Surprisingly Radioactive?

On April 26, 1986, the infamous explosion at a Chernobyl nuclear power plant unleashed large amounts of radiation into the atmosphere, an event that contaminated wildlife across country lines. The radiation levels seen in animals as a result has decreased in recent years — with the exception of one animal: the wild boar.

For years, scientists questioned why levels of a radioactive isotope known as cesium-137 have remained surprisingly high in wild boars rooting around Germany and Austria, while decreasing in other deer and roe deer. In a new study released last week, a team of researchers finally solved this “wild boar paradox.” They uncovered that the main radioactive source is not the Chernobyl accident but nuclear weapons testing from the 1960s.

“My mind was blown when I realized how relevant this source of radioactive contamination in general still is,” said Georg Steinhauser, a radiochemist at TU Wien and author of the new study. Steinhauser said people might not think that 60 years after a nuclear weapons explosion, wild boar populations would still be contaminated with radiation levels well above the regulatory food limit.

Researchers have previously proposed other explanations to no avail. Some thought the contamination levels in wild boar were actually decreasing, but the data didn’t show up in the limited number of studied samples. Steinhauser previously suggested that perhaps the cesium dissolves better in fat tissue in wild boars and stays present longer, but subsequent research did not support that assertion either.

In the new research, Steinhauser and his colleagues took a step back and reinvestigated the amount and origin of cesium in wild boars. Working with hunters collecting wild boar meat across southern Germany, they measured cesium levels with a gamma-ray detector.

Radioactive cesium results from both nuclear weapons explosion and nuclear energy production. The element comes in different isotopic composition, cesium-135 and cesium-137, depending on the source. By analyzing the ratio of these amounts, the researchers can pinpoint the source of the radiation. From previous literature, the team knew a higher ratio of cesium-137 indicated a nuclear weapons explosion but a lower ratio is linked with nuclear reactors.

In the nearly 50 collected meat samples, the team found 88 percent of the samples were above Germany’s regulatory limits for radioactive cesium in food. Calculating the ratio of cesium isotopes in the samples, they found that nuclear weapons testing accounted for 10 to 68 percent of the contamination.

Even if the Chernobyl accident had never happened, “some of the wild boars would actually still exceed the regulatory limits for food safety limits only because of the weapons tests today,” said Steinhauser. “I think this is pretty mind-blowing because they were 60 years ago.”

Steinhauser said the wild boars probably ingested the cesium from contaminated deer truffle mushrooms, which they dig up and eat during the winter when corn and acorns on the ground are scarce. Cesium seeps through the soil and is absorbed by the mushrooms, as if it were a nutrient. This also explains why observations show radioactivity levels in wild boar are higher in the winter. (Interestingly, the deer do not fancy deer truffles so much despite the name.)

While cesium from both the nuclear weapons testing and the Chernobyl accident spread through the soil, Steinhauser said, the mushrooms appear to have fully absorbed the source from the nuclear weapons testing so far. Cesium seeps very slowly through the soil, sometimes only one millimeter per year, he said. Deer truffles, located between 20 and 40 centimeters, have already absorbed the “older” cesium from six decades ago. The “younger” cesium from Chernobyl has likely not fully integrated or is just now integrating at the soil depths where the mushrooms are located.

More details: https://pubs.acs.org/doi/10.1021/acs.est.3c03565