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1444 MARIPOSA ROAD WKA No. 2938.07 a . January 18, 2001 Page 2 Of the analytes examined, Tert-Butanol was the only one identified during this sampling event. It was reported at a concentration of 13 ug/l in well MW7 This is consistent with findings from the April 18, 2000 sampling event when Tert-Butanol was reported in MW7 at a concentration of 11 µg/1. Tert-Butanol was not reported above detection limits in the water sample obtained from MW2 This finding represents an apparent improvement in water quality from the April 18,2000 sampling event when Tert-Butanol was reported in MW2 at a concentration of 8.4 ug/1 and MTBE was reported at concentration of 0 78 ug/l Dunng the most recent sampling event,the depth-to-groundwater measurements in wells MW2 and MW7 were recorded at 50.40 feet and 50.34 feet below top of casing,respectively. Free product, sheen or petroleum hydrocarbon odors were not observed during groundwater monitoring and sampling activities. The measured depth to groundwater is consistent with recent measurements for the site. However,it should be noted that while well MW7 is screened at a depth suitable to intersect the soil/groundwater interface,well MW2 is not. Since screening across the soil/groundwater interface is usually considered a requirement when evaluating a petroleum hydrocarbon site,the above situation wan-ants discussion if the site is to be considered for low risk status. In light of this present situation,we will provide a bnef history before discussing any adverse possibilities related to the well construction. Wells MWl through MW3 were constructed in October of 1993 at a time when extended draught condition prevailed in California. At the time of construction,the depth to groundwater was measured at approximately 74 feet below ground surface• the screen interval was accordingly set from 64 to 84 feet below site grade This construction practice was and is considered common and in most cases provides a more-than-adequate range within which groundwater can fluctuate. r However, since 1993, the local groundwater has risen approximately 20 feet beneath the site. Because of extreme nse in groundwater,the tops of the screened interval in wells MW 1 through MW3 are presently 14 or so feet below the water surface. This means petroleum hydrocarbons that primarily float on the water surface are unlikely to be drawn through the perforated casing during well purging and sampling Under a scenario where little or no site history exists,this is a very serious consideration on which to base final closure recommendations. The case of the above site,however, is somewhat less troublesome in that almost eight years of data exists for the three"inundated"wells.