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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 /> 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 53. <br /> 4.2.5.1 Background and Compliance Wells. During the first and second quarters of <br /> 2012, SO4 concentrations in background and compliance wells ranged from 15.1 milligrams per <br /> liter (mg/L) (MW-210 in April 2012) to 48.8 mg/L (MW-201 in April 2012). SO4 was not <br /> detected at concentrations above the WQO (250 mg/L) during the current reporting period. <br /> Table 6 presents the proposed upper background concentration limits for background wells in <br /> comparison to historical results and WQOs. While SO4 concentrations were generally below the <br /> proposed upper background concentration limits for the background and compliance wells, <br /> concentrations exceeded the proposed upper background limit concentrations at MW-310 (first <br /> and second quarter 2012), MW-325 (first and second quarter 2012), and MW-401 (first quarter <br /> 2012) (Table 6). <br /> 4.2.5.2 Injection Area Wells. During the first and second quarters of 2012, SO4 <br /> concentrations in injection area wells ranged from 8.4 mg/L (MW-361 in April 2012) to <br /> 1,470 mg/L (TW-17 in April 2012). SO4 was detected at concentrations above the WQO at <br /> MW-104 and TW-17 during first quarter 2012, and at MW-6, MW-104, TW-12, and TW-17 <br /> during second quarter 2012. The SO4 concentrations in MW-6, MW-104, TW-12, and TW-17 <br /> will continue to be monitored to determine if the concentration increase is sustained. <br /> Based on data from Phases I and II, and experience from other sites, SO4 concentrations <br /> typically naturally attenuate as a result of microbiological reduction of SO4 and precipitation of <br /> elemental sulfur. The carbon source (ethanol), supplied in the reductant solution, tends to <br /> (a mm" <br /> 4-8 <br />