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PROJECT BACKGROUND <br /> August 1986 Four underground storage tanks (two 10,000-gallon and one 5,000-gallon <br /> gasoline, and one 250-gallon waste oil) were removed from the site on August 19, 1991 <br /> Ground water was encountered in the tank cavity at approximately 7 feet below grade (fbg) <br /> KEI was retained by Mobil Oil Corporation to conduct an assessment of soil and ground <br /> water conditions One soil sample was collected at 6 8 fbg from the side wall of the waste <br /> oil tank cavity, an additional soil sample was collected beneath product delivery lines at <br /> 4 fbg, and a composite of three ground water samples was collected from the tank cavity <br /> and analyzed for total petroleum hydrocarbons (TPH), benzene, toluene, ethylbenzene, and <br /> total xylenes (BTEX), and halogenated volatile organic compounds (HVOCs) Analysis of <br /> the soil sample collected from the waste oil tank detected TPH at 61 parts per million <br /> (ppm) Analysis of the composite water sample detected 30,000 parts per billion (ppb) TPH <br /> HVOCs were not detected in any of the samples (Kaprealian 1986a) Subsequently, KEI <br /> installed four 2-inch-diameter ground water monitoring wells (MW-1 through MW-4) to total <br /> depths of 20 fbg (Kaprealian 1986b) <br /> September 2, 1986 During initial ground water sampling, no free product was observed in <br /> any of the monitoring wells, however, levels of TPH were detected in the samples collected <br /> from MW-3 at 320,000 ppb and MW-4 at 28,000 ppb (Kaprealian 1986) <br /> February 1987 Based on results of the preliminary investigation, KEI installed one 4-inch- <br /> diameter monitoring well, MW-5, and three 2-inch-diameter monitoring wells, MW-6 <br /> through MW-8 Monitoring Well MW-5 was installed onsite, and the other wells were <br /> installed offsite The highest TPH concentration detected in soil samples collected during <br /> installation of these wells was 24 ppm (MW-8 at 3 fbg) No free product or sheen was <br /> noted in any ground water samples collected from the new monitoring wells, however, <br /> analysis of the samples detected TPH concentrations ranging from 6,300 ppb in MW-7 to <br /> 41,000 ppb in MW-5, while concentrations of TPH were not detected at or above reported <br /> detection limits in MW-1 (Kaprealian 1987) <br /> September 1987- KEI initiated a monitoring and sampling program for the eight ground <br /> water morutonng wells, with a sampling frequency of 1 to 4 months Free product was <br /> observed in MW-3,while the presence of dissolved-phase gasoline was continuously detected <br /> in five of the wells, MW-4 through MW-8 (Kapreahan 1988). Based on results of the irutial <br /> field activities and laboratory analysis of soil and ground water samples, it was apparent that <br /> petroleum hydrocarbons had impacted subsurface soil and ground water, and additional <br /> characterization was necessary to define the nature and extent of affected ground water <br /> (KEI 1988). <br /> January 12, 1990: Alton Geoscience was retained by Mobil Oil to perform a supplemental <br /> site investigation. In addition to continuous monitoring and sampling of existing wells, four <br /> new ground water monitoring wells were installed; AMW-1 through AMW-4 AMW-1 and <br /> AMW-2 were installed on April 23, 1990, and AMW-3 and AMW-4 were installed on <br /> August 8, 1990 Analytical results of ground water samples showed concentrations of TPH <br /> as gasoline (TPH-G) in AMW-1 at 100 ppb and AMW-3 at 90 ppb It was concluded that <br />