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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 4.2.2.1 Background and Compliance Wells. During the third and fourth quarters of 2011, Fe concentrations in background and compliance wells ranged from 42J gg/L (MW-325 in July 2011) to 656 gg/L (MW-201 in July 2011). Fe was detected at concentrations above the WQO (300 gg/L) and proposed upper background limit concentrations at wells MW-201, MW- 202, MW-304, and MW-310 during both quarterly sampling events. The Fe concentration also exceeded the WQO and proposed upper background limit concentration in well MW-7 during the third quarter of 2011, and the WQO in well MW-7 during the fourth quarter of 2011. Table 6 presents the proposed upper background concentration limits for background wells in comparison to historical results and WQOs. 4.2.2.2 Injection Area Wells. During the third and fourth quarters of 2011, Fe concentrations in injection area wells ranged from 75J gg/L (MW-320 in July 2011) to 4,320µg/L (MW-105 in July 2011). Fe was detected at concentrations above the WQO (300 gg/L) in 28 injection area wells sampled during the third quarter of 2011, and 22 injection area wells sampled during the fourth quarter. These results are generally consistent with baseline concentrations or trends evident in the data collected since 2008 (see Graphs 1 through 53). The presence of ferrous Fe will serve as a continuing source of reduction to potential Cr+6 in these areas. Based on previous site data, Fe is expected to decrease in concentrations to near background levels over time. No additional action is recommended at this time regarding the observed Fe concentrations. 4.2.3 Manganese Concentrations of Mn commonly show a temporary increase following application of the reductant solution. Mn is temporarily mobilized as a result of conversion of ferric hydroxide on aquifer solids to mobile ferrous Fe, with the resultant reduction of Mn present as a co-precipitate in the ferric hydroxide to the soluble manganous form. Analytical results for Mn are presented in Table 5 and on Graphs 1 through 53. 4.2.3.1 Background and Compliance Wells. During the third and fourth quarters of 2011, Mn concentrations in background and compliance wells ranged from at or below the laboratory analytical reporting limit (RL) of 10 gg/L to 70 gg/L (MW-325 in July 2011). 0) MWH,, 4-4