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2011 Semiannual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site, Stockton,California January 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 Graphs 1 through 4. Cr concentration contours for the third-quarter (July 2011) monitoring and sampling event are shown in Appendix A, Figure 1 (Shallow Zone) and Figure 2 (Intermediate Zone). The fourth-quarter(October 2011) monitoring and sampling event total dissolved Cr concentrations are shown in Appendix A, Figure 3 (Shallow Zone) and Figure 4 (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 2011, the Cr concentration detections in background and compliance wells ranged from 3J gg/L, J refers to the value being detected but below the reporting limit; therefore, the result in an estimated concentration, (MW-302 in July 2011 and MW-202 in October 2011) to 17 gg/L (MW-304 and MW-310 in July 2011). Cr was not detected in the background and compliance wells at concentrations above the WQO (50 gg/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. Detected concentrations of Cr+6 in the background and compliance wells in the third- and foruth- quarter 2011 sampling events ranged from 0.61 gg/L (MW-211 in October 2011) to 9.74 gg/L (MW-201 in October 2011). 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 Cr+6 were detected at concentrations above the WQOs in 16 of the 44 wells sampled during both the third and fourth quarters of 2011 (not including the background and compliance wells). The highest reported 0) MWH , 4-2