Decades after the Chernobyl nuclear disaster, wild boar meat still exhibits surprisingly high levels of radioactivity. The answer is revealed: There is another important reason that has been ignored. The 1986 Chernobyl nuclear disaster severely affected the forest ecosystems of Central Europe. After the accident, high radioactive contamination led to years of recommendations against the consumption of mushrooms and game meat. Radioactivity in venison and roe deer meat decreased as expected over time, but levels in wild boar meat remained unexpectedly high, with some even exceeding safety limits.
Persistent radioactive contamination in wild boar meat in Central Europe caused by the Chernobyl disaster was also significantly affected by cesium produced by nuclear weapons testing in the 1960s, new research has found, highlighting complex ecological interactions and the key role of precise scientific analysis. Photo credit: Joachim Reddermann/TUWien
This persistent anomaly is known as the "wild boar paradox" and has baffled experts for years. However, recent detailed research from the Technical University of Vienna (TU Wien) and the Leibniz University of Hannover (Leibniz University of Hannover) has revealed the reason: it is a hangover from nuclear weapons testing in the 1960s.
Professor Georg Steinhauser from the Technical University of Vienna said: "The most important radioactive element in the sample is cesium-137, which has a half-life of about 30 years. Therefore, after 30 years, half of the material has decayed by itself. However, the amount of radiation in food usually drops faster."
After all, the cesium has been lost after Chernobyl, washed away by rain, bound to minerals, or perhaps migrated deeper into the soil, so that plants and animals no longer absorb it in the same amounts as they did after the reactor accident. Therefore, after a half-life, the radioactivity concentration in most food samples is not only half the original, but much lower.
However, the situation is different with wild boar: radiation levels there remain almost unchanged. They fall much more slowly than the natural radioactive decay of cesium - a result that, from a physical perspective, may at first seem completely contradictory.
To this day, samples of wild boar meat are measured that are unfit for consumption because their radiation levels clearly exceed the permitted limits. This may also be the reason why wild boars experience less hunting pressure in some areas, while their overpopulation often causes huge damage to agriculture and forestry.
Looking for Cesium Fingerprints
Professor Georg Steinhauser, who transferred from Leibniz University Hannover to the Technical University of Vienna in 2022, and his team set out to find the answer to this puzzle: By making new, more precise measurements, they not only determined the amount of radioactivity, but also its origin.
Dr. Feng Bin, who works at the Institute of Inorganic Chemistry at the Leibniz University of Hannover and the TRIGA Center Atominstitut at the Technical University of Vienna, explains: "This is possible because radioactive isotopes from different sources have different physical fingerprints. For example, they not only release cesium-137; Releases cesium-135, a cesium isotope with a longer half-life. The ratio of the two cesiums is not always the same - the radioactive isotope fingerprint of the Chernobyl disaster, for example, is different from that of the nuclear weapons tests of the 1960s, so measuring this ratio can provide information about the source of the radioactive material."
However, accurately quantifying cesium-135 is difficult. Georg Steinhauser says: "Because cesium-135 has a long half-life and rarely decays, it cannot be detected with radiation detectors alone. Mass spectrometric methods must be used and relatively great effort must be expended to accurately distinguish it from other atoms. We have now succeeded in doing this."
The findings show that while about 90% of the cesium-137 in central Europe comes from Chernobyl, the proportion in the wild boar samples was much lower. Instead, most of the cesium in wild boar meat can be attributed to nuclear weapons testing—as much as 68 percent in some samples.
Truffles are (probably) the culprit
The reason lies in the special food preferences of wild boars: they particularly like truffles dug out of the ground, and there is a long delay in the accumulation of radioactive cesium in these underground fungi. "Cesium migrates downward through the soil very slowly, sometimes only about a millimeter per year," Georg Steinhauser said.
Truffles grow in soil at a depth of 20-40 centimeters and are therefore only now beginning to absorb the cesium released by the Chernobyl accident. And the cesium from "old" nuclear weapons tests had gotten there some time ago.
There is therefore a complex interaction between different effects: both cesium from nuclear weapons testing and cesium from Chernobyl diffuse in the soil, so the truffles are reached by two different "cesium fronts" that gradually migrate through the soil. Cesium, on the other hand, decays over time.
Georg Steinhauser says: "If you add all these effects together, it explains why the radioactivity in truffles - and subsequently in pigs - has remained relatively stable over the years. Therefore, the contamination of wild boar meat is not expected to decrease significantly in the coming years either, since some of the cesium from Chernobyl is only now incorporated into truffles. Our work shows how complex the interrelationships are in natural ecosystems, but also just shows that answers to these mysteries can be found if the measurements are precise enough."