Healthy trees, clean soil | Roots, bacteria attack soil contaminants and help restore land
An experiment using trees to clean up heavily contaminated soil is working.
Federated Co-operatives Ltd. and the University of Saskatchewan’s soil sciences department have pioneered the process during the past 15 years to restore contaminated petroleum sites to their original state, leaving green spaces in five rural locations in Saskatchewan and Manitoba.
The groves of willows and poplars were planted as part of a process called phytoremediation at former petroleum sites in Sandy Lake, Man., and Colonsay, Young, Kelvington, Hendon and Wadena in Sask-atchewan.
Test results have surpassed original expectations, said Trevor Carlson, FCL’s environmental affairs director.
“We found the contamination was greatly reduced and literally absent in most of the test holes,” he said.
Former petroleum sites date back to the start of the co-operative movement in Saskatchewan and Manitoba. They were the original bulk plants, built to supply diesel and gasoline storage for delivery to farm customers.
Each of the two to three acre sites were designed for heavy road traffic with 15 to 20 centimetres of compacted gravel. Many sites were used for decades.
“Initial site assessment showed substantial contamination at these sites,” said Carlson.
He said FCL has learned through trial and error which trees are most tolerant of the conditions at these sites and how close it can plant them to the sites before they are inhibited.
The Colonsay site was the first test strip. Before planting trees, soil compaction was decreased by tilling with a ripper blade. Then they applied a thin layer of topsoil, which gave juvenile trees something in which to root. Irrigation was necessary for the first year. A layer of bark mulch helped control weed competition.
“These simple little things you would do to grow trees proved to be the critical things that really helped these sites take off,” he said.
“After Year 5, we did some strategic thinning. After that, it was basically let the trees do their thing.”
The trees have sent roots down more than five metres, which is below the depth of contamination. The far-reaching root systems of willows and poplars prevented further spread of the contamination.
They filter more than 10,000 litres of ground water daily, which remediates contaminants through natural metabolic processes and by stimulating micro-organisms that reside in the root zone. There’s no potential for contamination to migrate once the trees start to grow.
As well, trees pull in water, which keeps the contaminants on site. Micro-organisms then have time to break down while not being moved.
Bacteria and fungi will follow these roots as they move into the contaminated soil.
Poplar and willow roots give off simple sugars, called root exudates, as part of their growing process, which stimulate bacterial populations, improving the soil for the tree. Bacterial and fungal populations are also stimulated to degrade the hydrocarbons.
“It’s a situation where the tree acts as a delivery agent to foster the bioremediation,” he said.
FCL found a willing partner in the U of S soil science department, which had already begun a larger phytoremediation project with the Petroleum Technology Alliance of Canada.
“This project really kicked off a relationship and we’ve been able to build on that relationship over the years,” Carlson said.
Fourteen students have graduated on work completed during the project: five PhDs and nine at the master’s level. Ten undergraduate students used the project as part of their honours thesis.
“From our end, it was research,” said Richard Farrell, an associate professor in the department.
“Our goal was to train graduate and undergraduate students and figure out how the process worked and why it worked. One of the things you discover is that nature builds chemicals that look like contaminants,” he said.
“They’re naturally occurring hydrocarbon-like chemicals that are produced by plant roots and by trees and everything else. There are bacteria that naturally degrade these things. So when those bacteria come into contact with a contaminant, they don’t discriminate.”
Years of research have left Farrell in awe of nature’s evolutionary process and how it wastes little.
“The thing that really popped out is how resilient nature and soils are. Phytoremediation is not the fastest process, but it’s a very cost effective method,” he said.