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0 <br /> 2 <br /> Tosco Corporation A Subsidiary of Phillips Petroleum Company June 27 2002 <br /> Tosco Service Station No 4409 1502 81 Dorado Street,Stockton CA <br /> On December 9 and 10, 1999, four groundwater monitoring wells (MW-1 through <br /> MW-4) were installed to investigate the extent of petroleum hydrocarbons in soil and <br /> groundwater beneath the site Well MW-1 was completed at 50 feet bgs, and wells <br /> MW-2 through MW-4 were completed at 45 feet bgs Petroleum hydrocarbons were <br /> not detected in any soil samples from beneath the site Groundwater sampling from the <br /> new wells was initiated on January 5, 2000 Concentrations of total petroleum <br /> g), benzene, toluene, ethylbenzene, and xylenes (BTEX), <br /> hydrocarbons as gasoline (TPH <br /> and methyl tertiary butyl ether(MtBE) were generally not detected in the wells, with the <br /> exception of minor concentrations of benzene and/or MtBE (EPA Method 8260) <br /> detected in wells MW-3 and MW-4 Initial depth to water was approximately 28 to 29 <br /> feet bgs The direction of groundwater flow was established in the easterly direction, at <br /> an approximate gradient of 0 003 foot/foot (IT, February 25, 2000) <br /> According to IT's Site Assessment Report, dated February 25, 2000, IT performed a <br /> water supply well search to locate any water supply wells within 2,000 feet of the <br /> subject site Review of well logs at the California Department of Water Resources <br /> (DWR) indicated the presence of one water supply well within the 2,000-foot search <br /> radius, located at 1521 N Hunter Street Well construction details and usage of the well <br /> was not documented on the DWR well log The San Joaquin County Environmental <br /> Health Department (SJCEHD) attempted to locate the well, however the well was not <br /> found <br /> • On July 20 and 21, 2000, four Geoprobe borings (GP-I through GP-4) were completed <br /> by IT to a depth of 60 feet bgs to investigate the vertical extent of petroleum <br /> hydrocarbons in soil and groundwater Petroleum hydrocarbon-impacted soil and <br /> groundwater were encountered at depths of approximately 50 to 60 feet bgs Based on <br /> the results of this investigation, IT recommended an additional investigation to <br /> determine the vertical extent of petroleum hydrocarbons in soil beyond 60 feet bgs, and <br /> the installation of additional groundwater monitoring wells for assessment of deeper <br /> zone groundwater impacts (TT, August 30, 2000) <br /> On January 22, 2001, three deep on-site groundwater monitoring wells (MW-5, MW-6, <br /> and MW-7) were installed by IT to depths of 80 feet bgs to delineate the vertical extent <br /> of petroleum hydrocarbons in groundwater The highest concentrations of petroleum <br /> hydrocarbons were detected in soil samples collected between 60 and 70 feet bgs, at <br /> concentrations of TPHg, and benzene up to 4,780 parts per million (ppm) and 55 3 ppm <br /> (MW-6@60') Petroleum hydrocarbons were detected in groundwater samples <br /> collected from all three wells Maximum concentrations of TPHg, benzene, and MtBE <br /> in groundwater were reported at 2,300 ppb (MW-6), 22 ppb (MW-6), and 16 ppb <br /> (MtBE,EPA Method 8260) Based on the results of the investigation, IT concluded that <br /> the vertical extent of petroleum hydrocarbons in soil were generally defined to a depth <br /> of 80 feet bgs IT further recommended that additional assessment activities be <br /> performed to evaluate the lateral extent of dissolved petroleum hydrocarbon impacts <br /> between approximately 50 and 80 feet bgs (IT, April 16, 2001) <br /> 1\Tosco Marketing\4409\Work Plans\Additional Assessment WP doc <br />