If you talk to employees at one of the US Governments nuclear facilities long enough, you’re likely to hear a lot of interesting stories about the early days of dealing with nuclear materials. One, apparently originating in Oak Ridge, TN, involves an underground tank and a tree.

Apparently during a regular check of the site some radioactive contamination was found in the ground near one of the storage tanks. The area was promptly cleaned up, the soil carted away and replaced, but the people assigned to figure out how the ground became contaminated in the first place were stumped. They found no leaks, no spills, nothing that could have caused the radioactive material to escape. They checked over the spot for several months to see if the contamination recurred, which it did not, then finally shrugged and called it a mystery.

But a year later the contamination was back.

The second time was much like the first. The clean-up went smoothly, the investigation did not. It was not until the third time the same contamination recurred in the same place that the investigators found the culprit – an oak tree. The tree had sunk its roots into the tank, and was pulling up contaminated material. While one would expect the whole tree to become radioactive, that wasn’t the case. Instead the oak preferentially shunted the contaminants to its leaves. Every fall a new crop of radioactive leaves would hit the ground to mystify the workers at Oak Ridge, and every winter while they were investigating, there would be no sign of anything amiss with the tree.

It’s an amusing story, but the ability of that tree has become a major weapon in the efforts to clean up contaminated soil. You see, trees aren’t the only plants that can pull contaminants from the ground. There are a slew of others that can do it too, and they are becoming more and more valuable in places where the ground has become polluted or contaminated.

The process is called phytoextraction, and it’s one of a number of ways ecologists are beginning to use plants in bioremediation – the treatment of environmental problems. There are plants that can pull lead, uranium, arsenic, and any number of other contaminants out of the soil. They can then be harvested, a new crop planted, and the process repeated until the soil is sufficiently recovered.

The process has already been used with great success. At Chernobyl, genetically altered sunflowers have been used to clean up two ponds heavily contaminated with both cesium and strontium, while hemp plants are being used to clean up the soil. Various companies and institutions are beginning to jump on the bandwagon. Dr. Ilya Raskin of Rutger’s University, one of the field’s pioneers, has been working with members of the Brassica family (mustards), which tests have shown effective at removing a number of heavy metals. Dupont has found corn useful for lead, while Phytokinetics, a company in Ohio, is using trees to clean up deeper soil contamination.

All in all, the field of phytoextraction seems to be one of the most promising in the efforts to clean up the hundreds of thousands of sites worldwide (30,000 in the US alone, according to the EPA), that require hazardous waste treatment. Even if only modestly successful, the use of plants as contaminant removers could reduce cleanup costs considerably. Even more promising, phytoextraction is only one aspect of the whole field of phytoremediation, in which plants are being used not only to remove toxins, but sometimes to break them down (phytotransformation), enhance microbial activity (phytostimulation), or prevent leaching of contaminants in the first place (phytostabalization).

Perhaps if they had known where it would lead, the site workers at Oak Ridge might have been less frustrated when they finally found their answer.

Written by Cynthia Wood, posted on 24 February 2006. Cynthia is a contributing editor for DamnInteresting.com.
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