Post-mining Period Research Articles

Use of mining-contaminated sediment tracers to investigate the timing and rates of historical flood plain sedimentation

Changes in land use practices following European settlement in the 1830s produced accelerated sedimentation on virtually all valley floors in the Blue River Watershed, Wisconsin. The contamination of sediments by Pb and Zn mining allowed us to calculate cross-valley rates of flood plain sedimentation for three time periods: the pre-mining period (1830–1900), the mining period (1900–1920), and the post-mining period (1920–1997). Most of the eight valley floors examined contained multiple presettlement surfaces. Significantly higher rates of sedimentation occurred on the lower flood plain surfaces, while the terraces were high enough to prevent sedimentation from most floods. Higher rates of sedimentation on the lower surfaces eventually reduced valley floor relief and, consequently, lateral differences in sedimentation rates. Tributaries and larger valleys downstream exhibited differences in the timing and rates of historical flood plain sedimentation. While rates of sedimentation were high during the pre-mining period in tributary valleys, the lower valleys were receiving little or no pre-mining alluvium. Little pre-mining alluvium was found in mid-basin reaches, suggesting that most of the pre-mining sedimentation was limited to headwater locations. During the mining period, lateral channel migration and the development of meander belts increased the conveyance capacity of tributary and mid-basin channels, which decreased overbank flooding and produced lower rates of sedimentation during the post-mining period. The meander belt channels also had an effect on the lower portion of the watershed by increasing flood magnitudes and the transport of sediment downstream, thereby shifting the locus of sedimentation from the tributaries to the larger, lower valleys. Sedimentation rates in the largest, most downstream site were an order of magnitude higher during the post-mining period than any of the sites upstream.

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.