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The possibility of decreasing water levels after mining is completed is typically caused by increasing permeability of the water-bearing strata. The decrease of water level in most wells is compensated by an increased well yield. • Therefore, the slight decrease of water levels after mining in some wells does not materially affect the post-mining water availability. An extensive study of ground water inflow into longwall coal panels and of changes of hydraulic conductivities induced by longwall mining was conducted by researchers at the University of Nottingham in England (Singh, 1986; Singh, Hibberd, and Fawcett, 1986; and Whittaker, Singh, and Neate, 1979). Conclusions of their studies are summarized in the following points: The main zone of appreciable change of in-situ permeability was found to lie between the face of the longwall operation and 40 meters (131.2 feet) behind the face: Appreciable in-situ permeability change was observed to occur up to 40 meters (131.2 feet) above the extraction zone; and changes in ground water flow properties of the strata were found to be of stepped characteristics; this was thought to be due to opening and closing of fractures and separations. Studying effects of longwall mining in Japan, Nakajina (1976) found [hat just after mining at the Kushiro Mine, the ratio of vacant space to vertical height in the caved roof was about 30%. Data gathered from boreholes in the gob 2 years after mining, indicated that this ratio had dropped to an average of 6.2%. This indicates that much reconsolidating had taken place and that permeability was greatly reduced over the 2-year period. Conclusions from the referenced studies of the impacts of subsidence above a longwall coal mining operation on hydrology can be summarized in the following way: • 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 pre-mining 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 overburden of the Wadge Coal Seam, denominated as the Wadge Coal Seam Overburden, The Twentymile Sandstone, and the unconsolidated sediments in the Foidel and Middle Creeks' alluvium. Only the Twentymile Sandstone is considered a significant unit. However, the Twentymile Sandstone is separated from the mined coal seam by a 500 to 600 foot-thick layer of marine shale called "Tongue of Lewis Shale." In most situations, this . shale strata acts as a significant aquiclude above the mined coal. APPR®~r~D I=ra o ~ 2000 TR 99-32 2.05-148 1 (/15/99