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Technical Revision Permit No. M-1980-183 ' Page 2 of 4 • The proposed outflow to the South Platte River from the drainage ditch on the Frei property is at 4999.4 feet. ' A French drain is proposed to alleviate the mounding on the u radient (south) side of Pit 29, at the p p g pg ' Einspahr property. The plan view of the French drain system is shown on Figure 5. The drain would begin in the northeast corner of the Einspahr property, continue down gradient west to a maintenance manhole at the northwest corner of the Einspahr property, then continue approximately 1200 feet along the west side of Pit 29. The beginning and ending elevations of the drain will coincide with the ' estimated pre-mining water levels. Specifically, the high end of the drain invert will be at 2007 msl and the low end at 5002 msl. ' French Drain Design We estimated the pre-mining underflow in the aquifer in order to properly size the drain. The south ' face of the Pit 29 slurry wall is 950 feet in length. The pre-mining aquifer characteristics were obtained from the Blatchley map and the USGS publication: "Hydrologic Characteristics of the Valley-Fill Aquifer in the Brighton Reach of the South Platte River Valley, Colorado, by Hurr and Schneider, Open File Report, 1972. Darcy's Law was used to estimate the underflow,which states: Q=KIA Where: Q is flow in gallons per day ' K is the hydraulic conductivity in gallons per day per foot squared I is gradient in feet per feet, or dimensionless A is the cross sectional area in square feet Based on the above-referenced publications: • The aquifer transmissivity is 100,000 g/d/ft. • The saturated thickness of the aquifer was 20 feet prior to mining, resulting in a hydraulic ' conductivity of 5,000 g/d/ft2. • The gradient was 10 ft in 2660 ft or 0.0038 ft/ft The cross sectional area of the south face of the slurry wall is 950 x 20= 19,000 ftz