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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 448S and MW-448D) and its use as an active extraction well. Third quarter 2012 sampling results for I-11 indicate that Cr and Cr6+ concentrations remained stable at 440 and 438 µg/L, respectively. As outlined in the 2011 2nd Semi-Annual Groundwater Monitoring Report (Dudek, 2012), the water table elevation at EW-3 in the Shallow Zone, at the retort pit, experienced an approximate 20+-foot decline from fourth quarter 2006 to fourth quarter 2009. From fourth quarter 2009 to present, water table elevations have risen approximately 15 feet. The Cr and Cr6+ concentration fluctuations being observed at EW-3 and potentially in the area of MW-213 may be the result of rising ground water leaching soluble hexavalent chromium from the high concentrations immediately under the retort pit area. The newly installed wells MW-449S and MW-449D, installed in the North Yard, and MW-450S and MW-450D, installed in the South Yard, will help provide additional information on the distribution of Ct and Cr6+ concentrations in the intermediate zone. Data from these newly installed wells will be documented in the 2012 Annual Report, which is due by February 2013. 4.2.2 Iron Fe in the soluble ferrous form, which is formed when polysulfide reacts with ferric hydroxide coatings on aquifer grains, serves as a reductant of Cr+6. As a result, Fe is not generally detected in samples with elevated Cr+6, and only remains in solution once the Cr+6 has been reduced. Thus, Fe serves as an effective indicator of the reduction of Cr+6, and its presence serves as a continued reductant for Cr+6. Analytical data for Fe sampled during the program are summarized in Table 5 and presented on Graphs 1 through 53. 4.2.2.1 Background and Compliance Wells. During the first and second quarters of 2012, Fe concentrations in background and compliance wells ranged from 70 gg/L (MW-401 in April 2012) to 417 µg/L (MW-304 in January 2012). Fe was detected at concentrations above the WQO (300 µg/L) or proposed upper background limit concentrations at wells MW-201, MW-304, MW-310, and MW-325 during at least one quarterly sampling event in the first half of 2012. Table 6 presents the proposed upper background concentration limits for background wells in comparison to historical results and WQOs. (D MWH, 4-5