Evaluation of management options for intermine flow and associated impacts in the Central Witbank Coalfield

English: Research into the identification, quantification and impact assessment of the intermine flow on the groundwater and surface water quality of the Witbank and Highveld coalfields was undertaken as part of broader research initiatives The study area of this thesis included the coal collieries of the Central Witbank Coalfield. Intermine flow areas were previously identified in studies by Grobbelaar et al., 2001 and Grobbelaar, 2001. The aim of this thesis was to develop a quantitative prediction of long-term intermine flow in the central Witbank coalfield, using the available data collected in the project. From the quantification of these flows and different hydrochemical techniques, likely water quality profiles at these collieries were determined. The quantification on the intermine flow direction as well as the flow volume was predicted through numerical groundwater modelling using Modflow. Local and regional models were compared and it was found that regional models (relative to including the whole extend of the voids in the model area) yielded the most accurate answer of the two models in the study area. The quantification of the predicted impact on water qualities was done through the use of mass balance approximations and an empirical sulphate generation rate method (developed in this study for predicting future sulphate concentrations in underground workings) using available hydrochemical and acid base accounting data. It was calculated that the sulphate generation rate for the underground workings is in the order of 0.1 - 1.5 kg/ha/d depending on the degree to which the void is filled. An evaluation of water management strategies, which will minimise the long-term influence of intermine flow on the groundwater and surface water quality, was also done. A compartmentalization system to mining opencast pits has been suggested for opencast pits with low decant elevations, to inundate a maximum percentage of the spoils to limit AMD reactions. The implementation of artificially created, high recharge areas on opencast pits and underground workings also was investigated to reduce fill-up times of voids as to limit the exposure of oxygen to the acid generating materials. The research indicates that intermine flow can be potentially detrimental to the groundwater- and surface water systems depending on the flow volumes and quality of the intermine flow water in the Central Witbank Coalfield. Flow volumes predicted through numerical modelling was quantified to be considerable enough to be potentially detrimental to mining voids into which the intermine flow water flows. It was also found that the intermine flow could influence the fill up times of the mining voids (both opencast pits and underground workings). The evaluation of different management options identified viable alternatives to manage intermine flow in such a way as to yield future optimal water qualities for the Central Witbank Coalfield.