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Pollution Plants
October 05, 2000
Close-up of dirt and grass

For most people, crabgrass is a useless weed and for those with lawns it can be a downright nuisance. But researchers at the University of Arkansas see the scruffy grass as a useful tool in dealing with a far more pernicious nuisance–oil and gas spills.

Using plants to clean up toxic waste, a technique called phytoremediation, takes advantage of a plant’s natural soil conditioning properties whereby the microbes associated with the plants degrade certain contaminants.

Good crabgrass

While slow, it is a far cheaper, cleaner and greener way of cleaning up all but the most heavily contaminated soils. "It costs about 20 percent as much to phytoremediate the site as it would to dig it up and haul it to a landfill or incinerate it," says Duane Wolf, a pyhtoremediation expert at the University of Arkansas. "In addition you get the aesthetic advantage of vegetating the site."

Microbes among the roots clean the soil.

Oil and gas spills, often found around wellheads and old-fashioned manufactured-gas plants are unsightly and toxic, as oil can contain a number of toxins, some with the potential to cause cancer. But because oil contamination does not penetrate very deeply into soil, it is an ideal target for clean-up by plants like crabgrass, which have dense yet shallow roots.

Wolfe and colleagues Greg Thoma and Craig Beyrouty compared the survival of a number of different types of grass growing in crude oil-contaminated soil in both a greenhouse and at a site in southern Arkansas. While crabgrass had difficulty germinating in contaminated soil, the plants that did survive grew very fast and produced long dense roots—ideal for bio remediation.

Good bacteria

This study is the first comprehensive look at using crabgrass to clean up, or remediate, contaminated soil. Unlike some other plants used for phytoremediation, it’s not the crabgrass itself that sucks up pollutants from the soil. Bacteria that thrive around the roots of the grass and normally help the plants extract more food out of the soil are the ones which break down the oil into harmless by-products. "The contaminants are in the soil and the microbes are attached to the roots," says Kathy Banks, a toxic spill expert at Purdue University in West Lafayette, Indiana. "When the microbes are in contact with the contaminants they use the contaminants as a food source…and actually have the production of carbon dioxide and water as the end product."

Oil wellhead in Southern Arkansas
The Southern Arkansas site.

The focus of Wolfe’s research is to identify the types of plants that can break down the most contamination. "Our work is focused on trying to find those plants that will stimulate the largest number of petroleum degrading microorganisms around their roots," says Wolfe. And while it may sound crazy to anyone with a lawn, Wolfe’s team is also trying to find the best way to fertilize crabgrass. By increasing the size, or biomass, of crabgrass plants in a contaminated site, the grass will be able to break down more oil faster.

Wolfe’s aim is to demonstrate the use of crabgrass at cleaning up oil contaminated soil by restoring a disused oil wellhead in southern Arkansas to natural pineland. "The contaminated soil is asphalt-like," says Greg Thoma. "It’s hard and black with a thick crust layer covering a gooey, tar-like substance that can be several feet deep." Wolfe’s team hopes to clean up the soil to the point where pine seedlings will be able to survive.

How do you know if it’s working?

While employing Mother Nature to remediate soils, the best way to check that it’s working, says Banks, is to recruit another of her creations, the humble earthworm. "A standard way of evaluating the toxicity of soils is to evaluate earthworm health, or earthworm reproduction rate," says Banks. Because earthworms are in continual contact with the soil, they are particularly sensitive to contamination. "If we actually dig earthworms and place them in the contaminated soil and see that they grow well and reproduce, then we can probably say that the soil is not particularly toxic," says Banks. "If we find that earthworms die, or they don’t reproduce, or they lose weight, that indicates that the soil is probably still toxic and we need to do further remediation."

The main drawback of phytoremediation is that it takes a long time for plants to break down pollutants, but according to clean-up experts it pays to be patient in most cases. Faster methods require the removal of contaminated soil, but these cost significantly more than phytoremediation and can spread the contamination around. "It’s important to remember that this is an in-situ process—we do it in place—so you’re not exposing the surrounding public to contaminants as you dig-up, incinerate or landfill the contaminated material," says Banks.

Moreover, what’s left behind, especially in what used to be wasteland areas, has the potential to be of more use than what was there before. "In industrial areas that may not have an excessive amount of green space, it’s aesthetically pleasing," says Banks. "These areas can be used for public use, such as parks or golf courses, nature trails, wildlife reserves and more natural methods such as that."

Elsewhere on the web:

Find contaminated sites near you using the Environmental Protection Agency’s Superfund Webpage

A congressional research service report on the best ways to clean up superfund sites

Savannah River Ecology Laboratory’s comprehensive phytoremediation fact-sheet

by Tom Clarke

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