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2012 Annual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site, Stockton,California February 2013 in the ferric hydroxide to the soluble manganous form. Analytical results for Mn are presented in Table 5 and on Graphs 1 through 57. 4.2.3.1 Background and Compliance Wells. Background and compliance wells do not show an increase in Mn concentrations resulting from the reduction process in the treatment areas. During the third quarter of 2012, Mn concentrations in background and compliance wells were at, or below, the laboratory analytical reporting limit (RL) of 10 µg/L. Mn concentrations were below the WQO (50 µg/L) and the proposed upper background limit concentrations for all wells. Table 6 presents the proposed upper background concentration limits for background wells in comparison to historical results and WQOs. 4.2.3.2 Injection Area Wells. During the third and fourth quarters of 2012, Mn concentrations in injection area wells ranged from below the RL of 10 µg/L to a maximum detected concentration of 9,010 µg/L (TW-13 in July 2012). Mn was detected at concentrations above the WQO (50 gg/L) in 25 injection area wells sampled during the third quarter of 2012, and 23 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 (e.g., data from well MW-8 shows 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 1 through 57. 4.2.4.1 Background and Compliance Wells. During the third quarter of 2012, As concentrations in background and compliance wells remained below the WQO (10 gg/L) and the (D M W H.: 4-7