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Potential impacts to the hydrologic balance and the likelihood that these impacts will occur are given <br />below. <br />11 Containment of pit inflow and impacts on water puality. All runoff and pit pumpage from <br />disturbed areas will be routed through approved NPDES sedimentation pond(s). These pond(s) will <br />be designed and constructed to impound runoff and pit pumpage from areas disturbed by mining and <br />provide sufficient residence time to insure that the pond discharge water chemistry meets the effluent <br />requirements specified in the NPDES Permit. A review of the chemical and flow data indicates that <br />the potential forany discharge from any NPDES pond to exceed receiving stream orfederal standards <br />is minimal. Past history of mine operations at the Nucla Mine indicate very few exceedances of the <br />standards over the years of operations. <br />2) Interruption of groundwater flow and drawdown. In order to develop the impact assessment <br />for groundwater quantity, two different analyses techniques were utilized. First, pit inflow volumes <br />were determined on an annual basis using an analytical approach developed by McWhorter, 1982. <br />The second analysis involved the determination of annual pit inflow rates and annual drawdowns in <br />the adjacent overburden and coal aquifers as a result of the pit inflows. This analysis utilized the <br />USGS 3-dimensional finite-difference groundwater flow model MODFLOW. <br />• Transient simulations were performed for afive-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 are <br />expressed as a conical depression which results in decreased drawdown at further distance from the <br />mine. For the overburden, the pit drawdown was 5 feet for years 1 and 2, 8 feet during year 3, 15 feet <br />during year 4, and 30 feet during year 5. The drawdown for the coal simulation was 8 feet during year <br />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 <br />zero impact contour for the overburden and coal after five years of mining is approximately 4,000 feet. <br />The overburden and coal drawdown contours do not intersect any of the boundaries, therefore, no <br />impact of the San Miguel River from drawdown in the deeper part of the overburden or coal is <br />predicted. Shallow aquifer flow into Tuttle and Calamity Draws in the vicinity of the pit will be <br />decreased, but will be offset by pumpage from the pit. Simulated average daily pit inflow for the coal <br />and overburden aquifers varied from 1,255 cubic feet per day in year 1 to 5,604 cubic feet per day in <br />year 5. WFC's approach to these potential impacts is to monitor the aquifers and discharge from the <br />pit to determine the extent of drawdown. The hydrological monitoring program should provide <br />reasonably accurate measurements of effects of mining. Should the monitoring show that impacts to <br />the groundwater aquifers are precluding its use, WFC will provide alternate water sources of <br />comparable quantity and quality. As described in the water augmentation plan, WFC has a 114 acre <br />• (Revised 6/01) 2.05.6 (3) - 19 <br />