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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 /> Cr reduction. No additional action is recommended at this time regarding the observed <br /> 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 57. <br /> 4.2.4.1 Background and Compliance Wells. Arsenic samples were not collected from <br /> background and compliance wells in the first or second quarters of 2013 in accordance with <br /> MRP No. R5-2012-0814.. <br /> 4.2.4.2 Injection Area Wells. During the first and second quarters of 2013, the <br /> As concentrations in injection area wells ranged from below the RL of 5 µg/L to a maximum <br /> detected concentration of 19 µg/L (TW-13 in January 2013). As was detected at concentrations <br /> above the WQO in well TW-13 during the first and second quarters of 2013. As was also <br /> detected above the WQO in well TW-12 during the first quarter of 2013, and at well TW-17 <br /> during second quarter 2013. As concentrations in TW-12 had an increasing trend from 2009 <br /> through second quarter 2012, but decreased in third and fourth quarter 2012 then increase in the <br /> first quarter 2013 (see Graph 52). 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 /> 4.2.5 Sulfate <br /> SO4 is a common byproduct of the chemical reactions caused by the use of CaSX, a component of <br /> the reductant solution used for groundwater remediation. The reductant solution also includes <br /> ethanol, which acts as a carbon source to aid in the growth of sulfate-reducing bacteria (SRB). <br /> SRB have the ability to reduce SO4 back to the sulfide form, thus enhancing the reduction and <br /> © MWH 4-8 <br />