SITE INFORMATION AND CORRESPONDENCE_FILE 2

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4th Quarter 2006 Report Thrifty#172 Page 4 During the current sampling episode, TPHg, benzene, and MTBE were detected in offsite well MW-9 for the first time since the third quarter 2004. Prior to the third quarter 2004, TPHg, benzene, and MTBE were not detected in well MW-9 since 1996. The reason for the appearance and disappearance of these constituents is uncertain. Thrifty will continue to monitor well MW- 9. Since the third quarter of 1997, Thrifty has analyzed for oxygenates di-isopropyl ether(DIPE), ethyl-tert butyl ether(ETBE), tert-amyl methyl ether(TAME), and tert-butanol (TBA), and MTBE by method 8260B. The alcohols (ethanol and methanol) were analyzed by ASTM standard D285. The SJCEHD requested these analyses. Laboratory results indicate no alcohols or oxygenates (besides MTBE) were detected above the laboratory detection limit. In a letter dated December 9, 2004, the SJCEHD indicated that methanol may be discontinued as a sampling requirement, but that ethanol must continue to be analyzed for since it is a fuel additive at the site. The SJCEHD further required that ethanol must be analyzed by EPA Method 8015. Ethanol was not detected using EPA Method 8260B. Dissolved lead was detected at 0.005 mg/L in well MW-11. Possible New Release or Active Source In the 1st quarter of 2005, Thrifty noted dramatic increases in TPHg(130,000 ug/L) and MTBE (122,000 ug/L) concentrations in well MW-3 that may indicate that a new release occurred or an offsite active source is contributing dissolved hydrocarbons to the area of well MW-3. During the current quarter, MTBE was detected in wells MW-3 (14 ug/L), MW-4 (19 ug/L), MW-6 (2.6 ug/L), MW-9 (5.8 ug/L) and MW-I1 (109 ug/L). Further evidence of a recent release is provided by the use of BTEX ratios that are used as a means to compare the relative age of gasoline releases into the subsurface. The most common method is the cumulative BTEX ration that is described as B+T/E+X. Site investigations indicate that values between 1 and 6 or above are supportive of a recent release and that values less than 0.5 usually indicate a release older than about 8 to 10 years (Kaplan et. al. 1997, "Forensic Environmental Geochemistry: Differentiation of Fuel Types, Their Sources and Release Time;" Robert D. Morrison: "Forensic Techniques for Establishing the Origin and Timing of Contaminant Release"). The table provided below provides B+T/E+X ratios based on the groundwater samples collected during sampling events beginning in May 2005.