GROUNDWATER LEVEL RESPONSE TO FRACTURING ASSOCIATED WITH COAL MINE SUBSIDENCE AND METHANE GAS EXPLOSION

On November 15, 2015, methane gas exploded in the basement of a household located in southern Wyoming County of West Virginia. The house is situated on the top of a thick Lower Pennsylvanian age sequence of sandstone interlayered with shale, clay, and coal seams. The most prominent coal seams, Pocahontas 6, Pocahontas 4, and Pocahontas 3, are found at depths exceeding 200 m below the ground surface. Longwall mining of the Pocahontas 3 coal seam was conducted directly under the household at the depth of approximately 220 meters below the ground surface. Down-hole video performed in a14.9 m (48.8’) deep residential water well, ten days after the explosion, recorded the process of groundwater level re-equilibration; the groundwater table dropped from the pre-explosion episode static water level, at approximately 2.3 m (7.7’) below the ground surface, to the dynamic (temporal) level at the depth of 9.1 m (29.9’). Numerous fractures crossing diagonally the well walls were noted. Continuous seepages from fractures in the well wall were observed at the depth of 7.2 m (23.6’). Two weeks later, the quasi-static groundwater level was found at the depth of 7.9 m (26’) below the ground surface. This paper postulates that methane gas released during longwall coal mining operations migrated through deep fractures generated by the mine subsidence into the shallow phreatic aquifer and entered the household basement through the shallow residential water well. Upon reaching the flash-point concentration, the methane exploded, opening additional fractures in the shallow rock formations around the water well.