Response of surface springs to longwall coal mining Wasatch Plateau, Utah.
Proceedings of the International Land Reclamation and Mine Drainage Conference and Third International Conference on the Abatement of Acidic Drainage April 24-29, 1994, Pittsburgh, Pennsylvania. Pittsburgh, PA: United States Department of the Interior, Bureau of Mines, Special Publication SP 06C-94, 1994 Apr; :345-351
High-extraction longwall coal mining creates zones in the overburden where strata bend, fracture, or cave into the mine void. These physical alterations to the overburden stratigraphy have associated effects on the hydrologic regime. The U.S. Bureau of Mines (USBM) studied impacts to the local hydrologic system caused by longwall mining in the Wasatch Plateau, Utah. Surface springs in the vicinity of two coal mines were evaluated for alterations in flow characteristics as mining progressed. Fourteen springs located above the mines were included in the study. Eight of the springs were located over longwall panels, four were located over barrier pillars and mains, and two were located outside the area disturbed by mining. Flow hydrographs for each spring were compared to climatic data and time of undermining to assess if mining in the vicinity had influenced flow. Heights of fracturing and caving in the overburden resulting from seam extraction were calculated using common subsidence formulas, and used in conjunction with elevations of springs to assess if fracturing influenced the water-bearing zones studied. One spring over a panel exhibited a departure from a normally-shaped hydrograph after being undermined. Springs located over other mine structures, or outside the mine area did not show discernible effects from mining. The limited response of the springs was attributed to site-specific conditions that buffered mining impacts including the elevation of the springs above the mine level, and presence of massive sandstones and swelling clays in the overburden materials. Additional Key Words: underground coal mining, hydrology Introduction Longwall mining involves extracting coal in large blocks called panels using a mechanized shearer. With this method, the mine roof is supported with hydraulic supports that automatically advance as mining progresses. As the supports move, the mine roof is allowed to collapse into the mine void. Strata above the mine level are altered as the mine roof caves behind the shields, creating zones where blocks of rock fill the mine void, or fracture or deform as rock layers warp downward. Recognizing that these alterations in overburden characteristics produce associated changes in the hydrologic system, a study was undertaken to observe and quantify the response of water resources contained in the overburden to mining. Alterations to the hydrologic system resulting from longwall mining can include (1) increased permeability of the rock mass from fracturing and bed separation, (2) flow loss into mining-induced or dilated fractures or planes of bed separation, and (3) redirection of flow and hydraulic gradients. Studies in the Appalachian and Illinois coal regions indicate that rapid hydrologic response occurs in conjunction with the rapid subsidence induced by longwall mining (Coe and Stowe 1984, Trevits and Matetic 1991). In these studies, dramatic decreases in water levels in wells and flow from springs were observed as the longwall face passed underneath and was followed by subsequent total or partial recovery within months after mining. Hydrologic response to high-extraction coal mining has become an issue of concern in both Eastern and Western coalfields. Previous studies have examined the effects of longwall mining on aquifers in the Eastern U.S., but few have focused on western U.S. coalfields where the aridity of the region generates concern for the availability of surface and ground water resources. Public sentiment targets coal mining as threat to the available water resources.
Author Keywords: underground coal mining; hydrology
Proceedings of the International Land Reclamation and Mine Drainage Conference and Third International Conference on the Abatement of Acidic Drainage April 24-29, 1994, Pittsburgh, Pennsylvania