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Report: Groundwater-quality Monitoring—December 13,2001:7500 West Eleventh Street, Tracy, CA, Page b rr r` :I Fuel Oxygenates* EPA Method 8260A * Analyses were made for the fiieI 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). Ch-omalab is certified to perform the specified analyses by the California Department of Health Services (DHS). 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 December 13, 2001 with those from September 11, 2001 and earlier sampling rounds, it is noted that the concentrations of diesel and gasoline-range hydrocarbons in the groundwater sample recovered from monitoring well MW-3 continued to decline. However, there was a moderate increase in the concentration of MTBE in that sample. MW-3 is a short distance down gradient from the former location of the fueling station's pump islands where the preponderance of the leakage of fuels to the subsurface had occurred. Gasoline-range hydrocarbons reappeared in the sample from well MW-4 at a concentration of 740 µg/L, after having fallen to a less than detectable concentration when that well was sampled on May 30, 2001. Diesel in the sample from that well fell r>=:i slightly, from 290 pg/L to 170 pg/L, between September 11 and December 13, 2001. The concentration of MTBE rose significantly, from 170 �tgIL to 1300 µg/L. 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 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 f } gasoline) in groundwater samples recovered from it. MW-7 is located across West 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 December 13, 2001 continued to contain high concentrations of diesel, gasoline and the BTEX compounds. There were no detectable concentrations of MTBE or other fuel oxygenates in the groundwater samples =:. recovered from that well but, due to matrix interference from the presence of high concentrations of diesel and gasoline, the detection limits for those analytes were elevated. It is probable that MTBE, and possibly some other fuel oxygenates, are, in fact, .k present at concentrations below the detection limits. i- - sic l_J