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Ms Vicky McCartney ' September 27, 2005 Page 3 of 4 and the most current results from the site monitoring wells Previous phases of investigation determined that residual soil contamination is negligible ' Although there was a slight increase in the concentration of benzene detected in well MW1 since the previous sampling event, dissolved benzene levels have continued to decrease with time in ' wells MW 1 and MW2 The average concentration of benzene in MW 1 appears to be approaching the MCL of 1 ppb and benzene has not been detected in well MW2 for three consecutive sampling events Benzene levels in downgradient well MW3 were on an overall trend of increase from the inception of monitoring until 2001 Since that time, levels have risen and fallen from sampling event to event but the pace of overall increase has slowed substantially The most recent results from MW3 suggest that benzene levels may have peaked and will begin to decrease similarly to ' those in MW 1 Graphs depicting benzene concentrations over time in site wells are included in Appendix C ' The presence of 1,2-DCA has been confirmed in the vicinity of upgradient well MW2 However, the concentrations measured on three occasions have been only slightly above the MCL for this compound 1 r At the request of San Joaquin County Environmental Health Department (letter dated 6/13/05), ' Ground Zero prepared cross sections and maps depicting site lithology and extent of contamination, • calculated the contaminant mass in place, and modeled the future migration/extent of the plume of dissolved contamination The results indicate that the residual mass of the contaminants is ' negligible (185 pounds TPHg and 6 pounds of benzene in soil, <1 pound TPHg and benzene in groundwater) and the plume of dissolved benzene will not extend further east than the property line at a concentration above the MCL Figures, calculations and modeling results are included in ' A! Zp ndix D ,y Ak An e�f 5 ,�miiy�b�, Recommendation for Case Closure ' Based upon the evidence obtained from investigations conducted at the subject site, we believe that this is a low risk site and case closure is appropriate Enclosed in Appendix D is the required documentation as listed in the "Checklist of Required Data for No Further Action Requests at Underground Tank Sites"pursuant to Tri-Regional Board Staff Recommendations for Preliminary Investigation and Evaluation of Underground Tank Sites, Appendix A — Reports (formerly ' Appendix B) Included with the checklist are calculations of the mass of the contaminants remaining in place and the results of contaminant transport modeling G\GROiJ11rpZE\W-USAR\2005\2sAR05 doc