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<br />could be higher. <br />STREAM FLOW DEPLETION ANALYSIS <br />In order to determine the time required for dewatering to <br />significantly affect groundwater discharge to the St Vrain River, <br />the State of Colorado Analytical Stream Depletion Model {1987) was <br />used. Results of the stream depletion analysis (two sensitivity <br />runs) are included in attachment 1. The analytical stream <br />depletion model presents four options which consider various <br />boundary conditions. These options are as follows: <br />1. Semi-infinite aquifer bounded only by a stream. <br />2. Alluvial aquifer (aquifer bounded by a stream and a <br />parallel impermeable boundary). <br />3. Aquifer bounded by a steam and an impermeable boundary <br />perpendicular to the stream. <br />4. Boundary effects approximated by use of an effective <br />stream depletion factor (sdf). <br />Of the four available options, the second option (alluvial aquifer <br />bounded by a stream and a parallel impermeable boundary) was <br />selected. The following assumptions were made prior to applying the <br />model. <br />• The St Vrain River is 3200 feet from the southern most <br />mine cut. <br />• The southern-most mine face would act as a no-flow <br />boundary. <br />• The hydraulic conductivity of surficial sediments between <br />the southern-most mine face and the st vrain range from <br />10~ to 10-3 cm/s. This range in hydraulic conductivity <br />should be similar to the shallow Benton hydrogeologic <br />unit defined at Syntex. <br />• The alluvial aquifer is 30 feat thick. <br />• The St. Vrain is roughly 5o feet wide, averages 5 feet in <br />depth, and surface water moves at a rate of 3 feet per <br />second throughout the year. <br />• Discharge from the southern mine face will range between <br />15 and 125 gpm depending on actual in situ hydraulic <br />characteristics of surficial and bedrock material. <br />3 <br />