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Description
Wild boars in Europe exhibit an unexpectedly slow temporal decrease of the meat contamination with radiocesium originating from both nuclear weapon test fallout (NWT) and the Chornobyl accident (CA) – a phenomenon known as the wild-boar paradox. Previous estimates based on the Cs-135/Cs-137 isotopic ratios of Bavarian wild boar meat suggested that: 1. Due to different time spans available for downward migration, the older NWT radiocesium could have reached deeper soil layers than the younger CA radiocesium. Hence, NWT radiocesium was claimed to dominate the growth zone of deer truffles, one of the wild boars’ favorite food items, following that 2. CA fallout was reported to contribute a minimum of 1 % and NWT a maximum of 99 % (mean 31 %) of the total radiocesium in wild boar meat. To answer theses hypotheses, soil cores from multiple Bavarian regions were collected for depth-dependent specific activity determination using γ-spectrometry. The depth-dependent Cs-135/Cs-137 isotope ratios were measured in a representative core from the Bavarian Forest for source allocation. For this purpose, we optimized the chemical procedure for ultra-trace level environmental samples. The soil samples were leached four times (1. concentrated nitric acid; 2. - 4. concentrated aqua regia). After evaporation and redissolution, the cesium was extracted using ammonium molybdophosphate. Thereafter, interfering elements like Mo, Sb, Sn and Ba were removed by an anion and a cation exchange resin. The Cs-135/Cs-137 ratios were measured using a triple quadrupole mass spectrometer with nitrous oxide as reaction gas. We find Cs-137 specific activity in soil monotonously decreasing with depth, while the Cs-135/Cs-137 ratio and therefore, the contribution of NWT, increases slightly, as supported by a mathematical compartment model. Since the NWT radiocesium does not dominate the truffle growth zone and the increase is only small, it cannot sufficiently explain the disproportionate high NWT contribution in wild boar meat. However, we determined a new benchmark value for NWT derived Cs-135/Cs-137 ratio of 3.3 ± 0.3 at reference date 2011.03.11 in contrast to previously used value of 1.99 ± 0.19. Applying the new benchmark value to the wild boar data, the source allocation changes clearly; the minimum CA contribution changes from 1 % to 48 %, the maximum NWT contribution from 99 % to 52 % (mean 31 % to 16 %). Overall, this work answers open key questions of the wild-boar paradox.