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by Tom Vanderbilt
Heavy metals got you down? More industries are cleaning contaminated soil the natural way-with plants.
In April of last year, as gardeners across the country were readying their saplings and optimistically scanning the ground for incipient shoots of long-buried bulbs, a small group of Washington, D.C., gardeners gathered at the headquarters of the American Horticultural Society. There they were given samples of three ferns, licensed by the Dulles, Va.-based firm Edenspace Systems Corp., and asked to plant them in their backyards, particularly in areas adjacent to decks and fences built from pressure-treated lumber. In September, the volunteers presented samples of the soil in which the ferns (dubbed "Victory" ferns by the company) had grown. Researchers noted that levels of arsenic-one of the most common pollutants in residential landscapes (a significant source of which is CCA, a wood preservative found in lumber) - had dropped noticeably. The ferns had concentrated some 48 times the amount of arsenic as the soil in which they were planted-meaning each fern cleaned, on average, nearly 50 times its weight in soil.
The gardeners, beyond gaining the aesthetic benefits of graceful fronds, were domestic pioneers of an alternative technology called phytoremediation. This process can clean up polluted environments at a fraction of the cost of traditional techniques and is fast accruing adherents in scientific and commercial circles. Coast to coast, in the full spectrum of gardening zones, plants are being tested for their ability to remove heavy metals and other contaminants at a variety of former industrial sites and brownfields. From a shuttered Magic Marker factory in Trenton, N.J. to a ruptured oil pipeline on the southeast Texas plains, researchers are turning to such plants as Indian mustard and St. Augustine grass as cheaper, environmentally friendlier alternatives to a brutish old practice called "hog and haul": bulldozing contaminated soils and transporting them to specially zoned landfills.
Phytoremediation, more technically defined as the use of "plant-influenced biological, chemical and physical processes that aid in the remediation of contaminated substrates," actually refers to a variety of methods. "'Phyto' is a set of technologies rather than a thing," says Steven Rock, a scientist with the EPA's National Risk Management Research Laboratory in Cincinnati. "If someone calls and says, 'Can I use phytoremediation,' it depends-it's a fairly broad range of technologies." In rhizofiltration plant roots suck up water pollutants and transfer them to stems and leaves. For phytostabilization, plants proven to be tolerant of a given pollutant are used as defense mechanisms against the pollutant's migration by wind or water. Phytodegradation involves plants' microbial processes acting as catalysts for the breakdown of toxic compounds. And in phytoextraction, plants move soil pollutants like heavy metals to stems and leaves.
Aspiring toxin avengers can now choose from an abundant and diverse herbarium. As Rock points out, "There are over 200,000 species of plants. We've only looked at several hundred of them in phytoremediation." Generally, however, the best candidates are hardy weed-like natives. The heavy-hitters thus far include Alpine pennycress (Thalaspi caerulescens), a native of the Rocky Mountains, which "hyperaccumulates" such elements as cadmium and zinc; and Indian mustard (Brassica juncea), which is used in Southeast Asia as a raw ingredient for cooking oil and is particularly effective against selenium (a problematic byproduct of soil irrigation). The common sunflower also shows promising talents, ridding water of strontium and other radionuclides. No phytoremediation agent has been more celebrated, however, than the poplar tree. Its extensive and fast-growing roots nurture soil micro-organisms that help break down contaminants, and its vigorous water intake "transpires" significant amounts of pollutants. Poplars seem to actually react to their environments, Rock says: "If they're in the presence of certain contaminants, they'll exude specific proteins that will attack those contaminants."
The phytoremediation business itself is now growing like a weed. Niall Kirkwood, a phytoremediation researcher in the landscape architecture department at Harvard's Graduate School of Design, notes that more than 30 U.S. universities are currently experimenting with this technology. The market sector - estimated at between $50 million and $86 million in 2000 - will jump to between $235 million and $400 million by 2005, he adds. One doesn't have to dig deep to fathom the reasons for potential success: As of 2001, the EPA calculated that there are some 40,000 toxic sites in the United States, with a projected cleaning cost via conventional, nonorganic means as high as $750 billion. Edenspace, which supplied the Victory ferns for backyard gardeners' testing and is one of a handful of phytoremediation specialty outfits in the country, has demonstrated the processes' effectiveness and affordability in a high-profile project with DaimlerChrysler. In 1999, the car manufacturer wanted to repurpose a parts plant in Detroit into an axle factory. Lead had been accruing in the soil for decades. Rather than scraping and hauling off layers of contaminated dirt, Edenspace put in plots of sunflowers and Indian mustard. Within a year, lead concentrations dropped by a quarter, below the legal limit, and DaimlerChrysler had saved $1 million.
