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2011 Semiannual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site, Stockton,California January 2012 • 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 and fourth quarters of 2011, TOC concentrations in the background and compliance wells ranged from below the RL of 1 gg/L in 9 wells to 16 gg/L (MW-7 in July 2011). TDS concentrations ranged from 162 mg/L (MW-325 in July 2011) to 1,430 gg/L (MW-201 in July 2011). TDS concentrations exceeded the proposed upper background concentration limit in wells MW-7, MW-201, MW-202, MW-304, and MW-310 in the third and fourth quarters of 2011. TDS concentrations also exceeded the WQO and proposed upper background limit in wells MW-210 and MW-302 during the fourth quarter of 2011. The wells with TDS that exceeds the WQO and or proposed upper background concentration limit have relatively stable readings for each given well over time. 4.2.6.2 Injection Area Wells. During the third and fourth quarters of 2011, TOC concentrations in the injection area wells ranged from below the laboratory analytical RL of 1 mg/L at most wells to 59.6 mg/L (MW-104 (S-2) in October 2011). A WQO has not been established for TOC. The TOC is 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 2011, TDS concentrations in the injection area wells ranged from 138 mg/L (MW-361 in July 2011) to 1,900 mg/L (MW-104 (S-2) in August 2011). 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. 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 53). TDS concentrations are expected to decrease over time. No additional action is recommended at this time regarding the observed TDS concentrations. (a MWH 4-8