Mother nature may hold the keys to cleaning up toxic messes
Most trees planted along the edges of farms are intended to break the wind. But a mile-long stand of spindly poplars outside Amana, Iowa, serves a different purpose. It cleans pollution.
The poplars act like vacuum cleaners, sucking up nitrate-laden runoff from a fertilized cornfield before this runoff reaches a nearby brook--and perhaps other waters. Nitrate runoff into the Mississippi River from Midwest farms, after all, is a major cause of the large "dead zone" of oxygen-depleted water that develops each summer in the Gulf of Mexico.
Before the trees were planted, the brook´s nitrate levels were as much as 10 times the amount considered safe. But then Louis Licht, a University of Iowa graduate student, had the idea that poplars, which absorb lots of water and tolerate pollutants, could help. In 1991, Licht tested his hunch by plant-ing the trees along a field owned by a corporate farm. The brook´s nitrate levels subsequently dropped more than 90 percent and the trees have thrived, serving as a prime example of a promising new pollution cleanup method known as phytoremediation.
This method uses plants--many of them common species such as poplar, mustard and mulberry--that have an appetite for lead, uranium and other pollutants. These plants have genetic makeups that allow them to absorb and store, degrade or transform substances that kill or harm other plants and animals. "It´s an elegantly simple solution," says Licht, who now runs Ecolotree, an Iowa City phytoremediation company.
The simple treatment could help solve a complicated problem. According to the U.S. Environmental Protection Agency (EPA), there are more than 200,000 contaminated sites in the United States. Conventional "dig and dump" and "pump and treat" cleanup methods for these sites can be difficult and expensive. Phytoremediation may be a better alternative in many situations, says Walter W. Kovalick, Jr., director of the EPA´s technology innovation office. "It has an obvious ecological appeal," Kovalick says. "It is potential low energy, low maintenance and it has a ´natural´ appeal to citizens."
The idea behind phytoremediation isn´t new; scientists have long recognized certain plants´ abilities to absorb and tolerate toxic substances. But the idea of using these plants on contaminated sites has just gained support in the last decade. The plants clean up sites in two basic ways, depending on the substance involved. If it´s an organic contaminant, such as spilled oil, the plants or microbes around their roots break down the substance. The remainders can either be absorbed by the plant or left in the soil or water. For an inorganic contaminant such as cadmium or zinc, the plants absorb the substance and trap it. The plants must then be harvested and disposed of, or processed to reclaim the trapped contaminant.
Different plants work on different contaminants. The mulberry bush, for instance, is effective on industrial sludge; some grasses attack petroleum wastes; and sunflowers (together with soil additives) remove lead. Canola plants, meanwhile, are grown in California´s San Joaquin Valley to soak up excess selenium in the soil to help prevent an environmental catastrophe like the one that occurred there in the 1980s.
Back then, irrigated farming caused naturally occurring selenium to rise to the soil surface. When excess water was pumped onto the fields, some selenium would flow off into drainage ditches, eventually ending up in Kesterson National Wildlife Refuge. The selenium in ponds at the refuge accumulated in plants and fish and subsequently deformed and killed waterfowl, says Gary Bañuelos, a plant scientist with the U.S. Department of Agriculture who helped remedy the problem. He recommended that farmers add selenium-accumulating canola plants to their crop rotations. As a result, selenium levels in runoff are being managed. Although the underlying problem of excessive selenium in soils has not been solved, says Bañuelos, "this is a tool to manage mobile selenium and prevent another unlikely selenium-induced disaster."
Phytoremediation has also helped to clean up Superfund sites, in some cases at a fraction of the usual cost. Edenspace Systems Corporation of Reston, Virginia, just concluded a phytoremediation demonstration at a Superfund site on an Army firing range in Aberdeen, Maryland. The company successfully used mustard plants to remove uranium from the firing range, at as little as 10 percent of the cost of traditional cleanup methods. Depending on the contaminant involved, traditional cleanup costs can run as much as $1 million per acre, experts say.
Besides being cheap, phytoremediation has other advantages. It doesn´t involve heavy machinery that produces air pollution, and it can reduce the amount of contaminated material dumped in landfills. At a former manufacturing site in Detroit, for example, DaimlerChrysler engineers used sunflowers and mustard plants to reduce 5,750 cubic yards of lead-contaminated soil to a few cubic yards of plant material, which was then landfilled.
Phytoremediation does have its limitations, however. One of them is its slow pace. Depending on the contaminant, it can take several growing seasons to clean a site--much longer than conventional methods. "We normally look at phytoremediation as a target of one to three years to clean a site," notes Edenspace´s Mike Blaylock. "People won´t want to wait much longer than that."
It is also only effective at depths that plant roots can reach, making it useless against deep-lying contamination unless the contaminated soils are excavated. And phytoremediation won´t work on lead and other metals unless chemicals are added to the soil. In addition, it´s possible that animals may ingest pollutants by eating the leaves of plants in some projects.
Despite its shortcomings, experts see a bright future for this technology. David Glass, an independent analyst based in Needham, Massachusetts, predicts phytoremediation will grow from a $50-million to $86-million business this year to $235 million to $400 million by 2005. It´s a promising solution to pollution problems but, says the EPA´s Kovalick, "it´s not a panacea. It´s another arrow in the quiver. It takes more than one arrow to solve most problems."
Joe Bower lives in Kalamazoo, Michigan, and writes frequently on environmental issues.