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2013 Semiannual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site,Stockton,California August 2013 MW-4505, MW-450D, TW-14, TW-15, TW-16, TW-17, to obtain information on groundwater quality. 3.2 GROUNDWATER ELEVATION, ESTIMATED GRADIENT, AND LINEAR VELOCITY Depth-to-groundwater and groundwater-elevation measurements are presented in Table 4. Potentiometric surface maps were prepared by Dudek for the Shallow, 100-Foot Sand, and Intermediate zones using groundwater data collected in January and April 2013; those maps are presented in Appendix A. Interpretation of the groundwater gradient is complicated by the effects of the groundwater extraction (for treatment) in an area with an otherwise relatively flat water-table surface. Groundwater elevations, estimated gradients, and linear velocities during the first quarter (January) and second quarter (April) of 2013 are discussed by water-bearing zone in the following subsections. 3.2.1 Shallow Zone Potentiometric surface maps of Shallow Zone groundwater for the first and second quarters of 2013 are presented in Appendix A (Figures 1 and 4). In January 2013, groundwater elevations ranged from 15.62 feet (ft) relative to mean sea level (msl) at MW-400 to -22.91 ft msl at MW- 201. In April 2013, groundwater elevations ranged from 9.18 ft msl at MW-300 to -22.90 ft msl at EW-1. Note that the first and second quarter 2013 groundwater elevations at wells MW-400 and MW-300 are potentially anomalous; Dudek is investigating the reason for higher water level measurements at these locations. Groundwater in the Shallow Zone across the source area moved south in response to a gradient of approximately 0.00012 foot per foot (ft/ft) in January 2013 and 0.000018 ft/ft in April 2013, 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) (a MWH, 3-4