Technology Corridor
When Plants Attack
July 15, 2003
By Jennifer Alvey
Vegetation that helps break down toxins debuts at manufactured gas plant site.
Planting swaths of rye grass and mulberry trees and sowing the soil with bacteria are hardly standard operating procedure when it comes to cleaning up manufactured gas plant sites. But if Bill Bogan has his way, it just might be.
Bogan is an environmental microbiologist at the Gas Technology Institute. For the past year and a quarter, he has been working with Washington Gas in an experiment to see whether phytoremediation-using plants and biological materials to sequester or metabolize toxic materials from polluted soil and water-can work to clean up some of the 1,500 to 5,000 manufactured gas plant (MGP) sites scattered across the country.
If the experiment succeeds, remediation costs at some MGP sites could be halved, at least if Ford Motor Co.'s experience is any guide. Ford used phytoremediation at its River Rouge site to clean up 30 acres polluted with polyaromatic hydrocarbons (PAHs). Those PAHs, the byproduct of coke production on the site many years ago, are very similar to the waste produced by MGPs. Ford spent more than $900,000 on its phytoremediation project, but the company says it would have spent another $1 million on cleanup had it used traditional remediation methods. Rather than carting away 5,700 cubic yards of soil, Ford needed only to dispose of a few cubic yards of toxic plants.
It's too soon to say whether the Washington Gas site will see similar results, since the project won't be completed for nearly two years. Even assuming a highly successful experiment, Bogan points out that phytoremediation is not quite as cheap as the costs for plant material and labor for planting. "Still, phytoremediation is likely to be much cheaper" than traditional remediation methods, he says. There would, for example, be no need to pay for equipment to dig up tons of tainted soil and transport them to approved waste facilities, with the accompanying disposal fees.
The list of plants suited to phytoremediation is growing. Tomato plants, for example, absorb zinc, copper, and lead from surrounding soil. Sunflowers thrived in the radioactive waters near Chernobyl, and concentrated pollutants in their roots. The brake fern, native to the Southeast and California, drinks arsenic like a baby drinks milk. And many prairie grasses soak up hydrocarbons and speed up pollutant degradation.
Initial Success
Bogan's approach at the Washington Gas site was to try 10 different treatments combining various trees, grasses, and bacteria to see what, if anything, worked to reduce site toxicity. Midway into the second growing season, he says some combinations are having a significant impact on site toxicity.
But getting some plants and trees to grow at the site has been a problem in about half the experimental plots, Bogan says. That's not due to site toxicity, but the heavy clay soil that lines much of the mid-Atlantic, including the Washington Gas site. Clay soil is mineral rich, but without amendments of organic materials to help water and oxygen permeate it, many plants can't hack the dense ground. Bogan says the site's dogwoods have not fared well because they don't like the heavy clay soil. In fact, the only tree that has done well in the clay is the mulberry.
A combination of mulberry trees, rye grasses, and bacterial inoculum is producing the best results at the site, Bogan says. He hopes the bacteria eventually will degrade PAHs into carbon dioxide and water. Yet merely sowing the soil with bacteria alone rarely works, Bogan says, unless the bacteria is a type particularly adapted to the site soil. At the Washington Gas site, Bogan theorizes, the plant material secretes enzymes that create a more hospitable environment for pollutant-gobbling bacteria, so they persist in the soil.
Not for Everyone
Even if Bogan and Washington Gas demonstrate a sufficient reduction in toxicity, not every MGP site will become a candidate for phytoremediation. In part, that's due to the inherent nature of phytoremediation, which requires a few growing seasons for full effect. In contrast, a bevy of trucks can decontaminate a site much more quickly, in a matter of days. Sites that must be cleaned up quickly to address regulators' concerns are not likely to be good candidates for phytoremediation.
In addition, Bogan says that heavily polluted sites, especially those with high levels of non-aqueous phase liquids, are not strong candidates for a phytoremediation effort. "Generally, areas with a lower level of contamination to begin with" are the best sites to try, he says.
Another consideration is the depth of site contamination. At best, tree and plant roots penetrate the soil down to four feet. If coal tar pits, for example, are six or more feet deep, it's doubtful phytoremediation will provide enough of a remedy to be worthwhile.
Even for sites that are good candidates, there's still the hurdle of getting environmental regulators on board.
"It's not always an easy sell," Bogan notes. But, in his experience, if a site doesn't pose an immediate threat, regulators are often willing to consider a phytoremediation option.
For appropriate sites, phytoremediation offers a decided advantage-no exposure of PAHs or other contaminants to the open air. Opening up a 50,000-gallon, 60-year-old coal tar pit recently ended up costing AmerenCIPS a $3.2 million verdict, when a jury found (and an appeals court later agreed) that the company carelessly cleaned up the site, releasing coal tar thought to cause a rare form of cancer in four area children.
There's plenty going for phytoremediation at MGP sites, including lower cost and less site disruption. But so far there are no similar projects lined up after the Washington Gas phytoremediation is completed, Bogan says. Maybe as the economy picks up and more utilities want to push their MGP sites into re-use, that trend will change.
Jennifer Alvey is associate editor at Public Utilities Fortnightly.
A Phytoremediation Glossary
Phytoextraction. Plants pull toxic substances, frequently heavy metals, from the surrounding soil and concentrate the substances in their leaves and roots. The contaminant concentration in plant material is often thousands of times the level of that in surrounding soil.
Phytostabilization. Plants with large, dense root systems, often trees, prevent pollutant migration underground, either by preventing erosion and runoff, or by taking up tainted water and releasing moisture through leaves, keeping contaminants out of groundwater.
Phytotransformation. Plants secrete enzymes that break down pollutants in the soil, or enzymes within the plant structure break down contaminants taken up by the plant.
Rhizome biodegradation. Plants add oxygen to the soil and secrete enzymes that improve the growing environment for bacteria and fungi that break down contaminants. -J.A.
Articles found on this page are available to Internet subscribers only. For more information about obtaining a username and password, please call our Customer Service Department at 1-800-368-5001.
Public
Utilities Fortnightly & Spark
