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Quarterly Summary Report • Second Quarter 2002 Tosco Service Station #1205 16470 Cambridge Street, Lathrop, California City/County ID # Lathrop County San Joaquin BACKGROUND The site is located on northeast corner of Cambridge Street and Louise Avenue in Lathrop, California, and is an operating Circle K mim mart and Tosco service station 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 petroleum hydrocarbon-impacted soil was excavated from beneath the USTs and from the new UST excavation Additionally, approximately 17,000 gallons of petroleum hydrocarbon-impacted groundwater was removed from the new UST excavation Total petroleum hydrocarbons as gasoline (TPHg) were reported at concentrations ranging from 130 parts per million (ppm) to 5,800 ppm Methyl tert-butyl ether (MtBE) was reported in soil at concentrations ranging from 0 19 ppm to 3 6 ppm (PEG, 1998) In July 1998, one GeoprobeTm boring was advanced to a depth of 40 feet below ground surface (bgs) downgradient (north) of the former UST excavation Laboratory analysis of soil reported maximum concentrations of TPHg and MtBE of 1,900 ppm (GP-1-10') and 0 31 ppm (GP-1- 10'), respectively Laboratory analysis of groundwater reported concentrations of TPHg and MtBE of 52,000 parts per billion (ppb) and 61 ppb, respectively (PEG, 1999) 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 reported maximum concentrations of TPHg and MtBE of 6,800 ppm (MW-3-16') and 7 2 ppm (MW-2-11'), respectively Laboratory analysis of groundwater reported maximum concentrations of TPHg and MtBE of 29,000 ppb (MW-1) and 4,000 ppb (MW-2), respectively (PEG, 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 reported maximum concentrations of TPHg and MtBE of 473 ppm (MW-9-10') and 0 119 ppm (MW-6-20'), respectively Laboratory analysis of groundwater reported maximum concentrations of TPHg and MtBE of 72,000 ppb (MW-6) and 750 ppb (MW-6), respectively (IT, 2001) Groundwater monitoring has been performed since August 1998 Second quarter 2002 groundwater monitoring reported high concentrations ( 10,000 ppb) of TPHg in wells MW-5, MW-7, and MW-9 The highest concentrations of benzene (77 ppb) and MtBE (310 ppb) were