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1.4. INPUT PARAMETERS <br />1.4.1. Hydraulic Conductivities <br />Hydraulic conductivity for the lower gravel layer was set to 600 ft/day (Sy=0.2). <br />The value has been accepted by the Colorado Office of the State Engineer as a <br />representative value for gravel deposits on Boulder Creek. The upper unit was <br />set to 100 ft/day (Sy=0.1). This value was determined by consulting Freeze and <br />Cherry -Table 2.2 for a range of hydraulic conductivities for sandy silt and <br />picking a value near the upper range. Using a higher conductivity for this unit <br />would predict a greater influence of pumping on ditch flow. <br />1.4.2. Boulder-Weld Canal ' <br />The Boulder-Weld Canal occurs approximately 400 ft south of the southern edge <br />of the proposed gravel pit at an elevation of 4970 feet. It is approximately 40 ft <br />higher in elevation than the pit bottom at the southern edge. The canal was <br />simulated through application if the RIVER package in MODFLOW. <br />The average width of the ditch is 4 ft. the high water mark occurs approximately 2 <br />ft above the bottom. The model stage was therefore set to 2 ft above the ditch <br />bottom. Ditch bottom conductance calculation assumed a hydraulic conductivity <br />of 0.5 ft/day with a thickness of 1 ft. The ditch bottom conductance was <br />calculated to be 100 sq ft/day (50*4*.5/1). <br />1.4.3. Canal Seepage Drain <br />Some time ago a drain was installed downslope and parallel to the canal to <br />capture seepage and conduct it away from the area. The drain was simulated <br />using the DRAIN package of MODFLOW. The drain was modeled with a bottom <br />elevation of 4961 feet, 50 feet north of the canal. The drain conductance was <br />calculated to be 30,000 sq ft/day (k=600 ft/day,L=50 ft, W=1 ft, M=1ft) <br />1.4.4. Pit Dewatering/Pumping <br />Pit dewatering is simulated by pumping wells on the periphery of the pit using the <br />WELL package in MODFLOW. <br />Mining will ultimately result in the pit fully excavated with dewatering trenches <br />surrounding the toe of pit excavation. Total pumping from all wells was set to <br />1,970 gpm to dewater the entire pit. <br />1.4.5. Reclamation and Perimeter Drain <br />The reclamation plan will backfill the pits as shown in Sheet-4. Slopes will be <br />lined with a low conductivity compacted clay. A drain along the west perimeter of <br />the former pit will be installed for the purpose of preventing water table mounding <br />that could result from placement of the lower permeability fill in the former pit. <br />The drain is placed in the model at the top of the lower gravel and runs north <br />File Pit 112 <br />Groundwater Modeling Report <br />3of6 <br />