Rival automaker Ford is experimenting with phytoremediation on an even grander scale. The company has launched a 20-year, $2 billion overhaul of its famed River Rouge manufacturing complex in Dearborn, Mich. Architect William McDonough is overseeing the project along with landscape architect and artist Julie Bargmann (her Virginia-based D.I.R.T. Studio specializes in reimagining contaminated industrial sites). McDonough began by proposing a phytoremediation clean-up of 1,200 acres around River Rouge's coke manufacturing plants. As Roger Schickedantz, McDonough's project head, recalls, "Ford was preparing to do a 'dig and dump.' It was a little difficult to get them to reroute their momentum toward this new strategy." But the logic of McDonough's dual-purpose plan prevailed: The plants could simultaneously detoxify and beautify. "We wanted to revegetate the site and provide habitat," Schickedantz says. "We were looking for an aesthetic that wasn't too manicured, that provided seeds and berries.
McDonough sought botanical advice from Clayton Rugh, a biologist at Michigan State University. Rugh had previously worked at the University of Georgia on a groundbreaking effort to bioengineer plants that detoxify mercury (no plants do so naturally). With research funding from Ford, Rugh tested the anti-contaminant powers of more than 50 native plants in a university greenhouse. "About 20 of the species had pretty interesting and accelerated levels of rates of degradation of the compounds," Rugh notes. Among the top performers were New England asters, joe-pye weeds and bluestems. "These are big, robust plants, very striking, very pretty," he continues. "They're not aggressive. They won't take over a landscape." Last fall, Rugh planted a series of test beds on a one-acre plot adjacent to the old coke plants. Surveys had found heavy metals onsite, but of more pressing concern was the presence of polycyclic aromatic hydro-carbons (PAHs), a carcinogen that's a byproduct of coke production-a process so filthy it's no longer allowed in the United States. During nearly a century of coke production, PAHs had accumulated in the soil as a fine particulate. The phytoremediating plants, Rugh explains, have been stabilizing and decomposing PAHs in the soil before the pollutants can be blown away with eroding dirt or leach into the leaves. The plants' own roots and leaf litter, he adds, "are rehabilitating the soil, restoring nutrients and habitable conditions while the detoxification is ongoing."
But not all phytoremediation experiments reap such immediate harvests. The DaimlerChrysler clean-up took a year, notes Michael Blaylock, Edenspace's director of research. "That's about as fast as you can hope to do it with phytoremediation. You need at least one growing season, but it could take as long as five to 10 years. Our target is a one-to-three-year window." Results along the way, he adds, are difficult to predict: "It's an in situ technology. You measure the concentration of contaminants before; you come back and measure at the end of the year. You have a natural variability in just your soil measurements." This uncertainty, he says, "limits some of the acceptance from a regulatory standpoint."
The phytoremediation industry's growth could also be stunted by a force all gardeners know well: the simple fickleness of plants. Edenspace's ferns won't tolerate certain climates, so local alternatives must be found. Gardeners also know that fostering native species - which some would call weeds - is more art than science. "They grow if you don't want them to," Blaylock says. "But if you ever try to grow them on purpose, they don't like to grow." And each type of toxin-eating plant tends to act on only one pollutant, so they're ill suited to Superfund sites, which generally possess a cornucopia of poisons. Blaylock notes that for years many pesticides contained lead arsenate, leaving behind the residue of both lead and arsenic. The removal of each requires different systems. "It doesn't work to do both, so which do I do first? How much will it add to the time and cost to do both?"
In some cases, however, the plants' chemical specificity could help boost their popularity, and they could well move from niche technology to default procedure. Edenspace has begun a pilot project that involves using arsenic-loving ferns to purify groundwater for a small town in New Mexico. "Since we have nothing else that cleans up arsenic," notes the EPA's Rock, "if the plant Edenspace is working on develops into something that can be used widely, that'll be the first thing people reach for - it's cheaper, plus you get longevity benefits." Forests, he says, "last longer than pretty much anything we can build." In remote areas, he adds, phytoremediation offers solutions where virtually nothing else works: The national oversight organization Environment Canada is now advising lighthouse keepers to clean up their grounds with plants, at sites inaccessible to bulldozers.
There probably haven't been such heavy demands placed on plants since World War II, when the American homefront was encouraged to grow "victory gardens" as part of its contribution to the war effort. "Our Food Is Fighting!" trumpeted the posters. Once again, the garden is being enlisted in a battle, this time against a domestic enemy: the unseen, pervasive contaminants in our landscapes.
Tom Vanderbilt writes regularly for I.D. and many other publications. He's author of Survival City: Adventures Among the Ruins of Atomic America.
Reprinted with permission, I.D. Magazine, June 2003
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