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-57- <br />Dewatering of Coal, Roof, .and Floor <br />From field observations made by Mined Land Reclamation's staff hydrologists in the mines <br />and from information contained in their respective permit applications, water was noted <br />to seep from the coal, roof, and floor upon advance of mining in the B and C seams <br />of the Somerset mine, the D seam of the Orchard Valley mine, the E seam of the <br />Hawk's Nest mine and Blue Ribbon mine, and F seam of the Mt. Gunnison mine. This <br />source of mine inflows dries up one to two weeks after mining an area. The minor <br />amount of water resulting from dewatering of the coal, roof ,.:and floor indicates <br />that the coals and the overlying and underlying strata in the Mesa Verde Forms ti on <br />have very low permeabilities. Thus, these strata store and transmit very little <br />water. Therefore, the impacts of dewatering the coals and the strata immediately <br />above the below the coals within the Mesa Verde Formation will have an insignificant <br />impact on the hydrologic balance. Also, the very slow rates of inflow resulting <br />from dewatering strata indicate that these strata will not transmit significant <br />quantities of water which will result in depletion of surface water sources. <br />Subsidence <br />The impacts of subsidence due to mining can be expected to vary in magnitude and <br />extent. This variation will result from a combination of numerous natural phenomenon <br />and from the effects induced by underground mining. Surface water and ground water <br />can both be influenced by differences in timing of mining and the physical environment. <br />It is assumed that all mining will result in subsidence at some future point in <br />time, although surface manifestation may require many years to occur. Therefore, <br />control and prediction of subsidence are critical, necessary measures, which <br />should be implemented before and daring any mining activity. <br />Natural phenomena which influence subsidence are: <br />- Depth of overburden <br />- Thickness of the coal seam <br />- Stratigraphy, Zithology and structure of the overburden <br />- Topography <br />- Lithology of the floor strata <br />Mining activities which affect subsidence are: <br />- Size, shape, depth, location, and areal extent of workings <br />- Rate of development and extraction of resource <br />- Mining methods <br />The possible impacts of subsidence include: <br />- Modification of surface water hydrology - channel and watershed geometry <br />and sediment erosion rates <br />- Modification of ground water hydrology - changes in spring discharge, <br />aquifer characteristics and recharge <br />- Loss of water from surface impoundments due to disruption by surface cracking <br />- Increase in surficial mass movement activity - landsliding and rock falling <br />