Researchers use willows to decontaminate soil
In an east-end Montreal neighbourhood, a polluted piece of former industrial land has become a garden.
Willows sway in the breeze, creating a pleasant green space as the plants slowly reverse decades of industrial activity that has left the chemical-soaked soil of the Pointe-aux-Trembles site too contaminated to use.
The trees are part of a four-year natural decontamination project by the city and the Université de Montréal that uses the tall trees to rehabilitate former industrial “brownfields” that are left abandoned because they’re too expensive to redevelop.
Michel Labrecque, the head of research at the Montreal Botanical Garden and a biological sciences professor, said willows are “an excellent material” for decontamination because they ’re hardy, fast-growing, can survive Quebec winters and thrive in even the most polluted soil.
“I put willows in soil where there was some pollution and saw they were performing quite well, even when the soil was not only polluted by contaminants, but also poor and with a lack of organic materials,” he said.
As they grow, they absorb the pollutants in the soil, while their roots produce micro-organisms that further break down the chemical compounds that can’t be absorbed.
The contaminants become concentrated in the trees’ leaves and branches, which are then cut off and usually burned.
Labrecque said the method is favourable to traditional “dig and dump” methods of decontamination, which involve excavating and removing the polluted soil without removing any chemicals.
“(Excavating) doesn’t really address the problem, just moves it from one site to another,” he said.
“By using plants, we don’t have to deplace any soil or replace any soil, and you’re left with just a few bags of contaminants.”
Researchers have found that willows also have the potential to decontaminate waste water and could even replace treatment plants in some smaller towns.
Xavier Lachapelle-Trouillard, who recently completed a Masters from École Polytechnique, has been working since 2016 on a project that uses a 150-hectare willow marsh to filter waste water in a small Quebec town.
In St-Roch-de-l’Achigan, an hour north of Montreal, water from the town’s sewers are strained and fed directly to the plants, whose roots acts as natural purifiers, he said.
By absorbing the excess nitrogen, phosphorus and other materials, the fast-growing plants have the potential to treat waste water for towns of up to 5,000 people in an environmentally friendly way.
“We’re taking something that is basically garbage, and re-valorizing it,” he said in a phone interview.
While plant filtration technology is common in Europe, Lachapelle-Trouillard said North American scientists are still trying to make sure it will work in below-freezing winter conditions.
Researchers are playing with hydraulic systems so the swamp doesn’t freeze and the bacteria stays alive, and he noted the results appear promising.
Lachapelle-Trouillard said it takes between nine and 12 willows per citizen to filter waste water, or up to 100 willows per person to absorb the water completely.
Labrecque and Lachapelle-Trouillard said that using willows to treat soil or water has the advantage of being environmentally friendly, low-maintenance and much cheaper than traditional methods.
But they can take much longer to work and use much more space.
For that reason, both researchers said their work is focused on finding other commercial uses for the plants in the meantime, such as burning them for energy or transplanting cuttings to make sound barriers or green walls.
Labrecque said if the project in Montreal’s East End is allowed to continue, the city-owned land should be ready for new building after a period between five and 15 years.
The question, he said, is whether the citizens will want to give up their green oasis.
“The citizens really like it the way it is, so a challenge we face is that people like it as a green zone,” he said.