Tumbleweeds good for uranium clean-up

11/05/04

The lowly, ill-regarded tumbleweed might be good for something after all.

A preliminary study reveals that tumbleweeds, a.k.a. Russian thistle, and some other weeds common to dry Western lands have a knack for soaking up depleted uranium from contaminated soils at weapons testing grounds and battlefields.

"There is some use to what we consider noxious weeds," said geologist Dana Ulmer-Scholle of the New Mexico Institute of Mining and Technology in Socorro.

Depleted uranium (DU) is used in armor-piercing munitions. Although it produces only a low level of radiation, the metal poses a hazard in soils because it – like some other heavy metals – is toxic if ingested.

Other plants have been known to draw out DU from soils in wetter climes "but no one wanted to try doing it in arid regions," said Ulmer-Scholle.

Ulmer-Scholle's work is underwritten by the US Department of Defense, which is looking for innovative, cost-effective, and efficient ways of cleaning up soils at weapons testing areas and battlefields where DU has been used.

Ulmer-Scholle will be presenting the promising results of tumbleweeds and other weeds in arid lands on 10 November at the annual meeting of the Geological Society of America in Denver.

In her study, Ulmer-Scholle and her colleagues Bonnie Frey, Terry Thomas, and Michael Blaylock first sought out DU contaminated soils at an inactive munitions testing ground in New Mexico. Then they planted selected native and non-native plants in a test garden and in pots to see how much DU the plants absorbed from the soil.

Among the plants that sucked up lots of DU was Indian mustard (Brassica juncea), she reports. But that plant it is not well suited to deserts and needed irrigation. Better adapted to the dry environs, she said, were Russian thistle (Salsola tragus), the grain crop quinoa (Chenopodium quinoa) and purple amaranth (Amaranthus blitum).

"Our goal is to use plants with the least amount of water and the minimum amount of care," said Ulmer-Scholle.

They also found that sprinkling the ground with citric acid enhanced the plants' ability to absorb DU.

Russian thistle is a non-native plant to North America and is considered a nuisance in most parts of the western US. It springs up almost anywhere soils have been disturbed and each plant scatters its hundreds of seeds by detaching from its roots and tumbling along the ground in the wind.

Using tumbleweeds and other unpopular plants for DU clean-up needn't spread noxious weeds either, Ulmer-Scholle explained. It turns out that the plants tested do their best DU absorbing before they flower and long before they set seeds. So part of the trick to using weeds to clean up DU is to harvest the plants before they flower, she said.

The fact that plants absorb uranium is not news, since old uranium prospectors used to use Geiger counters on junipers to find buried uranium lodes. But finding a plant that grows fast on little water and can be easily harvested to carry away the depleted uranium – that's another story.

"We tried it here (in Southern New Mexico) and also in a natural uranium mine site in northern New Mexico," she said. The weeds picked up even more uranium in more contaminated soils. "So we got more where there was more in the soils."

As for why some plants absorb uranium, that's still a mystery, says Ulmer-Scholle. It could be that the plants use the metal to create pigments. One way she hopes to test that possibility is to grow native plants used for dyes, she said.

Phytoremediation of Depleted Uranium in an Arid Environment
Environmental Geosciences, Poster Session II
Wednesday, 10 November, 1:30 -5:30 p.m., CCC
Exhibit Hall

Abstract may be viewed at: http://gsa.confex.com/gsa/2004AM/finalprogram/abstract_80807.htm.

Source: Eurekalert & others

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