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1.0 INTRODUCTION j This report presents the findings of a supplemental site investigation, including the installation of additional monitoring wells and an aquifer pumping test at former Mobil Bulk Plant 04-343, located at 500 East Grantline Road, in Tracy, California (Figure 1). This work was performed in accordance with the San Joaquin County Public Health Services, Environmental Health Division (PHS/EHD) requirements (SITE CODE 1968) and the Alton Geoscience work plan dated March 22, 1993. The objectives of this investigation were to: • further characterize the aquifer beneath the site; and • provide data for a site-specific, quantitative health risk assessment. Three observation wells (AW-10, AW-11, and AW-12) were drilled and installed for use during the aquifer pumping test. Data collected from these wells, existing pumping well AW-9, and ground water Monitoring Well AW-1 were used to estimate aquifer parameters for this study, and will be used to perform the human health risk assessment for petroleum hydrocarbons in ground water. 2.0 SITE DESCRIPTION Present Site Use: The site is currently a fenced, vacant lot (Figure 2). Past Site Use: The site was a bulk distribution plant and in August 1988, was abandoned by Mobil Oil Corporation. All structures were removed from the property including one underground gasoline storage tank and four aboveground diesel fuel storage tanks. Future Site Use: The site is slated for redevelopment into a commercial/industrial complex. Adjacent Properties: The site is bound by vacant lots to the east and south. Doane Products Company is located to the west, across the Southern Pacific Railroad (SPRR); Leprino Foods is to the north, across East Grantline Road. Geography: The site is located about 3,500 feet south of Highway 205 at an elevation of approximately 30 feet above mean sea level (NGVD-1929). Topography in the vicinity of the site is relatively flat, dipping slightly to the north. I 1