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r 80 feet(i.e.,extends 6 feet into the hard clay aquitard).A 16-inch-diameter'/4-inch-steel casing was installed through the conductor casing from the surface to a depth of 450 feet. Groundwater is pumped from the 16-inch-diameter casing which is perforated from 209 to 426 feet below ground surface. As discussed above in the SRS section, the emergency backup well is reportedly turned on for periodic sampling but is otherwise not operated. Groundwater Flow Groundwater flow was simulated using the USGS Modflow 2000 engine incorporated in the Visual Modflow Pro software package. Modflow uses a block-centered finite-difference approach to simulate three-dimensional groundwater flow. Average groundwater flow direction from the Site is 830 to the east at a magnitude of 0.00.11 ft/ft. Based on the depths to groundwater observed at the Site,we assume that residual impacted groundwater resides primarily in the upper sandy clay and light sand materials. We created a 450-foot-thick, three-layer model with a horizontal domain of 19,140 feet by 19,140 feet (see Figure 6). The horizontal model domain had 99 cells in the."X" direction and 81 cells in the "Y" direction. Cell sizes ranged from 1,000 feet along the outside boundary of the model domain to 20 feet in the center of the model (see Figure 7). Constant head boundaries of-5.398 feet(at top of model domain) and-25.352 feet(at bottom of model domain)were incorporated into all model layers to reproduce initial site groundwater elevations of approximately -15.1 feet and maintain an initial hydraulic gradient of 0.0011 ft/ft in Layer 1 (see Figure 8). Default input parameters were used except where site-specific parameters were available or assumed. The site-specific input parameters were as follows: Laver l (sandy clay and light sand) • Thickness;=74 feet • Top elevation=22 feet • Bottom elevation=-52 feet • Total porosity=351/o • Effective porosity=20% • Hydraulic conductivity=5x10-3 cm/sec La er2hard clay) • Thickness=53 feet • Top elevation=-52 feet • Bottom elevation=-105 feet • Total porosity=50% . • Effective porosity=20% • Hydraulic conductivity= 1x10-5 cm/sec Stockton Yard Maintenance Station,TO No.1 B Caltrans Contract No.06A 1895,EA No.43-910206 Geocon Project No.S9800-01-01B -5- September 30,2013