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-2a- <br /> The dimensions of surface cracks appear to relate to the type of rocks in <br /> which they occur, the thickness of coal removed, the depth of the overburden, <br /> and the location of crack occurrence. Cracks in competent rocks tend to <br /> remain open while cracks in incompetent rocks and soils are likely to fill. <br /> In light of these observations, a range of subsidence impact scenarios are <br /> plausible. Subsidence related cracking could increase surface and overburden <br /> infiltration rates, diverting water from streams and impoundments above the <br /> effected areas. Related changes in local piezometric surfaces could result <br /> in the modification of springs and seeps. Waters, having entered the sub- <br /> surface strata, could migrate towards the underground workings, eventually <br /> permeating the strata and gradually migrating downdip beneath the Grand Mesa. <br /> Possible Effects of Subsidence Upon the Stevens Gulch and East Roatcap Creek <br /> Drainages <br /> Over the life of the mine, subsidence from Orchard valley's underground mining <br /> operations could effect surface water hydrology in the North Fork of the Gunnison <br /> watershed. Subsidence effects would reduce the surface flow from an affected <br /> watershed in several ways: <br /> 1. by increasing infiltration which would, in turn, decrease runoff, <br /> 2. by lowering the piezometric surface in local perched water tables and <br /> causing springs associated with the perched groundwater to dry up, and <br /> 3. by direct infiltration from streams and their alluvium. <br /> In areas not covered by soils, subsidence fractures would directly divert runoff <br /> into the groundwater system. In such an area, numerous open fractures could <br /> significantly reduce surface runoff. In areas covered with soils (which would <br /> not maintain an open fracture) , any reduction in runoff would be less severe. <br /> Subsidence fractures would increase deep percolation of soil water, but the <br /> infiltration characteristics of the soil would still control the amount of <br /> runoff. infiltration might still increase slightly, because deep percolation <br /> might more rapidly deplete soil moisture and decrease antecedent moisture. <br /> A lowering of the piezometric surface in local , perched groundwater tables might <br /> cause springs to go completely dry. Map 43 identifies several springs in or <br /> adjacent to the area to be mined. Although current data is not conclusive and <br /> will need to be verified by periodic monitoring, these springs do not appear <br /> to contribute significantly to the amount of flow in the watersheds, and even <br /> in a worst case scenario where all the springs dry up, this would affect only <br /> a small percentage of the low flow runoff. In reality, it is probable that <br /> not all the springs would be effected and, therefore, reduction of surface flow <br /> would be less than the worst case scenario. <br /> Most mines in the drainage basin of the North Fork of the Gunnison produce in- <br /> sufficient water through inflows to sustain their mining operations. This is <br /> evident in the need for these mines to import water into their mines for dust <br /> suppression and fire control. The only mine which must discharge water from <br /> it's workings is the U.S. Steel Somerset mine. <br /> C-1 <br />