2010-12-17_REVISION - C1981019 (153)

Home Browse Search

72 Groundwater Mine dewatering /slope depressurization measures will be important in the north area. The deeper units including the F and G coal sequences are under significant pressures and will need to be depressurized to improve slope stability. 5.3.5 Gradients In the vicinity of the minesite, water level elevations drop from 7,800 to about 7,000 ft in the F sandstone and from about 7,600 to 6,900 ft in the F/G sequence. In the F sandstone, direction of groundwater flow is north- northwest (Figure 5.11). In the F/G sequence, the direction of groundwater flow is north through Phase 1 and northeast across Phase 2. The direction of groundwater flow in the H and I sandstone is northeast. Horizontal groundwater gradients in the F sandstone and F/G sequence are approximately 0.3 in the F/S sequence and 0.14 in the F/G sequence on the southern half of the site (Figures 5.11 and 5.12). The groundwater gradients in the central area flatten in the F sandstone and F/G sequence compared to gradients to the south. Gradients in the F sandstone and the F/G sequence are both 0.04. Gradients in the H sandstone range from 0.02 to 0.08 from south to north. The limited water level data for the I sandstone shows a gradient of 0.03. To the south, groundwater gradients in the individual units reflect perched zones. In the central portion of the pit, these units intersect the area -wide groundwater table and are no longer perched. Thus, the central portion of the pit represents a transition from perched to water table conditions. This transition zone is represented in Figure 5.5 by the point where the Gc seam intersects the area -wide water level. Vertical groundwater gradients increase with distance downdip, starting at approximately the location where the horizontal gradients start to flatten. Buildup of head, as listed above, appears to be due to the numerous mudstone and siltstone units between the coal seams and coarser sand interburdens. The degree of hydraulic connection between a coal seam and the sandstone, mudstone or siltstone units above and below it vary considerably. For example the interburden layer above the Gb coal showed much more drawdown than the interburden immediately above the Fab coal. 5.3.6 Groundwater recharge and discharge Groundwater recharge areas are located in the higher elevations to the south where sandstones and coal seams crop out. Recharge to groundwater was estimated at 0.2 to 0.35 inches per year, based on a comparison of precipitation and base flow in Spring, Taylor and Wilson Creeks (Leonard Rice Consulting Water Engineers, Inc., 1979, Recharge may also occur along the upper (i.e., southern) sections of the stream channels from the valley fill. Conceptually, significant recharge occurs as a result of the spring snowmelt and spring storms. During the summer, recharge is minimal due to the dry soil, high evaporation rate and general precipitation pattern of short, intense storms. Flow proceeds down dip (northeastward) towards the axis of the syncline. The discharge area is to the north where the steeply dipping northern limb of the syncline is intersected by the stream drainages at about elevation 6,500 ft. This system is illustrated conceptually in the north -south cross sections (Figures 5.2, 5.3, and 5.5). 2572 -R2 Colowyo Coal Company Water Management Consultants