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The hydraulic conductivity of alluvial sediments in the Denver Basin range from <br />100 to 2000 ft/day. The State Engineer recommends that we use 600 ft/day in <br />this area. <br />The transmissivity is the product of aquifer thickness and conductivity so where <br />the gravel is 10 feet thick the transmissivity is 6000 sq.ft/day. The specific yield is <br />assumed to be 0.20 and the storage coefficient for confined conditions is 1 E-5. <br />2.4. Input Parameters <br />2.4.1. 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 the Big Thompson River. The <br />conductivity of reclamation fill materials to be placed after mining is completed <br />was set to 10 ft/day (Sy=0.1). The clay liner in the model has a hydraulic <br />conductivity of 0.003 ft/day as is required by the State Engineer. <br />2.4.2. Big Thompson River <br />The Big Thompson River is simulated through application of a Constant Head <br />boundary in MODFLOW. <br />The Big Thompson River has an average width of 10 ft of through the model <br />domain. The riverbed and banks were surveyed for a separate Floodplain <br />Modeling effort by WI. This river survey data was used as the basis for the river <br />water elevations and constant head elevations. <br />2.4.3. West Pit Dewatering <br />Dewatering of the pit is simulated through application if the DRAIN package in <br />MODFLOW. Each model cell on the perimeter of the pit is simulated as a.drain <br />cell. Mining will proceed in two phases: first a west pit will be mined and <br />reclaimed then an east pit will be mined and reclaimed. <br />Mining will ultimately result in the west pit fully excavated with dewatering <br />trenches surrounding the toe of pit excavation. In the simulation, the elevation of <br />the drain was set at 1.5 feet above the bottom of the pit wall. The dewatering <br />drain yields approximately 900 gpm. These values are consistent with <br />observations of seepage faces (2 to 4 feet above toe of wall) and seepage yields <br />for other similar gravel pits in the area. <br />Gardels Pit <br />Revised Groundwater Modeling Report <br />3 of 9 <br />