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a 1444 MARIPOSA ROAD WKA No. 2938.07 . January 18, 2001 Page 3 Specifically,the historic depth to groundwater measurements obtained for well MWl indicate that the well screen was not submerged until after the August 1996 sampling event. Similar measurements revealed that the wells screens in MW2 and MW3 were submerged by May 1996. This means that the wells were appropriately screened for petroleum hydrocarbon sampling for approximately two and a half years prior to submergence A significant observation is that samples obtained from well MW3 between 1993 and 1996 revealed only one trace lit of gasoline(68 ug/17/18/94),two trace hits of benzene(31 ug/l 4/11/94 and 16 ug/l 7/18/94), and one trace hit of toluene(3.4 ug/l 8/17/95) in this two and a half year period prior to well screen inundation. Even more importantly,the trace hit of toluene was the only constituent reported in the well for the preceding four sampling events Using standards that date back well before the 1993/1996 period,this well would have likely been considered free of the problematic constituents gasoline and benzene because the previous four sampling events had occurred over at least one complete hydrologic cycle. Based on the above sampling history,it is our opinion that predominately downgradient well MW3 was free of contaminants by February 1996 and submersion of the well screen should not be a significant consideration when evaluating the adequacy of tins well for petroleum hydrocarbons. Wells MWl and MW2 are somewhat more problematic in that contaminants were reported during the early sampling period prior to well screen inundation. In both cases,however,the presence of contaminants continued to be reported after the screens were submerged. This suggests that even when the screens were submerged,it was possible to obtain representative samples. Further, we were unable to correlate any consistent pattern of reported sample concentrations to the depth of the water surface below the screened interval of the well. For example,there was not a consistent drop in gasoline concentrations as the well screens became increasingly inundated. Rather,the gasoline concentrations bounced around for several years from below detection to several hundred parts per billion. MTBE is somewhat less problematic than gasoline or benzene for the inundated wells in that it tends to dissociate in water more readily. This property,though detrimental from the;standpoint of contaminant migration, facilitates the dissolution of MTBE and allows it to migrate in groundwater more readily that say benzene. With this property in mind, the fact that wells MWl I1 '1' SRfi