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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 Mn concentrations were below the WQO (50 gg/L) and the proposed upper background limit concentrations for respective wells, with the exception of July 2011 detection at MW-325. Table 6 presents the proposed upper background concentration limits for background wells in comparison to historical results and WQOs. Background and compliance wells do not show an increase in Mn concentrations resulting from the reduction process in the treatment areas. 4.2.3.2 Injection Area Wells. During the third and fourth quarters of 2011, Mn concentrations in injection area wells ranged from below the RL of 10 gg/L in 16 wells to a maximum detected concentration of 9,110 gg/L (TW-13 in July 2011). Mn was detected at concentrations above the WQO (50 gg/L) in 22 injection area wells sampled during the third quarter of 2011, and 21 injection area wells sampled during the fourth quarter. Many of the wells where Mn concentrations exceed the WQO show a concentration increase after the injections of reductant solution in the vicinity. These elevated concentrations are expected to be temporary; data from wells MW-8 and MW-208 show that Mn concentrations decrease over time. Elevated Mn concentrations reflect groundwater conditions that are conducive to Cr reduction. No additional action is recommended at this time regarding the observed Mn concentrations. 4.2.4 Arsenic Concentrations of dissolved As are naturally low, as this element is typically present as a solid, bound to aquifer solids in the pentavalent As (As V) oxidation state (arsenate, bound to solids similar to phosphate). However, under reducing conditions, As V (as the arsenate anion) can be reduced to the more soluble trivalent As (As III) (as the arsenite anion) form. Analytical data for As collected for the full-scale in situ Cr+6 reduction treatment implementation are summarized in Table 5 and presented on Graphs I through 53. 4.2.4.1 Background and Compliance Wells. During the third and fourth quarters of 2011, the As concentrations in background and compliance wells remained below the WQO (10 gg/L) and the proposed upper background concentration limit (10 to 13 gg/L), with the exception of 19 gg/L detection at MW-325 in July 2011. The detection reduced to 4J gg/L in (a MWH,, 4-5