Samantha Andrews

Marine biologist/ecologist and a science and environmental writer. She can be found talking or writing about our Earth in all its splendour—including the people and other animals who live here —and achieving a more sustainable future.

Rooting out the risks of plant recolonization on mine waste

June 8, 2020 | 2 minute read

If not carefully stored, mining operation waste—finely crushed ore known as tailings—can contaminate the environment. Tailings are typically stored in large pits in the ground. The pits are sealed with covers that block out air to prevent a chemical reaction between tailings and oxygen; a reaction that leaches toxic components into water flowing naturally in and out of pits. Covers come in several flavours, one of which is known as a Cover with Capillary Barrier Effects (CCBE)—multiple layers of carefully selected soils that act as an oxygen barrier.

One of the benefits of CCBEs is that plants in the surrounding area can colonize the soil layers. But, there’s a catch to the restorative abilities of CCBEs: roots make holes in the layers and oxygen seeps in. “We want[ed] to know if the natural evolution of the reclaimed mine site towards a temperate forest could impact the cover performance in the long term,” explained Dr. Marie Guittonny, co-author of a new study in Canadian Geotechnical Journal, led by her PhD student Alex Proteau at the University of Quebec at Abitibi-Témiscamingue.

As it happens, their research shows that plant roots are a mixed blessing for the cover’s performance.

The researchers studied a 17-year-old tailings storage site spanning some 15.5 hectares in Quebec, Canada, which was naturally recolonized by woody plants and vegetation growth managed with a one-time excavation. The researchers took multiple samples of the CCBE, calculating oxygen fluxes and measuring the number of roots and how deep they went.

While the roots did indeed let oxygen into the CCBE, the researchers found the roots also contributed to the cover’s performance.

Plants are famed for consuming carbon dioxide and producing oxygen during photosynthesis, but they also consume oxygen, sucked up by their roots from air trapped inside tiny spaces in the soil. Despite allowing some oxygen to penetrate the CCBE, the roots consumed enough oxygen to result in a net-positive benefit.

The results are promising for CCBE users, though Guittonny notes that that the balance between the benefits of roots and their negative impacts may change over time and differ between sites.

Guittonny offers some advice to those who are considering using a CCBE. “Consider the root characteristics of the plant species surrounding the site to anticipate an appropriate thickness for the protection layer and avoid too much root colonization in the core of the cover,” she said. “If this root colonization remains limited in the core (in intensity and depth), the CCBE should continue to perform well even with vegetation.”

For Guittonny, mixing plant ecologists, mine engineers, and hydrogeologists is key to developing effective covers. “This research is interdisciplinary,” Guittonny explained. “It is not always easy to understand each other across different disciplines. But when we succeed, we get integrated and more complete images of the studied processes that are worth the effort.”

Read the open access paper: Oxygen migration through a cover with capillary barrier effects colonized by roots in Canadian Geotechnical Journal.

Banner image: Aerial view of a coal mine | iStock 

Samantha Andrews

Marine biologist/ecologist and a science and environmental writer. She can be found talking or writing about our Earth in all its splendour—including the people and other animals who live here —and achieving a more sustainable future.