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6.1.4 Analytical Results—Groundwater Groundwater impacts were only observed in the samples collected from Monitoring Well TW-5 during the initial phase of investigation at the Site. This well is located immediately downgradient of the former gasoline UST. The highest TPH concentration reported in the samples collected from TW-5 was 350 ug/L for TPH-d. In addition, low level naphthalene concentrations were reported (— 6 ug/L). No benzene, toluene or ethylbenzene was detected in groundwater; however, other VOC detections included: 1,2,4-trimethylbenzene, 1,3,5- trimethylbenzene, n-butylbenzene, methyl tertiary butyl ether (MTBE), m,p-xylenes and o- xylenes. Results from the second round of sampling at TW-5 showed no detectable concentrations of the constituents of concern. A grab groundwater sample collected from Boring E3-2 (E3-2GW) showed only one detection (2.5 ug/L of acenaphthene). All other analytes from this grab groundwater sample were non- detect. 6.2 CONCLUSIONS The following conclusions were generated based on the results of the RI/SSI conducted at the Site: • The former Tracy MGP operated for a very short period of time (— 3 years). The RI/SSI conducted at the Site identified minimal impacts, which is consistent with the short-term operations of the former MGP; • Previous investigations at the Site identified areas impacted with known releases from past MGP operations and/or the gasoline UST that required further investigation. These constituents included PAHs, TPH and VOCs in soil, soil gas and groundwater. • Based on data generated during the RI/SSI, the known releases are confined to the upper zones of subsurface soils. Lateral and vertical delineation of MGP and/or UST related impacts has been achieved in all three environmental media (soil, soil gas and groundwater); • Soil beneath the Site is consistent with descriptions provided in previous investigations and also with the descriptions of regional geology; • Groundwater depths, flow direction and gradient are consistent with historic data for the Site. Groundwater was encountered at depths between approximately eight and 10-feet bgs and was found to be moving to the north-northeast at an approximate gradient of 0.003 ft/ft; • Excavation activities conducted at the new SPCC structure are complete and all impacts beneath this area were removed as outlined in the report. The no further action letter is provided in Appendix N; and • Six AOIs have been identified at the Site as having impacts from either former MGP operations and/or the release from the gasoline UST. These areas were delineated in the lateral and vertical horizons using the criteria identified in Section 4.4 and will be further evaluated in the streamlined HRA/FS/RAW. The following bullets provide a summary description of impacts in these AOIs, and it is anticipated that these areas may require some form of remediation: PARSONS 47 FINAL RI REPORT-FORMER TRACY MGP APRIL 2010