1986-06-13_GENERAL DOCUMENTS - C1981018

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• -33- Subsidence effects include fracturing of the material overlying the coal for a distance of 30 to 40 feet due to caving of the roof material and surface subsidence and possible development of tension cracks on the surface due to differential settling. Ground water inflows during mining are expected to be minimal : 5 to 10 gpm. In 1 ractured areas, increased flows of 100 to 300 gpm may be encountered for a short period of time. These flows will diminish as equilibrium is reestablished. Due to the low transmissivities of the aquifers, if caving should create a hydraulic connection from the mine to an aquifer within 30 to 40 feet of the coal , the additional inflow will be minimal . It is not probable that fracturing from the mine to. the surface will occur as a result of subsidence and cause the flow of surface hater to the mine. Subsequent to completion of mining, the ground water will slowly reestablish its premining water levels. Due to the caving of the roof materials into the mine, an area of increased permeability will exist within the mine which will act as a "sump area" until equilibrium is established. However, due to the small quantity of water in the aquifers being discharged to the White River and its tributaries, the lack of ground water usage in the area, and the poor quality of ground water, potential impacts to flow in the White River will be very minor. It could be possible for water to discharge from the portals once the mine workings have become completely flooded. The portals are up-dip in the mine and drainage will tend to move down the dip of the mine and coal seam, thereby increasing the time to flood the workings; however, it is uncertain if the drainage out of the mine area through the coal seam will exceed the drainage into the mine from overlying strata. If outflow does not exceed inflow, some discharge from the portals may occur. The permittee has proposed backfilling of the portals and construction of concrete bulkheads upon closure of the mine, minimizing the possibility for discharge of water. In the northern sections of the mine plan area, a large coal refuse disposal area is proposed. Drill hole data indicates that there is no ground water within 44 feet of the ground surface in the disposal area, and recharge to ground water-An the Mesaverde Group is minimal . Infiltration of water through the refuse pile will be minimized by covering the compacted refuse with four feet of the best available nontoxic material . Impacts to the ground water system are not expected to be significant due to the relatively impermeable nature of the refuse and the low water table. The development and reclamation of the cuts and fills in the portal areas should have no impact on the bedrock aquifer system. Recharge of the aquifers does not occur in these areas due to surface water control systems, so possible degradation of the ground water by infiltrating waters is unlikely. The fill material itself is generally of suitable quality as a topsoil substitute except for SAP, values which are up to twice as high as defined in the Wyoming Guidelines for suitability. As water flows through the fill , • total dissolved solids may increase due to soluable salts; however, this occurs naturally within the existing ground water region. The fill does not contain materials which create a toxic leachate. Water which infiltrates the fill may reach Scullion Gulch. However, due to the small area of disturbance relative to the entire drainage basin for Scullion Gulch, this will be a very minor contribution of flow in the creek.