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Cleaning up polluted industrial sites may not require billion-dollar government programs. Instead, scientists suggest, plant a poplar tree.
Laboratory-designed hybrids of the fast-growing poplar tree have been found to act like 100-foot straws that suck contamination from soil and groundwater.
In a process known as phytoremediation, the tree either safely stores the chemicals in its tissues or metabolizes them into apparently less volatile compounds. Then the tree releases these byproducts through its leaves as vapor into the atmosphere.
This natural cleanup takes several years to complete. But tests show the method is inexpensive and might work at least as well as high-tech soil roasting and groundwater filtering, while keeping the site green and attractive.
Researchers say phytoremediation eventually may transform the way industry and government agencies treat long-term pollution problems, which are estimated to top $200 billion nationwide.
Still unknown, is whether the chemical byproducts generated by the poplars really are less harmful, or if diluting them in the atmosphere only creates another hazard.
“We may soon be using trees to heal the hurt inflicted on the Earth,” said David E. Salt, an environmental chemist at Northern Arizona University in Flagstaff. “But would we simply be exchanging soil pollution for air pollution?”
In one study, published in the October issue of the journal, Nature Biotechnology, researchers at the University of Georgia took a gene from a strain of bacteria that enables it tolerate high levels of ionic mercury, a highly toxic version of the heavy metal.
They modified the merA gene and inserted it into the genetic code of the yellow poplar. Laboratory tests showed the genetically engineered poplar had a tenfold increase in mercury resistance and also its ability to transport it through its roots and tissues, reduce it to a less volatile form and release it into the air. The genetic hybrid has not yet been planted in field tests.
The yellow poplar, also known as the tulip poplar, is one of the largest and most commercially valuable hardwood trees. It grows primarily east of the Mississippi River.
Environmental scientists favor it for phytoremediation for the same reasons that make the tree popular with commercial foresters, plywood manufacturers and neighborhood landscapers.
It grows up to 15 feet per year and absorbs 25 gallons of water a day. Its broad green leaves measure 6 inches square, providing plenty of leaf surface to release processed contaminants. It has an extensive root system. And it’s resistant to everything from gypsy moths to toxic wastes.
In a study published in the October issue of the Journal of Environmental Engineering, scientists at the University of Missouri-Rolla and the University of Iowa report that more conventional poplar trees effectively reduce high soil levels of atrazine, a farm herbicide.
Other remediation technologies strip the soil of vital nutrients and microorganisms, so nothing can grow on the site even if it has been decontaminated, they said.
“Phytoremediation simultaneously restores soil health and revegetates during the cleansing process,” said co-author Joel G. Burken of the University of Missouri-Rolla.
One of the nation's largest phytoremediation experiments is being conducted in Oregon. In 1984, a tanker truck skidded on icy Interstate 5 near Central Point, spilling hundreds of gallons of the solvent TCE.
Conventional cleanup efforts spanning 13 years failed to remove the solvent, a suspected carcinogen, from the surrounding soil and groundwater. In 1997, University of Washington scientists planted 800 hybrid poplars.
The results will not be available for several years. But in laboratory simulations, the Washington team said, poplars reduced TCE concentrations by 97 percent.
