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REG 603 UPDATE-8 7/17/92 4 0 DISCUSSION AND CONCLUSIONS As stated in earlier reports, the soil is contaminated with gasoline range hydrocarbons beneath the tank field (MW10) and along the capillary fringe of the ground water, MW1, MW2, MW3 and MW10. Ground water beneath the site contains the dissolved constituents of gasoline range hydrocarbons and has spread out from beneath the site to the north MW-6, to the south MW-7 to the west MW-9 and to the east MW-8. The latest ground water samplings, March 10 and June 18, 1992, have shown a decline in- the lateral extent of the plume, see Figures 5A (12-17-91) and 5B (6- 18-92) Vapor extraction commenced on October 17, 1991 and up until March 18, 1992 has operated for 2611 hours out of a possible 3674 for a 71% run time, destroying 16,156 pounds of gasoline range hydrocarbons. Since the last status report, February 3, 1992, a total of 4884 pounds of gasoline range hydrocarbons have been removed/destroyed DCE, TCE and PCE screening indicates that these compounds are not present in the vapor stream at this site. . Ground water elevation measurements show the ground water is being mounded at the lower K value wells MW3 and MW10, while the overall flow direction is to the northeast and southeast. Ground water sample results obtained on December 17, 1991, approximately 1473 hours after commencement of vapor extraction compared to the ground water sample results obtained on June 18, 1992, 5885 hours after commencement of vapor extraction, show a decline in the extent of the dissolved constituents in all of the monitoring wells with the exception of MW1 and MW3 which have floating product and MW10 which was dry, see Table 1 and Figures 5A and 5B. It is felt that once the air dilute has been completely closed and all vacuum recovery can be directed to MW1, MW2, MW3, MW10 and EX- 4", an increase in the vacuum influence will be obtained and a more substantial reduction in the contaminants, found in the other wells, will be noted The permit to dump treated ground water to the sewer has been applied for and once approved, ground water pumping will occur from monitor wells MW1 and MW3. Treatment will be tank stripping, utilizing the "ConVault" as a 1000 gallon tank stripper to batch treat approximately 300 gallons every 5 hours. All vapors driven off of by the air stripping from the tank will be processed into the thermal oxidizing unit for destruction, see Appendix D - WaterDischarge Permit Application. • PAGE 11