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Stant@C II <br /> SECOND QUARTER 2009 OPERATIONS MAINTENANCE AND MONITORING REPORT <br /> SHOP 10 MAINTENANCE STATION <br /> Conclusions and Recommendations <br /> July 31,2011 <br /> 50 <br /> Conclusions and Recommendations <br /> f <br /> Results from May 2009 groundwater sampling indicated declining 1,2-DCA levels above the California <br /> �r Maximum Contaminant Level {MCL}for drinking water of 0.5 pg/L in,.three of twelve wells on-site. No <br /> other concentrations exceeding PQL levels of dissolved phase petroleum hydrocarbons, BTEX, other fuel <br /> oxygenates, or lead scavengers were detected in any of the wells. <br /> Levels of hydrocarbon constituents have been reduced to non-detect levels in all monitoring wells by a <br /> combination of vapor removal, AS related volatilization, and natural biodegradation processes. In <br /> addition, hydrocarbon levels in groundwater did not rebound appreci{ably following three rebound periods. <br /> Vapor samples continue to show declining concentrations in system influent and vapor extraction wells. <br /> Mass removal rates and influent vapor concentrations have decreased to a point where continued <br /> operation of the system is no longer warranted. Based on four consecutive monthly influent vapor results <br /> below 200 ppmv, indicating the end of cost-effective removal, and supported by low and declining <br /> groundwater concentrations, the system was shut down on June 25; 2009,`to begin the Site closure <br /> process. Site closure may include further soil confirmation borings,,vapor sampling, and submittal of a <br /> I. formal written request for closure to the SJCEHD. <br /> L� <br /> f�. <br /> I <br /> is 5\us1329-f011seroAproject-055caltrans%stockton-shop 1 DYomBm and gw reporting=OM2nd qtr 2DMg2_49 omm fnl.doc 5.1 <br />