SITE INFORMATION AND CORRESPONDENCE

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'"00'' GEOMATRIX Mr. Robert Mihalovich Chevron Environmental Management Company May 9, 2002 Page 3 The vertical extent of petroleum-affected soil was addressed during the investigation performed in September 1999. As indicated in the January 2000 report, Soil and Groundwater Investigation and Screening Health Risk Assessment, prepared by Geomatrix,petroleum stained-soil was observed in nine on-site borings to a depth up to approximately 19 feet below ground surface (bgs). Using the borings logs developed for the Site, a cross-section(Figures 1 and 2) depicting the subsurface lithology and the vertical extent of petroleum-affected soil is attached to this letter. It should be noted that petroleum-affected soil in the western portion of the Site, in the vicinity of borings GMX-7 and EB-7, was removed during construction of an approximately 50,000 square foot percolation/retention pond in August 2001. Soil was excavated to a depth of approximately 15 feet bgs and used in the adjacent parcel as fill material beneath an asphalt-paved parking lot. The ponds are intended to be used for retention and percolation of site storm water on an interim basis (two to four years)until the City of Tracy storm drain system is constructed in the area. Confirmation soil samples were collected from the base of the excavation. Discrete soil samples were collected on 20-foot centers and four discrete samples were composited by the analytical laboratory into a single composite sample for chemical analysis. Composite soil samples were analyzed for TPH quantified as crude (TPHc)using U.S. EPA Method 8015 Modified. The discrete soil samples also were analyzed individually if TPHc was detected at a concentration exceeding 100 mg/kg. Confirmation soil sample results indicated that TPHc was detected above laboratory reporting limits in two of eleven composite samples at concentrations of 120 and 130 mg/kg. The discrete soil sample analysis for the two soil composites indicated that TPHc was detected in two of eight samples at concentrations of 80 and 250 mg/kg. These results indicate that most of the affected soil within the footprint of the retention pond had been removed. The pertinent table and figures from Geomatrix's October 8, 2001 report, Confirmation Sampling at Proposed Percolation/Retention Ponds, are attached in Attachment A. 2. The source of the BTEX has not been identified, though the former gasoline station is implicated. Soil from GMX-13 was the only environmental sample analyzed from the area of the former gasoline station. It is likely that above ground or underground leaks or spills associated with the former gasoline station are the source of BTEX. Potential contamination by the former gas station may exist toward the southern boundary where the former gasoline station is depicted in Figure 2 of the December 2000, Report of Additional Soil and Groundwater Investigation." Analytical results of a soil sample collected from GMX-13 at a depth of 12.5 feet bgs indicate that ethylbenzene and xylenes were detected at concentrations of 1.3 and 0.55 mg/kg. Benzene and toluene were not detected above the laboratory detection limit. In addition, benzene has not been detected in other soil samples collected from the site. As indicated in the response to general comment number 2, our investigations have focused on evaluating the potential impacts associated with the residual heavy petroleum