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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 South yard. The residual concentrations at MW-204 and MW-205, appears to be contained by the injection barrier due continued relative stability of concentrations in MW-301, maintaining concentrations below the WQO in MW-201 and MW-304. The estimated lateral extent of the Cr plume in the Intermediate Zone has decreased by an estimated 67 percent since initiation of the full-scale in situ remediation in 2008. The greatest change occurred between April 2010 and April 2011 in response to supplemental treatment efforts in 2010 that targeted the Intermediate Zone. Cr concentrations are above the WQO in portions of the South Yard and on the western property boundary of the North Yard (at the margin of the Intermediate Zone treatment area), and the retort pit area. Cr reduction in these areas may be delayed due to environmental conditions, such as finer grained aquifer material with relatively low hydraulic conductivity, or the speed and direction of groundwater movement. Cr reduction in the Intermediate Zone will continue to be monitored. 6.4 DECISION LOGIC FOR PERFORMANCE MONITORING DATA EVALUATION Three years have elapsed since completion of the initial full-scale in situ treatment program (January to June 2008). Data collected during quarterly (and supplemental) sampling events demonstrates that Cr and Cr+6 concentrations across large portions of the former groundwater plume have been reduced to below the WQO (50 gg/L) without a lasting accumulation of undesirable byproducts (i.e., SO4, As, or other metals). Figure 5 is a decision logic diagram by which in situ treatment monitoring data are evaluated to plan for future actions (such as sampling, monitored natural attenuation, or additional remediation). The decision logic diagram is consistent with the MNA approach identified in the Remedial Action Implementation Work Plan (RAIWP) (MWH, 2007b). 6.5 GROUNDWATER EXTRACTION AND TREATMENT The electrochemical system/ion exchange treatment plant was operated during the second half of 2011 to remove Cr and other chemicals from groundwater removed by the extraction well network. A summary and evaluation of groundwater extraction and treatment activities is provided in a separate report prepared concurrently by Dudek. 1) MWH, 6-4