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2012 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California October 2012 <br /> 4.2.2.2 Injection Area Wells. During the first and second quarters of 2012, Fe <br /> concentrations in injection area wells ranged from non-detect (MW-323 in January 2012) to <br /> 4,810 µg/L (MW-206 in January 2012). Fe was detected at concentrations above the WQO <br /> (300 µg/L) in 20 injection area wells sampled during the first quarter of 2012, and 17 injection <br /> area wells sampled during the second quarter. These results are generally consistent with <br /> baseline concentrations or trends evident in the data collected since 2008 (see Graphs 1 through <br /> 53). The presence of ferrous Fe will serve as a continuing source of reduction to potential Cr+6 <br /> in these 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 /> in the ferric hydroxide to the soluble manganous form. Analytical results for Mn are presented <br /> in Table 5 and on Graphs 1 through 53. <br /> 4.2.3.1 Background and Compliance Wells. Background and compliance wells do not <br /> show an increase in Mn concentrations resulting from the reduction process in the treatment <br /> areas. During the first and second quarters of 2012, Mn concentrations in background and <br /> compliance wells were below the laboratory analytical reporting limit (RL) of 10 µg/L. <br /> Mn concentrations were below the WQO (50 gg/L) and the proposed upper background limit <br /> concentrations for all wells. Table 6 presents the proposed upper background concentration <br /> limits for background wells in comparison to historical results and WQOs. <br /> 4.2.3.2 Injection Area Wells. During the first and second quarters of 2012, <br /> Mn concentrations in injection area wells ranged from below the RL of 10 µg/L to a maximum <br /> detected concentration of 9,610 gg/L (TW-12 in April 2012). Mn was detected at concentrations <br /> above the WQO (50 gg/L) in 17 injection area wells sampled during the first quarter of 2012, <br /> and 21 injection area wells sampled during the second quarter. Many of the wells where Mn <br /> (D MWH. <br /> 4-6 <br />