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

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Summary and Conclusions Reconnaissance mapping of subirrigated alluvial valley floors was conducted to determine how much land might be involved if special legislation evolved to protect areas of agricultural importance, and, indirectly, how much coal might he affected if mining of alluvial valley floors was determined to he environmentally unacceptable. This reconnaissance effort defines the limits of subirrigated alluvial valley floors adequately enough to serve as a basis for making first estimates of surface mineable coal that may be affected if the alluvial valley floors cannot be mined. A recon- naissance study does not provide sufficient information to estimate how much of the coal that lies outside the alluvial valley floor might be affected. Th lysis appears possible only with additional experience in mines located . a to tions, including those outside alluvial valley floors, or with detail , .-y i g forma- tion that accurately describes the alluvial valley floor al source, and the effect of the reclamation procedures propo ;s' sis does confirm the suggestions of others that about three percent ppable coal resources of the western United States underlies subirrigate uvial valley floors. The impact of alluvial valley floor protection requirements on the amount of surface mineable coal available may be expected to be about 14% 1f` reclamation is not possi- ble. In the judgement of the authors, only where there is a considerable density of alluvial valley floors is it likely that mining plans cannot be developed to exclude mining of such areas. It is likely that the critical functions of some alluvial val- ley floors can be reestablished except where complex surface and groun3 water systems are present and when shallow ground water quality is severely degraded as it passes through spoils. This reconnaissance identification of the topographic (with some regard for geo- morphology) and vegetative characteristics of alluvial valley floors within lands leased for coal extraction by surface methods provides a basis for estimating that three percent of the leaseholds in the interior western United States are overlain by alluvial valley floors. These alluvial valley floor designations encompass a land unit with a geohydrologic and biologic interrelationship that may be critical to the viability of an agricultural economy. However, the economic importance of the alluvial valley floor has not been assessed. Alluvial valley floors are quite common in the Montana, Wyoming, and Colorado 'areas of coal development, but are less common in the coal regions of North Dakota, southwestern Wyoming, Utah, New Mexico, and Arizona. "Less common" refers to the size and amount of land identified as alluvial valley floors. For example, the mines examined in Arizona included areas of alluvial valley floors, but the area involved was less than 0.5 percent of the land area examined. Identification of these allu- vial valley floors was accomplished through the use of aerial imagery, topographic maps, previous studies, and knowledge gained from past site visits. Though it has been found to be a unique circumstance when any existing western coal mine is located entirely within, an alluvial valley floor, alluvial valley floors may be supported by a coal seam aquifer system* that extends out from the alluvial valley. Thertfore, the effects of mining an area of recharge may extend to an allu- vial valley floor. In this case, the amount of coal affected could easily be raised if reclamation were not possible (e.g., if shallow ground water quality were de- graded so as to interfere with use in the alluvial valley). In alluvial valley floors containing relatively thick coals overlain by thin overburden as well as in areas where alluvial aquifers are in a sense "perched" over the aquifer surface mine- able coal, all coal may have to be withheld at least until the details of the natu- ral system and the proposed' mining plan provide the necessary assurance that areas may be reclaimed, at least to pre - mining productivity (and land uses). * A moderately fractured coal seam which, often in combination with sandy strata, transmits sufficient water to be classified as an aquifer.