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2012 Annual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site, Stockton,California February 2013 for the background and compliance wells, concentrations exceeded the proposed upper background limit concentrations at MW-310, MW-325, and MW-401 (Table 6). 4.2.5.2 Injection Area Wells. During the third and fourth quarters of 2012, SO4 concentrations in injection area wells ranged from 2.6 mg/L (I-11) to 1,600 mg/L (TW-17). SO4 was detected at concentrations above the WQO at MW-6, MW-104 (S-2), MW-105, TW-12, TW-14, and TW-17 during the second half of 2012. Based on data from Phases I and II, and experience from other sites, SO4 concentrations typically naturally attenuate as a result of microbiological reduction of SO4 and precipitation of elemental sulfur. The carbon source (ethanol), supplied in the reductant solution, tends to promote the growth of SRB and the subsequent reduction of SO4. Attenuation of SO4 has been noted at similar sites when a carbon source is added to aid in the growth of SRB. No additional action is recommended at this time regarding elevated SO4. 4.2.6 General Chemistry Several general chemical parameters were included in the laboratory analytical suite, and the results are summarized in Tables 4 and 5 and shown on Graphs 1 through 57. Table 6 presents the proposed upper background concentration limits for background wells in comparison to historical results and WQOs. The general parameters relevant to the in situ performance monitoring are: • TOC, which is introduced as a component of the reductant solution (a carbon source for the SRB). The presence of TOC in groundwater indicates the potential for SO4 and associated Cr+6 reduction by SRB. • TDS, which may increase relative to baseline concentrations following the injection of the reductant solution(a strongly buffered mixture of calcium bicarbonate and sulfide). 4.2.6.1 Background and Compliance Wells. During the third quarter of 2012, TOC concentrations in the background and compliance wells ranged from below the RL of 1 µg/L to 1.1 µg/L (MW-210). TDS concentrations ranged from 324 mg/L (MW-401) to 906 µg/L (MW- 201). TDS concentrations exceeded the proposed upper background concentration limit in wells MW-201, MW-202, MW-304, MW-310, and MW-325 in the third quarter of 2012. Wells with (D MWH 4-9