Researchers get to root of soil remediation

The researchers found that grasses like this fescue work well for petroleum phytoremediation.

July 11, 2000
Web posted at: 1:53 p.m. EDT (1753 GMT)

By Environmental News Network staff

A research team at Purdue University has pioneered the use of plants to help clean up soil contaminated with petroleum products. The team is currently using their methods to clean several petroleum spill sites across the United States with help from the Environmental Protection Agency and the petroleum industry.

Using plants to clean up contaminated soil, phytoremediation, is not new, but the Purdue team is the first to use grasses to clean up petroleum contaminated areas. While it takes years for phytoremediation to clean a moderately contaminated site, green space is usually the outcome.

And it's a socially accepted and inexpensive way to clear contaminants from the soil and groundwater, said Kathy Banks, a member of the husband wife research team.

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"We're using natural methods to clean up soil pollutants," said Banks, a professor of civil engineering. She and her agronomy professor husband, Paul Schwab, were also one of the first research teams to develop methods for field-testing phytoremediation.

"Using plants to clean contaminated soil is best used for moderately contaminated soil, or low risk soil that doesn't pose a high health hazard to the public," said Banks. "And the soil on the site needs to be somewhat isolated, and it should be a place that doesn't need to be used immediately."

Other biological cleanup methods can do the job faster, but phytoremediation costs much less and leaves the soil structure intact, explained Schwab.

"With standard methods you have to dig the soil out and then incinerate, compost or landfill the contaminated material," said Schwab. "Using phytoremediation, we can treat the soil at the spot where a spill occurred."

Banks says her expertise solving hazardous waste problems combined with Schwab's strong plant and soils background is a marriage that makes their research in phytoremediation work.

Near Bedford, Indiana, Banks and Schwab are working with the EPA and Indiana Gas Co. to compare the efficiency of several bioremediation methods at a contaminated site at a coal-to-natural gas refinery. Their efforts have become a demonstration project for natural gas manufacturers nationwide.

The researchers planted grasses and poplar trees on one part of the site to hasten the degradation of the soil pollutants. EPA is treating other parts of the site by composting soil, land farming (adding nutrients to soil with tillage), or letting natural processes work to degrade the contaminants. Over the next few years both EPA and Purdue will compare the cost and speed of each clean-up treatment.

Banks' graduate student Tom Spriggs oversees these poplar trees at the Bedford phytoremediation project

The biggest challenge at any site comes in finding the right plant for the job, said Schwab. Part of the challenge is matching plants to climate. For example, plants that thrive in southern Indiana may not survive the heat of a Florida summer.

Another part of the problem is finding plants that survive in contaminated areas while at the same time encourage microbial growth. Banks and Schwab worked with crop scientists to find plants that work best with soil microbes to break down petroleum.

"Soil microbes are actually the ones that break down the petroleum contaminants," said Schwab. "But the plants accelerate the microbes' action in the soil. They stimulate microbes to degrade contaminants by getting more oxygen into the soil and by supplying nutrients through their roots."

There is not a complete list of the best plants for the job, said Banks, although researchers have identified some characteristics that make plants good at phytoremediation.

"For this method to work, we've got to get the roots in contact with the contamination," said Banks. "Sod-forming grasses work well in certain situations, because they have a large root surface in contact with the soil."

"Because contaminants are highly absorbed into the soil, we need to make sure we get roots in contact with as much soil as we can get," said Banks. "We chose grasses because they are very fibrous and have an intense root-forming system."

In field tests, the researchers found that fescue and Bermuda grass work well. Clovers and alfalfa also look promising in certain situations, because their root systems stimulate microbe growth.

"Petroleum isn't very mobile, it adheres tightly to components of the soil," said Schwab. "Phytoremediation works well with compounds like that because the contaminants stay in the top six feet of the soil and are in direct contact with plant roots."

Banks and Schwab have used phytoremediation to help clean up a Texas oil pipeline spill, contamination at an Indiana manufactured-gas plant, an industrial sludge site in California and diesel spills on naval bases in Virginia and California.

Copyright 2000, Environmental News Network, All Rights Reserved

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American Society of Agronomy
Purdue University
An article from The Scientist
Ecolotree
The Missouri Botanical Garden
The EPA's Citizen's Guide to phytoremediation
report on phytoremediation

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