The mining industry has made significant progress at improving mine closure technologies. In Canada, as in most jurisdictions, a closure plan is now required as part of a mine development application, and progressive reclamation and closure must now be achieved at rates acceptable to regulators. Closure landforms and drainage systems should be designed to handle extreme events. Acid Rock Drainage (ARD) must be controlled and mitigated. But there is still more work to do before mine closure practices can truly be called sustainable.
Many miners still gravitate toward an optimal “benefit/cost” solution that involves dams containing water impoundments in the closure landscape. These structures require perpetual monitoring, so that any deterioration can be remedied, and the impact of extreme events corrected – and this presents an enormous problem. The problem with perpetual maintenance schemes, particularly those with dams, is that perpetual maintenance means forever.
The idea of perpetual maintenance is based on the assumption that the security for funding maintenance is safe for hundreds and thousands of years, irrespective of political upheaval, economic collapse and human error. How likely is that? To me, it is inconceivable that perpetual maintenance, supported by some sort of security in a bank, has any hope of preventing a dam from failing in the far future.
What gives us the right to expect future generations to maintain thousands of closed mines that are vulnerable to failure with catastrophic consequences? Will any provision to protect vulnerable closed mines be heeded or remembered after a thousand, or even a hundred years?
I believe the mining industry should adopt solutions that provide alternatives to permanent water impoundments contained by dams. However, I realize that this is a controversial point, and I need to stress that this is my own opinion, not that of the company I work for.
In some jurisdictions, the mining industry has already progressed a long way towards maintenance-free closure. The government of Alberta requires all mine closure plans to target maintenance-free performance, after transitional monitoring. Dams containing fluid impoundments are not allowed in the mine closure landscape.
This regulation has forced the decommissioning of several coal mines in Alberta; others are in the process of being closed. We see similar compliance in Alberta’s oil sands mines, where a dry landscape, now hosting diverse wildlife, has already replaced Suncor’s Pond 1.
More improvements are resulting from technology advances, stakeholder participation and information-sharing among mine operators:
Treatment of acid producing waste rock: Technologies are improving for separating out the most acid-generating waste rock to enable relocation back into the pit after mining. With this approach, the pit lake will provide a cover without a dam for containment. Paste tailings can be used to control ARD, as demonstrated at Osisko’s Canadian Malartic Mine. Paste/rock is another technology that reduces the impact of ARD without creating permanent tailings ponds. It involves mixing coarse rock with fine tailings to create a saturated deposit that minimizes ARD. A decade-long research project at the Copper Cliff mine in Sudbury demonstrates the successful application of this new technology.
Geomorphic landscapes: Historically, many post-closure landscapes were designed with straight lines and uniform slopes. This makes them vulnerable to erosion and gullying, which releases sediment and contaminants into natural watercourses. Such features require ongoing maintenance. Fortunately, there is a growing interest in designing geomorphic landscapes with variable topography that mimics nearby natural landforms and watercourses with configurations that match nearby natural streams. Good examples of this technology are illustrated at TransAlta’s Whitewood Mine in Alberta and at the Centralia Mine in Washington State.
Walk-away closure may be an elusive goal, and one that is not yet fully tested by the vagaries of nature. But we must not yield to the “optimal solution” of benefit/cost modelling if we are to avoid unfairly burdening future generations with closed mines that cannot endure the tests of time.
Les Sawatsky is principal, director engineering division, Golder Associates, Calgary, AB. Contact: 1.403.299.5600; email@example.com