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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 concentration of total Cr and Cr+6 was 12,400 gg/L (for both constituents) in well EW-3 in October 2011. Well EW-3 is located at the former retort pit (source area) and was sampled at SPX's discretion (sampling at this well is not required by the MRP). A summary of the concentration trends for wells where Cr concentrations exceeded the WQO in 2011 is presented below. Wells where Cr concentrations have decreased to below the WOO. Quarterly sampling in the second half of 2011 showed that concentrations of total dissolved Cr or Cr+6 in well MW- 207, which were slightly above the WQO in April 2011, have now decreased to below the WQO again(see Graph 18). Concentrations in this well have historically fluctuated near the WQO. Wells where Cr concentrations remain stable above the WOO. Quarterly sampling in the second half of 2011 showed that concentrations of total dissolved Cr and Cr+6 remain relatively stable at concentrations above the WQO in wells MW-4, MW-8, MW-102, MW-106, MW-204 (S-8), MW-205 (S-7), MW-212, MW-301, MW321, MW-447S, MW-447D, MW-448S, MW- 448D, EW-3, and I-11 (see Graphs 3, 6, 9, 12, 15, 16, 22, 26, 32, 39, 40, 41, 42, 44, and 47). Recent data show concentrations increased at wells I-11 and EW-3 during the current period, but remain below baseline concentrations before full-scale in situ remediation was initiated. Recent data show Cr concentrations are decreasing at MW-321 (1-9), likely in response to supplemental injections near these wells in 2010. Well MW-213 concentrations increased from below the WQO in first and second quarter to above the WQO in third and fourth quarter (see Graph 23). This well will continue to be monitored and verify if the concentration is actually rebounding. 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. (a MWH' 4-3