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Technology Corridor

Vegetation that helps break down toxins debuts at manufactured gas plant site.
Fortnightly Magazine - July 15 2003

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.

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