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Dewatering Evaluation Report <br />Varna Gravel Operation <br />Greeley, Colorado <br />Page 3 <br />PrOleCt ASSUmptlOnS <br />The following are assumptions used in estimating the effects of mine dewatering <br />operations. <br />- The aquifer within the model boundary is homogeneous and isotropic. <br />- The saturated thickness of the aquifer prior to mine operations was 26 feet (six <br />feet below ground surface). <br />- The average hydraulic conductivity of the sand and gravel deposits is 250 feet per <br />day. <br />- Other aquifer stresses (such as pumping of domestic water wells and drought) in <br />the adjacent area will have an impact in the water level in the Carbaugh well and <br />other domestic welts; however, this analysis is for only for dewatering operations. <br />- There are no allowances for aquifer recharge from precipitation or irrigation. <br />- All groundwater solutions are steady state. <br />- The location of the Carbaugh well is 235 feet from the south seepage face of the <br />mine and the static depth to water in the well is 17 feet below grade (as reported <br />by the well owner). <br />- The Pierre Shale which underlies the coarse alluvial deposits is an impermeable <br />barrier and the average depth from ground surface to the shale is 32 feet. <br />Overview of Drawdown Estimates <br />A review of well log and well test data indicate that the hydraulic conductivity of the coarse <br />gravel deposits is on the order of 250 feet per day, which is consistent with published <br />values and pump test evaluations conducted by the author in similar geologic settings. <br />The average specific yield of the local sand and gravel deposits is estimated at 0.25. <br />McWhorter and Sunada, 1977, provide solutions to determine the location of the water <br />table surface for one dimensional unconfined flow. Hand calculations were performed to <br />estimate the expected head at a point 235 feet from the southern most seepage face <br />(approximate distance from the Carbaugh well to the southern most seepage face). Based <br />on a well log for the well of concern the saturated thickness of the coarse alluvial deposits <br />(based on static water level conditions measured in 1991) was 26 feet. The saturated <br />