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• <br />Groundwater Effects <br />As described under Rule 2.05.3, the Foidel Creek Mine will be developed by <br />the room-and-pillar method. Facilities for three portals will be installed at <br />the highwal! in Area 1. Five entries will be developed north under Foidel <br />Creek to a point beneath the Twentymile Sandstone At this point, sub-mains <br />will be developed from the main entries in areas where the Wadge Coal Seam <br />is overlain by 1100-1300 feet of competent strata. Panels will be developed <br />from the sub-mains. Furthermore, there are no structures or renewable <br />resources above the panels or production sections which will be affected by <br />mining or subsidence. <br />Groundwater Flow Model <br />in order to simulate the flow regime and artesian head distribution in the <br />overburden aquifer, a two~imensional, finite~ifference digital model was <br />developed for the Twentymile Park basin. The GRWATER model, developed at <br />Colorado State University, was formatted utilizing at 2,000 x 2,000 ft. node <br />pattern, as shown in Figure 12c, Groundwater Model Grid System. The model <br />was initiated by inputting the following information for each node: <br />1) Hydraulic conductivity (ft./day); <br />2) Elevation of base of aquifer (ft.); <br />3) Elevation of top of aquifer (ft.); <br />4) Confined aquifer storage coefficient; <br />5) Unconfined aquifer specific yield; <br />6) Net recharge (+) or discharge (-) (acre-ft./day); and <br />7) Steady-state piezometric elevation (ft.). <br />Boundary conditions were simulated by special coding of grids where negative <br />boundaries exist (discharge areas, exposed outcrop, impermeable barriers) or <br />where positive boundaries exist (recharge areas, submerged outcrop, faults <br />contributing water to the aquifer). The model was calibrated by executing the <br />steady-state model simulation over long time increments. The piezometric <br />surface map thus derived, shown in Figure l3, Calibrated Piezometric Surface <br />Contour Map, was compared to the map drawn from empirical data, shown in <br />Figure 14c, Piezometric Surface Contour Map from Observed Data. <br />Adjustments were made in those input parameters where empirical data are <br />not available, and the simulation run was repeated. By trial-and~rror, the <br />calibrated artesian head distribution in the Foidel Creek Mine Plan area was <br />made to match the piezometric surface shown on Map 16b, Piezometric <br />Surface. A comparison of empirical data and calibrated model values at <br />specific locations is given in Table 18c, Actual vs. Model Permeabilities. <br />2.05-47c <br />