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

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78. Hypothetical Coal Mining Situations in Alluvial Valley Floors In order to better describe the amount of strippable coal affected by restric- tions on mining of alluvial valley floors, this section investigates selected hypo- thetical mining situations in alluvial valley floors. The situation is examined in which maintenance of pit wall stability adjacent to alluvial valley floors may re- quire setting aside additional amounts of coal. Also examined are situations in which hydrologic complexity appears to preclude successful reclamation of affected alluvial valley floors. It must be remembered that the analyses that follow are based not on exhaustive field investigations, but rather on attempts to generalize surface and subsurface conditions according to principles of hydrology and geology. While there is confidence that situations such as these exist, field investigations may show them to be rare, oversimplified, or even too complex. But whatever the con- ditions are, they cannot be ignored or their analysis postponed since judgements as to their compatibility with surface mining of coal must be made now. We have assumed for purposes of calculations that an "average" alluvial valley floor is 200 meters wide and, as noted above, that any coal under the floors must be left undisturbed. In certain cases, coal may be removed up to the border of the alluvial valley floor. Such cases might exist if the coal were relatively thin (three to six meters for western coal), covered by thin yet sufficient overburden to permit regrading to a similar elevation and topography, and in relatively simple hy- drologic situations such as a thick alluvial aquifer that can be reestablished through selective placement of spoiled overburden. In other cases, pit wall stability or perhaps maintenance of a stream may require leaving a coal barrier outside the allu- vial valley floor, thereby increasing the amount of "lost" coal. We have assumed the pit wall stability situation to be one where pit wall stability requires main- tenance of a 45 slope (1:1) on the active highwall such that 60 meters of the coal seam extending away from each side of the alluvial valley could not be mined. The 60 -meter value is obtained by assuming that the mine pit will be 60 meters deep and that a 1:1 highwall slope will be maintained. Since the coal lies along the base of the right triangle (45 ° ) created by the 1:1 slope, at least 60 meters of the coal seam would be left in place. This increases "lost" coal from the 200 -meter width lying directly under the alluvial valley floor to 320 meters, or a 60 percent in- crease in the amount of coal that would not be mined in this situation. Based then on the three percent value calculated in the initial analysis of alluvial valley floors overlying coal lands (Table 3), this "pit wall stability" situation would increase the amount of coal "lost" or "withdrawn" to about five percent. Additional coal may also be lost if the coal in an alluvial valley floor is covered by relatively thin overburden, since this coal may offset the higher extrac- tion costs of coal lying outside the alluvial valley floor and covered by thicker overburden. That is, the coal overlain by thin overburden in the alluvial valley is relatively cheap to mine and can be used to balance the higher expense of extracting coal lying under thick overburden in the highlands. If this overburden factor enters into consideration when marketing coal from a leasehold and the "pit wall stability" situation is also encountered, the amount of coal "lost" could double to about ten percent. Note again, that these calculations are hypothetical. Actual field situa- tions would have to be studied before firm estimates of the amount of coal lost could be made. In other extreme cases, mining in the entire drainage area could be prohibited unless it were deemed possible to maintain and reestablish critical geohydrologic characteristics such as effective aquicludes* and water quality. Such would be the case if reclamation could not reestablish the essential functions of an alluvial val- ley floor supported by a perched water table (thus lowering the water table), or if * Aquicludes: Relatively impermeable rock capable of absorbing water slowly, but functions as an upper or lower boundary to an aquifer. Does not transmit ground water rapidly enough to supply a well or spring. 1 79. mining caused the plant growth medium to change significantly in permeability and chemistry, thus affecting plant productivity, ground cover density, or species com- position. Of course, if the impact of mining were one of deterioration of water qua- lity to the point of adversely affecting water use, no coal seams that are hydrolo- gically upgradient of alluvial valley floors, or which are related to the subirri- gated system could be mined without reducing the productivity and use of the alluvial valley floor to a degree proportional with the watershed area affected. It would be inappropriate to mine such areas until pollution control technology was available, regardless of other potentials for successful grading, revegetation, or local sub - irrigation of the mined area. Finally, if a number of individual alluvial valley floors occurred on one mine tract, it is likely that avoidance of these areas would result in isolated, small blocks of coal that might be uneconomical to mine. In these examples, the entire leasehold could be determined uneconomical to mine. A better estimate of the impacts of mining alluvial valley floors might be made by examining a number of f situations that appear characteristic of the western United States coal regions. Emphasis in the examination is on the geologic and hy- drologic situation existing in, and in the immediate vicinity of, the subirrigated alluvial valley floor. Figure 1 portrays the hydrologic cycle in a simplified form. It depicts how water may become available to vegetation as precipitation, as overland flow in the unsaturated and the capillary zones, in the saturated zone beneath the water table, or as locally "perched" water. Figure 2 shows the concept of multi- aquifer systems where waterbearing strata are separated by confining beds or aquicludea containing semi - permeable strata. The aquifers may have completely independent flow systems providing significantly differ- ent piezometric surfaces (i.e., height to which water will rise in a well) and flow directions. Aquicludea permit minor amounts of water to pass. The permeability of aquifers may involve connected pore space formed during deposition and /or secondary permeability (i.e., fracture permeability) formed after deposition. The shallow coals of the interior western United States gain most of their permeability, and therefore their role as part of the shallow aquifer system, from fracturing. Blast- ing of overburden and coal during mining may increase the permeability both of aquifers and aquicludes. This artificial fracturing is important when aquicludea immediately beneath the coal separate aquifers with significantly different water pressures or piezometric heads, or strata containing water of differing quality. It appears likely that in many cases subirrigation in an alluvial valley floor occurs through a single, shallow, unconfined aquifer. This single aquifer includes stream alluvium comprised of gravels, sands, silts, and thin clays. In the coal re- gions of the western United States, this shallow aquifer may also include a coal seam and associated sandy strata. The first case, shown in Figure 2, depicts a situation where ground water is discharged to a stream (i.e., a gaining or effluent stream). Figure 3 illustrates a case of recharge where the stream recharges a ground water aquifer (i.e., a losing or influent stream) and hence provides shallow waters for the alluvial valley floors. Figures 4, 5, and 6 illustrate three simplified geohydrologic situations that could exist in alluvial valley floors. Using these hypothetical situations, we have attempted to assess the impact of mining and reclamation on the hydrologic system and, based upon this analysis, tried to ascertain whether mining would be compatible with the objective of mining only where reestablishment of vegetative productivity and species composition was assured. Figure 4 represents a situation with a single coal seam which serves as the bottom portion of a shallow aquifer system. The aqui- fer includes sandstone, siltatone, gravel, and alluvium. The aquifer is unconfined and is assumed to discharge some water to the stream through the alluvium. Reclama- tion of this area after surface mining of the coal at a mine of typical size (less than 1,000 hectares) should result in the water table returning to pre - mining levels. However, if multiple mining operations cause the entire coal- sandstone aquifer to be disturbed throughout the regional watershed so as to drastically change the permea-