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Report:Groundwater-quality Monitoring—September 11,2001: 7500 West Eleventh Street, Tracy, CA. Page 6 f-. Fuel Oxygenates* EPA Method 8260A f * Analyses were made for the fuel oxygenates tertiary-Butyl alcohol (TBA), Methyl-tertiary butyl ether (MTBE), Di-isopropyl ether (DIPE), Ethyl tertiary-butyl ether(ETBE), and tertiary-Amyl methyl ether(TAME). Chromalab is certified to perform the specified analyses by the California Department of Health Services (DHS). k.. r The results of the analyses of samples of groundwater are presented in Table 2, which also includes the results of analyses of groundwater samples recovered from the previous sampling rounds. Comparing results of analyses of samples of groundwater recovered on September 11, 2001 with those from May 30, 2001, it is notable that the concentrations of diesel and ' gasoline-range hydrocarbons in the groundwater sample recovered from monitoring well MW-3 continues to decline significantly. MW-3 is a short distance down gradient from the former location of the fueling station's um islands where the preponderance of the F:==; g pump P P leakage of fuels to the subsurface had occurred. However, diesel range hydrocarbons reappeared in the sample from well MW-4 at a concentration of 290 gg/L, after having fallen to a less than detectable concentration when that well was sampled on May 30 2001. MW-4 is just down gradient of the former site of underground fuel tanks. These tanks had not leaked but floating product flowed into their pits from the area of the pump k ' islands when they were removed. That product was pumped from the pits at that time and disposed at a permitted off-site facility. Since well MW-7 was installed, there have been high concentrations of diesel, gasoline and Benzene, Toluene, Ethyl benzene and Xylene (the BTEX compounds associated with gasoline) in groundwater samples recovered from it. MW-7 is located across West a. Eleventh Street and directly down gradient from the former site of the fueling station's pump islands, the principal location on the site where fuels had been lost into the _ subsurface. The sample gathered from that well on September 11, 2001 contained frank floating product, as is reflected by the very high concentrations of analytes of concern measured by the laboratory analyses of the MW-7 sample. (Note: The sample of groundwater recovered from MW-7 on September 11, 2001 continued to be free of any traces of fuel oxygenates, including MTBE.) The groundwater in monitoring well MW-5, which is to the west of the principal plume of affected groundwater, continued to be free of any detectable analytes of concern. The sample recovered on September 11, 2001 from well MW-2, which is also interpreted to i be beyond the western limit of the plume, contained 52 µg/L of hydrocarbons in the diesel range. This is the first occurrence of either diesel or gasoline in the groundwater in that well, although traces of some of the BTEX compounds and MTBE were detected in the first sample recovered from it on May 11 2000. ! f sic r -i z