2020-03-24_PERMIT FILE - C1981019A (2)

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RULE 2 PERMITS 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 I3 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. The South Taylor pit is likely to be within the watersheds for these springs: GSCS-1, WFS-1, WFS-2, WFS-4, WFS-5 and 5A, and WFS-7 and 7A, and 3-93-29-234. These springs collectively contribute about 20 gpm to the base flow and about 130 gpm to the peak flow of Good Spring Creek, the majority of this flow originating in the WFS-2 complex. This is equivalent to 0.04 cfs contribution to the base flow and about 0.3 cfs contributed to the peak flow. The WFS-2 spring complex is located in the bottom of the drainage and therefore is likely to obtain most of its water from areas outside of the South Taylor pit area. If all the contributions from these springs were terminated by South Taylor mining, the West Fork Good Spring Creek would lose 0.04 cfs of its base flow, and about 0.3 cfs of its peak flow. This amounts to a calculated loss of about 5% of the base flow of 0.85 cfs and about 3% of the peak flow of 11 cfs (as measured at NUGSC). However, since much of the recharge is from undisturbed areas outside of the South Taylor pit, the probable reduction is likely to be less than half of this amount and not expected to be measurable or statistically significant. Once the mining has been completed and the pit has been saturated, the contributions to surface water from springs originating from infiltration into the South Taylor pit would return to normal. South Taylor Pit Hydrology — The South Taylor pit will have a reclaimed surface area of approximately 1004 acres and a pit bottom that inclines predominantly towards the Good Spring Creek drainage (Figure 2.05.6-1). Assuming resaturation would raise a pit aquifer level to 7,500 feet AMSL (the elevation of the lowest point on the southeastern pit boundary) and considering the pit topography, the volume of materials that must be resaturated is calculated to be 6.92 x 108 cubic feet (ft). Assuming 20% effective porosity, 1.38 x 108 ft3 of water (3,178 acre-feet) must infiltrate from the surface and from the Williams Fork Formation to fill the pit to this level. Prior to flow from a pit, resaturation of the materials in the pit must occur. The time necessary for the resaturation of the backfilled pit can be estimated by utilizing the volume of the pit, the infiltration rate, South Taylor/Lower Wilson — Rule 2, Page 81 Revision Date: 4/7/17 Revision No.: RN -07