2012-06-20_PERMIT FILE - C2010089 (88)

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Alluvial Valley Floor 84. wafer fable Perched on clay lenses Figure 7 Detailed Section Across Alluvial Valley Floor And Alluvial Aquifer Showing Locally Perched Water Tables. slllsbne soil 85. of permeabilities expected of poor aquifers or aquicludes (which may have permeabi- lities of 1.0 to 0.01 gpd /ft or 0.04 to 0.0004 meters /day), it is suggested that more compaction than that performed at Big Horn is necessary to achieve lower per - meabilities. This compaction over and above that performed at the Big Horn Hine appears necessary to recreate zones of "perched" water. The water supply to the alluvial valley floor (as shown in Figure 7) could also be the alluvial aquifer (i.e., underflow) and this would require that the area of recharge located upstream would have to be protected. This locally complex situa- tion could also exist in'any of the situations shown in previous figures. Of the three hydrologic situations shown in Figures 4, 5, 6, and the more de- tailed perched system of Figure 7, the multiple aquifer system shown in Figure 5 appears the most difficult to reestablish. The situation shown in Figure 6 is also difficult to reestablish. Both cases require the recreation of an effective semi - confining layer, a reclamation practice not yet attempted on such a scale. The perched aquifer system of Figure 7 could be difficult to reestablish for the same reason. The single unconfined aquifer system shown in Figure 4 appears to have the highest probability for reestablishment. Based upon many assumptions which are given below, we suggest that in the situa- tion shown in Figure 4 no coal would be removed from production because the cri- tical features of the hydrologic system could be reestablished. The situation shown in Figure 5 would necessitate leaving the second, deeper coal seam in place if pres- sures of ground water were sufficiently out of balance and the combined post- mining pressure (or water table elevation) was deemed to adversely affect the subirrigation system. The situation portrayed in Figure 6 would require leaving the clayey shale in the vicinity of the alluvial valley floor in place and thus would require leaving sufficient coal under the shale to maintain the confining bed. The upper coal would be mineable. In the case of the alluvial valley floor in Figure 7, we suggest that the hydrologic system could be reestablished and that all the coal shown could be extracted if very selective placement and additional compaction of spoil in a manner not now used in western surface mining were required. In order to judge the probability for successful reclamation of these hypothe- tical alluvial valley floors and reestablishment of their essential functions, we have made the following assumptions: 1. The post - mining topography and elevation would be very similar to that existing prior to mining such that the approximate distance from the ground surface to the water table was retained; 2. The post - mining stream gradient (and flow) achieved the same quasi - stable equilibrium state existing prior to mining in terms of erosion and sedimentation; 3. A suitable plant growth medium was reestablished after mining; 4. Vegetative productivity and species composition were reestablished after mining; 5. The areas of ground water recharge were not modified; 6. Water quality was maintained; and 7. It would not be economically feasible to reestablish aquicludes over large areas. The first two assumptions would not be correct in instances where the overburden thickness is thin relative to coal thickness. In such cases, post - mining topo- graphy would be lowered by many maters. Those situations have not been examined on a mine -by -mine basis but must instead be examined on a regional basis that encom- passes entire basins.