An hour’s drive south of San Francisco, a stand of several hundred poplars grows in a Y-shape — a rather unusual sight wedged between two baseball fields. The trees were planted in 2013 to suck carcinogens out of a 1,500-acre Superfund site contaminated by the U.S. Navy, which disposed of toxic waste generated from developing military aircraft into ponds and landfills.
The Naval Air Station at Moffett Field is one of more than 1,000 Superfund sites in the U.S., the legacy of decades of industrial pollution. Cleanup of these sites is expensive, often owing to the specialized machinery and tools needed to excavate and dredge the land. And the moving of contaminated soil to landfills or the pumping and filtering of systems used to decontaminate water can themselves be disruptive to the environment.
But just by living and continuing to grow, the poplars, in Mountain View, Calif., can slurp up about 50 gallons of toxic water a day and break it down into innocuous byproducts such as carbon dioxide and chloride.
The poplars are part of a wave of advances in phytoremediation, the process of using plants to clean up toxic soil or water. They arise from work by Sharon Doty, a plant microbiologist at the University of Washington, who identified the microbes that naturally colonized poplars. She then licensed those strains of microbes to Intrinsyx Environmental, which gave the poplars in Mountain View a boost to enable the trees to survive and even thrive in a toxic landscape.
Intrinsyx Environmental and Phytoremediation and Phytomining Consultants United have since applied the same method at more than 20 polluted groundwater sites across the country in Texas, Kentucky, New York and the Midwest.
The microbe-boosting method “will prove to be a very useful green biotechnology with wide application in the very near future,” said Guy Lanza, an environmental biologist at the State University of New York in Syracuse who is not involved with the companies.
Phytoremediation of toxic soil and water dates back to the 1970s. As researchers with the U.S. Department of Agriculture began visiting farm plots ravaged by fertilizers, they noticed that some plants, such as soybeans, survived.
“That told us right away that plants are not just these immovable creatures,” said James Landmeyer, a groundwater geochemist with the U.S. Geological Survey and the author of “Introduction to Phytoremediation of Contaminated Groundwater.”
Initially, researchers called these outliers “volunteer plants.” In the right circumstances, Dr. Landmeyer said, “the plants can act as a vacuum cleaner to concentrate the contaminants.”
Still, Dr. Landmeyer estimates that probably less than 10 percent of Superfund sites in the country use phytoremediation as a stand-alone cleanup method.
One reason for the limited success is that trees that were planted with good intentions could not survive in areas that were highly polluted. “People were losing faith in phytoremediation because it got expensive to replace dead or sick trees,” said John Freeman, chief scientific officer at Intrinsyx. “It was viewed to be cost prohibitive.”
If the trees remained healthy, phytoremediation would cost about half as much, saving perhaps a half-million dollars per project, compared to traditional engineering methods such as drilling and extracting toxins from soil or groundwater, Dr. Landmeyer said. Trees might take a bit longer for the same job — about five years, he estimates — “but the cleanup goal would be the same.”
Before planting the poplars, Intrinsyx inoculate the poplar cuttings with high concentrations of a microbe called PDN3. This microbe not only gives the poplars an added survival advantage, but helps the trees withstand toxic groundwater by breaking down trichloroethylene, an industrial pollutant from solvents and degreasers.
The technique was developed by Dr. Doty, who in 1995, as a post-doctoral researcher, began studying whether genetically modified poplar trees could clean up pollutants. Poplars are fast-growing and have an extensive root system, which allows them to quickly take up whatever is in the soil groundwater. Poplars could be genetically modified, she reasoned, to speed up their ability to break down chemicals and toxins.
Dr. Doty found that poplars that were engineered with an abundance of one enzyme seemed to make the carcinogen trichloroethylene disappear. The results were “so beautiful” in eliminating trichloroethylene, she said. But because of public concern about genetically modified organisms — for instance, that engineered genes might “jump” into neighboring wild plants — Dr. Doty feared that her technique might never catch on.
Dr. Doty noticed that when she was engineering her poplars, other bacteria in the plant tissue seemed to be contaminating the results. At the time, little was known about the microbiome; Dr. Doty considered these bacteria a nuisance.
A few years later, when Dr. Doty started her own lab, she and her graduate student Jun Won Kang speculated that one of those bacteria might be responsible for the poplar’s anti-pollutant superpowers.
Dr. Kang isolated more than 100 strains of Endobacteria that colonized poplar trees, and grew each one in flasks to see if it could survive in high concentrations of trichloroethylene. In 2012, they identified one strain, called PDN3, that could do so. When the researchers inoculated poplars with PDN3, the microbe not only helped take up trichloroethylene, but degraded it into two benign products: carbon dioxide and salt.
This combination is powerful — and politically acceptable, Dr. Doty said: There is no genetic engineering, because PDN3 probably evolved with poplar trees for millions of years.
Dr. Doty and her colleagues have also found that when paired with willows and grasses, another bacteria, PD1, can degrade toxins called polycyclic aromatic hydrocarbons. Intrinsyx has licensed strains of PD1 and PDN3 for their phytoremediation projects.
“These are biologically enhanced systems,” said Dr. Freeman of Intrinsyx. “And the neat thing is, we’re doing all this with naturally occurring microbes.”
Elsewhere, phytoremediation is regaining popularity.
Other companies — among them Ecolotree, Sand Creek and GeoSyntech — are treating contaminated groundwater with plants, but are not using microbes to enhance the trees’ ability to degrade toxins.
Christian Bako, a graduate student in Jerald Schnoor’s civil and environmental engineering lab at the University of Iowa, and his colleagues are using poplar cuttings inoculated with a bacteria called LB400 to clean up polychlorinated biphenyls, chemicals used in hydraulic fluids, plasticizers, and lubricants that are now banned.
Scientists in Nanjing, China, are finding ways to degrade toxic chemicals known as polycyclic aromatic hydrocarbons from industrial sites, using plants in the clover and sunflower family and their respective bacterial pairings.
“The plant microbiome can be a powerful tool,” Dr. Doty said.