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2011 Annual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site, Stockton,California January 2012 2.0 BACKGROUND The Site was formerly used for cooling tower component manufacturing, which included the use of arsenic (As), chromium (Cr), and copper for wood treatment and preservation. Previous Site investigations have identified soils located beneath the former wood treatment retort area (Figure 3) containing elevated concentrations of As, Cr, and copper (Dudek and Associates, Inc. [Dudek], 2000; Black & Veatch, et al., 1990). Groundwater beneath the Site is impacted by Cr+6 (Dudek, 2000), a component of the primary groundwater constituent of concern total Cr, due to its solubility and mobility in soil and groundwater. The concentration of total dissolved Cr is in excess of the California maximum contaminant level (MCL) of 50 micrograms per liter (gg/L) for total Cr. Impacted groundwater extends approximately 2,000 feet south from the original source (the retort pit), and beyond the facility boundary. Impacted groundwater occurs primarily within three identified water-bearing zones beneath the Site: • The Shallow Zone extends from approximately 60 to 85 feet below ground surface (bgs) • The Intermediate Zone extends from approximately 120 to 180 feet bgs • The Deep Zone extends from approximately 180 to 225 feet bgs (Cr concentrations are below 50 gg/L). In addition to the three primary water-bearing zones listed above, a 100-Foot Sand Zone has been identified as a potential pathway for horizontal contaminant migration. Although the 100- Foot Sand Zone thickness is not uniform (generally approximately 5 to 10 feet thick), the unit appears to be consistent throughout much of the Site. The original remediation system at the Site was a groundwater extraction system with a network of extraction wells screened in the Shallow, Intermediate, and Deep water-bearing zones. Extracted groundwater was treated with ion exchange for low concentrations of Cr+6, and an electrochemical reduction/precipitation system (ANDCOTM) capable of treating Cr and other metals at higher concentrations. Residual solids from the ANDCOTM system were dewatered and disposed of off site at a licensed facility. Treated groundwater was and is discharged to the Stockton Diverting Canal north of the Site under a National Pollutant Discharge Elimination System permit. (a MWH 2-1