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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 /> 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. SO4 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 /> 4.2.5.2 Injection Area Wells. During the first and second quarters of 2013, SO4 <br /> concentrations in injection area wells ranged from 9.1 mg/L (MW-105) to 1,610 mg/L (TW-17). <br /> SO4 was detected at concentrations above the WQO at MW-6, MW-104 (S-2), TW-12, and TW- <br /> 17 during the first and second quarters of 2013. <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 5O4. 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 5O4. <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 57. 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 /> • TDS, which may increase relative to baseline concentrations following the injection of <br /> the reductant solution(a strongly buffered mixture of calcium bicarbonate and sulfide). <br /> © MWH 4-9 <br />