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the Site. Locations of soil borings, soil gas probes and the monitoring wells are shown in Figure 9. The results showed the presence of PAHs, TPH, metals, and volatile organic compounds (VOCs) in soil, soil gas and/or groundwater. The highest carcinogenic PAH (CPAH) concentration detected in soil was 15,000 micrograms per kilogram (ug/kg) of chrysene at B 1-1-1. To delineate impacts identified in Boring B 1-1, an additional step out boring (B 1-2) was installed at the property boundary as part of the Supplemental Site Investigation (SSI) and results in the samples collected from B1-2 were significantly lower than those in Boring B1-1. The highest concentration of TPH in soil (13,000 mg/kg) was encountered in the motor oil range at E3-1-10D. Based on results from this sample, a step out boring (E3-2) was installed as part of the SSI and TPH results in soil were all non- detect. The highest concentration of any metal detected in soil during this investigation was barium at a concentration of 1,160 mg/kg in sample BI-1-5. All metals results in soil appear to be within the range of concentrations identified in background samples collected as part of the RI. A3-2A-1D had the highest soil VOC concentration (naphthalene at 8,800 ug/kg); however, soil gas samples collected at this location showed no detectable concentrations of naphthalene. The highest soil-gas concentration [230,000 micrograms per cubic meter (ug/m3) of 1,2,4- trimethylbenzene] was observed at A3-SG1 where constituents appeared to be associated with residual impacts from a former gasoline UST that was removed from this area. Other elevated VOCs in soil gas were BTEX, trimethylbenzenes, propylbenzenes and other gasoline related constituents associated with the gasoline UST. Based on elevated concentrations observed in A3-SG1, an additional five soil gas points (A3-SG2 through A3-SG6) were installed to delineate impacts identified in A3-SG1 and to provide data to assess potential indoor air impacts. Results from these additional soil gas points were several orders of magnitude below those detected in A3-SG1 and below residential California Human Health Screening Levels (CHHSLs) in shallow soil gas; therefore, delineation of soil gas impacts in this location is complete. In addition, during the RI, a detection of tetrachloroethene (PCE) was observed in soil gas point AI-SG-1 at a concentration of 1,500 ug/m3, which is above the commercial CHHSL of 603 ug/m3 for PCE in shallow soil gas. As such, two additional soil gas points (A1-SG2 and A1- SG3) were installed approximately 10 feet downgradient and upgradient (relative to groundwater flow direction), respectively, to determine the extent of PCE impacts. Results from these two locations showed PCE concentrations of 250 and 93.0 ug/m3, respectively, and that PCE concentrations rapidly decrease in a relatively short distance away from Al-SG1. In groundwater, the only elevated concentrations detected were observed in Well TW-5 (located immediately adjacent to the former gasoline UST), the highest of which was a detection of TPH as diesel (TPH-d) at 350 micrograms per liter (ug/L). In addition, low level naphthalene detections (5.8 to 6 ug/L) were observed at this location. Title 22 Metals concentrations in the groundwater samples collected from the on-site wells were relatively low and only three of the metals (barium, vanadium and zinc) were observed in all samples. Chromium, molybdenum and nickel were detected sporadically. VOCs in groundwater were detected only in samples from TW-5 and detections were attributable to a former gasoline UST that was removed from this location. Detected constituents included: 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, n- butylbenzene, methyl tertiary butyl ether (MTBE), m,p-xylenes and o-xylenes. These constituents were detected at relatively low concentrations. No benzene, toluene or ethylbenzene was detected in groundwater. PARSONS 4 FINAL RI REPORT-FORMER TRACY MGP APRIL 2010