When Rochelle Young first started working at Syncrude 20 years ago, she heard about plans to turn the west in-pit tailings pond into a lake.
“I remember wondering, ‘How is that actually possible? I was working as an environmental scientist and it was hard to imagine.”
Today, Rochelle works as a Regulatory Affairs Advisor and that tailings pond is now Base Mine Lake, an eight-square-kilometre pit lake that borders Highway 63 to the west as you travel south from the Mildred Lake site.
Syncrude stopped using it as a tailings facility close to 10 years ago – on Dec. 31, 2012. The lake now plays a critical role in Syncrude’s reclaimed landscape as the first commercial-scale demonstration of water-capped tailings technology and the oil sands industry’s first pit lake.
But, why pit lakes? Surface mining results in large pits that must be reclaimed. Some mine pits are filled with mine materials such as overburden or tailings to form a solid surface, which is then reclaimed to forests and wetlands. Other mine pits are reclaimed to lakes.
Due to the nature and timing of mine material movement and use, the overburden that was removed to initiate the pit is reclaimed as soon as practical on Syncrude’s site. This is part of our commitment to progressive reclamation and meeting our goal of minimizing the effect of disturbance.
However, this leaves insufficient material available to fill one or more of the mine pits at the completion of mining. These pits may be partially or fully filled with water (mine and/or fresh water) and reclaimed as a lake (pit lake). Pit lakes reduce the need for material re-handling, which would result in increased emissions from material transportation (such as greenhouse gases or nitrogen oxides), lengthen reclamation timelines and success, and increased costs.
Pit lakes are used at open-pit mine sites around the world, and are a global mining industry best practice for reclamation and closure. Pit lakes support a variety of plants and animals typical of aquatic ecosystems and are integrated into the reclaimed landscape.
“Syncrude has included water-capped fluid tailings in pit lakes since its earliest closure plan designs,” says Rochelle. “The Alberta regulator gave our company approval to have Base Mine Lake as a demonstration to prove that pit lakes are viable.”
Developing a successful pit lake requires planning, monitoring and research to learn and guide lake management.
Pit lake designs and plans have changed over time to reflect the state of knowledge of oil sands mine waters and tailings, technology advances, changing regulations and inputs from local stakeholders and Indigenous communities.
Base Mine Lake’s water capping technology includes filling the empty mine pit with fluid tailings (a mixture of clay, fine solids, water and residual bitumen), and then capping it with water to form a lake. The demonstration intends to show that the water quality will improve over time while the tailings solids remain isolated at the bottom of the lake.
Rochelle, who is working on several tailings projects, explains pit lakes are just one of a suite of reclamation methods that are used today and that like with any reclamation project, there are challenges. However, she believes that this is a challenge that Syncrude will meet.
“Results of the water-capped tailings technology are very positive so far. Syncrude is demonstrating we have physically sequestered the tailings from the water body,” says Rochelle. “We are also showing the water quality is improving over time, which is supporting natural biological communities to develop.”
With roughly 40 years of rigorous research on pit lakes among Canadian oil sands producers, Rochelle is confident and optimistic about Syncrude’s reclamation that includes Base Mine Lake and future pit lakes to come.