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refuse disposal area will be a maximum of 120 acres at any one time. All <br />runoff from the disturbed area will be routed to properly sized sediment <br />ponds. <br />Subsidence from underground mine workings could potentially impact both <br />Red Wash and Scullion Gulch. The effect on Red Wash is of greatest <br />importance as it drains approximately 122 square miles at the location where <br />undermining will occur. Although predictions indicate that maximum <br />vertical subsidence could approach six feet, large crevasses or collapse of the <br />surface should not occur due to the fact that there is approximately 600 to <br />1200 feet of overburden material above the coal seam in that area. <br />Furthermore, the swelling of shale and overburden from the caved roof <br />should effectively fill the open mine cavity. An ongoing monitoring program <br />was implemented to detect subsidence on the first longwall panel and first <br />room - and - pillar section to be mined during the initial years of production. <br />The permittee originally predicted that tension cracks from subsidence may <br />develop in formations directly underlying Red Wash and that these cracks <br />may extend upwards for tens of feet. These cracks were predicted to be of <br />limited extent; therefore, a minimal amount of surface water was predicted to <br />be siphoned into the cracks. The permittee predicted that the sediment <br />accumulating in the cracks and clay in the overburden would ultimately seal <br />the tension cracks from the surface inflow. <br />Continued monitoring of Red Wash has confirmed the permittee's <br />predictions. As projected, subsidence in Red Wash initially resulted in water <br />ponding in the stream - bottom. The high content of silt and clay carried by <br />Red Wash rapidly fills these stream - bottom ponds and seals any cracks that <br />formed in the alluvium as a result of the subsidence. Water continues to flow <br />in the surface channel at Red Wash while piezometric levels in the basal <br />alluvial gravel have dropped, indicating that the basal alluvium is not <br />recharged by surface flow infiltration. <br />Minimal subsidence impacts are expected in Scullion Gulch, similar to those <br />observed in Red Wash. The overburden under Scullion Gulch is less than <br />that of Red Wash. Tension cracks could cause several stock ponds in <br />Scullion Gulch basin to lose impounded water. Although the stock ponds are <br />normally dry, a loss of water could occur where water is present as a result of <br />damage to the dams or pond bottom that would allow seepage loss. Prior to <br />the advancement of mining that could potentially impact the ponds, <br />monitoring is required to be initiated, and the ponds repaired or replaced by <br />the operator if water retention capability of the ponds is diminished due to <br />subsidence. The operator will haul water for stock in the event of pond repair <br />or construction. <br />2. Ground Water Impacts <br />33 <br />