Quantification and management of intermine flow in the western Witbank coalfield: implication for mine water volumes and quality
English: The collieries in the Western Witbank Coalfield have geometries such that there are several areas where this intermine flow is possible. Identification of intermine flow areas in the Witbank Coalfield was previously done by Grobbelaar, (2001). Research into the identification, quantification and impact assessment of the intermine flow on the groundwater- and surface water quality of the Western Witbank Coalfield was undertaken as part of broader research initiatives sponsored by COALTECH 2020. The larger project aims to provide water management strategies for optimal water quality and quantities in South Africa's coalfields. The focus in this thesis is on the Western Witbank coalfield. The main aims were to quantify the intermine flow in this study area and determine the impacts on regional water qualities and quantities. The impact of different management options was evaluated. The research showed that the paleogeologic conditions of coal deposition and formation can explain currently observed hydrochemical and hydrogeological phenomena. Numerical flow modeling and analytical or empiric approaches were used to quantify flow directions, flow volumes and filling times. This research showed that numerical models such as Modflow can be fruitfully used to understand the hydrologic interactions that occur in typical intermine flow areas Comparison between the approaches yielded similar answers in several cases, but numerical models allow the evaluation of changing conditions and resolve complex situations satisfactorily. This thesis has highlighted certain instances where the answers obtained must be used with circumspection. A specific example dealt with, occurs in some underground mining situations which are overlain by saturated media. Due to moving boundary conditions in such situations conventional models can produce erroneous results. Refilling times for underground models using saturated flow models, should be checked for consistency using volumetric/inflow calculations. Comparison of mass transport approaches and mixing cell approaches using a geochemical model provided similar results, with mass transport more insightful in intermine flow assessment due to the spatial variation in concentration change. By using PHREEQC to evaluate almost 2000 samples at two collieries, it is clear that sulphate concentrations are often limited by the saturation of gypsum. An upper limit of around 3000 mg/I is suggested by these evaluations. Intermine flows have been shown to vary considerably in the area with values ranging from more than 1.5 Mild (Wolwekrans/Kleinkopje and Greenside/Kleinkopje) to less than 10 m3/d (Kriel /Tavistock). Evaluation of the situation around the Rietspruit pit showed that due to the immense perimeter and thin unmined barrier, significant flows are expected into the adjacent underground workings. This leakage results in very long projected fill up times of more than 200 years for N3 pit. An evaluation of management options has shown that an inter-colliery option of piping the decant from Kriel to Tavistock's 4 Seam workings will have mutually beneficial results. Among the other options investigated, it was illustrated that impermeable barriers will not be effective in preventing flow between Rietpsruit and the adjacent underground workings. From the research done recommendations for future work include more detailed water balances at the collieries, site - specific investigations into critical considerations such as aquifer parameters, effective recharge and localized intermine management plans.
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