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

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 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 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 operational, making it difficult to enforce compliance,” adds Lake.

He believes that it is best to take a catchment-level approach to tackling potential water issues when dealing with a mine closure.

“Mines need to consider water stewardship for the mine’s entire life cycle. The first step is defining and understanding what problems mine closure may bring. This requires a substantial amount of investigative work.”

There are a huge range of treatment options; some include neutralising agents for acidity and high-tech solutions like reverse osmosis and ion exchange for salts.

“Water treatment often generates a waste product, creating a further waste management problem, so it is not always the best and only answer when tackling water issues and mine closures,” says Lake.

Passive treatment

Passive water treatment methods can be used. They are based on natural chemical and biological reactions with little or no nutrient and energy addition. They use mainly naturally available energy sources such as microbial metabolic energy, photosynthesis and chemical energy.

In addition, the nutrients needed in passive treatment are commonly available in nature and only compost and/or limestone addition may be needed. Since passive treatments are self-sustaining processes, they require infrequent maintenance, and function well with minimal human interference.

“Over the past few years, I’ve been pleased to witness a much more responsible attitude in mining. Mines allocate substantial resources into understanding and managing their risks. Mining companies typically allocate mine-closure-related key performance indicators to their management team. They also undertake studies needed to create a closure plan and regularly update closure plans to changes within the environment and regulations. The proactive management of environmental impacts is required from the outset of mining activities and the word ‘closure’ is now used from greenfield projects. Early mine closure planning and concurrent rehabilitation during the operation of the mine can save a lot of time, effort and money,” concludes Lake.

SRK Consulting considers the entire life cycle of a mine. The firm offers a wide range of mine closure services that include:

• Closure plan and concept development, assessment and management

• Environmental and social impact assessments, and health and safety and economic studies, including mitigation planning, as well as community and government stakeholder engagement

• Waste characterisation and geochemical assessment

• Seismic assessments

• Earthworks design and stability assessments, such as landform engineering for waste dump and tailings storage facilities

• Soil cover and reclamation design, as well as sediment and erosion control

• Hydrologic assessments, including water quality and acid rock drainage, and water treatment system design

• Revegetation management