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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 /> concentrations exceed the WQO show a concentration increase after the injections of reductant <br /> solution in the vicinity. These elevated concentrations are expected to be temporary (e.g., data <br /> from wells MW-8, MW-101 show that Mn concentrations decrease over time). Elevated Mn <br /> concentrations reflect groundwater conditions that are conducive to Cr reduction. No additional <br /> action is recommended at this time regarding the observed Mn concentrations. <br /> 4.2.4 Arsenic <br /> Concentrations of dissolved As are naturally low, as this element is typically present as a solid, <br /> bound to aquifer solids in the pentavalent As (As V) oxidation state (arsenate, bound to solids <br /> similar to phosphate). However, under reducing conditions, As V (as the arsenate anion) can be <br /> reduced to the more soluble trivalent As (As III) (as the arsenite anion) form. Analytical data for <br /> As collected for the full-scale in situ Cr+6 reduction treatment implementation are summarized in <br /> Table 5 and presented on Graphs 1 through 53. <br /> 4.2.4.1 Background and Compliance Wells. During the first and second quarters of <br /> 2012, As concentrations in background and compliance wells remained below the WQO <br /> (10 µg/L) and the proposed upper background concentration limits (10 to 13 µg/L). Table 6 <br /> presents the proposed upper background concentration limits for background wells in <br /> comparison to historical results and WQOs. <br /> 4.2.4.2 Injection Area Wells. During the first and second quarters of 2012, the <br /> As concentrations in injection area wells ranged from below the RL of 5 µg/L to a maximum <br /> detected concentration of 25 µg/L (TW-12 in April 2012). As was detected at concentrations <br /> above the WQO in wells TW-12 and TW-13 during the first and second quarters of 2012; <br /> As was also detected above the WQO in MW-213 during the first quarter of 2012 but not the <br /> second quarter. As concentrations in TW-12 have an increasing trend since 2009 and are <br /> relatively stable at TW-13. However, as the groundwater becomes re-oxidized, the As III is <br /> anticipated to convert to less-soluble As V. No additional action is recommended at this time <br /> regarding the observed As concentrations. <br /> (D MWH.: <br /> 4-7 <br />