A generic study of strip mining impacts on groundwater resources

Abstract

This report evaluates the influence of strip mining features, commonly found in the Northern Great Plains Coal Region, on ground water hydrology. The features examined are: reclaimed mine geometry, relative transmissivity between the reclaimed spoil and the surrounding unmined coal bed aquifer, anisotropy, the gravity sorted rubble layer, coal wedges left between trench cuts, and the position and size of an operational mine in the regional flow system. A finite element computer model was used to simulate the groundwater flow field from three frames of reference: a local plan view of the mine site (local hydrology), a cross sectional view of flow through the reclaimed mine interior (interior flow), and a cross sectional view of a regional flow system (regional hydrology). The simulation model solved for the piezometric head distribution in each system. For each simulation the piezometric head contours were plotted, and in some cases, the increase in flux induced by the mine properties, and the contact time of water passing through the reclaimed spoil were calculated. Although only the flow of water through the spoil was modeled, water quality effects were inferred through a set of indices dealing with the reclaimed mine size and amount of water passing through it. The effects of a reclaimed mine were studied in the local and interior flow simulations. The influence of an operational mine was examined in the regional simulation. In all cases, the long term impacts were simulated by solving for the steady state condition. Regional location is found to be the most important factor in the influence of an operational mine on groundwater resources. Relative transmissivity is the most important factor in determining the influence of a reclaimed mine. When present, the rubble layer dominates the flow pattern through the mine spoil. The coal wedges are apparently of little hydrologic consequence. Equidimensional mine shapes are preferred to elongated shapes because they induce the least amount of flow through the spoil per unit extracted coal.