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Former Beacon Station 494 October 8, 1996 <br /> Stockton, Califortua Appendix B Closure Report <br /> Initial Distribution of Petroleum Constituents in Groundwater <br /> Five groundwater monitoring wells were installed at the site to monitor for the presence of <br /> gasoline-range hydrocarbons in the groundwater Separate-phase hydrocarbons have not <br /> been detected on the groundwater table underlying the site Groundwater monitoring has <br /> been performed quarterly at the site from April 1992 through May 1996 Tables prepared <br /> by AMV and Horizon listing the results of this monitoring are included within Appendix C <br /> Groundwater flow was initially evaluated as northwesterly with a relatively flat gradient <br /> (0 01 to 0 008) The depth to groundwater was initially measured at approximately 75 feet <br /> bsg and since that time has risen to approximately 55 feet bsg (Groundwater Tables- <br /> Appendix C) <br /> Dissolved benzene in the groundwater was initially reported (April 1992) in MW-2 and <br /> MW-3, and then (December 1992) in MW-4 After February 1993, benzene was detected in <br /> all five monitoring wells A copy of the July 1993 "Inferred Benzene Iso concentration <br /> Contour Map" (AMV) is included within Appendix E This map is representative of the <br /> initial distribution of benzene in groundwater prior to groundwater pumping and <br /> remediation <br /> Current Distribution of Petroleum Constituents in Soil <br /> In June 1995, Horizon, advanced four confirmation soil bonngs at the site to evaluate the <br /> effectiveness of the VES in removing hydrocarbons from the subsurface The VES had been <br /> operating from November 1993 until June 1995 and had removed an estimated 13,401 lbs <br /> of TPHg and 212 lbs of benzene (Vapor extraction Data Table, Appendix D) or <br /> approximately 2,127 gallons of gasoline The bonngs (CB-1 through CB-4) were drilled on <br /> 1494 APB <br /> 8 HORIZON ENVIRONMENTAL INC <br />