2020-06-25_REVISION - C1981019 (6)

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RULE 2 PERMITS limited perched water, and no saturated conditions, in the Williams Fork Formation. In the Williams Fork Formation, the low permeability and depositional nature of the strata restrict the ability of the bedrock to store and transmit water. There are no continuous non-coal beds in the Danforth Hills. Groundwater movement is mainly controlled by fractures of varying orientation. The Williams Fork Formation is not a significant water supply source in the Danforth Hills. It is not used as a source of water where the alluvial and surface waters are accessible. Where wells yield water, the water quality in the Williams Fork Formation is generally good. Very few registered wells for domestic, agricultural, or industrial purposes are completed in the Williams Fork Formation in the vicinity of the South Taylor pit. Drilling by Colowyo and other parties encountered no significant water in the South Taylor pit area in the litholgic sequence which is planned to mined. This is based on the drilling and geophysical logs. It should be noted that the current East and West Pits at the Colowyo Mine do not intersect any significant aquifers. Perched aquifers have been encountered which drain rapidly. Once drained, they do not produce any significant water to the current pits. Since the South Taylor pit is higher in elevation than the two current pits, and also up dip of the current pits, no significant aquifers should be encountered in this Pit. The Trout Creek Sandstone aquifer is separated from the lowest coal seam (G8) to be mined by approximately 590 feet in the South Taylor pit area. Between this coal seam and the Trout Creek Sandstone is a mudstone/shale, sandstone, siltstone, and coal sequence of the Williams Fork Formation. About 165 feet above the Trout Creek Sandstone, a two-foot thick smectite clay layer (known as the Km bed) exists that is found throughout the Danforth Hills area. This layer has low permeability and therefore would be an additional impediment to downward or upward groundwater flow. To determine the potential for the operations to encounter substantial groundwater and thus to require dewatering, elevations of groundwater and the depth of the pits were compared. The elevation of the potentiometric surface in well 84-0-OB was 7,054 feet above mean sea level (AMSL) in October 1984 (CDM 1985a). This well was completed in the sandstone in the above the 13 seam of the Williams Fork Formation (as correlated by Colowyo). The lowest projected depth of the South Taylor pit is approximately 7,320 feet AMSL. The Trout Creek Sandstone aquifer has a potentiometric elevation of between 7,050 and 7,100 feet AMSL beneath the South Taylor mining area (CDM, 1985a). This indicates that the pit bottom is above the saturated bedrock. Since the base of the pit will be above the elevations of the potentiometric surfaces in bedrock and alluvial aquifers, no impacts to the quantity of groundwater available in the Williams Fork Formation or the Trout Creek Sandstone are anticipated. Pit Inflow and Pit Surface Water Recharge Impacts The minor springs located on the hill slopes adjacent to the South Taylor Pit (Map 10), which flow four months of the year or less, are the springs likely to experience diminished flow. Springs 3-93-20-212 and 3-93-17-142, -143, -144, and -432 (South Taylor) and 3-93-20-213, -214, and -215 (West Fork Good Spring Creek) are located within the pit boundary and will be eliminated by the pit. Taylor Creek would potentially lose about 20 gpm of its peak flow (0.04 cfs), which is about 2%of its 1.9 cfs peak flow. The West Fork Good Spring Creek would potentially lose about 5 gpm (0.01 cfs) of its peak flow which is 0.5%of its 2.1 cfs peak flow. Since these springs only flow seasonally, neither creek would lose any base flow by the elimination of these springs. South Taylor/Lower Wilson—Rule 2,Page 79 Revision Date: 6/22/20 Revision No.: MR-220