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2012 Annual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California February 2013 <br /> proposed upper background concentration limits (10 to 13 gg/L). Table 6 presents the proposed <br /> upper background concentration limits for background wells in comparison to historical results <br /> and WQOs. <br /> 4.2.4.2 Injection Area Wells. During the third and fourth quarters of 2012, the <br /> As concentrations in injection area wells ranged from below the RL of 5 gg/L to a maximum <br /> detected concentration of 38 gg/L (TW-17 in October 2012). As was detected at concentrations <br /> above the WQO in wells MW-363 and TW-13 during the third and fourth quarters of 2012. <br /> As was also detected above the WQO in TW-12 and TW-14 during the third quarter of 2012, and <br /> at MW-322 and TW-17 during fourth quarter 2012. As concentrations in TW-12 had an <br /> increasing trend from 2009 through second quarter 2012, but decreased in third and fourth <br /> quarter 2012 (see Graph 52). As the groundwater becomes re-oxidized, the As III is anticipated <br /> to convert to less-soluble As V. No additional action is recommended at this time regarding the <br /> 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 <br /> of 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 /> the ultimate precipitation as elemental sulfur. During Phase II, no significant long-term increase <br /> in the SO4 concentration was noted. SO4 concentrations in groundwater were monitored before <br /> and after injection. Analytical data for SO4 sampled during the injection program are <br /> summarized in Table 5 and presented on Graphs 1 through 57. <br /> 4.2.5.1 Background and Compliance Wells. During the third quarter of 2012, SO4 <br /> concentrations in background and compliance wells ranged from 16.6 milligrams per liter(mg/L) <br /> (MW-210) to 51.0 mg/L (MW-202). SO4 was not detected at concentrations above the WQO <br /> (250 mg/L) during the current reporting period. Table 6 presents the proposed upper background <br /> concentration limits for background wells in comparison to historical results and WQOs. While <br /> SO4 concentrations were generally below the proposed upper background concentration limits <br /> (D MWH. <br /> 4-8 <br />