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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 TDS concentrations that exceed the WQO and/or proposed upper background concentration limit have relatively stable readings (see attached graphs). 4.2.6.2 Injection Area Wells. During the third and fourth quarters of 2012, TOC concentrations in the injection area wells ranged from below the laboratory analytical RL of 1 mg/L at most wells to 12 mg/L (MW-321 (I-9) in October 2012). A WQO has not been established for TOC, which originates from the ethanol component of the reductant solution, and is designed to support growth of SRB and the subsequent microbiological reduction of SO4 and associated reduction of Cr+6. As a result, the TOC concentrations are expected to decline over time as SRB consumes the carbon and removes Cr+6 from solution. No additional action is recommended at this time regarding the observed TOC concentrations. During the third and fourth quarters of 2012, TDS concentrations in the injection area wells ranged from 144 mg/L (MW-361 in July 2012) to 2,750 mg/L (TW-17 in July 2012). Concentrations in all but nine wells exceeded the WQO of 450 mg/L. TDS concentrations have been relatively stable at most wells since the baseline sampling event. However, several wells show relatively high increases and decreases in TDS concentrations which appear to correlate with periods of treatment solution injection or impact (for example, reductions in Cr concentrations) (see Graphs 1 through 57). TDS concentrations are expected to decrease over time. No additional action is recommended at this time regarding the observed TDS concentrations. 4.3 FIELD PARAMETERS Field parameters were measured at each well before sample collection, and results are summarized in Table 4. The pH, temperature, EC, ORP, and DO measurements are discussed below. Field sampling logs are provided in Appendix B. Field parameter measurements for pH, EC, and ORP are also presented on Graphs 1 through 57. Field measurements during the third and fourth quarters of 2012 showed the pH in the wells was generally within the range of 6.5 to 8 standard units. Results from the current period are generally consistent with trends since the baseline sampling event in 2008. Injection of the alkaline CaS,, solution can cause a rise in pH in influenced wells. This effect has been observed (a MWH. 4-10