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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 • SO4 (may be formed as calcium polysulfide [CaSX] reacts with Cr+6 or other reducible species) • General parameters (indicator of redox conditions in the treatment zone) 4.2.1 Hexavalent and Total Chromium Analytical results for total dissolved Cr and Cr+6 collected for the injection program are summarized in Table 5 and presented on the graphs. Cr concentration contours for the third- quarter (July 2012) monitoring and sampling event are shown in Appendix A (Figure 1 [Shallow Zone] and Figures 2 and 3 [Intermediate Zone]). The fourth-quarter (October 2012) monitoring and sampling event total dissolved Cr concentrations are shown in Appendix A (Figure 5 [Shallow Zone] and Figures 6 and 7 [Intermediate Zone]). 4.2.1.1 Background and Compliance Wells. Initial background sampling for the full- scale in situ Cr+6 reduction treatment implementation was completed in January 2008. During the third and fourth quarters of 2012, Cr concentration detections in background and compliance wells ranged from non-detect (MW-401 in October 2012) to 18 gg/L (MW-201 in July 2012). Cr was not detected in the background and compliance wells at concentrations above the WQO (50 µg/L) during the current reporting period. Table 6 presents the proposed upper background concentration limits for background wells in comparison to historical results and WQOs. Cr concentrations were below the proposed upper background concentration limits for the background and compliance wells. Concentrations of Cr+6 in the background and compliance wells in the third quarter 2012 sampling event ranged from 0.93 gg/L (MW-210) to 8.48 gg/L (MW-201). Third quarter 2012 Cr+6 concentrations in background and compliance wells were below the WQO and the proposed upper background concentration limits for each well. Fourth quarter 2012 sampling for the background and compliance wells did not include Cr+6, consistent with the revised MRP. 4.2.1.2 Injection Area Wells. Total dissolved Cr and/or Cr+6 were detected at concentrations above the WQOs in 18 of the 44 wells and 19 of the 44 wells sampled during the third and fourth quarters of 2012, respectively, (not including the background and compliance wells). The highest reported concentration of total Cr and Cr+6 was 12,600 and 12,500 µg/L, (a MWH" 4-2