Plutonium aided reconstruction of caesium atmospheric fallout in European topsoils

Katrin Meusburger,Lionel Mabit,Christine Alewell,Cristiano Ballabio,Giorgia Cinelli,Kristof Van Oost,Panos Panagos,Olivier Evrard,Pasquale Borrelli,Michael Ketterer

doi:10.1038/s41598-020-68736-2

Abstract

Global nuclear weapon testing and the Chernobyl accident have released large amounts of radionuclides into the environment. However, to date, the spatial patterns of these fallout sources remain poorly constrained. Fallout radionuclides (137Cs, 239Pu, 240Pu) were measured in soil samples (n = 160) collected at flat, undisturbed grasslands in Western Europe in the framework of a harmonised European soil survey. We show that both fallout sources left a specific radionuclide imprint in European soils. Accordingly, we used plutonium to quantify contributions of global versus Chernobyl fallout to 137Cs found in European soils. Spatial prediction models allowed for a first assessment of the global versus Chernobyl fallout pattern across national boundaries. Understanding the magnitude of these fallout sources is crucial not only to establish a baseline in case of future radionuclide fallout but also to define a baseline for geomorphological reconstructions of soil redistribution due to soil erosion processes.

Highlights

Global nuclear weapon testing and the Chernobyl accident have released large amounts of radionuclides into the environment
As radionuclides were emitted in lower atmospheric layers, the Chernobyl radiocaesium deposition was much more heterogeneous across space than the global fallout because it originated from few distinct precipitation events that occurred late in April and early in May 1986 when the radioactive cloud travelled across the European continent
The 240Pu/239Pu ratios reflect the exclusive global fallout origin of the 239+240Pu found in soils of Western Europe since the analysed ratios remain in excellent agreement with those found in reference locations worldwide reported in the literature (Table 1)

Summary

Results

239+240Pu sources in European topsoil samples: 240Pu/239Pu atom ratios. Since the start of the atmospheric nuclear weapon tests in the twentieth century, anthropogenic plutonium has become a ubiquitous element in the environment, it is generally found naturally at trace levels in uranium ores. The Chernobyl derived 137Cs fallout contribution varies greatly within the studied western part of Europe with values higher than 80% (decay-corrected to August 1, 2009) for southeast Germany, the mid-mountain ranges and along the Alpine Arch (Fig. 5). By 2009, the contribution of the global fallout (1.4 ± 0.3 PBq) was lower than that of the Chernobyl accident (1.8 ± 0.3 PBq) it supplied 44% ± 18.3% of the total 137Cs inventory found in soils of the western part of Europe (Fig. 5B). Multiplying the activity ratio with the 239+240Pu activities provides a way to calculate the proportion of the 137Cs fallout activities originating from the global fallout, which were converted into inventories and subsequently regionalised Following this calculation method, the resulting spatial pattern of the prior Chernobyl global-derived 137Cs fallout shows a substantial similarity with the 239+240Pu inventory map (Fig. 6). There was virtually no Chernobyl fallout in soils of northwestern France

Discussion

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Findings

Materials and methods