2020-06-25_REVISION - C1981019 (6)

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RULE 2 PERMITS The South Taylor pit is likely to be within the watersheds for these springs: GSCS-I, WFS-I, 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 10' cubic feet(ft3). Assuming 20% effective porosity, 1.38 x 10' 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, and the porosity of the materials within the pit. Published infiltration rates for the area are 0.5 inches per year(Rice, 1979) and 3 inches per year (Williams & Clark, 1992), for an average value of 1.8 inches per year. Calculated inflows, in the above equations, indicate an inflow rate 92 gpm (approximately 150 acre-feet/year from 1.8 inches infiltration over 1,000 acres) from infiltration due to precipitation. (No other water is expected to flow into the reclaimed pit materials since the South Taylor Pit is on a topographic and structural high). The volume of water needed to fill the reclaimed pit divided by the infiltration rate equals the time to fill the pit to form an aquifer necessary for sufficient outflow. The result of this calculation is approximately 45 years for pit resaturation to the elevation of the lowest point of the pit boundary where water could be discharged. This assumes no water infiltrates into the undisturbed Williams Fork Formation on the limits of the reclaimed pit, and the entire pit fill becomes saturated. It is possible that the pit fill will be anisotropic and heterogeneous in a way that can allow a pit spring to form prior to complete saturation of the pit fill. It is also possible that most or all of the pit water will enter the Williams Fork Formation (see discussion below) thereby reducing the time to reach saturation or preventing the full thickness from becoming saturated. Groundwater from the reclaimed South Taylor pit will eventually discharge into Good Spring Creek at the drainage that is above the Sturgeon Flume (the unnamed tributary to West Fork Good Spring Creek in Section 21). This would result in a pit spoil spring and/or discharge through colluvial and shallow bedrock groundwater infiltration. This water would likely have the same characteristics as the water in the Streeter Fill well or the Streeter pond or in similar spoil springs (Williams and Clark, 1994). Analytical data for these sampling points are summarized on Table 2.04.7-31. If all of the water that infiltrates into the pit discharges into Good Spring Creek, then 150 acre-feet per year or 92 gallons per minute (0.21 cfs) of pit spoil water will enter the Good Spring Creek drainage. South Taylor/Lower Wilson—Rule 2,Page 80 Revision Date: 6/22/20 Revision No.: MR-220