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2012 Annual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California February 2013 <br /> 4.2.2 Iron <br /> Fe in the soluble ferrous form, which is formed when polysulfide reacts with ferric hydroxide <br /> coatings on aquifer grains, serves as a reductant of Cr+6. As a result, Fe is not generally detected <br /> in samples with elevated Cr+6, and only remains in solution once the Cr+6 has been reduced. <br /> Thus, Fe serves as an effective indicator of the reduction of Cr+6, and its presence serves as a <br /> continued reductant for Cr+6. Analytical data for Fe sampled during the program are summarized <br /> in Table 5 and presented on Graphs 1 through 57. <br /> 4.2.2.1 Background and Compliance Wells. During the third quarter of 2012, Fe <br /> concentrations in background and compliance wells ranged from 69 gg/L (MW-401)to 445 gg/L <br /> (MW-201). Fe was detected at concentrations above the WQO (300 gg/L) or proposed upper <br /> background limit concentrations at wells MW-201, MW-202, MW-304, MW-310. Table 6 <br /> presents the proposed upper background concentration limits for background wells in <br /> comparison to historical results and WQOs. Iron samples were not collected from background <br /> and compliance wells in fourth quarter 2012 in accordance with the revised MRP. <br /> 4.2.2.2 Injection Area Wells. During the third and fourth quarters of 2012, Fe <br /> concentrations in injection area wells ranged from 58 (MW-363 in July 2012) to 9,060 gg/L <br /> (MW-104 in October 2012). Fe was detected at concentrations above the WQO (300 gg/L) in <br /> 23 injection area wells sampled during the third quarter of 2012, and 29 injection area wells <br /> sampled during the fourth quarter. These results are generally consistent with baseline <br /> concentrations or trends evident in the data collected since 2008 (see Graphs 1 through 57). The <br /> presence of ferrous Fe will serve as a continuing source of reduction to potential Cr+6 in these <br /> areas. Based on previous site data, Fe is expected to decrease in concentrations to near <br /> background levels over time. No additional action is recommended at this time regarding the <br /> observed Fe concentrations. <br /> 4.2.3 Manganese <br /> Concentrations of Mn commonly show a temporary increase following application of the <br /> reductant solution. Mn is temporarily mobilized as a result of conversion of ferric hydroxide on <br /> aquifer solids to mobile ferrous Fe, with the resultant reduction of Mn present as a co-precipitate <br /> (0 MWH. <br /> 4-6 <br />