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2012 Semiannual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site, Stockton,California October 2012 • 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 first- quarter (January 2012) monitoring and sampling event are shown in Appendix A, Figure A-1 (Shallow Zone) and Figures A-2 and A-3 (Intermediate Zone). The second-quarter (April 2012) monitoring and sampling event total dissolved Cr concentrations are shown in Appendix A, Figure A-5 (Shallow Zone) and Figures A-6 and A-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 first and second quarters of 2012, Cr concentration detections in background and compliance wells ranged from non-detect (MW-302 and MW-401 in January and April 2012) to 17 µg/L (MW-201 in January 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 first and second quarter 2012 sampling events ranged from non-detect (MW-401 in January 2012) to 10.7 µg/L (MW- 201 in April 2012). Cr+6 concentrations in background and compliance wells were below the WQO and the proposed upper background concentration limits for each well. 4.2.1.2 Injection Area Wells. Total dissolved Cr and/or Cr+6 were detected at concentrations above the WQOs in 14 of the 37 wells and 12 of the 36 wells sampled during the first and second quarters of 2012, respectively, (not including the background and compliance wells). The highest reported concentration of total Cr and Cr+6 was 5,410 and 5,120 gg/L, respectively, at well MW-213 in April 2012. MW-213 is located in the North Yard QD MWR, 4-2