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-38- <br />amount of water resulting from dewatering of the coal, roof, and floor <br />indicates that the coals and the overlying and underlying strata in the <br />Mesa Verde Formation have very low permeabilities. Thus, these strata <br />store and transmit very little water and are not considered aquifers. <br />Therefore, the impacts of dewatering the coals and the strata <br />immediately above and below the coals within the Mesa Verde Formation <br />will have an insignificant impact on the hydrologic balance. Also, the <br />very slow rates of inflow resulting from dewatering strata indicate <br />that the quantity of water transmitted by these strata will not result <br />in depletion of surface water sources. <br />Subsidence <br />The impacts of subsidence due to mining can be expected to vary in <br />magnitude and extent. This variation will result from a combination of <br />numerous natural phenomenon and from the effects induced by mining. <br />Surface water and ground water can both be influenced by differences in <br />timing of mining and the physical environment. It is assumed that all <br />mining will result in subsidence at some future point in time, although <br />surface manifestation may require many years to occur. Therefore, <br />control and predication of subsidence is critical, necessary measures <br />to be implemented before and during any mining activity. <br />Natural phenomena which influence subsidence are: <br />- Depth of overburden, <br />- Thickness of the coal seam, <br />- Stratigraphy, lithology and <br />structure of the overburden, and <br />Topography. <br />Mining activities which affect subsidence are: <br />- Size, shape, depth, location and <br />real extent of workings, <br />- Rate of development and extraction <br />of resource, and <br />- Mining methods. <br />The possible impacts of subsidence include: <br />- Modification of surface water hydrology - channel and watershed <br />geometry and sediment erosion rates, <br />- Modification of ground water hydrology -changes in spring <br />discharge, aquifer characteristics and recharge, <br />r <br />