MINE WATER MANAGEMENT
After the dewatering pumps are switched off and when the mines close, the open pits, underground workings and mined voids will gradually fill with surface water or groundwater
MINE CLOSURE: IT’S ALL ABOUT THE WATER
One thing all mines have in common is that, at some point, every single one of them will have to shut down. Water is a particularly important element of mine closure, and mistakes in its management can have devastating consequences. By Kirsten Kelly
D
uring mining operations, water is often dewatered from the mine pits and underground workings. At closure, when the dewatering pumps are switched off, the open pits, underground workings and mined voids may gradually fill with groundwater and in some cases surface water where the water balance is positive. This rewatering can last from several months to many decades. Water that comes into contact with the side walls of an open-pit or underground mine may be contaminated, depending on the mining operations. It may result in salty water, water rich in metals, acidic water, or water that is rich in nitrates (from explosives). Eventually, the water table stabilises, often to levels reached before mining took place. From there, the water could move out of the workings (decant) and overflow/discharge into the environment and even receiving water courses. Decants occur because the mine void and openings connected to it, such as shafts, occur at a variety of elevations. Water will flow into the void
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in higher areas (called recharge areas) and decant at low (discharge) points. There is a head of water pushing the water to the surface. In addition to contamination risks, decants also pose erosion risks and can cause instability of the mine itself. Mine waste storage facilities are another cause for concern. At the time of closure, many mines have tailings, waste rock, low-grade ore stockpiles and/or slags. Rainwater falling on this mine waste percolates and mobilises contaminants, which can be toxic and corrosive. This then joins and pollutes surface water, groundwater or both. Cover systems are used over the waste material to reduce the percolation of rain into the mine waste, which reduces effluent seepage volumes. Backfill in pits often has the same effect as covers on residues, in that backfill can reduce infiltration to workings, as long as the backfill is geochemically inert.
Other solutions
“Simplistically put, most mine closure strategies revolve around separating
James Lake, principal environmental scientist, SRK Consulting
the ‘dirty/contact water’ that has come into contact with mine residue from the ‘clean/non-contact water’ and keeping the latter clean,” says James Lake, principal environmental scientist, SRK Consulting. The existing legal framework in South Africa is such that the environmental responsibilities and financial implications of mine closure remain with the mine until a closure certificate is issued by the Department of Mineral Resources and Energy, upon which the environmental liabilities can be transferred to a competent person. However, a number of large underground mines, particularly within the Witwatersrand Basin, are interconnected. “Therefore, it is very difficult to work out who is responsible for what. Furthermore, acid mine drainage (AMD) is the result of a century and more of environmental damage, and hundreds of mining companies have long closed – so which companies must be held accountable for centuries of pollution? Mines in many affected areas are no longer
