Disproportionately High Contributions of 60 Year Old Weapons-(137)Cs Explain the Persistence of Radioactive Contamination in Bavarian Wild Boars

[Image: see text] Radionuclides released from nuclear accidents or explosions pose long-term threats to ecosystem health. A prominent example is wild boar contamination in central Europe, which is notorious for its persistently high (137)Cs levels. However, without reliable source identification, the origin of this decades old problem has been uncertain. Here, we target radiocesium contamination in wild boars from Bavaria. Our samples (2019–2021) range from 370 to 15,000 Bq·kg(–1)(137)Cs, thus exceeding the regulatory limits (600 Bq·kg(–1)) by a factor of up to 25. Using an emerging nuclear forensic fingerprint, (135)Cs/(137)Cs, we distinguished various radiocesium source legacies in their source composition. All samples exhibit signatures of mixing of Chornobyl and nuclear weapons fallout, with (135)Cs/(137)Cs ratios ranging from 0.67 to 1.97. Although Chornobyl has been widely believed to be the prime source of (137)Cs in wild boars, we find that “old” (137)Cs from weapons fallout significantly contributes to the total level (10–68%) in those specimens that exceeded the regulatory limit. In some cases, weapons-(137)Cs alone can lead to exceedances of the regulatory limit, especially in samples with a relatively low total (137)Cs level. Our findings demonstrate that the superposition of older and newer legacies of (137)Cs can vastly surpass the impact of any singular yet dominant source and thus highlight the critical role of historical releases of (137)Cs in current environmental pollution challenges.