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2012 Annual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site, Stockton,California February 2013 6.0 PERFORMANCE EVALUATION This section presents an evaluation of the full-scale in situ Cr+6 reduction treatment implementation performance, as required in the WDR. Performance is evaluated based on the extent of groundwater influenced by the injection, degree of Cr+6 reduction, and degree of accumulation of undesirable byproducts (i.e., SO4, As, or other metals). 6.1 EXTENT OF TREATMENT ZONE IN GROUNDWATER 6.1.1 Phase I and Phase II of the Pilot Study Results for Phase I and Phase 11 of the pilot study are summarized in the 2008 Annual Groundwater Monitoring Report(MWH, 2009). 6.1.2 Full-Scale In Situ Groundwater Remediation Results of Cr and Cr+6 analyses during the second half of 2012 are presented in Tables 5 and 7 and shown in Appendix A (Figures 1 through 8). The right column in Table 7 lists the reduction in Cr concentrations (as a percent) by comparing the most recent concentration with the highest concentration since the baseline sampling event in 2008. This approach was taken because in some cases, the remedial process caused temporary increases in Cr concentrations in groundwater that may not be reflected in the baseline data. The percent reduction is not considered meaningful if Cr has not been detected above the laboratory analytical RL. The list below summarizes the effectiveness of the remedial action program, as evidenced by the reduction in Cr and Cr+6 concentrations in groundwater at 39 injection area wells (background and compliance wells are not included). Bold font indicates wells where the most recent concentration of Cr or Cr+6 exceeded the WQO. New wells MW-4495, MW-449D, MW-450S, and MW-450D, which only have results from two sampling events, were not considered for the lists below. Wells with 80-to more than 99-percent reduction in Cr concentrations • Shallow Zone wells MW-6, MW-9, MW-101, MW-104 (S-2), MW-105, MW-204, MW- 206, MW-208, MW-301, MW-303, EW-1 (a Shallow/Intermediate Zone well), TW-12, TW-14, and TW-17 • 100-Foot Sand Zone wells TW-13 and TW-16 (D MWH,, 6-1