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Mr Ron Rowe <br /> Page 4 of 5 <br /> ' <br /> sampling event and increased significantly in MW3 Benzene toluene total xylenes and MTBE <br /> ' were detected in MW7, located north of well MW3 <br /> Dissolved contaminants present in groundwater fi om wells MW3, MW4 and MW7 appear to be <br /> ' related in large part, if not entirely, to offsite releases The sudden appearance of MTBE in these <br /> wells, the discernable downgradient movement of the MTBE plume and the increased <br /> concentrations of BTEX—rich gasoline suggests an offsite contributing source It is suspected that <br /> the ARCO station at 130 S Wilson Way, the formei Top Filling Station at 101 S Wilson Way, or <br /> both may the source of this encroaching contamination In addition, we understand that a release <br /> of MTBE was recently documented at the ARCO facility This provides further indication that <br /> ' ARCO is the source of the MTBE and fresh gasoline on at least the southern portion of the <br /> subject property <br /> 1 Hydrocarbon concentrations downgradient of the former onsite USTs remain within the low <br /> historic range in wells MW2 and MW8 and continue to decline in well MW 13 Concentrations in <br /> well MW5 are stable Overall, the plume of contamination is diminishing <br /> ' VAPOR EXTRACTION SYSTEM ACTIVITIES <br /> VES Operation <br /> The soil vapor extraction system was shut down intentionally during the first quarter of 2000 <br /> tpending direction from PHS/EHD i egai ding fui ther operation of the system <br /> ' Vapor Sample Analytical Results <br /> No samples were collected during the fii st quarter of 2000 <br /> ' System Evaluation <br /> ' As of January 3, 2000 the system had been in operation for 12,225 hours Based on graphical <br /> interpolation between measured influent vapor concentrations, an estimated 12,670 pounds of <br /> hydrocarbon contamination have been removed from the soil Table 5 summarizes flow and vapor <br /> ' concentration averages as well as the total hydrocaibon mass removed from the soil The most <br /> recent hydrocarbon removal rate was appi oaimately 2 pounds per day <br /> ' RECOMMENDATIONS <br /> Develop Closure Critei is and Evaluate Cui i ent Conditions <br /> ' As proposed in our workplan (09117199), we recommend di filling soil borings to evaluate the <br /> effect vapor extraction has had on residual soil contamination and to further define the vertical <br /> roek\q=r 1gn12000 <br />