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2012 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California October 2012 <br /> retort pit), at the margin of the treatment areas, and confined between injection barriers in the <br /> South Yard. <br /> The estimated lateral extent of the Cr plume in the Intermediate Zone has decreased by an <br /> estimated 78 percent since initiation of the full-scale in situ remediation in 2008. The greatest <br /> change occurred between April 2010 and April 2011 in response to supplemental treatment <br /> efforts in 2010 that targeted the Intermediate Zone. Cr concentrations are above the WQO in <br /> portions of the South Yard and on the western property boundary of the North Yard (at the <br /> margin of the Intermediate Zone treatment area), and the retort pit area. Cr reduction in these <br /> areas may be delayed due to environmental conditions, such as finer grained aquifer material <br /> with relatively low hydraulic conductivity, or the speed and direction of groundwater movement. <br /> Cr reduction in the Intermediate Zone will continue to be monitored. <br /> 6.4 DECISION LOGIC FOR PERFORMANCE MONITORING DATA <br /> EVALUATION <br /> Four years have elapsed since completion of the initial full-scale in situ treatment program <br /> (January to June 2008). Data collected during quarterly (and supplemental) sampling events <br /> demonstrates that Cr and Cr+6 concentrations across large portions of the former groundwater <br /> plume have been reduced to below the WQO (50 µg/L) without a lasting accumulation of <br /> undesirable byproducts (i.e., SO4, As, or other metals). Figure 5 is a decision logic diagram by <br /> which in situ treatment monitoring data are evaluated to plan for future actions (such as <br /> sampling, monitored natural attenuation, or additional remediation). The decision logic diagram <br /> is consistent with the MNA approach identified in the Remedial Action Implementation Work <br /> Plan (RAIWP) (MWH, 2007b). <br /> 6.5 GROUNDWATER EXTRACTION AND TREATMENT <br /> The electrochemical system/ion exchange treatment plant was operated during the first half of <br /> 2012 to remove Cr and other chemicals from groundwater removed by the extraction well <br /> network. A summary and evaluation of groundwater extraction and treatment activities is <br /> provided in a separate report prepared concurrently by Dudek. <br /> (D MWH., <br /> 6-4 <br />