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2011 Annual 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 /> 6.0 PERFORMANCE EVALUATION <br /> This section presents an evaluation of the full-scale in situ Cr+6 reduction treatment <br /> implementation performance, as required in the WDR. Performance is evaluated based on the <br /> extent of groundwater influenced by the injection, degree of Cr+6 reduction, and degree of <br /> accumulation of undesirable byproducts (i.e., SO4, As, or other metals). <br /> 6.1 EXTENT OF TREATMENT ZONE IN GROUNDWATER <br /> 6.1.1 Phase I and Phase II of the Pilot Study <br /> Results for Phase I and Phase 11 of the pilot study are summarized in the 2008 Annual <br /> Groundwater Monitoring Report(MWH, 2009). <br /> 6.1.2 Full-Scale In Situ Groundwater Remediation <br /> Results of Cr and Cr+6 analysis during the second half of 2011 are presented in Tables 5 and 7 <br /> and shown in Appendix A (Figures 1 through 4). The right column in Table 7 lists the reduction <br /> in Cr concentrations (as a percent) by comparing the most recent concentration with the highest <br /> concentration since the baseline sampling event in 2008. This approach was taken because in <br /> some cases, the remedial process caused temporary increases in Cr concentrations in <br /> groundwater that may not be reflected in the baseline data. The percent reduction is not <br /> considered meaningful if Cr has not been detected above the laboratory analytical RL. The list <br /> below summarizes the effectiveness of the remedial action program, as evidenced by the <br /> reduction in Cr and Cr+6 concentrations in groundwater at 44 injection area wells (background <br /> and compliance wells are not included). Bold font indicates wells where the most recent <br /> concentration of Cr or Cr+6 exceeded the WQO. <br /> Wells with 80-to more than 99-percent reduction in Cr concentrations <br /> • Shallow Zone wells MW-6, MW-9, MW-101, MW-104 (S-2), MW-105, MW-206, MW- <br /> 207, MW-208, MW-303, EW-1 (a Shallow/Intermediate Zone well), TW-12, TW-14, and <br /> TW-17 <br /> • 100-Foot Sand Zone wells TW-13, TW-15, and TW-16 <br /> • Intermediate Zone wells MW-1 (Cr), MW-213 (I-7), MW-214 (I-8), MW-320, MW-321 <br /> (I-9), MW-322, MW-323, and EW-4 <br /> MWH <br /> 6-1 <br />