SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Harold and Dena Knowles 102 S.Wilson Way Page 2 of 4 concentration decline from 6,400 milligrams per kilogram (mg/kg) to less than 1 mg/kg; however, the low concentrations of the saturated zone samples in CB4 may indicate that the boring soil samples may not be fully comparable to VW1NW2 samples. Overall, the analytical data from the vadose zone soil samples collected from the confirmation borings are suggestive that the SVE system has had a significant effect on the impacted soil plume. GZA has recommended removal of the SVE system while retaining the wells for use in the event that ground water elevation declines enough to make additional soil vapor extraction beneficial. EHD believes that a mass removal rate of 5.5 pounds per day is still of benefit to the site and is protective of ground water, although the extraction rate may be declining. EHD believes that additional system testing should be conducted prior to removal of the system. Specifically, the hydrocarbon vapor concentrations recovered from each well during continuous system operation should be determined to assess for `hot spots' that may still benefit from additional SVE without wasting system energy on areas that no longer yield significant hydrocarbon vapor. Additional information required for evaluation of the effectiveness of the SVE remediation includes an estimate of the original sorbed mass in place, the mass removed, and an estimate of the maximum mass that may be removed prior to reaching asymptotic conditions. Be aware that asymptotic and steady state are not the same condition. Please have your consultant prepare these estimations. During review of the site status, EHD noted that 1,2-dichloroethane (1,2-DCA) has been detected in vapor and monitoring wells extending from VW-4 to MW- 15A, B, and C, and from MW-10 to MW-11. The highest concentration, 940 micrograms per liter (µg/I) was detected in a water sample from monitoring well MW-13B. The vertical and lateral extent of the 1,2-DCA must be delineated and its distribution in the subsurface described. GZA has identified two sand intervals in the subsurface that appear to serve as contaminant migration pathways; Sand A, lying between 40 and 55 feet below surface grade (bsg) and Sand B, lying between 60 and 80 feet bsg. GZA interprets Sand A and Sand B to be in vertical contact in the area of MW-11 and MW-12; however, soil samples were only collected from the Sand A interval in either well. As this interpretation may have a profound influence on contaminant- migration, the interpretation should be verified through sampling. It appears to EHD that the lateral extent of impacted ground water may have been delineated in Sand B, but not in Sand A. Also, the vertical extent of impacted ground water has not been demonstrated. The vertical and lateral extent of impacted ground water must be determined.