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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 The estimated lateral extent of the Cr plume in the Shallow Zone has decreased by an estimated 94 percent since initiation of the full-scale in situ remediation in January 2008. Cr concentrations remain above the WQO only in relatively small areas near the source (former retort pit), at the margin of the treatment areas, and confined between injection barriers in the South Yard. The estimated lateral extent of the Cr plume in the Intermediate Zone has decreased by an estimated 56 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 Four and a half 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 NINA 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 2012 to remove Cr and other chemicals from groundwater removed by the extraction well (D MWH, 6-4