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PROBABLE HYDROLOGICAL CONSEQUENCES <br />A discussion of the probable hydrologic consequences and reclamation plan are contained in Section <br />2.05.6(3) and Section 2.05.6(3)(b)(v) of the permit application document. The following discussion <br />is intended to supplement the description of potential impacts of mining and mitigation of these <br />potential effects. The determination of significance has been made considering the impact on the <br />quality of the human environment, existing water uses, and the intended post mining land use of the <br />area. <br />Interruption of groundwater flow and drawdown. In order to develop the impact assessment for <br />groundwater quantity, two different analyses techniques were utilized. First, pit inflow volumes were <br />determined on an annual basis using an analytical approach developed by McWhorter, 1982. The <br />second analysis involved the determination of annual pit inflow rates and annual drawdowns in the <br />adjacent overburden and coal aquifers as a result of the pit inflows. This analysis utilized the USGS <br />3-dimensional finite-difference groundwater flow model MODFLOW. <br />Transient simulations were performed for alive-year period, using the maximum drawdown estimates <br />for the overburden and coal. These drawdown results are expressed as a maximum at the pit and <br />are expressed as a conical depression which results in decreased drawdown at further distance from <br />the mine. For the overburden, the pit drawdown was 5 feet for years 1 and 2, 8 feet during year 3, <br />15 feet during year 4, and 30 feet during year 5. The drawdown for the coal simulation was 8 feet <br />during year 1, 5.8 feet during year 2, 6 feet during year 3, 7.3 feet during year 4, and 8 feet during <br />year 5. The zero impact contour for the overburden and coal after five years of mining is <br />approximately 4,000 feet. The overburden and coal drawdown contours do not intersect any of the <br />boundaries, therefore, no impact of the San Miguel River from drawdown in the deeper part of the <br />overburden or coal is predicted. Shallow aquifer flow into Tuttle and Calamity Draws in the vicinity <br />of the pit will be decreased, but will be offset by pumpage from the pit. Simulated average daily pit <br />inflow for the coal and overburden aquifers varied from 1,255 cubic feet per day in year 1 to 5,604 <br />cubic feet per day in year 5. WFC's approach to these potential impacts is to monitor the aquifers <br />and discharge from the pit to determine the extent of drawdown. The hydrological monitoring <br />program should provide reasonably accurate measurements of effects of mining. Should the <br />monitoring show that impacts to the groundwater aquifers are precluding its use, WFC will provide <br />• (REVISED 9/99) 2,04.7-46 <br />