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2011 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California January 2012 <br /> Though the original remediation system has been effective in reducing Cr concentrations in soil <br /> and groundwater, monitoring and modeling data indicate that compliance with cleanup <br /> standards, presented in the WDR, would require more than 2 decades of additional operation. <br /> The full-scale in situ Cr+6 reduction groundwater remediation was implemented under the <br /> direction of the DTSC and the CVRWQCB (formerly the RWQCB) in accordance with WDR <br /> Order No. R5-2007-0126, with the goal of reducing dissolved Cr concentrations such that the <br /> groundwater extraction and treatment system could be shut down in a shorter time than was <br /> projected with only pumping and treating. <br /> The full-scale In Situ Cr+6 reduction treatment implementation included a series of reactive <br /> barriers strategically placed to treat the plume. These barriers were placed throughout the Site, <br /> as presented on Figure 3. Additionally, the most severely impacted areas, such as the former <br /> core of the plume (with concentrations in excess of 5,000 gg/L of Cr+6) located primarily beneath <br /> the northern site parcel (North Yard), were targeted for direct treatment in a grid injection <br /> approach to eliminate these principal sources of contamination in an expeditious manner. In <br /> addition, the soil flushing program was reinstated with improvements made to the distribution <br /> network, using horizontal well technology to better address residual Cr+6 in the vadose zone in <br /> the retort pit area. Horizontal wells were also employed to deliver reductant solution into the <br /> deepest portion of the vadose zone immediately overlying the water table at the source area. <br /> This enabled delivery of treatment chemicals at the location of contaminant entry to the aquifer, <br /> such that the treatment chemicals would follow the original migration path of Cr+6. Supplemental <br /> treatment injections were completed in the fall of 2010 to expedite the removal of Cr+6 <br /> surrounding the former retort pit and in the Intermediate Zone beneath the North and South <br /> yards. <br /> 0) MWH. <br /> 2-2 <br />