2009-10-30_PERMIT FILE - C1982056A (5)

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(1983) performed a study of subsidence due to mining in the Pittsburgh seam in Greene County in southwestern Pennsylvania. During his study, he monitored a well located above a longwall panel. The water level in the . monitored well declined during undermining, but returned within 3 feet of the premining level when mining advanced beyond the well. The U.S. Bureau of Mines (Walker, 1988) monitored surface subsidence and water level fluctuations in 10 shallow observation wells above a series of four adjacent longwall panels in southern Pennsylvania for about 4 years. Results of this study indicate that wells are generally unaffected by mining of a preceding panel. The most important conclusion of this study was that nine out of ten wells investigated recovered to their pre- mining water level after mining was completed. Similar observations were made during several studies of longwall mine subsidence impacts on hydrology in Ohio and West Virginia completed by Hydro -Geo Consultants, Inc. (1988 and 1991). In these studies, monitoring of water levels and water quality before, during, and after undermining by longwall operation indicated that impacts due to undermining were noticeable only in wells or surface water bodies within approximately 200 feet of the mined coal. Typically, in wells installed within Zones 2 and 3, water levels declined during undermining and returned to levels somewhat lower than pre- mining. An explanation of the slight decline of the water table after longwall mining was presented in studies where the permeability of the water - bearing strata was measured before and after mining. A study of subsidence effects due to longwall mining on hydrology in Marshal County, West Virginia was published by the U.S. Geological Survey (Schultz, 1988). In this study, ten observation wells underlain by longwall panels were monitored before and after mining. Three of the wells were tested for transmissivity before and after mining. It was found that transmissivity increased substantially after mining in two out of the three wells. The study also concluded that changes in water levels prior to and after mining exceeded ten feet in only two of the ten monitored wells. • The potential for water levels to decrease after mining is completed, typically results from increasing permeability of the water- bearing strata. The decrease of water level in most wells is compensated for by an increased well yield. Therefore, the slight decrease of water levels after mining in some wells does not materially affect postmining 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 that just after mining at the Kushiro Mine, the ratio of vacant space to vertical height in the caved roof was about 30 percent. Data gathered from boreholes in the gob 2 years after mining, indicated that this ratio had dropped to an average of 6.2 percent. 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 as follows: • PR09 -08 2.05 -147 04/27/09