Quantification and management of intermine flow in the western Witbank coalfield: implication for mine water volumes and quality

[摘要] English: The collieries in the Western Witbank Coalfield have geometries such that there are several areaswhere this intermine flow is possible. Identification of intermine flow areas in the Witbank Coalfieldwas previously done by Grobbelaar, (2001). Research into the identification, quantification andimpact assessment of the intermine flow on the groundwater- and surface water quality of theWestern Witbank Coalfield was undertaken as part of broader research initiatives sponsored byCOALTECH 2020. The larger project aims to provide water management strategies for optimalwater quality and quantities in South Africa's coalfields. The focus in this thesis is on the WesternWitbank coalfield. The main aims were to quantify the intermine flow in this study area anddetermine the impacts on regional water qualities and quantities. The impact of differentmanagement options was evaluated.The research showed that the paleogeologic conditions of coal deposition and formation can explaincurrently 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 canbe fruitfully used to understand the hydrologic interactions that occur in typical intermine flow areasComparison between the approaches yielded similar answers in several cases, but numericalmodels allow the evaluation of changing conditions and resolve complex situations satisfactorily.This thesis has highlighted certain instances where the answers obtained must be used withcircumspection. A specific example dealt with, occurs in some underground mining situations whichare overlain by saturated media. Due to moving boundary conditions in such situations conventionalmodels can produce erroneous results. Refilling times for underground models using saturated flowmodels, should be checked for consistency using volumetric/inflow calculations.Comparison of mass transport approaches and mixing cell approaches using a geochemical modelprovided similar results, with mass transport more insightful in intermine flow assessment due to thespatial variation in concentration change. By using PHREEQC to evaluate almost 2000 samples attwo 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 morethan 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 immenseperimeter and thin unmined barrier, significant flows are expected into the adjacent undergroundworkings. 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 decantfrom Kriel to Tavistock's 4 Seam workings will have mutually beneficial results. Among the otheroptions investigated, it was illustrated that impermeable barriers will not be effective in preventingflow between Rietpsruit and the adjacent underground workings. From the research donerecommendations for future work include more detailed water balances at the collieries, site -specific investigations into critical considerations such as aquifer parameters, effective recharge andlocalized intermine management plans.

[发布机构] University of the Free State