2020-06-04_PERMIT FILE - C1981008A (2)

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effects. The determination of significance has been made considering the impact on the quality of the human environment, existing water uses, and the intended post mining land use of the area. Interruption of Groundwater Flow and Drawdown In order to develop the impact assessment for groundwater quantity, two different analyses techniques were utilized. First, pit inflow volumes were determined on an annual basis using an analytical approach developed by McWhorter, 1982. The second analysis involved the determination of annual pit inflow rates and annual drawdowns in the adjacent overburden and coal aquifers as a result of the pit inflows. This analysis utilized the USGS 3-dimensional finite -difference groundwater flow model MODFLOW. Transient simulations were performed for a five-year period, using the maximum drawdown estimates for the overburden and coal. These drawdown results are expressed as a maximum at the pit and are expressed as a conical depression which results in decreased drawdown at further distance from the mine. For the overburden, the pit drawdown was 5 feet for years 1 and 2, 8 feet during year 3, 15 feet during year 4, and 30 feet during year 5. The drawdown for the coal simulation was 8 feet during year 1, 5.8 feet during year 2, 6 feet during year 3, 7.3 feet during year 4, and 8 feet during year 5. The zero impact contour for the overburden and coal after five years of mining is approximately 4,000 feet. The overburden and coal drawdown contours do not intersect any of the boundaries, therefore, no impact of the San Miguel River from drawdown in the deeper part of the overburden or coal is predicted. Shallow aquifer flow into Tuttle and Calamity Draws in the vicinity of the pit will be decreased, but will be offset by pumpage from the pit. Simulated average daily pit inflow for the coal and overburden aquifers varied from 1,255 cubic feet per day in year 1 to 5,604 cubic feet per day in year 5. New Horizon's approach to these potential impacts is to monitor the aquifers and discharge from the pit to determine the extent of drawdown. The hydrological monitoring program should provide reasonably accurate measurements of effects of mining. Should the monitoring show that impacts to the groundwater aquifers are precluding its use, New Horizon will provide alternate water sources of comparable quantity and quality. As described in the water augmentation plan, New Horizon has a 114 acre foot consumptive use right on the Highline Canal which would be used to mitigate the potential 26 acre foot impact on surface water right users from pit inflow drawdown. Impact on Groundwater Rh!hts No surface or groundwater rights have been identified within the New Horizon 2 mine area. Therefore, there will be no direct impact from the approved mining plan on any local water rights. Impact of Spoil Material on Groundwater Flow and Recharge The mine pit will remain open only until the coal has been removed. Following the short-term water level decline on the groundwater system as a result of pumpage of groundwater inflow to the pit, a potential long-term impact to the local groundwater flow is the period of time necessary for renaturation of the spoil material and reestablishment of a flow gradient. Spoil material at the New Horizon 2 mine will be replaced using techniques which will restore permeabilities of the material. Thus, the mining operation will not diminish vertical or horizontal permeabilities but may increase these permeabilities. As a result, there should be no significant impacts from the mining operation on groundwater flow and recharge rates. The time period required for the spoil material to resaturate should be greatly reduced because of the irrigation recharge as the mined area will be revegetated for an irrigation type of post mining land use. Page 2.04.7-31 March 2017 (TR -77)