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1 <br /> ' In April and November 1991, eight additional soil borings were dnlled by RESNA Industries, <br /> two of which were converted to groundwater monitoring wells and four of which were converted <br /> to vapor extraction wells (Figure 4) Two groundwater recovery wells were also installed for <br /> the purpose of hydraulic control when groundwater remediation commences Laboratory <br /> ' analyses determined that benzene concentrations, above method detection limits, was present in <br /> only soil samples from borings VP1 and VP3, in concentrations which ranged from 0 008 ppm <br /> to 0 076 ppm Total petroleum hydrocarbons as gasoline (TPHG) were detected only in borings <br /> ASB1 and VP1 in concentrations which ranged from 2 4 ppm to 3300 ppm <br /> 4.1 Extent of Soil Impaction <br /> ' Based on the laboratory analyses of soil samples, the lateral extent of hydrocarbon impacted soils <br /> is restricted to the former storage tank excavation and beneath the eastern dispenser island The <br /> ' vertical extent of hydrocarbon impacted soils has been constrained by the shallow water table <br /> at approximately 30 feet below grade <br /> 5.0 VADOSE ZONE REMEDIATION <br /> To remediate hydrocarbon impacted soil within the vadose zone, a vapor extraction system was <br /> installed The off-gas treatment technology has been broken down into two phases, with Phase <br /> I utilizing an Internal Combustion Engine/Thermal Oxidizer and Phase II a Carbon Adsorption <br /> System The remediation system consists of extracting hydrocarbon vapors from the four <br /> existing vapor extraction wells These vapors are routed, below grade, to the treatment <br /> ' compound beneath the eastern dispenser canopy The hydrocarbon vapors removed from the <br /> subsurface are destroyed by the use of an Internal Combustion Engine/Thermal Oxidizer and the <br /> ' Carbon Adsorption System <br /> ' 5.1 Phase I - Internal Combustion Engine/Thermal Oxidizer <br /> The vapor extraction system utilizing an Internal Combustion Engine became operational on <br /> January 24, 1992 The Internal Combustion Engine extracted hydrocarbon vapors from the four <br /> existing vapor extraction wells (Figure 4) <br /> R070792 TLP 3 <br />