To this day, the meat of wild boar is sometimes so radioactively contaminated that it cannot be eaten. Since the Chernobyl reactor disaster, the burden due to natural radioactive decay alone should now be significantly lower. Researchers have now got to the bottom of this “wild boar paradox”. According to this, a large part of the radioactivity found in wild boar meat comes from nuclear weapons tests in the 1940s and 50s. The culprit is probably a type of truffle popular with wild boars, which stores radioactive cesium-137.
After the reactor accident in Chernobyl in April 1986, the radioactive fallout ensured that the radioactive isotope caesium-137 got into the environment and accumulated in mushrooms and game meat, among other things. For years, the consumption of these foods was therefore discouraged. While the level of contamination in rabbits, deer and the like has long since dropped to harmless levels, wild boar have puzzled scientists up to now: Because their meat still has some questionable radiation levels, which are significantly higher than given the natural radioactive decay and the dilution in the environment would be expected.
On the trail of the paradox
“Cesium-137, with a half-life of around 30 years, is decisive for the radioactivity of the samples,” explains Georg Steinhauser from the Vienna University of Technology. More than 30 years after Chernobyl, the pollution should have been reduced by at least half. “Since 1986, however, the values in wild boar meat have not decreased significantly.” He and a team led by first author Felix Stäger from Leibniz University Hannover have now got to the bottom of this phenomenon, the so-called “wild boar paradox”. To do this, the researchers analyzed numerous samples of wild boar meat provided by the Bavarian Hunting Association over a period of two years. “Some of the samples we tested exceeded the legal limit of 600 becquerels per kilogram of meat by up to 25 times,” reports the team.
In search of the cause, Stäger and his team investigated the source of the radioactivity in wild boar meat. “This is possible because different sources of radioactive isotopes each have a different physical fingerprint,” explains Stänger’s colleague Bin Feng. “For example, not only cesium-137 is released, but also cesium-135, a cesium isotope with a significantly longer half-life.” Depending on the ratio of these two isotopes, it can be determined whether the radioactivity comes from a nuclear power plant or from the explosion of an atomic bomb comes from.
Aftermath of the Cold War
The result: “Although it is generally accepted that Chernobyl is the main source of cesium-137 in wild boar, we find that ‘old’ cesium-137 from weapon fallout contributes significantly to the total content,” reports the team. According to this, up to 68 percent of the contamination in some samples comes from nuclear weapons tests in the 1940s and 50s. “In some cases, the weapon cesium-137 alone was enough to exceed the limit value,” write Stäger and his team.
But how can a radioactive isotope released up to 80 years ago during weapons tests during the Cold War still cause such significant contamination today? According to the researchers, the answer probably lies with an inconspicuous fungus that wild boars dig deep out of the ground to eat: the deer truffle. This fungus, which is not very tasty for humans but is popular with wild boar, stores the radioactive cesium from its environment.
Burden for decades to come
However, it takes decades for the cesium to reach the fungus after a fallout. “The cesium migrates down through the soil very slowly, sometimes only around one millimeter per year,” explains Steinhauser. “The deer truffles, which can be found at a depth of 20 to 40 centimeters, are now only absorbing the cesium that was released in Chernobyl. The cesium from old nuclear weapons tests, on the other hand, arrived there a long time ago.” The cesium-137 naturally also decays over the years in the soil and in the deer truffles. However, since new radioactive material is constantly seeping in from above, the load has remained largely constant so far.
The research team has not yet examined the deer truffle itself. In order to prove the hypothesis, corresponding analyzes should follow. If the assumption is confirmed, the wild boar paradox is solved – and since the uptake of cesium-137 from Chernobyl has only just begun, it can be expected that the wild boar will be contaminated with radioactivity for many more years to come.
Source: Felix Stäger (Leibniz Universität Hannover) et al., Environmental Science & Technology, doi: 10.1021/acs.est.3c03565