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2013 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site,Stockton,California August 2013 <br /> 4.2.2.2 Injection Area Wells. During the first and second quarters of 2013, Fe <br /> concentrations in injection area wells ranged from <50 gg/L (EW-1 in April 2013) to 10,100 <br /> gg/L (MW-104 in January 2013). Fe was detected at concentrations above the WQO (300 gg/L) <br /> in 28 injection area wells sampled during the first quarter of 2013, and 11 injection area wells <br /> sampled during the second 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 /> in the ferric hydroxide to the soluble manganous form. Analytical results for Mn are presented <br /> in Table 5 and on Graphs 1 through 57. <br /> 4.2.3.1 Background and Compliance Wells. Manganese samples were not collected <br /> from background and compliance wells in the first or second quarters of 2013 in accordance with <br /> MRP No. R5-2012-0814. <br /> 4.2.3.2 Injection Area Wells. During the first and second quarters of 2013, <br /> Mn concentrations in injection area wells ranged from below the RL of 10 gg/L to a maximum <br /> detected concentration of 8,040 gg/L (TW-13 in January 2013). Mn was detected at <br /> concentrations above the WQO (50 gg/L) in 24 injection area wells sampled during the first <br /> quarter of 2013, and 14 injection area wells sampled during the second quarter. Many of the <br /> wells where Mn concentrations exceed the WQO show a concentration increase after the 2012 <br /> supplemental injections of reductant solution in the vicinity. These elevated concentrations are <br /> expected to be temporary (e.g., data from well MW-8 shows that Mn concentrations decrease <br /> over time). Elevated Mn concentrations reflect groundwater conditions that are conducive to <br /> © MWH 4-7 <br />