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.
During 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 n 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.
