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• III IIIIIIIIIIIIIIII • <br />999 <br />MEMORANDUM <br />TO: Greg Roush -Leonard Rice Consulting Water Engineers, Inc. (LRCWE) <br />File: 1057SDR01 <br />~~ <br />FROM: Dennis McGrane (LRCWE) <br />DATE: Mazch 23, 2001 <br />RE: Potential Ground Water Resource Impacts Associated with Starter Pit Dewatering <br />BACKGROUND <br />Tom Sharkey requested that we evaluate the potential impacts to nearby well owners associated <br />with digging a starter pit for a wet mine dredge at the S&H mine site near Gilcrest, Colorado. <br />Our analysis is based on the following scenario. A one-acre starter pit will be excavated below <br />the water table to a depth of around 25 feet (assumed 15 feet below the water table). It could <br />take around 1 yeaz to excavate the pit depending on the gravel sales of the volume excavated. <br />During this time, water entering the pit will be removed by pumping from the low points where <br />water accumulates. Once the pit has been mined to an adequate size, a dredge would be <br />constructed in the pit. Dewatering would then cease, and the water levels would be allowed to <br />recover. Dredge mining operations would begin from that point in time. <br />We were asked to estimate potential effects of this scenario on ground water levels and wells <br />located outside the mine property. <br />METHOD <br />There are several engineering equations that can be used to help solve this problem. In this case, <br />we believe that the Theis Nonequilibrium Equation is the most applicable because: <br />Time/drawdown relationships can be determined; and <br />Radial flow conditions (converging flow to a dewatering well or small pit) exist. <br />The Modified Theis Nonequilibrium Equation is a simplified evaluation of the Theis integral that <br />takes the form: <br />s = 264Q/T' log [(.3Tt')/(r^2;S)] <br />LEONARD RICE CONSULTING WATER ENGINEERS, INC. <br />2000 CLAY STREET, SUITE 300. DENVER. COLORADO 8021 1-51 1 9 • PlIONE (303) 455-9589 • FAX (303) 455-01 15 <br />