2011-05-23_REVISION - C1981019 (2)

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Colowyo Response - PR3 adequacy No. 2 21 May 6, 2011 time, some pitwall seepage would occur thus increasing the time for a potential spoil aquifer to develop. Pitwall seepage will be predominantly through the coals and to a lesser extent sandstones. Some seepage will occur along fracture planes in the mudstones and siltstones but will be minimal. Groundwater movement in the bedrock is largely controlled by the existence of joints or fractures rather than by permeability within the rock strata. The typically low permeability and discontinuous and lenticular nature of the strata restrict the ability of the bedrock to transmit water. No hydraulic conductivities (K) have been obtained for these units since they 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. Since the coals and sandstones will transmit the majority of water from the spoils the estimate ofpitwall 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 sandstonewith 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 (1.0x10-2) 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 2.52x105 square feet. With Q=KA(h/l), K =1.0x10-2 ft/day A = 2.52x105 ft2 h1l=0.1 This equals pit seepage volume of 2016 ft3/day. Based on the 5000 foot length of the highwall, this equals a potential pitwall seepage of less than 0.5 ft lday (3.7 gallons) per linear foot of pitwall over a height of 125 feet. However, this pitwall seepage will only occur if a spoil aquifer is developed. 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 - -