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4.3.4 Volatile Organic Compounds A total of eight groundwater samples were analyzed for VOCs using EPA Method 8260 during the RI/SSI. There were only six detected VOCs, all of which were observed in samples collected from monitoring well TW-5. The constituents, including their concentrations, are as follows: • 1,2,4-trimethylbenzene at 37 and 38 ug/L, respectively [in TW-5 and its duplicate (TW- 5D) during RI only]; • 1,3,5-trimethylbenzene at 18 and 18 ug/L, respectively [in TW-5 and its duplicate (TW- 5D) during RI only]; • n-butylbenzene at 1.7 and non-detect (<1.0) ug/L, respectively [in TW-5 and its duplicate (TW-5D) during RI only]; • MTBE at 1.1 and 1.2 ug/L, respectively [in TW-5 and its duplicate (Dup-1) during SSI only]; • m,p-xylenes at 13 and 13 ug/L, respectively [in TW-5 and its duplicate (TW-5D) during RI only]; and • o-xylenes at 3.8 and 4.0 ug/L,respectively [in TW-5 and its duplicate (TW-5D) during RI only]. The highest VOC concentration detected(38 ug/L of 1,2,4-trimethylbenzene)was encountered in sample TW-5D (duplicate sample collected from TW-5). There were no detections of benzene, toluene and/or ethylbenzene in any of the groundwater samples collected at the Site. VOC analytical data for groundwater is presented in Table 26. 4.4 DELINEATION OF MGP/UST RELATED IMPACTS After the RI was conducted at the Site in July/August 2008, the data was evaluated and data gaps were identified. The data gaps were conveyed to DTSC in a Final Technical Memorandum dated March 6, 2009 along with a plan for delineation of these data gaps (Parsons, 2009). The technical memorandum is provided in Appendix M. After receipt of the DTSC's approval of the memorandum, an SSI was conducted and the results were incorporated into this section. Based on these results, the requirements for adequately characterizing the nature and extent of impacts have been met; thus it is anticipated that this Site can move forward with the risk assessment and development of the RAW. The following subsections provide a description of the delineation of MGP/UST related impacts based on previous investigation data as well as data generated during this RI/SSI. Based on the results of the RI/SSI sampling conducted at the Site in July/August 2008 and April 2009, a total of six areas of impact (AOI) have been identified with distinct characteristics. The location of these AOIs is provided in Figure 14. These six areas have been delineated using the following criteria: • B(a)P equivalent concentrations in soil greater than 0.9 mg/kg; • TPH concentrations in soil greater than 1,000 mg/kg; • TPH concentrations in groundwater greater than 100 ug/L; and • VOC concentrations in shallow soil gas related to CHHSLs. PARSONS 37 FINAL RI REPORT-FORMER TRACY MGP APRIL 2010