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degradation. The size of the non-reclaimed disturbed area at the refuse disposal <br /> area will be a maximum of approximately 120 acres at any one time. All runoff <br /> from the disturbed area will be routed to properly sized sediment ponds. <br /> Subsidence from underground mine workings could potentially impact both Red <br /> Wash and Scullion Gulch. The effect on Red Wash is of greatest importance <br /> because it drains approximately 122 square miles at the location where mining <br /> will exist beneath it. Although predictions indicate that maximum vertical <br /> subsidence could approach six feet, large crevasses or collapse of the surface <br /> should not occur because there is approximately 600 to 1200 feet of overburden <br /> above the coal seam in that area. Furthermore,the swelling of shales and <br /> overburden from the caved roof should effectively fill the open mine cavity. An <br /> ongoing monitoring program was implemented to detect subsidence on the first <br /> longwall panel and first room-and-pillar section to be mined during the initial <br /> years of production. Predictions of probable disruption of the hydrologic regime <br /> from subsidence were based on an analysis of those data. The permittee <br /> originally predicted that tension cracks from subsidence may develop in <br /> formations directly underlying Red Wash and that these cracks may extend <br /> upward for several tens of feet. These cracks were predicted to be of limited <br /> extent; therefore, a minimal amount of surface water was predicted to be <br /> siphoned into them. Furthermore,the permittee predicted that sediment <br /> accumulating in the cracks, and clays in the overburden would ultimately seal <br /> them off from surface inflow. <br /> Continued monitoring of Red Wash has confirmed the permittee's predictions. <br /> As projected, subsidence in Red Wash initially resulted in water ponding in the <br /> stream bottom. The high content of silt and clay carried by Red Wash rapidly <br /> fills the ponds. The silt and clay layers have sealed any cracks that formed in the <br /> alluvium as the result of subsidence. Water continues to flow in the surface <br /> channel at Red Wash while piezometric levels in the basal alluvial gravel have <br /> dropped, indicating that the basal alluvium is not recharged by surface flow <br /> infiltration. <br /> Minimal subsidence impacts are expected on Scullion Gulch, similar to those <br /> observed on Red Wash. The overburden is much less under Scullion Gulch than <br /> Red Wash. Tension cracks could cause several stock ponds in Scullion Gulch <br /> basin to lose impounded water. Although the stock ponds are normally dry, a loss <br /> of water could occur where water is present as a result of damage to the dams or <br /> pond bottom that would allow seepage loss. Prior to the advancement of mining <br /> that could potentially impact the ponds, monitoring is required to be initiated, and <br /> the ponds repaired or replaced by the operator if water retention capability of the <br /> ponds is diminished due to subsidence. The operator will haul water for stock in <br /> the event of pond repair or construction. <br /> 2. Groundwater Impacts <br /> xxix <br />