2011-08-09_REVISION - C1981019 (10)

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• • RULE 2 PERMITS do not contain enough water to perform' long term hydrologic tests. Therefore, the hydraulic conductivities used in any pit wall seepage calculation are derived from tests in wells (WMC, 2005) where saturated water table and /or confined conditions exist. As the pit is reclaimed to post mining topography, it is likely that the discharge point of any spoil water will be at the lowest topographic point on the northern pitwall. This is the drainage channel of Little Collom Gulch. The elevation of this potential discharge point is approximately 7275 feet (based on the current mining plan and topographic data). Using an estimated saturated water table /piezometric surface of approximately 7150 feet and a discharge elevation of 7275 feet, any water in the spoils would have to rise a minimum of 125 feet before any water could be discharged into the Little Collom Gulch drainage. During this time, some pitwall seepage would occur thus increasing the time for a potential spoil aquifer to develop. As previously noted, since the coals and sandstones will transmit the majority of water from the spoils, the estimate of pitwall seepage will be based on the hydraulic conductivities and estimated total thickness of the sandstones and coals in the northern pitwall. The majority of the geologic beds above the saturated zone are siltstones and mudstones with lesser amounts of sandstones, than coals. The coal seams have hydraulic conductivities ranging from approximately 0.002 to 0.77 ft/day, with a geometric mean of 0.14 ft/day. The sandstone units have geometric mean hydraulic conductivity values ranging from approximately 0.002 ft/day (H sandstone) to 0.008 ft/day (F sandstone), with a geometric mean of 0.006 ft/day. Thus, using a single hydraulic conductivity (K) for pit wall seepage is difficult. For further discussion, a K of 0.01 (1.0x10 ft per day will be used. The length of the northern pitwall is approximately 5000 ft. The thickness of the sandstones and coals along this highwall (using actual core measurements and geophysical logs from three drill holes) varies from a high of 84 feet to a low of 31 feet. The geometric average is 50.3 feet. Thus the cross - sectional area (A) for the northern pitwall sandstones and coals is 2.52x10 square feet. With Q= KA(h/1), K = 1.0x10" ft /day A = 2.52x10 ft h /1= 0.1 This equals a mimimum pit seepage volume of 2016 ft /day (0.05 acre -ft /day) over a height of 125 feet. However, this pitwall seepage will only occur if a spoil aquifer is developed. The probable impact of the seepage from the highwall to the non - saturated portion of the area down gradient of Collom Lite mining area is considered minimal. In addition, spoil spring discharges will be unlikely due to the geological characteristics of the Collom Lite permit area. Since the dip of the geologic units is greater than the topographic gradient to the north, any pitwall seepage will follow the geologic units to a depth which is greater than that found in the pitwall. Also since the most probable discharge point for potential spoil aquifer is at the intersection of Little Collom Gulch with the northern pitwall (elevation 7275 feet), all pitwall discharges will be below this elevation and thus below the Little Collom Gulch drainage to create spoil springs. Using the above spoil aquifer and the pit seepage calcualtions above, the time for a spoil aquifer to reach the most probable discharge point has increased slightly due to the pit seepage. Collom — Rule 2, Page 138 Revision Date: 8/5/11 Revision No.: PR -03