By Diane Banegas
Special varieties of trees are increasingly the “go-to” tool for cleaning up pollution from closed landfills, dumps, and similar waste sites. Fast growing poplars and willows are the most widely employed but other species can serve, depending on the problem and its location.
“Phytoremediation is about finding the right variety of the right species for the particular contaminant we are addressing,” said Ron Zalesny, a research plant geneticist with the Forest Service’s Northern Research Station. Although the practice of phytoremediation has been around for three decades, Zalesny said the science is still in its infancy; ultimately, he believes the knowledge base will be robust enough that any waste site anywhere in the country will have a green tool ready for it.
“We develop varieties of trees for phytoremediation to offer solutions for environmental problems but also with people in mind,” Zalesny said. “A small village in a rural state isn’t going to have the same kind of budget that a big city does. Also, waste managers from big corporations are always looking for solutions but may not have the luxury to try something promising because they have to pay attention to the bottom line. We want to help all of them.”
Using trees as “green tools” for environmental cleanup costs about 25 to 33 percent of what other remediation technologies would cost and there is much less disturbance to the site. Special trees can easily be planted in densely packed urban areas or in tiny rural villages and everyplace in between.
Poplars and willows are ideal for phytoremediation because they grow quickly and have deep and extensive root systems. They also use water effectively, Zalesny noted, as they have the potential to take up a lot of waste water at water-rich sites but they can also work without a lot of water on water-limited sites. The faster a tree grows and the bigger it grows, the harder it works to take up pollutants from soil and nearby water sources such as surface streams and belowground aquifers. “Bigger trees are like bigger straws, they can suck up more contaminants much faster than smaller, slow-growing trees, Zalesny said.” Planting shade-tolerant hardwood species like oak and maple beneath mature poplars or willows is a way to transition a brownfield into something like a green park as the phytoremediation process winds down, he added.
There are limitations to the benefits of phytoremediation. At some sites there is very little real estate between a landfill and a body of water, so the scientists are limited in how much phytoremediation can be installed. If a site is considered “closed,” meaning it’s been lined and capped to prevent the contents from seeping out, trees cannot be planted on top of the cap or anywhere that their roots would likely breach the vault walls, so to speak. Most closed waste sites pre-date current environmental regulations, therefore the contents are not well known.
The researchers test different varieties of tree species in the greenhouse to discern their affinity for various contaminants before using them on-site. Some species work best for organics such as petroleum hydrocarbons, others take up metals such as cadmium and chromium. Of the more than 100,000 varieties of poplars developed since the 1950s, about 300 can serve as green tools.
Once the trees are planted, the scientists monitor their success with the help of their research partners: the Missouri University of Science and Technology (S&T), the University of Missouri, the University of Novi Sad in Serbia, and Waste Management Corporation. Partners at S&T have patented phytoforensic technologies that use plants as monitoring tools of remediation efforts, plus as a way to delineate the pollutants’ boundaries.
The research partners and clients play an important role in long-term monitoring of green tools as Forest Service scientists can’t always project what their resources will be long into the future due to short-term budget cycles and evolving public policy. “We’ve got near term data,” Zalesny said. “To build the phytoremediation knowledge base into what it needs to be, we need data across several decades.”
The Forest Service is a member agency of the Great Lakes Restoration Initiative, which is led by the U.S. Environmental Protection Agency to protect and restore the largest system of fresh surface water in the world. The initiative has five focus areas that include addressing toxic substances and mitigating pollution impacts on nearshore health.
As part of this initiative, Forest Service scientists are using a tool called “phytorecurrent selection” for choosing plant varieties capable of remediating a broad range of contaminants; or, for choosing varieties that specialize in remediating specific pollutants. In addition, as a component of a more comprehensive green tool, Forest Service scientists and partners are projecting the volume of runoff captured and treated at waste sites.
With funding from the Great Lakes Restoration Initiative, Zalesny and his colleagues planted more than 10,000 trees for runoff reduction and phytoremediation of 10 waste sites in southeastern Wisconsin and Michigan’s Upper Peninsula, and have identified superior varieties for use at five other sites in 2019.