Pit lakes are seen as a solution for two environmental requirements of oil sands mines. The first is to support the full reclamation of all disturbed lands. The second is managing fluid tailings.
A way of dealing with tailings
A by-product of the oil sands mining process is tailings. Oil sands ore contains up to 15 per cent bitumen. The remainder becomes tailings, of which there are two main types. The first consists primarily of sand and is used mainly as a construction material The second, known as fluid tailings, is a byproduct consisting of sand, silt, clay, residual bitumen and water left over following bitumen extraction.
“Managing tailings as a component of reclamation has been a big challenge for oil sands producers,” explains John Brogly, Water Director for Canada’s Oil Sands Innovation Alliance (COSIA), an alliance of producers focused on accelerating the pace of environmental performance through the development of shared technologies.
Left unmanaged, fluid tailings can form a yogurt-textured mixture that could take many decades to fully consolidate (ie. Separate the water from the solids). This can delay reclamation. It’s important to remember that when an oil sands mining operation reaches its end of life, the operator is required to fully reclaim its lease to a state of equivalent capability.
Land is typically reclaimed by refilling excavated areas with treated tailings, then capping with material such as sand or clay and soil. The surface is re-vegetated with native seedlings, trees, shrubs and plants.
Another approach is to convert the mine pit into a lake. Here’s how it works: an operator could fill the base of the mining pit with tailings and other solids s once the ore in the pit has been mined out. Then water, some left over from mining operations and some fresh water, is placed over the tailings mixture—a process called water capping. The tailings are physically isolated under water, where they consolidate over time, forming the lake bottom. Meanwhile, nature takes over within the water and at the surface. Water quality in these lakes will improve over time, allowing natural biological communities to develop. A number of technologies are being developed that could significantly speed up this natural process.
It’s a potentially elegant solution: tailings are reclaimed and an area of the mine is reclaimed to be a lake habitat that can sustain plants and animals and be enjoyed by local residents.
Pit lake advantages
“The concept of water capping for oil sands mines was developed here at Syncrude,” says Carla Wytrykush, an ecologist who works on and researches pit lakes at Syncrude. “We’ve done almost 40 years of research: at the laboratory bench scale and at the pilot scale. We’ve built and studied the technique through a series of test ponds, ranging in size from half of a hectare to four hectares. Now we are studying a full-scale 800-hectare lake called Base Mine Lake”
Wytrykush says all this research means they know a lot about water capping tailings and pit lakes. “Water capping is much less energy intensive than other tailings management technologies. It also allows us to leave behind an aquatic feature in the reclaimed landscape.”
Another advantage of the pit lake approach is that holes in the ground created during the mining process can be stabilized and made part of the overall closure lands by filling them with tailings and water.
Pit lakes in the oil sands are relatively small in geographic size—about five per cent of the total oil sands mine closure landscape. However they can serve an important role in terms of helping to manage water quantity and quality as the land transitions back to a natural state.
A not-so-new innovation
Pit lakes are not new. They’ve been a best practice in conventional mining operations around the world for more than a hundred years—as a way of handling process water and tailings from those operations.
“Pit lakes are a typical conventional mining outcome and a way to manage tailings, and not unusual at all,” says Wytrykush.
Indeed, many successful pit lakes, mostly from past coal mining operations, already exist in Alberta. Some have become popular trophy fishing lakes (Teck’s Sphinx Lake, East Pit Lake, Lovett Lake and Silkstone Lake) and even incorporated into golf courses (Coal Creek Golf Resort).
Another key fact is the pit lakes proposed for oil sands mines are below-grade, making them more safe and secure. “There is soil and rock between any pit lake and river so, geotechnically, it’s a much safer place to put tailings than if they were above grade,” adds Brogly.
Demonstrating pit lakes in the oil sands
While pit lakes are a proven approach in mine closure across the world, oil sands producers have researched them for 40 years with the rigour that typifies Canada’s world-leading environmental standards and regulations.
This includes the aforementioned research conducted by companies such as Syncrude. The research continues today at Syncrude’s 800-hectare commercial demonstration pit lake, Base Mine Lake.
Commissioned in 2012, the lake has made significant progress. “We are kicking off our seventh year of research and monitoring of the lake and we’ve seen water quality has improved,” says Wytrykush.
“Base Mine Lake is the result of the research we’ve done in the laboratory and test ponds over the past 40 years,” Wytrykush says. “In the future, Base Mine Lake is going to be an important part of Syncrude’s reclaimed lease as the boreal landscape is mixture of upland forests, wetlands and lakes.”
Another project that is funded by all oil sands mining members is the pit lake mesocosms at the Innotech Alberta facility in Vegreville. The mesocosms are a much smaller scale than Syncrude’s Base Mine Lake; each mesocosm is only 14 cubic metres in size. However, there are thirty of them. So, what they lack in size they make up for in replication, enabling simultaneous testing of several tailings and water treatment technologies along with control mesocosms (no treatment of any sort) for comparison.
Addressing concerns: science and planning
Critics say that long-term impacts of pit lakes in the oil sands aren’t known. They question, for example, whether the tailings from the bottom of pit lakes will seep into the land.
Concerns have also been raised stemming from examples of pit lakes in the global mining industry that have been poorly managed.
“That’s not how we do things here in Canada’s oil sands,” says Brogly. “If you look at the research that Syncrude and Suncor have done, it includes decades of laboratory studies and field-based test ponds. We are doing the right kind of work and planning to be successful. In fact, these are the most researched pit lakes on earth.”
“We have 40 years of research that shows pit lakes can be effective in oil sands mining,” notes Brogly, “And we are continuing by monitoring larger-scale demonstration pit lakes—so that operators and stakeholders can be confident that oil sands surface mine pits are being reclaimed using the best possible science and technology.”
And what makes the Canada’s oil sands pit lake research so robust and unique is collaboration and interdisciplinary research. Scientists from different companies are working together and sharing their data. Academia is involved too: six different Canadian universities have research programs focused on Syncrude’s Base Mine Lake.
“When you have people working together like that, your answers become much better and much more complete than if you had a bunch of independent work happening,” says Wytrykush.
The promise of pit lakes
As pit lake research and monitoring continues in the oil sands, the findings continue to be promising.
“The lake water mixes like a natural lake,” says Wytrykush of the Base Mine Lake. “The water quality is improving. The water has oxygen in it. There’s algae in it. There’s a variety of insects. And there’s a range of plants that are growing. We’re seeing all of this already, and expect it will only improve with time”
Looking to the future, industry is aware that oil sands mines will likely be long gone in 200 years. But that doesn’t mean there can’t be a lasting positive impact.
Says Brogly: “The people who are still in this northern Alberta region will have assets in pit lakes, which will be water features that function as part of the overall boreal forest ecosystem. Pit lakes also have inherent value in the landscape as part of a mosaic of wetland, forests and lakes. You see all kinds of long-term potential here. We just need to give these lakes time, because nothing this good happens overnight.”