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• measurement of groundwater Ievels and one round of groundwater sampling for <br /> . TPH-d, <br /> • installation of a groundwater monitoring well within 10 feet of the former waste oil <br /> tank, <br /> 0 drilling and sampling of two additional soil borings for TPH-d, <br /> • analysis of five soil samples for Waste Extraction Test (WET) extractable TPH and <br /> serm-volatile compounds to evaluate the concentration and mobility of the <br /> hydrocarbons remaining in the soils, <br /> • evaluation of the data using geochemical modeling, and <br /> • soil and groundwater analysis of volatile halogenated organic compounds at MW-5 <br /> The remaining sections of this report presents the results of the above activities <br /> 20 BACKGROUND <br /> Two underground storage tanks(UST) were removed from the site in 1988 under supervision <br />. by PHS-EHD AFPC submitted a Site Remediation Report prepared by Dames & Moore to the <br /> PHS-EHD and the California Regional Water Quality Control Board (RWQCB) on May 28, 1992 <br /> The report presented a summary of the site history, corrective action activities, soil remediation, and <br /> groundwater monitoring undertaken at the site, and requested site certification and closure approval <br /> As presented in that report, no petroleum hydrocarbon constituents were detected in any of the <br /> groundwater samples collected from monitoring wells on the site between 1987 to 1991 <br /> In December 1993, the RWQCB responded that a potential threat to groundwater existed at <br /> the site and forwarded the matter to PHS-EHD in a January 10, 1994 letter from PHS-EHD to <br /> AFPC, the PHS-EHD concluded that site certification and closure was not merited and that the <br /> maximum allowable TPH-d concentration to remain in place without detriment to water quality was <br /> 100 milligrams per kilogram (mg/kg) <br /> In March 1994, the RWQCB issued new UST closure guidance in their "Tri-Regional Staff <br /> Recommendations for Preliminary Investigation and Evaluation of Underground Tank Site, <br /> Appendix B - Requests for Closure " This most recent guidance document specifies that conditional <br /> closure may be obtained if human health risks, feasibility and technical considerations support site <br /> closure with limited additional action AFPC then submitted to the PHS-EHD a Draft Amended <br /> Closure Request requesting conditional closure (Dames & Moore, 1994a) Based upon an analysis <br /> SAC156 14 2 <br /> DAMES &MOORE <br />