Simulation of changes in ground-water levels associated with strip mining

Coal strip mining can influence ground-water levels both spatially and in time in the vicinity of a mine. The magnitude, extent, and timing of such changes are not, however, well known. Through the use of a finite element model that was developed specifically to study these problems, it is possible to solve for the hydraulic-head distribution within a two-dimensional region and for the position of the water table and seepage faces in a mined area as a function of time. Sensitivity analyses conducted with the model show that strip mining not only influences water levels during the mining and early post-mining period but also show that the most significant effects are observed during the re-establishment of a steady-state flow system following mining. The magnitude and extent of changes in ground-water levels are controlled by at least four important hydrogeological parameters: (1) the presence of local discharge areas or sources of recharge, (2) the character of the post-mining landscape, (3) changes in the rate of recharge through the spoil, and (4) changes in the hydraulic conductivity of the spoil relative to the undisturbed geologic materials. The timing of water-level changes depends upon features of the flow system, such as how efficiently water is able to move to the low in hydraulic potential, caused by mining, and also upon the storativity of the mine spoil. All of the simulations indicate that long times are required for the flow system to re-establish steady-state conditions following mining. Such results imply that the maximum impact of mining at some sites may not be felt until many years following the cessation of mining.