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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 /> promote the growth of SRB and the subsequent reduction of SO4. Attenuation of SO4 has been <br /> noted at similar sites when a carbon source is added to aid in the growth of SRB. No additional <br /> action is 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 /> • 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 /> 4.2.6.1 Background and Compliance Wells. During the first and second quarters of <br /> 2012, TOC concentrations in the background and compliance wells ranged from below the RL of <br /> 1 gg/L to 1.7 gg/L (MW-210 in January 2012). TDS concentrations ranged from 245 mg/L <br /> (MW-401 in January 2012) to 975 gg/L (MW-201 in January 2012). TDS concentrations <br /> exceeded the proposed upper background concentration limit in wells MW-201, MW-304, MW- <br /> 310, and MW-325 in the first and second quarters of 2012. TDS concentrations also exceeded <br /> the WQO and proposed upper background limit at MW-202 during second quarter 2012. Wells <br /> with TDS concentrations that exceed the WQO and/or proposed upper background concentration <br /> limit have relatively stable readings (see attached graphs). <br /> 4.2.6.2 Injection Area Wells. During the first and second quarters of 2012, TOC <br /> concentrations in the injection area wells ranged from below the laboratory analytical RL of <br /> 1 mg/L at most wells to 81.0 mg/L (MW-9 in January 2012). A WQO has not been established <br /> for TOC, which originates from the ethanol component of the reductant solution, and is designed <br /> to support growth of SRB and the subsequent microbiological reduction of SO4 and associated <br /> reduction of Cr+6. As a result, the TOC concentrations are expected to decline over time as SRB <br /> (D MWH. <br /> 4-9 <br />