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DPE SYSTEM <br /> The DPE system began operation during April 2007. The DPE system was shut off on <br /> February 2, 2009 due to de minimus vapor recovery rates which indicated the system <br /> had reached its effective limit. The DPE system removed in total 2,543 pounds of vapor <br /> phase TPHg and 35.5 pounds of vapor phase benzene. A total of 527,530 gallons of <br /> groundwater were extracted during DPE system operation which equates to an <br /> estimated removal of 10.4 pounds of dissolved phase TPHg and 0.29 pounds of <br /> dissolved-phase benzene. Vapor-phase mass removal rates decreased to low levels and <br /> could not be increased despite well array adjustments and other optimization efforts, <br /> thus indicating that DPE had reached its effective limit of operation. <br /> INSITU CHEMICAL OXIDATION(ISCO) <br /> This proposed remedial alternative was determined to be infeasible, based on a bench <br /> scale test performed by CRA at another site in Stockton(2575 Country Club Boulevard). <br /> We determined that the naturally elevated levels of iron and sodium present in <br /> groundwater would be dramatically increased if ISCO was applied at this site. Based on <br /> the water quality testing performed at the subject site (Site Investigation Report and Fourth <br /> Quarter 2010 Gmundroater Monitoring Report,March 10,2011), CRA concluded that ISCO <br /> is not an appropriate remedial approach. <br /> CONSIDERED REMEDIATION ALTERNATIVES <br /> CRA reviewed multiple remedial alternatives for this site. <br /> • As discussed in the April 2009 Remediation System Shutdozon Request, SVE and DPE <br /> reached de minimus levels of effectiveness and will not effectively remediate the <br /> source area. <br /> • ISCO is not considered applicable due to naturally high levels of sodium and iron in <br /> groundwater that would be increased if ISCO was applied. <br /> • Air sparge with SVE could potentially decrease concentrations of dissolved-phase <br /> hydrocarbons, but would have limited effectiveness at treating soils in the source <br /> zone. A full description of CRXs assessment of air sparge with SVE is presented in <br /> the December 9, 2009 Corrective Action Plan and Site Investigation Work Plan <br /> (CAP). <br /> 240864(�) 4 CONESTOGA-ROVERS&ASSOCIATES <br />