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1.0 INTRODUCTION This report presents the findings of an additional site investigation performed at former Mobil Bulk Plant 04-343, located at 500 East Grantline Road, in Tracy, California (Figure 1). The 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 28, 1994. The workplan was submitted to the PHS/EHD for review; approval to proceed was received from the PHS/EHD on April 12, 1994. The objective of this investigation was to further characterize residual soil contamination which may remain after the 1990 remedial excavation (Alton Geoscience, 1990). 2.0 SITE DESCRIPTION The site is currently a fenced, vacant lot (Figure 2). 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. 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. 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, sloping slightly to the north. Regional Geology and Hydrogeology: The site lies in the Central Valley of California. Geologically, the valley is a large asymmetric trough that is bounded by pre-Tertiary granitic, metamorphic, and marine sedimentary rocks. The trough has been filled with as much as 30,000 feet of sediment in the San Joaquin Valley to the south and as much as 60,000 feet of sediment in the Sacramento Valley to the north. These sediments range in age from Jurassic to Holocene and include both marine and continental sedimentary deposits (USGS 1986). The scope of this investigation includes only the uppermost deposits within the Central Valley. Deposits of Tertiary and Quaternary age compose a number of units; in total they constitute the major aquifer of the Central Valley and yield large quantities of water to wells. Lithologically. these deposits are heterogeneous mixtures of poorly sorted clay, silt, sand, and gravel, and in places some beds of claystone, siltstone, sandstone, and conglomerate. The maximum thickness reported for these units is 3,000 feet. Continental deposits of Quaternary age crop out primarily along the major rivers and streams of the valley as well as other low-lying areas; the deposits include river and flood-basin deposits, and sand dunes. River deposits, including channel and flood-plain deposits, are considered to 1