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Gre~~slnne De~~rlupmenl Consultants, Inc. <br />• <br />June 10, 1986 <br />Page 3. <br />Sediment Pond Evaporation <br />As projected in the OSM December 3, 1984 memorandum, <br />sediment pond evaporation has been determined to he insignificant <br />because of the small area of surface disturbance and limited pond <br />surface area. <br />Subsidence Effects on Springs and Seeps <br />The principal effect of subsidence is disruption of bedrock <br />aquifers which supply springs and seep flows. Analysis of the <br />depth of overburden indicates that between 1,000 to 1,500 feet <br />exists from the coal seam to the surface over the majority of the <br />area to be mined. As indicted in the probable hydrologic <br />consequence section of the mine plan application, effects of <br />subsidence should not reach the Twentymile Sandstone Aquifer <br />• (major aquifer above the coal seam) or surface alluvial aquifers. <br />Thus, insignificant depletions of springs and seeps should occur <br />as a result of mining and associated subsidence. <br />Alluvial Well Pumpaqe <br />Twentymile Coal Company does not propose to pump any <br />alluvial wells for water supply to either surface or underground <br />operations. Therefore, this category of depletion will not occur <br />as a result of proposed mining operations. <br />Alluvial Aquifer Abstractions into Mines <br />Such abstractions may occur where mine affected aquifers <br />subcrop beneath valley-fill alluvial aquifers. These effects <br />could change the direction of ground water flo~•~ which may result <br />in a decrease of discharge to alluvium and surface streams. <br /> <br />Revised 10/24/86 <br />