Investigation of water decant from underground collieries in Mpumalanga

English: This investigation was intended: 1. To investigate and describe the status quo in terms of mining methods, scheduling, geology, geohydrology, hydrochemistry, water and salt balances at six underground collieries that are in the process of decanting or where decanting is imminent. 2. To investigate management options whereby the quality of the mine water can be influenced in operating underground collieries, thus minimising the long-term salt load that will be released into the environment. 3. To document the six case histories to be used for future reference, demonstrating the time-dependency of these systems. 4. To extract and discuss management options that should be applied in operating collieries to achieve the long-term goal of minimising the salt load to the environment. Six collieries where mining has ceased have been selected for detailed geohydrological and hydrochemical investigation. The scope is to provide sufficient time-related information with which the industry and authorities can associate to promote standardisation of water management methodologies. The mines selected for this investigation are: This investigation was intended: 1. To investigate and describe the status quo in terms of mining methods, scheduling, geology, geohydrology, hydrochemistry, water and salt balances at six underground collieries that are in the process of decanting or where decanting is imminent. 2. To investigate management options whereby the quality of the mine water can be influenced in operating underground collieries, thus minimising the long-term salt load that will be released into the environment. 3. To document the six case histories to be used for future reference, demonstrating the time-dependency of these systems. 4. To extract and discuss management options that should be applied in operating collieries to achieve the long-term goal of minimising the salt load to the environment. Six collieries where mining has ceased have been selected for detailed geohydrological and hydrochemical investigation. The scope is to provide sufficient time-related information with which the industry and authorities can associate to promote standardisation of water management methodologies. The mines selected for this investigation are: • Minnaar -Small underground mine; compartments artificial recharge; mine water irrigation;- Bord-and-pillar • Ermelo - Large underground mine, partially stooped; in Ermelo the process of filling up with water; flushing option considered.- Bord-and-pillar stooping • TNC – Complex arrangement of underground and opencast mining; partially filled with water; water quality management is possible through mixing – Bord-and pillar stooping opencast • New Largo – underground mine with very little subsidence; water balance calculations and seepage losses – Bord-and-pillar limited stooping • Schoongezicht – Underground mine; currently decanting; water and salt balance studies – Bord-and-pillar opencast • Kromdraai – Underground mine currently being reworked by opencast methods; impact of change in mining method – Bord-and pillar secondary opencast The main conclusions are: • The mining method and associated mining geometry dictates the end result in terms of water influx and water quality. • On a regional scale, there is no significant variation in the geology, mineralogy and geohydrology of the collieries in Mpumalanga (Pinetown, 2003). • Water balances are of overriding importance in determining areas of recharge, water loss and reaction rates. These vary from mine to mine. Overriding factors are the method of mining, depth of mining and the surface hydrology. High extraction methods invariably disturb the overlying strata more than bord-and-pillar methods. A summary of the percentage influx to be expected for the various mining methods is as follows: • Shallow bord-and-pillar 5-10% of the rainfall. • Deep bord-and-pillar with no subsidence - 1% of the rainfall. • Stooping 4 - 12% of the rainfall. • Longwall 6 - 15% of the rainfall. • Opencast 14- 20% of the rainfall. The longwall mining value (Hodgson and Krantz, 1998) was not covered in this investigation, but is included for reference purposes. e Daily sulphate production rates range from 0.4 - 2.7 kg/ha in the mines. Sulphate constitutes about 50% of the salt load and concentrations typically ranging from 400 - 2 000 mg/L. • Flooding of the mines should minimise oxygen ingress and sulphate production will decline under alkaline conditions. Four of the six collieries currently have alkaline water. Long-term projections indicate that three of these collieries should remain alkaline. • It is suggested in this document that more should be done in terms of planning mine-water handling, during and after mining. Projections indicate that flushing is a viable option to reduce salinity in the mine water. Mine water from the northern collieries is low in sodium content, which makes it usable for the irrigation of crop. In the southern part, the Mpumalanga Collieries generally have high sodium concentrations, which limits the possible applications of this water. • Management options for mine water during and after mining can be reduced to a few simple concepts. Endless branching into more sophisticated, sometimes more complex, procedures is possible. The following is a list of simple management options that include all the important issues: • Select the mining method based on environmental considerations. • Mine from low-lying areas to high ground. • Flood the mine workings as soon as possible. • Flush the mines after being flooded.