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Trees can do the Dirty Work of Waste Cleanup

Posted by Diane Banegas, Research and Development, USDA Forest Service in Forestry
Jul 29, 2021
David Karlsson (left) and Aleksander Peqini
David Karlsson (left, Sweden) and Aleksander Peqini (Albania) conducting physiological measurements at a landfill in southeastern Wisconsin. Both are international fellows sponsored by the Forest Service and as part of its International Forestry Fellowship Program (IFFP). Photo credit: Ron Zalesny, USDA Forest Service.

When it comes to ridding the earth of pollution leaking from dumps, closed landfills, and other waste sites, specific types of trees are quietly and efficiently absorbing the toxins.

Through a process known as phytoremediation, green plants are used to remove, degrade, or stabilize pollutants and contaminants, such as toxic metals, from soil or groundwater. The practice of using trees as waste cleanup tools has been around for many decades and its early promise as a low-cost alternative to other cleanup methods has borne out.

The Lake Michigan and Lake Superior watersheds map
As part of a Forest Service research study, 20,000 specific tree types, referred to as “special trees” were planted in 16 phytoremediation sites in the Lake Michigan and Lake Superior watersheds, including the 16 landfills featured here. Map developed by Liz Rogers, USDA Forest Service.

So with the science maturing, and the success stories racking up, phytoremediation is becoming the solution of choice for many communities and corporations looking to cleanup polluted waste sites.

For instance in the Great Lakes Region, phytoremediation work is expanding on a massive scale mainly due to word-of-mouth endorsement, according to Ron Zalesny, a research plant geneticist with the Forest Service’s Northern Research Station. Altogether, there are 20,000 trees planted in 16 phytoremediation sites in the Lake Michigan and Lake Superior watersheds.

Poplars and willows
Poplars and willows grown for runoff reduction and phytoremediation. Willows and poplars are ideal for phytoremediation because they grow quickly and have deep and extensive root systems. Photo credit: Ron Zalesny, USDA Forest Service.

The trees are mostly fast growing willows and poplars, which are ideal for phytoremediation because they grow quickly and have deep and extensive root systems. 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.

In fact, the faster and bigger a tree 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,” Ron said. Other species will work as well, depending on the problem and its location. It’s all about finding the right variety of the right species of tree for the problem at hand. Ron and his colleagues are really seeing the results of their life’s work.

“We’re in our fourth year of annual funding from the Great Lakes Restoration Initiative,” Ron said. “We work in partnership with cities, counties and corporations to install the phytoremediation sites.”

In addition to sites created by human waste, Ron sees a growing role for phytoremediation in cleaning up pollution from animal waste produced by pig farms and chicken growers. Waste from these intensive farming practices is monitored closely by federal agencies to protect surrounding real estate and local water sources.

Trees are even playing a role in blighted urban areas to address empty lots and “brown fields” that look unattractive and often serve as a dumping ground for trash. Studies suggest that these criteria go hand-in-hand with local crime.

In these areas, it’s a matter of “greening” not “cleaning,” Ron said. He is working with Forest Service ecologist Richard Hallet to develop hardy, pioneer species of trees planted to mature in phases, so when the fast growing poplars or willows begin to die off, slower-growing oaks and maples will begin maturing.

“They will green the brown fields, offer shade, and work hard to purify the air and the ground,” said Hallet. These are great benefits that trees give us humans.”

Poplars at a landfill in northern Wisconsin
Poplars grown for phytoremediation at a landfill in northern Wisconsin. Photo credit: Ron Zalesny, USDA Forest Service.
Category/Topic: Forestry

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Gary Makus MPH
Sep 04, 2019

Does this mean my use of any trees for food (sap, mushrooms, leaves, tea) will not be as healthy as I might think?
How do I avoid problems. any recommendations.

Jon Heal
Sep 05, 2019

What do you do with the trees once they are full of nickel and dioxins?

Walt Thompson
Oct 23, 2019

You might want to look at the phytoremediation project that the Forest Service has been working on at the Savannah River in SC for over 15 years.

Oct 23, 2019

Does the USFS view this as a permanent solution? i appreciate the views below that question the products that come from the trees, and what happens when they die? the toxins return to the soil, right?
Is seems like what is needed is for us to cease and desist.
Next we need solutions that neutralizes these toxic substances.

Adam Clarke
Apr 29, 2020

Are these trees as healthy as other individuals not planted near a landfill?

What happens to birds or animals that might eat the seeds or leaves of these trees?

Sharon A Fritz
Apr 11, 2021

Are there any lasting negative effects on the tree from absorbing all the toxins?

Try planting beautiful Catalpa trees; they can grow as high as 70 feet with huge spade shaped leaves. It flowers for two weeks in the first half of June. Beautiful.

Jean Nichols
Jun 03, 2021

Which trees are best for cleaning nitrates from the water. Our town is on wells and the nitrate levels are going up and not from crops . We live in Colorado 25 miles ne of Denver. I would like to present something to the board as a solution. RO plant cannot be the only solution.

Susan Shillingford
Jul 31, 2021

Are the tree leaves contaminated and do they affect wildlife health?

Susan Shillingford

Ben Weaver
Aug 09, 2021

@Susan Shillingford - thank you for your comment. Answer provided by Ron Zalesny Jr., supervisory research plant geneticist with the Forest Service’s Northern Research Station:

Trees take up two broad categories of contaminants. The first category contains heavy metals, salts, and other elements/nutrients from the period table of elements. These pollutants are absorbed through the roots and they either remain in the roots or they move to the wood or leaves. These are not broken down but rather are stored or used for tree growth and development. The second category contains human-made contaminants such as petroleum-based chemicals and industrial solvents. While this second category is much more complex from a chemistry perspective, the trees are able to remediate it easier because they create an environment in the root zone where micro-organisms live that “eat” the pollutants or the pollutants enter the roots, wood, and leaves and are broken down into non-dangerous forms. As far as the leaves are concerned, contaminants from this second category are typically broken down and then released through the leaves in a “clean” form that is safe for the environment (from a technical standpoint – the contaminants are volatilized).

It is difficult to answer the question about whether the tree leaves are contaminated and whether they affect wildlife health because it depends on the type of contaminants, what tissues (root, wood, leaves) they end up in, and how the tree breaks them down (based on the two categories above) – which always depends on the particular site. At the Forest Service, we have developed a special green tool for choosing special trees that remediate both categories of contaminants. A substantial component of our research is to match specific tree varieties with the contaminants and where the trees put those contaminants (i.e., root, wood, leaves). Overall, impacts to wildlife health not only depend on the ultimate contaminant levels in the leaves but also the frequency and duration of contact with the leaves. And, obviously, whether the wildlife are eating the leaves. If there is no consumption, there is typically very little concern for health impacts. An example of wildlife eating the leaves would be where people let their cows graze in tree plantings – the cows can eat the ground cover but also the tree leaves. However, I do not know of any example where people grew their trees on contaminated soils and then let their cows eat the leaves. I’m just giving this as an example.

Andres Restrepo
Sep 05, 2021

Very interesting approach.
A great Avenue to produce biomass out of waste.
Any study regarding bioacumulation of toxic compounds ?
I would like to have better information for considering this initiative for landfill reclamation projects