Soil can trap radioactive toxins

Soil particles lock up contaminants hundreds to thousands of times faster under the caustic conditions found beneath leaking toxic waste tanks at the Hanford Nuclear Reservation than under normal soil conditions, researchers report. Understanding more about how contaminants such as radioactive cesium and strontium move through the soil under real-world conditions will help clean-up efforts at Hanford and other sites fraught with nuclear waste. Previous work on the movement of soil contaminants had not replicated the extreme conditions found in the state of Washington.

At Hanford, high-level nuclear waste from the manufacture of weapons is stored in 177 tanks buried in the soil. The waste contains toxic and highly radioactive elements, including plutonium, cesium 137, and strontium 90 mixed in with other materials that make the liquids in the tanks extremely caustic. Weapons production began at Hanford in 1944. Since then, 67 of the tanks have leaked, releasing liquid radioactive waste into the soil. Some of the contaminants have been found in the groundwater. Cost estimates for cleaning up the venue run into the tens of billions of dollars.

Figuring out how the wastes move through the soil is difficult because the tanks' contents are complex mixtures of chemicals. Moreover, the contaminated soils and toxic waste materials are too dangerous for the scientists to handle. "If we were working with the concentrations of radioactivity that are out there, it would be lethal doses," indicates Jon Chorover, an associate professor of environmental chemistry at the University of Arizona, Tucson. "It would kill everyone in the lab."

He and his team are doing the next best thing. They have created nonradioactive chemical mixtures that match those at Hanford and are studying how those chemicals move through uncontaminated soil collected near the tanks. In addition, to better understand how the different components of soil interact with the various chemicals, the team manufactured some model soils to test with the Hanford-like chemical mixtures.

To the researchers' surprise, over time the contaminants in the experimental set-ups were bound by newly formed clays, forming unusual minerals called zeolites and feldspathoids. The mineral particles, about 1,000 times smaller than a grain of sand, do not dissolve easily and therefore keep the contaminants trapped in the soil.

Although learning that contaminants get sequestered in solid particles seems like good news, Chorover is cautious. "We really don't know the lifetimes of these particles. We've shown it in the lab, but we don't know what's happening in the field. We do know contaminants are migrating through the soil in the field, so these laboratory results don't explain all of what we see at Hanford."

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