2012-06-20_PERMIT FILE - C2010089 (87)

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the mapped alluvial valley floor. In cross section B -B', the gravels are recharged by discharges from the underlying McKay coal, indirectly by discharges from the Rosebud coal, and by surface flow along the channel. Farther downstream and out of the surface- mineable coal area (cross section C -C'), the creek receives discharge from the gravels and adja ent consolidated materials. Thus the stream elluvltan loses water in the upstream rea hes and gains water from the bedrock materials through most of the all: :al vall :ey floor areas. The clay layers shown in cross sections B -B' and C -C' have not been thoroughly described but appear to have potential to hold some water in the overlying silt so as to be available for subirrigation. Mining of the coal seams shown outside the alluvial valley floor in Figure 9 will interrupt the flow of water from the coal seam aquifers to the alluvium during mining. Intercepted water of acceptable quality could be discharged back into the stream (downstream) during mining. Assuming that spoiled overburden removed outside the alluvial valley floor is placed by draglines and /or scrapers (without additional compaction) to approximate the pre - mining configuration of the surface and that water quality is maintained, we concur with Van Voast's and Hedges' preliminary findings that the hydrologic system could be reestablished along the margins of the East Fork of ArmeUe Creek. However, the geologic system in the creek itself should probably not be disturbed since the reestablishment of the semi - confining clay layers would he more difficult. Final opinions in this matter would be dependent on more detailed field investigations. Alluvial valley floors overlying a confined coal aquifer appear to create a most difficult situation to mine and reclaim if it is necessary to recreate the confining bed or aquiclude. Thus these situations pose problems for the extraction of under- lying and nearby shallow coals. Substantial lowering of the land surface (elevation) as would occur when mining thick coals under thin overburden could also create situa- tions (shallow water tables and steep surface channels) creating impediments to the reestablishment of subirrigated alluvial valley floors. The greater the area of alluvial valley floors on a Leasehold, the greater the likelihood that it will not be possible to mine shallow coal by surface methods without adverse impacts on the alluvial valley floors. • While it is obviously necessary to assess the impacts of alluvial valley floor designations on the coal reserves in a site - specific manner, it is suggested that the average amount of coal affected would be about ten percent on a lease tract con- taining coal under an alluvial valley floor. This ten percent was developed earlier in this section by assuming the need to maintain highwall stability along the allu- vial valley floor and the concurrent loss (for economic reasons) of coal under thick overburden. Further refinement of this estimate necesaitates additional geohydro- logic and specific mining plan information. Such refinement, presently necessary on a site - specific basis, could decrease or increase this estimate. Obviously there can be situations in which little coal could be mined in a single leaseable coal unit since water quality deterioration will be assured. It is believed that at least a few places exist where essential functions of subirrigated alluvial valley floors can ho recreated After mining. A subsequent section of this report suggests infor- matiim requirements necessary to make pre - mining evaluations of such impacts.