It’s long been known that plants are excellent at taking up pollutants through their roots. This process is called phytoremediation.
And while many plants can do this work, poplars are especially well suited for the job. Their fast growth and well-developed root system is capable of taking up large amounts of groundwater, along with contaminants present.
“Poplars act as ‘water pumps’,” explained Andrej Pilipovic, manager of NRRI’s Hybrid Poplar Research Program. “They remediate soils and prevent the spread of contaminants through groundwater.”
And NRRI is putting this tree “super power” to the test with a demonstration of around 300 fast-growing hybrid poplars at a site near the moderately sulfate-laden St. James Pit in Aurora, Minnesota. The hybrid trees will be irrigated with the lake’s water and evaluated for sulfur uptake.
The five-year project is partnering with the City of Aurora, with funding from the U.S. Forest Service.
Sulfate is the most abundant form of dissolved sulfur on earth, though it is rare in many freshwater environments. When sulfate is introduced to originally low-sulfate and poorly oxygenated waters, it can produce toxic methylated mercury and hydrogen sulfide. Natural sulfate sources exist, but elevated levels in freshwater ecosystems often come from human sources, like wastewater treatment plants, industrial waste streams, agricultural runoff, or mine site runoff.
NRRI, with project partners at the U.S. Forest Service Northern Research Station, will test specially selected hybrid poplars for their ability to efficiently and cost effectively reduce sulfate levels with water management.
"Sulfate is an essential nutrient for plants, and we want to understand how much sulfate these hybrid poplars absorb, where they store it within the tree, and how we can optimize their performance," said Chan Lan Chun, NRRI environmental engineer and project co-leader. "This is exciting, collaborative research that can have many ecological benefits, not to mention many trees for people to enjoy, too."
The water in St. James Pit Lake is clear, with low vegetation growth. It’s a popular trout and whitefish lake, and presently serves as the city’s drinking water source. Due to the geology surrounding the pit lake, formed by iron mining activities, the moderate sulfate levels have the potential to influence a sensitive freshwater ecosystem downstream, including a wild rice wetland.
The trees planted at the site were selected using Phyto-Recurrent Selection, a process developed by the USDA Forest Service that was endorsed as a “Good Practice” by the United Nations Decade on Ecosystem Restoration Task Force on Best Practices in 2023. Specific genotypes were chosen based on their survival, growth, and physiology when irrigated with high sulfate waters during greenhouse selection cycles.
To learn more about NRRI’s fast-growing trees visit: innovatree.umn.edu.
PHOTO TOP: Chan Lan Chun and Andrej Pilipovic provide water from St. James Pit lake to planted hybrid poplar cuttings in early June 2025.