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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 /> 4.2.6.1 Background and Compliance Wells. TOC and TDS samples were not collected <br /> from background and compliance wells in the first or second quarters of 2013 in accordance with <br /> MRP No. R5-2012-0814. <br /> 4.2.6.2 Injection Area Wells. During the first and second quarters of 2013, TOC <br /> concentrations in the injection area wells ranged from below the laboratory analytical RL of <br /> 1 mg/L at most wells to 13 mg/L (MW-447D in April 2013). 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 /> consumes the carbon and removes Cr+6 from solution. No additional action is recommended at <br /> this time regarding the observed TOC concentrations. <br /> During the first and second quarters of 2013, TDS concentrations in the injection area wells <br /> ranged from 90 mg/L (MW-105 in January 2013) to 2,820 mg/L (TW-17 in April 2013). <br /> Concentrations in all but eight wells exceeded the WQO of 450 mg/L. TDS concentrations have <br /> been relatively stable at most wells since the baseline sampling event. However, several wells <br /> show relatively high increases and decreases in TDS concentrations which appear to correlate <br /> with periods of treatment solution injection or impact (for example, reductions in Cr <br /> concentrations) (see Graphs 1 through 57). TDS concentrations are expected to decrease over <br /> time. No additional action is recommended at this time regarding the observed TDS <br /> concentrations. <br /> 4.3 FIELD PARAMETERS <br /> Field parameters were measured at each well before sample collection, and results are <br /> summarized in Table 4. The pH, temperature, EC, ORP, and DO measurements are discussed <br /> below. Field sampling logs are provided in Appendix B. Field parameter measurements for pH, <br /> EC, and ORP are also presented on Graphs 1 through 57. <br /> Field measurements during the first and second quarters of 2013 showed the pH in the wells was <br /> generally within the range of 6.5 to 8 standard units. Results from the current period are <br /> generally consistent with trends since the baseline sampling event in 2008. Injection of the <br /> © MWH 4-10 <br />