A Little Help From Below: Naturally Occurring Microbes Ready To Lend A Hand Trapping Radioactivity Underground

Microbes isolated from groundwater drawn at the lab site break down the compound urea through a process called urea hydrolysis, researchers from INEEL, Idaho State University, and the University of Toronto reported today at the American Geophysical Union meeting in San Francisco. The INEEL specializes in subsurface science as part of its environmental mission.

That means the naturally occurring microbes may be able to change the chemistry of the groundwater so calcite, a mineral found naturally in the rock the water flows through, will accumulate faster than normal. The growing calcite deposits will trap radioactive strontium-90 in their crystal structures.

Researchers hope to inject urea into a contaminated area and let the native microorganisms go to work breaking it down. The resulting buildup of calcite should keep the strontium-90 from spreading. The buildup will remain limited to the area around the injection well and will not interfere with groundwater flow.

"If we can trap strontium-90 in the subsurface, it's not going to keep moving with the groundwater," said INEEL microbiologist Yoshiko Fujita, who reported the results in the Hydrology: Environmental Geochemistry poster session. "We want to stop it in place."

Trapping strontium-90 underground should be cheaper and safer than extracting it, said INEEL geochemist Bob Smith who leads the project. "If you were to do some sort of treatment where you brought it to the surface, you'd have to expose workers, you'd have to ship it, you'd have to pack it," he said.

The researchers hope to contain the contaminant until it has decayed away. Strontium-90 decays with a half-life of 29 years. That means a sample containing strontium-90 will be only half as radioactive after 29 years, only one-quarter as radioactive after 58 years, and so on. After 300 years, 99.9 percent of the radioactivity will have disappeared.

Strontium-90 is a toxic, radioactive substance produced in nuclear reactors. In past decades, some waste-disposal methods have left pockets of low-level radioactive contamination in and above the groundwater at INEEL and other DOE sites.

Proving that naturally occurring microbes break down urea and promote the formation of calcite is the first step in a 3-year, $900,000 DOE-funded project to develop the technique for trapping strontium and other metals in calcite deposits in arid regions of the western United States.

The groundwater in arid regions is often saturated with calcium carbonate. Since the water cannot hold any more of the substance, additional calcium carbonate gradually drops out of solution and forms calcite in the surrounding rock in the same way deposits build up in the pipes of houses with hard water.

Microbes that break down urea should increase the rate at which calcite builds up. When urea is hydrolyzed, the groundwater will become less acidic and more basic, which will reduce the amount of calcium carbonate the water can hold. More calcium carbonate will then precipitate out as calcite.

Growing calcite deposits should draw strontium and other metals out of the groundwater because strontium atoms and other contaminants can replace calcium atoms in the mineral's crystal structure.

The existence of the urea-hydrolyzing microbes suggests nature is willing to help the researchers with their plan. "For every sample of groundwater we tested, there was some hydrolysis of urea," Fujita said. "I think that's important because it shows this is a very common activity. If you rely on a reaction that is rare, your chances of going out in the field and having the method work are slim."

When grown in a medium rich in both urea and calcium carbonate, the isolated microbes precipitate calcite rapidly as they multiply, said microbial geochemist F. Grant Ferris of the University of Toronto. "In our medium, all of the microbes started the precipitation within the first half-hour of growth," he said.

Electron microscope images of the precipitated calcite suggest the mineral grows in globules around the microbes themselves. By serving as seeds for crystal growth, the microbes might further accelerate the rate at which calcite forms.

The researchers hope to begin field tests of the technique in two years. Before then, they must determine the rate at which calcite forms when the microbes are present, whether they should supply the microbes food as well as urea, and whether the precipitation of calcite will be as efficient if the concentrations of calcium carbonate and strontium are reduced from the artificially high levels the researchers have used in the laboratory so far.

The INEEL is managed and operated by Bechtel BWXT, Idaho, LLC (BBWI) for the U.S. Department of Energy.