Radiation Redux: Forest fires remobilize fallout from bomb tests

A sensitive instrument installed in the Canadian Arctic to monitor fallout from modern nuclear tests has detected small amounts of radioactive cesium produced by bomb tests decades ago. The material, which during the Cold War was spread across northern latitudes by high-altitude winds, is still being redistributed far and wide by forest fires, researchers say.

Scientists use a worldwide network of sensors to ensure compliance with the 1996 Comprehensive Nuclear-Test-Ban Treaty. While some devices are on the lookout for the telltale seismic vibrations generated by nuclear tests, others sniff the air for radioactive fallout (SN: 7/14/01, p. 25: Available to subscribers at The Silence of the Bams).

Beginning in May 2003, a sniffer in Yellowknife, Northwest Territories—a device that had been switched on for the first time in January of that year—collected radioactive particles that included cesium-137, says Gerhard Wotawa, a meteorologist with the Comprehensive Nuclear-Test-Ban Treaty Organization in Vienna. That particular isotope of cesium, which has a half-life of about 30 years, is generated when atoms of uranium-235 and plutonium-239 undergo fission within bombs or nuclear reactors.

The Yellowknife sensor regularly detected cesium-137 until mid-September 2003. In 2004, the radioactive particles showed up sporadically between late June and mid-September. Detectors at two other high-latitude sites—one in Iceland, the other on the remote Norwegian island of Spitsbergen—have detected cesium far less often.

Using computer models and weather reports, Wotawa and his colleagues pinned down the source of the cesium: the fires that typically rage unchecked through the boreal forests of Siberia, Alaska, and northern Canada. The concentrations of cesium measured by the Yellowknife sensor during a given month strongly correlate with the sizes of boreal forest fires then burning upwind, the team reports in the June 28 Geophysical Research Letters.

Air samples taken in previous studies near forest fires have contained cesium-137, says Wotawa, but this is the first time that scientists have detected long-range redistribution of the radioactive isotope.

The researchers aren’t sure how the radioactive element makes its way from fallout-tainted soil into the atmosphere. Cesium, a chemical relative of potassium, is readily taken up by plants, so ash derived from wood and leaves could contain traces of the element. Another possibility is that because cesium has a boiling point of 670°C, some of the radioactive atoms may be vaporized from the ground by fires and then condense on airborne ash and soot, says Wotawa.

The cesium-137 lofted during a forest fire is diffusely distributed. “This isn’t a health risk, but it’s interesting,” Wotawa notes. Scientists will have to account for the presence of wildfires when they’re interpreting the readings from radiation sniffers, he says.

“[This finding] isn’t too surprising, but I hadn’t thought of it before,” says Mark Fuhrmann, a geochemist at Brookhaven National Laboratory in Upton, N.Y. Scientists might use the cesium-137, strontium-90, and other radioactive isotopes in fallout to track nutrient cycles in forests, he notes.

---