existing wells was measured at 14 31 to 15 40 feet below ground surface (bgs) on April 11, 2000 The historical groundwater flow direction generally ranges from north to northeast at a gradient of 0 001 to 0 009 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 MW-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 14024503-1 2