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2011 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California January 2012 <br /> the proposed upper background concentration limits for background wells in comparison to <br /> historical results and WQOs. SO4 concentrations were generally below the proposed upper <br /> background concentration limits for the background and compliance wells; concentrations in <br /> MW-310 in July 2011 (47.0 mg/L) and October 2011 (44.2 mg/L) exceeded the proposed upper <br /> background limit concentration of 21.5 mg/L for this well. <br /> 4.2.5.2 Injection Area Wells. During the third and fourth quarters of 2011, the SO4 <br /> concentrations in injection area wells ranged from 0.3J mg/L (MW-208 in October 2011) to <br /> 835 mg/L (TW-17 in October 2011). SO4 was detected at concentrations above the WQO in <br /> wells MW-6, MW-104, TW-14, TW-16, and TW-17 during the third quarter of 2011, and wells <br /> MW-104, MW-206, TW-14, and TW-17 during the fourth quarter of 2011. The SO4 <br /> concentrations in well TW-14 are consistent with results from the past several quarterly sampling <br /> events. The SO4 concentrations in MW-104, MW-206, and TW-14 will continue to be <br /> 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 typically <br /> naturally attenuate as a result of microbiological reduction of SO4 and precipitation of elemental <br /> sulfur. The carbon source (ethanol), supplied in the reductant solution, tends to promote the <br /> growth of SRB and the subsequent reduction of SO4. Attenuation of SO4 has been noted at <br /> similar sites when a carbon source is added to aid in the growth of SRB. No additional action is <br /> recommended at this time regarding elevated SO4. <br /> 4.2.6 General Chemistry <br /> Several general chemical parameters were included in the laboratory analytical suite, and the <br /> results are summarized in Tables 4 and 5 and shown on Graphs 1 through 53. Table 6 presents <br /> the proposed upper background concentration limits for background wells in comparison to <br /> historical results and WQOs. The general parameters relevant to the in situ performance <br /> monitoring are: <br /> • TOC, which is introduced as a component of the reductant solution (a carbon source for <br /> the SRB). The presence of TOC in groundwater indicates the potential for SO4 and <br /> associated Cr+6 reduction by SRB. <br /> MWH <br /> 4-7 <br />