Researchers using willows to decontaminate soil, groundwater

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 Universite de Montreal that uses the tall plants to rehabilitate former industrial “brownfields” that are left abandoned because they’re too expen- sive 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 plants’ 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 wastewater and could even replace treatment plants in some smaller towns.

Xavier Lachapelle-trouillard, who recently completed a Masters’ from Polytechnique University, has been working since 2016 on a project that uses a 150-hectare willow marsh to filter wastewater in a small Quebec town.

In Saint-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 wastewater 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.