Decant calculations and groundwater: surface water interaction in an opencast coal mining environment

[摘要] English:Acid mine drainage is by far the most significant long term groundwater quality impact associated with both opencast and underground coal mining, in both a local and international context. The modern day geohydrologist has access to numerous tools, which can be used to determine important decant issues �?issues ranging from when decanting will begin to occur, and the volumes of water that are expected to decant. The continuous development and improvement of numerical groundwater flow models is steadily leading to an increasing dependence on them. The main aim of the thesis was to determine whether there exists any correlation between modern day numerical groundwater flow models and analytical calculations, and the presentation of a toolbox of tools that may be used for decant related issues. The following conclusions were drawn after numerous numerical and analytical scenarios and statistical correlations were performed: �?Given the amount of uncertainty regarding aquifer heterogeneity, there do exist a good correlation between the numerical and analytical groundwater decant volume estimations, �?An increase in the effective porosity of the backfilled opencast pits cause an increase in the time-to-decant, as more water is required to fill the pits to their decant elevations, �?An increase in the effective aquifer recharge cause an increase in the decant volumes and a decrease in the time-to-decant, because more water is available to fill the pits to their decant elevations, �?The effective aquifer recharge is a very sensitive parameter (more so than specific yield, storage coefficient, and transmissivity), as significant decreases in the time-to-decant were simulated with an increase in the aquifer recharge, as were significant increases in decant volumes simulated with an increase in recharge, �?The volumes of groundwater decant are more sensitive to variations in the transmissivity of the surrounding aquifer/s compared to the transmissivity of the backfilled opencast pits, �?During the numerous flow model scenarios it was found that the groundwater contribution to pit water is far less compared to the recharge component. The above conclusions prove that there are still applications for analytical calculations in modern day geohydrology, despite the continuous development of numerical groundwater flow models. Based on experience in similar coal mining operations within the Mpumalanga coal fields, the results of both the analytical decant volume and time-to-decant estimations correspond well with actual figures. One must however understand and master the various equations and keep in mind that an aquifer is a highly heterogeneous system. The results of both numerical groundwater flow model simulations and analytical calculations are only as good as the understanding of the geohydrological environment and the data they are based on.

[发布机构] University of the Free State