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2012 Semiannual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site, Stockton,California October 2012 vertical treatment injections were completed in the fall of 2010 to expedite the removal of Cr+6 surrounding the former retort pit and in the Intermediate Zone beneath the North and South yards. The reactive barrier locations are shown on Figure 3, and program details are summarized in Table 1. Historical reports prepared for the Site can be accessed at the public information repository established at the Stockton City Library located at 605 N. El Dorado Street. These reports detail Site conditions, Site investigation history, and implementation of remedial activities performed under the Final RAP (California Department of Health Services, 1990). This report presents the results of groundwater monitoring and sampling performed for wells in the injection program area from January through June 2012, including quarterly sampling events in January and April. The groundwater monitoring program wells, analytes, and sampling frequencies are presented in Table 2. The remainder of this report is organized as follows: • Section 2.0 — Background. Presents a brief overview of Site history and describes the full-scale in situ groundwater remediation activities. • Section 3.0 — Groundwater Monitoring and Sampling. Describes the groundwater monitoring and sampling plan associated with the full-scale in situ groundwater remediation and presents and discusses the results of the field and laboratory analyses performed during the first and second quarters of 2012. • Section 4.0 —Laboratory and Field Sample Results and Discussion. Summarizes and discusses laboratory and field sample results for the different Site areas. • Section 5.0 — Groundwater Treatment Activities. Summarizes treatment activities conducted in the first and second quarters of 2012 as part of the full-scale in situ Cr+6 reduction treatment implementation. • Section 6.0 — Performance Evaluation. Presents an evaluation of the full-scale in situ Cr+6 reduction treatment implementation performance, as required by the WDR. • Section 7.0 — Summary and Conclusions. Summarizes monitoring results and presents conclusions. • Section 8.0 — Recommendations. Presents the proposed activities recommended to enhance and monitor the full-scale in situ groundwater remediation. MWH, 1-2