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• <br />• <br />Memo to Wallace Erickson 5 May 3 2001 <br />accordance with the following calculation, given that hydraulic conductivity and flow are directly <br />proportional under Darcy's Law': <br />6 <br />x 20.4cfs = .01224cfs <br />10,000 <br />To be conservative, i.e., to err on the side of overestimation of drawdown potential, DMG multiplied <br />this flow rate by ten (10) for analytical purposes and used 0.0035 cubic meters per second for the <br />simulation. As a check on the suitability of this hypothetical flow rate, consider ground water <br />inflows to the mature, dewatered pit occurring as seepage through the three feet of gravel <br />immediately above the pit floor. With a pit perimeter of4107 feet, a seepage face with an area of <br />12,321 square feet exists. Plugging this information along with the hydraulic conductivity of six (6) <br />gallons per day per square foot into Darcy's Law, and conservatively assuming a gradient of unity, <br />inflows to the pit are estimated to be: <br />0 = KiA = 6 x 12321 = 73,926gallons / dny <br />which is very close to the estimated 80,000 gallons per day inflows input to the DMG simulation and <br />further demonstrates that the DMG model is conservative with regard to the predicted drawdown in <br />the Robinson Well. <br />.~ The transmissivity T is a derived parameter developed primarily for the analysis of well hydraulics in <br />confined aquifers and is defined as T=K6 where K is hydraulic conductivity and b is aquifer <br />thickness. For two-dimensional horizontal flow toward a well in a confined aquifer the teen is well <br />defined. In an unconfined aquifer, such as the one at the Line Camp Pit location, the term is not as <br />well defined, but can still be used in analysis. Transmissivity in an unconfined aquifer is defined by <br />the same equation T=Kb but b is now the saturated thickness of the aquifer or the height of the water <br />table above the underlying aquitard that bounds the aquifer. At the Line Camp Pit location, <br />discussion in the permit application indicates that the Morrison Formation underlies the alluvial <br />aquifer at a depth of approximately 20 feet and the water table is approximately five (5) feet below <br />ground surface. Hence b=15 feet or 4.572 meters in the transmissivity equation. As discussed <br />above, the hydraulic conductivity K of the aquifer is assumed to be six (6) gallons per day per square <br />foot. Calculating the transmissivity using these values yields T=0.0000129 square meters per <br />second. <br />•: The radius r is the distance from the pumping well, in this case the dewatering Line Camp Pit, to the <br />location where the magnitude of drawdown in the water table is to be determined. It is estimated in <br />the permit application that the Robinson Wel] is 1190 feet from the edge of the proposed pit. The <br />applicant used a distance of 1000 feet from the pit pumping point as input to their drawdown <br />calculations, which is equivalent to 960 feet from the pit wall. Since most pit dewatering operations <br />use perimeter trenches to direct ground water inflows to a pumping sump, it the DMG's position the <br />parcy~s Law is the foundation of ground water hydrology as a quantitative science and may be expressed as Q=/"i,4 where n is <br />Ihnv, /" is hydraulic conductivity, r is gradiznl, and .4 is cross sectional area <br />