2010-06-04_PERMIT FILE - C1982056A (10)

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• Longwall operation induced subsidence creates a zone of increased permeability above the mined - out area and within a 15 to 25 degree angle of draw • • Cracking and opening of fissures at the ground surface can occur within the zone of tensile strain. However, the depth of such subsidence fractures is limited. • The zone of increased permeability above the longwall operation is typically 30 to 50 times the thickness of the removed strata • The thickness of the zone of increased permeability is dependent on the thickness of the removed strata, speed of mining, and geologic character of the overburden; and, after completion of mining, the permeability of the disturbed strata returns to close to premining conditions. Subsidence Impacts on Alluvial Ground Water in the Foidel and Middle Creek Alluvial Valleys The bedrock in the study area is composed of Lewis Shale in the central part of the Foidel Creek valley, and of the Williams Fork Formation on the east and southeast parts of the Foidel and Middle Creek valleys. The Williams Fork Formation dips southwest. The potentiometric surface of ground water in the Wadge Coal Seam Overburden is between elevations of 6,650 and 6,700 feet within these AVF designated areas. Ground water flows toward the northeast. The average hydraulic conductivity of the Wadge Coal Seam Overburden, verified by several tests, is 0.32 ft /day. The potential water- bearing strata of the study area, above the Wadge Coal Seam, are the sandstones in the Wadge Coal Seam Overburden, The Twentymile Sandstone, and the unconsolidated sediments in the Foidel and Middle Creek alluvium. Only the Twentymile Sandstone is considered a significant unit. The Twentymile Sandstone, however, is separated from the mined coal seam by a 500 to 600 foot -thick layer of marine shale designated in the geology description as, "Tongue of Lewis Shale." In most situations, this shale strata acts as a significant aquiclude above the mined coal. The Foidel and Middle Creek alluvium is composed mostly of fine- grained sediments, silts, silty clays, and clays. The geologic characteristics of the study area are shown on Figure 7, Cross - Section of Alluvial Valley Lower Foidel/Middle Creeks Hay Meadows. This cross - section is located downstream of the proposed mining. Only the area near the confluence of Foidel and Middle Creeks, designated as AVF, contains more consistent gravel strata. The thickness and composition of the alluvial sediments was verified by the drilling of numerous alluvial wells and excavation test pits within the study area. The alluvial sediments are saturated and the water table, as measured in the existing monitoring wells along the Foidel Creek drainage area upstream and downstream of the study area, fluctuated during the annual seasons from 3 to 9 feet below the surface. The longwall operations in the EMD were 500 to 1,200 feet below the ground surface and the total thickness of the coal removed was 8.5 to 9.5 feet. In the NMD, the proposed longwall operations will be approximately 700 - 1,600 feet below the ground surface; and the total thickness of the coal removed will be 8.0 to 8.5 feet. Based on experience from the Appalachian and other coalfields, the extent of a zone with increased permeability above the longwall panel will reach approximately 30 to 40 times the thickness of the extracted coal seam. This conclusion is supported by information from the Foidel Creek Mine, where eleven ground water monitoring points were installed in the area of the Alluvial Valley Floor to monitor effects of subsidence. The monitoring wells and piezometers installed in the alluvium indicated no significant changes in water level or water quality. One monitoring well installed in a shallow bedrock sandstone, however, had to be replaced after casing failure and a drop in water level. This was attributed to the casing being pinched off at the interface between sandstone and shale after undermining. This well failure is attributed to near - surface cracks in Subsidence Zones 4 and 5. The geologic conditions of the EMD, NMD, and WMD are very favorable for limiting the impacts of land subsidence on hydrology. The presence of 600 feet of marine shale above the mined coal and the limited extent and fine character of the unconsolidated sediments in the local valleys would reduce any potential for subsidence impacts. The potential for flow interruption in Foidel Creek was previously discussed. The same considerations appear to be true for any near - surface fracturing in Subsidence Zone 5, where additional pore space would fill with water allowing ground water levels or the creek to return to "base" flow conditions. In addition, some sealing of cracks would occur from in- filling with fine grained sediments. PR09 -08 2.05 -148 11/15/99