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2013 Semiannual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site,Stockton,California August 2013 Though the original remediation system was effective in reducing Cr concentrations in soil and groundwater, monitoring and modeling data indicated that compliance with cleanup standards, presented in the WDR, would have required more than 2 decades of additional operation. The full-scale in situ Cr+6 reduction groundwater remediation was implemented under the direction of the DTSC and the CVRWQCB (formerly the RWQCB) in accordance with WDR Order No. R5- 2007-0126,with the goal of reducing dissolved Cr concentrations such that the groundwater extraction and treatment system could be shut down in a shorter time than was projected with only pumping and treating. The full-scale in situ Cr+6 reduction treatment implementation included a series of reactive barriers strategically placed in the Shallow Zone to treat the plume. These barriers were placed throughout the Site, as presented on Figure 3. Additionally, the most severely impacted areas, such as the former core of the plume (with concentrations in excess of 5,000 gg/L of Cr+6) located primarily beneath the northern site parcel (North Yard), were targeted for direct treatment in a grid injection approach to eliminate these principal sources of contamination in an expeditious manner. In addition, the soil flushing program was reinstated with improvements made to the distribution network, using horizontal well technology to better address residual Cr+6 in the vadose zone in the retort pit area. Horizontal wells were also employed to deliver reductant solution into the deepest portion of the vadose zone immediately overlying the water table at the source area. The deeper portion of the vadose zone under the retort pit area has been documented to contain high concentrations of hexavalent chromium in leachable form. The horizontal wells enabled delivery of treatment chemicals at the location of contaminant entry to the aquifer, such that the treatment chemicals would follow the original migration path of Cr+6 Supplemental vertical treatment injections were completed in (1) the fall of 2010 to expedite the reduction of Cr+6 surrounding the former retort pit and in the Intermediate Zone beneath the North and South yards and (2) in the summer of 2012 to expedite Cr+6 reduction from the intermediate zone of the South Yard. Experience at the Site during the two-phase pilot test (Phase 1 conducted in 2003 and Phase 2 conducted in 2005), confirmed experience at other sites related to the migration of the calcium polysulfide reductant solution: (a MWH, 2-2