Wild Boar Paradox

On April 26, 1986, the Number Four RBMK reactor at the nuclear power plant at Chernobyl, Ukraine, went out of control during a test at low-power, leading to an explosion and fire that demolished the reactor building and released large amounts of radiation into the atmosphere.

The explosion had a major impact on the forest ecosystem in Central Europe. While the contamination of deer and roe deer decreased over time as expected, the measured levels of radioactivity in the meat of wild boar remained almost constant. The limit values are still being exceeded by a significant margin in some samples. For years, this Wild Boar Paradox was considered unsolved. Until now[1].

To identify the source of radioactive caesium in Bavarian wild boar meat, the scientists turned to a different caesium isotope with a much longer half-life: ₁₃₅Cs. A nuclear explosion yields a relatively high ₁₃₅Cs/₁₃₇Cs-ratio, whereas a reactor yields a low ratio.

After analysing the ratio of Caesium isotopes in samples of wild boar meat from eleven districts of Bavaria, Germany, scientists concluded that global fallout from nuclear weapons tests is still responsible for a significant fraction of the contamination, even though Bavaria also experienced heavy fallout from the Chornobyl reactor meltdown.

Although Chernobyl has been widely believed to be the prime source of ₁₃₇Cs in wild boars, the team found that “old” ₁₃₇Cs from weapons fallout significantly contributes to the total level (10–68%) in those specimens that exceeded the regulatory limit. 

In the paper, scientists note that this paradoxical non-decline is often attributed to the boars’ tendency to root up and eat underground fungi such as deer truffles. Under the 'right' soil conditions, these organisms act as a repository for ₁₃₇Cs, which migrates downwards through the soil very slowly, sometimes only about one millimetre per year.

This result suggests that there are, in effect, two separate downward-migrating Caesium “fronts” contaminating the boars’ winter food supply: one from atmospheric nuclear weapons tests, which peaked in 1964, and one from Chernobyl in 1986.

[1] Stäger et al: Disproportionately High Contributions of 60 Year Old Weapons-137Cs Explain the Persistence of Radioactive Contamination in Bavarian Wild Boars in Environmental Science & Technology – 2023. See here.