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4.2.1 Volatile Organic Compounds A total of 27 soil gas samples, including duplicates, were analyzed for VOCs including naphthalene using analytical method TO-15, with an additional analysis for isobutane, which was the leak-check compound utilized on-site during the RI (July/August 2008). Due to proposed changes in the Cal/EPA Soil Gas Advisory, the leak detection compound used for the SSI was helium. As such, soil gas samples collected during the SSI (April 2009) were analyzed for VOCs including naphthalene using method TO-15, and helium using ASTM Method D1946. Select analytical soil gas data is shown in Figure 12 and tabulated soil gas analytical data is presented in Table 22. Laboratory analytical data for soil gas samples is presented in Appendix U. A total of 25 VOCs were detected in the soil gas samples collected during the RI/SSI. Naphthalene was non-detect below laboratory reporting limits in all samples, and isobutane was below DTSC-mandated leak-check compound detection limit of 10 ug/L (DTSC, 2003) in all but one sample (A3-SGID). In A3-SG1D (duplicate sample of A3-SG1), isobutane was encountered at 12 ug/L. At that location, the primary sample (A3-SG1) did not exceed the DTSC leak detection limit, and it is therefore presumed that the vapor probe column is sealed and the isobutane entered through another means (i.e., a connection at the surface). In addition, the results for the primary sample (A3-SG1) and the duplicate sample (A3-SGID) were within 5% of each other, which is within acceptable variance levels for data quality. There were no helium leaks detected in the sampling system for the SSI locations (A1-SG2, Al-SG3, and A3-SG2 through A3-SG6). The highest VOC detections consistently occurred in sample A3-SG1 and its duplicate sample A3-SG1D (Figure 12), and the identified constituents are those typically associated with residual TPH impacts (i.e. BTEX, trimethylbenzenes, butylbenzenes and propylbenzenes). The A3-SG1 location is directly adjacent to the former gasoline UST that was previously removed. At this location, the highest concentration was 230,000 ug/m3 of 1,2,4-trimethylbenzene. Other constituents with elevated concentrations detected in these two samples (A3-SG1 and A3-SG1D) included the following: BTEX, 1,3,5-trimethylbenzene, 2,2,4-trimethylpentane, 4-ethyltoluene, cumene, cyclohexane, heptane, hexane, and propylbenzene. Based on results indentified during the RI, additional soil gas points A3-SG2 through A3-SG6 were installed to delineate impacts identified in A3-SG1 and to evaluate the potential for vapor intrusion in service center buildings. Results from these additional five soil gas points showed concentrations several orders of magnitude below the results exhibited in A3-SG1 and were below residential CHHSLs in shallow soil gas. Therefore, soil gas delineation has been achieved in this location. Beyond AG-SGI and A3-SG1D, the highest concentration was encountered at Al-SG1 where PCE was detected at a concentration of 1,500 ug/m3. Additional soil gas points Al-SG2 and AI- SG3 were installed approximately 10 feet downgradient and upgradient (relative to groundwater flow direction), respectively, to delineate the extent of PCE in soil gas. Results from these samples showed that concentrations of PCE in Al-SG2 and AI-SG3 were 250 and 93.0 ug/m3, respectively, and that PCE concentrations rapidly decrease in a relatively short distance away from Al-SG 1. The source of the PCE is not known, and PCE is not associated with MGPs and/or gasoline. However, it is a common solvent. PARSONS 35 FINAL RI REPORT-FORMER TRACY MGP APRIL 2010