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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 third quarter (July) and fourth quarter(October) of 2012 are discussed by water-bearing zone in the following subsections. Appendix A (Figures 4 and 8) present the groundwater elevations and chromium concentrations in the deep zone in the third and fourth quarters of 2012, respectively, and are provided for informational purposes. 3.2.1 Shallow Zone Potentiometric surface maps of Shallow Zone groundwater for the third and fourth quarters of 2012 are presented in Appendix A (Figures 1 and 5). In July 2012, groundwater elevations ranged from -10.4 feet (ft) relative to mean sea level (msl) at MW-300 to -23.77 ft msl at MW-302. In October 2012, groundwater elevations ranged from -2.60 ft msl at MW-300 to - 24.67 ft msl at MW-302. Note that the third and fourth quarter 2012 groundwater elevations at well MW-300 are potentially anomalous; Dudek is investigating the reason for higher water level measurements at this location. Groundwater in the Shallow Zone across the source area moved south in response to a gradient of approximately 0.0008 foot per foot (ft/ft) in July 2012 and 0.0004 ft/ft in October 2012, as calculated between wells MW-210 and MW-9. The average linear velocity of groundwater (velocity equals hydraulic conductivity multiplied by hydraulic gradient divided by effective porosity), in the Shallow Zone was estimated using: • A hydraulic conductivity of 25.6 feet per day (ft/day), which was averaged from hydraulic conductivities estimated from aquifer tests conducted at wells MW-202, MW-203, MW-206, MW-207, MW-208, and MW-209 (Hargis +Associates, Inc. (H+A), 1989a and 1989b) • An effective porosity of 20 percent (H+A, 1990) • The hydraulic gradient calculated between wells MW-210 and MW-9 The average linear velocity of groundwater in the Shallow Zone across the source area was approximately 0.10 ft/day in July 2012 and 0.06 ft/day in October 2012. 3.2.2 100-Foot Sand Zone In July 2012, groundwater elevations ranged from -22.15 ft msl at TW-13 to -22.97 ft msl at MW-310. In October 2012, groundwater elevations ranged from -22.76 ft msl at TW-16 to -23.27 ft msl at MW-310. Groundwater in the 100-Foot Sand Zone across the source area (0 MWH. 3-4