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Mr. Alex Schatz March 30, 2007 Page 4 of 7 Fountain Creek, the model should be adjusted as appropriate and new predictive results should be produced to reflect this. The model was initially run to steady state without any pits in place and calibrated so that the water table level in the alluvium was recharged by the river and the saturated thicknesses agreed with those indicated in the site boring logs. These heads were then used as the starting heads in the remaining model runs simulating pit dewatering. Fountain Creek was modeled using MODFLOW's river package with the hydraulic conductance of the riverbed set to 20,000 square feet per day in each river cell, and the river bottom one foot below the head in the river, which ranged from an elevation of 5,330 feet at the north end of the mode] to 5,250 feet at the south end of the model. Riverbed elevations were based on USGS topographic maps. Modeling of Pits Based on information obtained from Lafarge, the gravel pits will be mined essentially to bedrock, which is approximately 40 feet below ground surface. Consequently, the pits were modeled with perimeter drains set at a depth approximately one foot above bedrock. This will allow the cells to be dewatered to within approximately one to two feet of bedrock. The drain hydraulic conductance was set to 50,000 square feet per day for each drain cell. The mining cells were modeled one at a time in accordance with our current understanding of Lafarge's plan to mine each cell individually. 1. Cell t was modeled with perimeter drains as the site reservoir in the northwest portion of the site was modeled as an open body of water. 2. Cell 1 was then partially converted to a site reservoir (approximately 8 acres) and silt pond (approximately 10 acres) while Cell 2 was modeled with perimeter drains. The site reservoir and silt pond within Cell 1 were maintained for the remainder of the modeling. 3. Cells 3 and 4 were then modeled individually each with perimeter drains. 4. Cells 5 and 6 were then inactivated because ground water flow will not be allowed through the cells once the slurry walls are constructed around each cell. A model run was performed to determine the shadow and mounding effects associated with the slurry walls. 5. Cell 7 was then modeled with perimeter drains and with a recharge system that extended along the western side of Cell 6, across the bottom of Cells 7 and 8, and up the eastern side of Cell 8. 6. Cell 8 was then modeled with perimeter drains and a recharge system while Cell 7 was modeled as an open body of water. Areas modeled as reservoirs, silt ponds, or open bodies of water were assigned a specific yield of 100 percent to simulate open bodies of water. The recharge system was adjusted so that 5:\536 -Banks and Gesso, LLC\536.4 - Sundance Pi[\Repori\Final Letter Report.doc Martin and Wood Water Consultants, Inc. ~~ -. - _ -._..:-c-. _~.~.~ ~- _ _ ---..