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i . Mr Ron Rowe Page 4 as a result of vapor retention in the sampling pump) prompting the March 27 resampling which confirmed the 96% destruction efficiency of the system A blind blank sample (Efflu-A) was also collected and analyzed on March 27, 1998 The results confirm a source of cross contamination ether in the sampling pump or the copper effluent sampling tube Analytical results of influent samples indicated reduced vapor concentrations in January as compared to fourth quarter 1997 Influent vapor concentrations measured in March were higher than in January, presumably because of the hiatus in extraction during the preceding month System Evaluation The VES has been removing hydrocarbons from vapor wells VW2, VWS, and VW6 since II November 19, 1997 and from VW3 since January 5, 1998 at combined flow rates ranging from 260 to 340 cubic feet per minute (cfm) As of March 30, the system had been in operation for 1,890 hours Based on graphical ' interpolation between measured influent vapor concentrations, as of March 30 an estimated 5,100 pounds of hydrocarbon contamination had been removed from the soil Table 5 summarizes flow and vapor concentration averages as well as the total hydrocarbon mass removed from the soil f i RECOMMENDATIONS r Ground Zero recommends continuing quarterly sampling to monitor the extent of gasoline r hydrocarbons in groundwater Analysis for oxygenates will continue for two additional quarters I The next sampling event is scheduled for May 1998 Your office will be provided 48-hours notice of any field work i Ground Zero also recommends continuing weekly monitoring of the VES and collecting monthly I vapor samples for laboratory analysis Effluent sampling procedures will be modified to minimize false positive readings i 0 f V rmklgmrllgm1998