Trees inoculated with probiotics could clean up America’s contaminated land

In 1980, a federal law identified the most hazardous sites around the United States, those that contained toxic contaminants in urgent need of cleanup. These “Superfund sites,” named for the initial funding allotted to the cleanups, are chiefly old industrial sites contaminated with pollutants from substances such as metals, oils, hydrocarbons, and explosives, and which are dangerous to human health. On top of the 1,327 Superfund sites, there are more than 400,000 Brownfields sites across the country that are not under federal purview but are also perilously contaminated.

The cleanup process for this contaminated land is energy-inefficient and expensive—it often relies on hauling the toxic soil away or mechanically pumping it out of the ground to treat it. Now, a pilot program is testing out a greener and cheaper solution by planting trees to soak up the contaminants, and inoculating those trees with probiotics that make them resilient against the toxins. It’s a twist on an old technology known as phytoremediation, and with the promise that the new innovation will keep the trees healthy, an environmental nonprofit and a startup are collaborating to plant these contaminant-cleaning trees in municipalities nationwide, and get more cities on board with embracing the largely natural solution to a man-made problem.

The U.S. has a “legacy of contamination,” says Kyle Kornack, manager of social innovation and enterprise at the Arbor Day Foundation, a nonprofit that has planted 400 million trees since the 1970s in order to tackle “pervasive global issues” like poverty and climate change. Areas across the country are contaminated for a variety of reasons; most are former industrial facilities, gas stations, military bases, and dry cleaners. The contaminants, which have been found to cause cancer, birth defects, and cognitive problems, are released into the air and can be inhaled, but the majority are leaked into groundwater—the source of drinking water for 51% of the general U.S. population and 99% of the country’s rural population.

Traditionally, contaminants have been cleaned up by tedious, time-consuming methods, including soil excavation, which essentially just takes the hazardous material elsewhere, usually to landfills; and “pump and treat,” whereby groundwater is heaved out of the soil into a treatment system via giant pumps. These methods are incredibly energy-intensive, which makes them expensive. “They’re not really solving it at the root,” Kornack says.

That’s where trees could help, via phytoremediation, which lets “nature do the dirty work,” he says. Essentially, a tree works as a “solar-powered pump,” drawing contaminated groundwater up through its roots, naturally breaking the compounds down into their harmless atom components. For instance, TCE, or trichloroethylene—a carcinogenic industrial solvent primarily used to degrease metals—is broken down into ethylene, which the tree can use as a carbon source, and chlorides, which are released from the roots in the form of innocuous salts.

These “hyperaccumulating” properties of plants have been observed for centuries, but it was only in the final decades of the 20th century that much more research was commissioned into using these attributes as a method of toxic disposal (the term phytoremediation was coined in the early ’90s). The process of using plants to clean up contamination has been around since approximately 1990 but hasn’t taken off on a wide scale, largely because, in many cases, the trees themselves can’t survive once they absorb the nasty chemicals.

New findings are changing that. After noticing some trees that were flourishing in toxic areas including sites of oil and gas spills, Sharon Doty, a plant biochemistry professor at the University of Washington, found that a handful of microbes living inside the trees provided symbiotic benefits. Just like humans need a healthy balance of microorganisms in their microbiomes, so do trees—and, it turns out, those microorganisms can make trees resilient to some toxins.

Doty isolated the microbes that were best-performing in breaking down the compounds, and licensed the use of the strains to Intrinsyx Environmental, a firm that carries out “endophyte-assisted phytoremediation,” referring to the microbes that do the biodegrading, explains Intrinsyx’s chief science officer, John Freeman. When the company plants trees, it inoculates them with these endophytes, either by dunking the roots in a probiotic solution before planting, or by sprinkling probiotic-filled beads in the soil while the tree is a seedling. The company has now planted trees in about 30 sites nationwide, and reports a 95% survival rate within the first year of planting (versus 70% among the non-inoculated). “The endophytes provide an added level of insurance that the trees will survive on the site,” Freeman says.

One such location is the MEW Superfund site in Intrinsyx’s hometown of Mountain View, California, where a nearly 1-square-mile area is infected with “extensive plumes of TCE in the groundwater,” says former mayor Lenny Siegel. (Mountain View is home to a former Navy base and, as “the heart of Silicon Valley,” is a semiconductor manufacturing hub.) Mountain View used the “pump and treat” method from the 1980s up to 2018, when Intrinsyx planted 1,000 poplars—tall, deciduous trees that have 30-foot roots to absorb contaminated water deep in the ground.

Siegel praises the new method for its sustainability and relative affordability to maintain (it’s generally between 50% and 90% cheaper than traditional methods, according to Arbor Day). The contamination levels are monitored periodically to ensure they’re dipping, and so far, Siegel says, the poplars have been effective. Plus, they’re pretty: The trees now line the alleyway along the Google campus in the city, and there are plans to plant some along the freeway as well. In this way, the trees act as part of urban redevelopment.

“We believe the field of phytoremediation is at this breakthrough moment,” Kornack says. “It’s jumped out of the lab, and it’s begun to commercialize.” The Arbor Day Foundation is now hoping it can start implementing phytoremediation at scale; it’s entered into a partnership with Intrinsyx to expand the use around the country. In this pilot program, Arbor Day is offering free assessments to contaminant-affected municipalities to help design bespoke tree-planting strategies. Kornack hopes those assessments spread awareness and help “break through the fog” of phytoremediation and its benefits—both environmental and monetary. Despite the misleading name, financing and planning cleanups of Superfund sites is now the responsibility of cities.

Planting trees, of course, also has extensive environmental and human benefits: They sequester carbon, reduce runoff, and provide a canopy for cooler temperatures. Planting trees in low-income areas—where many Superfund sites are located and where there’s a relative sparsity of trees—could help vulnerable populations deal with oppressive heat. Siegel recalls the eyesores of “big tanks in the middle of parking lots” when his city pumped and treated hazardous sites. Now, rows of poplars do that job while benefiting the public’s mental health and well-being. Quite simply, he says, “Everybody loves trees.”