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S E C ® R k iL Mr Mike Infurna <" February 26, 2004 11 Page 2 i za established in the easterly direction, at an approximate gradient of 0 003 foot/foot (IT, February 25, q 2000) " On July 20 and 21, 2000, four Geoprobe borings (GP-1 through GP-4) were completed by IT to a depth of 60 feet bgs to investigate the vertical extent of petroleum hydrocarbons in soil and groundwater Petroleum hydrocarbon-impacted soil and groundwater were encountered at depths of approximately 50 to 60 feet bgs Based on the results of this investigation, IT recommended an additional investigation to determine the vertical extent of petroleum hydrocarbons in sod beyond 60 feet bgs, and the installation of additional groundwater monitoring wells for assessment of deeper zone groundwater impacts(IT, August 30, 2000) On January 22, 2001, three deep on-site groundwater monitoring wells (MW-5, MW-6, and MW-7) were installed by IT to depths of 80 feet bgs to delineate the vertical extent of petroleum hydrocarbons in groundwater The highest concentrations of petroleum hydrocarbons were detected in sod samples collected between 60 and 70 feet bgs, at concentrations of TPHg, and benzene up to 4,780 parts per million (ppm) and 55 3 ppm (MW-6@60') Petroleum hydrocarbons were detected in ~ groundwater samples collected from all three wells Maximum concentrations of TPHg, benzene, and MtBE in groundwater were reported at 2,300 ppb (MW-6), 22 ppb (MW-6), and 16 ppb (MtBE, EPA Method 8260) Based on the results of the investigation, IT concluded that the vertical extent of petroleum hydrocarbons in sod were generally defined to a depth of 80 feet bgs IT further ' recommended that additional assessment activities be performed to evaluate the lateral extent of 'ri a dissolved petroleum hydrocarbon impacts between approximately 50 and 80 feet bgs (IT, April 16, 2001) 1� On August 30, 31, and September 5, 2001, IT installed three additional on-site deep groundwater monitoring wells (MW-8 through MW-10) at locations upgradient of the UST complex, in the vicinity of the pump islands and downgradient of the pump islands Wells were completed at depths of 79 d, feet (MW-8) and 80 feet bgs (MW-9 and MW-10) Petroleum hydrocarbons were present in soil " samples collected between 60 and 80 feet bgs, with concentrations of TPHg and benzene reported f, up to 4,400 mg/kg and 77 mg/kg, respectively TPHg was detected in wells MW-8 through MW-10 at concentrations up to 22,000 ug/l, and benzene was detected in wells MW-9 and MW-10 at ; concentrations up to 600 ugll Fuel oxygenates were not detected in any soil and groundwater samples collected from the site Based on the results of this investigation, SECOR recommended N the installation of additional off-site deep zone groundwater monitoring wells for further delineation of the dissolved petroleum hydrocarbon plume between approximately 60 feet and 80 feet bgs (SECOR, May 29, 2002) Between June 6 and July 11, 2003, SECOR install four deep groundwater monitoring wells (MW-11zj through MW-14) Each of the wells was in an offsite location and the purpose of the work was to delineate the lateral extent for the petroleum plume in the deeper zone Petroleum hydrocarbons ; were delineated with the exception of 1,2-DCA 4 The historical groundwater gradient direction within the shallow zone at the site has been to the northeast at gradients ranging from 0 001 to 0 008 feet per foot The historical groundwater gradient direction within the deep zone at the site has been to the northeast at gradients ranging from 0 003 to 0 007 feet per foot "s conocoPhdlips144091work plans/WP For Vertical Assessment 5 7 r