Wild boar and the wooden platforms used for hunting them are a common sight in European forests, but in some areas their meat contains such high levels of radioactive caesium that it cannot be sold for human consumption. Worse, radiation levels in wild boar have remained constant for the past 30 years, even though the well-understood physics of half-lives and a countervailing trend in other forest animals suggest the levels ought to be falling.
This puzzle is known as the wild boar paradox, and a team of chemists and radiation experts at Leibniz Universität Hannover, Germany and TU Wien, Austria, has now come up with an explanation for it. After analysing the ratio of caesium isotopes in samples of wild boar meat from 11 districts of Bavaria, Germany, the team concluded that global fallout from nuclear weapons tests is responsible for a significant fraction of the contamination, even though Bavaria also experienced heavy fallout from the Chornobyl reactor meltdown. Though the exact ecological mechanism for the ongoing contamination remains unclear, the researchers say their result adds to a growing body of evidence indicating that decades-old nuclear tests continue to have a significant impact on the environment.
From the mid-20th century until 1980, the US, the Soviet Union and other nuclear-weapons states conducted a total of 528 nuclear tests in the Earth’s atmosphere, showering the globe with fission products such as caesium-137 (137Cs). Nuclear accidents such as the ones at Chornobyl, Ukraine in 1986 and Fukushima, Japan in 2011 also released 137Cs, and while the fallout from these events was more localized than the fallout from weapons testing, it still affected large areas. The Chornobyl disaster, for example, spread 137Cs across much of Europe due to a combination of prevailing winds and poorly-timed rainfall.
Many countries responded to the health risks of this fallout by adopting regulations that limit the amount of 137Cs in foodstuffs. In Europe, the limit is 600 Bequerels (Bq) per kilogram; in Japan, it is 100 Bq/kg. But while food testing and other forms of surveillance provide useful information about levels of 137Cs contamination, they are silent on its source. This is because “reactor-137Cs” and “weapons-137Cs” are indistinguishable, with the same chemistry and the same 30-year half-life.
To identify the source of radioactive caesium in Bavarian wild boar meat, the Leibniz and TU Wien scientists turned instead to a different caesium isotope with a much longer half-life: 135Cs. Because the parent nuclide of 135Cs, 135Xe, has a large cross-section for thermal neutron capture, the high neutron flux density within a reactor core tends to transmute it into other substances, limiting 135Cs production. In contrast, the neutron flux during nuclear explosions is intense but brief, meaning that more 135Xe “survives” to decay into 135Cs. Hence, a nuclear explosion yields a relatively high 135Cs/137Cs ratio, whereas a reactor yields a low ratio.
The wild boar paradox
In the immediate aftermath of the Chornobyl disaster, levels of 137Cs in Bavaria’s surface soil ranged from 102 to 105 Bq/m, and concentrations in local wild boar meat exceeded the regulatory limit by 1–2 orders of magnitude. Other forest species such as deer were also heavily contaminated, but whereas their 137Cs levels declined substantially over time, levels in wild boar did not. In fact, the decline in wild boar has in some locations been slower than the physical half-life of 137Cs.
Fallout problems: Caesium-137 activity concentrations in wild boar and the respective contributions from weapons fallout and the Chornobyl nuclear accident. The light yellow and blue “pies” show the contribution percentages of weapons-137Cs and Chornobyl-137Cs, respectively. Redder areas of the map indicate higher levels of total caesium-137 contamination. (Courtesy: Environ. Sci. Technol. 2023, https://doi.org/10.1021/acs.est.3c03565)
In a paper published in Environmental Science and Technology, Felix Stäger, Dorian Zok, Anna-Katharina Schiller, Bin Feng and Georg Steinhauser 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,” Steinhauser explains.
The long shadow of weapons testing
As for where that 137Cs came from, the Leibniz-TU Wien researchers’ isotope ratio measurements show that it varies by location (see image). Overall, though, about 25% of their wild boar meat samples contained enough weapons-137Cs that they would have exceeded the European regulatory limit even if Chornobyl hadn’t happened. 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 Chornobyl 22 years later.
For the Bavarian hunters who supplied the team with samples, this is bad news. With caesium levels at deer-truffle level being continually replenished by this two-front downward migration, the natural decay of 137Cs into stable barium-137 is unlikely to solve the problem of wild boar meat contamination any time soon – especially in areas where caesium from Chornobyl is only now reaching the truffles that boar consume so, well, piggishly. “Strategic decisions to conduct atmospheric nuclear tests 60–80 years ago still impact remote natural environments, wildlife, and a human food source today,” the team conclude. “A similar, long-lasting consequence can be expected from Chornobyl-137Cs deposited in central Europe.”
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