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Previous Site Investigations In June 1986, a PA was conducted by PG&E for the Site and included the collection of one background soil sample (PG&E, 1986). Results from the background soil sample showed detections of polycyclic aromatic hydrocarbons (PAHs), lead, and arsenic. The reported total PAH concentration was 0.9 milligrams per kilogram (mg/kg), lead was 150 mg/kg and arsenic was 15 mg/kg. The PA made the determination that the Site did not warrant interim management measures or immediate in-depth characterization. Three investigations were conducted at the Site after the PA that focused on a former 1,500 gallon gasoline underground storage tank (UST), located at the northwest corner of the Service Center building (beneath the area currently labeled as the emergency generator in Figure 2). The gasoline UST was installed in the early 1960's (PG&E, 1989). The first of the three investigations was conducted in November 1986 and found floating gasoline product on the groundwater table in two borings drilled within 5 feet of the gasoline UST. Benzene, toluene, ethylbenzene and xylenes (BTEX), and lead were found in soil and groundwater samples collected from the two borings. The gasoline UST was removed in October 1987. During the gasoline UST removal effort, the impacted soil and product, identified in the two borings,were removed. The second investigation conducted in June 1987 consisted of the installation and sampling of thirteen vapor monitoring points (V1 through V13) surrounding the gasoline UST as shown in Figure 7. Samples were collected from each of these vapor monitoring points and analyzed using a portable gas chromatograph (GC) calibrated to benzene and toluene. Results from portable GC showed concentrations of benzene and toluene greater than the detection limit of 1 part per million (ppm) in vapor monitoring points V2, V5 and V7. In December 1987/January 1988, a third investigation was conducted after the removal of the gasoline UST to further quantify the lateral and vertical extent of the hydrocarbons in soil and groundwater in the vicinity of the former gasoline UST. As shown in Figure 8, this investigation consisted of the drilling and sampling of six observation wells (OW-1 through OW-6), which were placed within 80 feet of the former gasoline UST location. During the installation of wells OW-1 and OW-2, it was noted that a layer of black odorous material was encountered near the surface that was suspected to be residue from the former MGP. However, no samples of these materials were submitted to the laboratory for analysis. Groundwater samples were collected from the OW wells on a quarterly or semi-annual basis between January 1988 and November 1995 as part of a long term groundwater monitoring program. Results from the sampling of these wells showed minor detections of the constituents of concern, total petroleum hydrocarbons as gasoline (TPH-g) and BTEX, in OW-2. Additionally, sporadic detections of TPH-g, toluene and xylenes were observed in wells OW-3 and OW-6. Well locations and constituent result summaries are provided in Figure 8. Based on the results from the groundwater sampling events conducted at the Site and because the impacted soil and product was removed during the removal of the gasoline UST, the case was closed per Regional Water Quality Control Board concurrence letter dated January 2, 1997 and by the San Joaquin County Department of Environmental Health (SJCDEH) in a letter dated January 15, 1997. In the SJCDEH closure letter, the SJCDEH stated that no additional monitoring was needed and that the wells must be destroyed in order for the Site to be issued a PARSONS 2 FINAL RI REPORT-FORMER TRACY MGP APRIL 2010