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ICOR 1 SIECOR • a� 2.0 SiTE HISTORY 2.1 Site Location The site is located on the northeast corner of Cambridge Drive and Louise Avenue in Lathrop, California (Figure 1) and is an operating Circle K Mini Mart and 76 Service Stations. An aerial photograph of the site is shown in Figure 2. 2.2 Previous investigations In November 1997, Pacific Environmental Group (PEG) performed oversight and sampling during the removal of one 4,000-gallon and two 10,000-gallon underground storage tanks (USTs), product dispensers, and associated product piping from the site during facility upgrade activities. Approximately 1,241 tons of hydrocarbon-impacted soil was excavated from beneath the USTs and from the new tank pit excavation. ' Additionally, a a approximately 17,000 gallons of hydrocarbon-impacted groundwater was Pumped from the new tank pit excavation. Soil samples collected from the excavated former tank pit contained total petroleum hydrocarbons as gasoline (TPHg) at ' concentrations ranging from 130 ppm to 5,800 ppm. Methyl tert-butyl ether (MtBE) was reported in the excavation soil samples at concentrations ranging from 0.19 ppm to 3.6 ppm (PEG, 1998). The new UST complex is located directly north of the dispenser ' ! islands. In July 1998, PEG supervised the advancement of one Geoprobe B boring (GP-1) to a ' depth of 40 feet below ground surface (bgs) downgradient (north) of the former UST excavation. Laboratory analysis of soil samples collected from GP-1 at 10 feet bgs indicated maximum concentrations of TPHg and MtBE at 1,900 ppm and 0.31 ppm, ' ' respectively. Laboratory analysis of grab groundwater samples collected from GP-1 indicated concentrations of TPHg and MtBE of 52,000 ppb and 61 ppb, respectively (PEG, March 17, 1999). Historical Figures are presented in Appendix C. ' From August through September 1998, IT Corporation (IT) drilled and installed four groundwater monitoring wells (MW-1 through MW-4) to approximate depths of 26 to 28 feet bgs. During the installation of well MW-4, the drill auger broke through a City of ' Lathrop water main, releasing approximately 600,000 to 1,000,000 gallons of water. Laboratory analysis of soil boring samples reported maximum concentrations of 6,800 r. ppm TPHg in MW-3-16' and 7.2 ppm MtBE in MW-2-11'. Groundwater samples collected from newly installed wells contained maximum concentrations of TPHg and MtBE of 29,000 ppb (MW-1) and 4,000 ppb (MW-2), respectively (PEG, March 17, ° 1999). In December 2000, IT drilled and installed five groundwater-monitoring wells (MW-5 through MW-9) to approximate depths of 24 feet bgs. Laboratory analysis of soil samples indicated maximum concentrations of TPHg and MtBE of 473 ppm (MW-9-10') s. . and 0.119 ppm (MW-6-20'), respectively. Groundwater samples collected from the 1205 SCM.doc 2 iF