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2013 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site,Stockton,California August 2013 <br /> 6.0 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 2013 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 conductive to <br /> natural attenuation of Cr, which was a component of the RAIWP (MWH, 2007b). Supplemental <br /> in-situ treatment injections implemented in 2012 have resulted in additional reduction of Cr+6 <br /> concentrations at wells MW-204 (which has been reduced to below the WQO), MW-447S, MW- <br /> 447D, and MW-4495. It is expected that the continued reducing effect will be observed within <br /> and downgradient from the injection areas. <br /> (a MWH" <br /> 6-1 <br />