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. • September 1987: KEI initiated a monitonng and sampling program for the eight <br /> ground water momtonng wells, with a sampling frequency of 1 to 4 months Free <br /> product was observed in MW-3, while the presence of dissolved-phase gasoline was <br /> continuously detected in five of the wells (MW-4 through MW-8), (Kaprealian 1988). <br /> Based on results of the uutial field activities and laboratory analysis of soil and <br /> ground water samples, it was apparent that petroleum hydrocarbons had impacted <br /> subsurface soil and ground water, and additional charactenzation was necessary to <br /> define the nature and extent of affected ground water (Kapreahan 1988). <br /> • January 12, 1990: Alton Geoscience was retained by Mobil Oil to perform a <br /> supplemental site investigation. In addition to continuous monitoring and sampling <br /> of existing wells, four new ground water monitoring wells were installed, AMW-1 <br /> through AMW-4 AMW-1 and AMW-2 were installed on Apn123, 1990, while <br /> AMW-3 and AMW-4 were i istalled on August 8, 1990 Analytical results of ground <br /> water samples showed detectable concentrations of TPH as gasoline (TPH-G) in <br /> AMW-1 (100 ppb) and AMW-3 (90 ppb), (Alton Geoscience 1990) It was concluded <br /> that potentially three hydrocarbon plumes exist in the vicinity of the site onsite on <br /> the northeast comer of the Mobil property, adjacent to the Exxon property; and <br /> adjacent to the Chevron property <br /> • April 26, 1990: Alton Geoscience initiated a ground water monitoring and sampling <br /> program <br /> • September and December 1991: The two most recent ground water sampling and <br /> monitoring events are discussed in this report. <br /> 4.0 FIELD METHODS AND PROCEDURES <br /> 4 1 Remedial Installation Objectives <br /> The location of the ground water recovery wells, which were installed in utility trench <br /> backfill (Figures 2, 3, and 4), was based on the following observations. <br /> • The flow of shallow ground water (general ground water gradient direction to the <br /> north) appears inhibited by the location of the intersecting underground utility <br /> trenches (Figures 5 and 6) <br /> • The extent of free product and the dissolved-phase hydrocarbon plume appears <br /> restricted to the northeast corner of the property (Figures 7 and 8) <br /> • The possibility that the utility trench backfill either could or has acted as a conduit <br /> for further migration of the hydrocarbon plume needed to be addressed. <br /> 3 <br />