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2012 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California October 2012 <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 April 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). Groundwater conditions across much of the former plume <br /> area continue to be conducive to natural attenuation of Cr, which was a component of the <br /> RAIWP (MWH, 2007b). It is expected that the continued reducing effect will be observed <br /> within and downgradient from the injection areas. <br /> (D MWH.: <br /> 7-1 <br />