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AWALAW Quik Stop Market No. 132 Workingo Res##or ��yyature Project No. F7132.93 - lune 8, 193 �1 Results of the March 17, 1993 sampling found that TPHG and BTEX were not detected in groundwater samples from monitoring wells MW-1R, MW-6, and MW-7. Wells MW-3 and J MW-4 were not sampled due to historical nondetectable results. TPHG was also not detected in monitoring well MW-2 but trace amounts of toluene (0.07 parts per billion 4 [ppb]) were detected. MW-5 contained detectable levels of TPHG (280 ppb), and trace amounts of eth lbenzene (0.6 ppb) and total xylenes (0.7 ppb). In the March 1993 analysis, unknown solvent peaks were reported by the laboratory for g samples groundwater les from MW-1R and MW-5. An examination of the chromatogram revealed that these solvent peaks closely resembled those associated with methyl-tert- butyl-ethane (MTBE). MTBE is a non-metallic (lead-free) antiknock compound used in lead-free gasoline. In our sampling and analysis of September 22, 1992, samples from MW-1R and MW-5 were analyzed for MTBE. MTBE was detected in the groundwater samples from MW-1R and MW-5 at 61 ppb and 920 ppb,respectively(see Table 1). MTBE is a very light and highly mobile compound. In light of the past history of excavation at the site, the presence of such a compound is not unexpected. The next quarterly groundwater sampling will be conducted in June 1993. Analysis of the samples collected before and after the March 2, 1993 pumping of MW-5 resulted in TPHG at concentrations of 79 ppb prior to pumping and 200 ppb after the pumping was completed. BTEX constituents were not detected in the sample collected prior to pumping, but a trace amount (1.0 ppb) of ethylbenzene was detected after the pumping was completed (see Table 1). Regulatory Actions During a telephone conversation on Tuesday, February 2, 1993 Mr. Michael Collins of the San Joaquin County Public Health Services (SJCPHS) discussed RESNA's recently completed, Expanded Soil and Groundwater Investigation and December 1992 Quarterly Groundwater Sampling and Analysis (RESNA, January 1993) with Mr. Richard Garlow of RESNA. Mr. Collins stated that he generally agreed with RESNA's recommendations and that sampling of wells MW-3 and MW-4 could be discontinued but may be resampled in the future. He also agreed that fecal coliform analysis could be discontinued. Mr. Collins then requested that this quarterly report include a discussion of the extent of any gasoline related ` contamination that may be present in the soil. As briefly discussed in the Site Background section(Appendix A), in October and November 1990, the three underground storage tanks were removed and contaminated soil above the groundwater was removed. A more detailed discussion of this operation was included in the Exceltech report, 'Termination of Excavation" (Exceltech, November 1990). A summary of our findings on this subject follows. 3