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2012 Annual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California February 2013 <br /> 7.0 SUMMARY AND CONCLUSIONS <br /> The results presented in this report demonstrate the continued success of the full-scale in situ <br /> Cr+6 reduction treatment implementation in reducing Cr+6 concentrations in groundwater. Data <br /> collected through October 2012 show that Cr+6 concentrations at multiple locations have been <br /> reduced to below the laboratory analytical detection limit in wells screened within the Shallow, <br /> 100-Foot Sand, and Intermediate zones. Concentration contour maps (see Appendix A) show the <br /> plume of groundwater with Cr concentrations above the WQO is shrinking as a result of the <br /> reductant injection program(Table 9). <br /> Groundwater conditions across much of the former plume area continue to be conducive to <br /> natural attenuation of Cr, which was a component of the RAIWP (MWH, 2007b). Supplemental <br /> in-situ treatment injections implemented in 2012 are anticipated to further expedite cleanup in <br /> the intermediate zone at the South Yard. It is expected that the continued reducing effect will be <br /> observed within and downgradient from the injection areas. It can be seen with the data <br /> presented in Section 4 for MW-447S, MW-447D, and MW-448D that Cr and Cr+6 concentrations <br /> are decreasing indicating that the 2012 supplemental injection is working as desired. <br /> (a MWH <br /> 7-1 <br />