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Discriminating the contribution of global fallout and Chernobyl to anthropogenic fallout radionuclide inventories in soils of Europe

Fallout radionuclide inventories are increasingly used to derive soil redistribution rates in watersheds across the world. However, one of the main prerequisites of this assessment method consists in establishing a baseline fallout map based on the inventories measured at reference undisturbed sites. This prerequisite is particularly difficult to obtain at continental scales, because of the extensive work required to collect and analyse the samples. Moreover, in Europe, an additional difficulty arises from the potential contribution of two main sources to the fallout radionuclide inventories: the global fallout and the Chernobyl fallout. To overcome these difficulties, we used a selection (n=156) of topsoil (0-20 cm) samples collected in 2009 at undisturbed locations under permanent grassland in the framework of the European LUCAS Survey. The selected samples were analysed for Cs-137 by gamma-spectrometry (n=156) and for Pu-239+240 (n=106) by ICP-MS. Preliminary results show that the use of the Cs-137/Pu-239+240 activity ratios provide a reliable proxy to discriminate between both sources of radioisotopes. The spatial variability of Cs-137 Chernobyl fallout across Europe can be re-assessed based on the updated map produced using advanced geostatistical methods. The measurement of Am-241 as a surrogate of plutonium was also explored through the gamma spectrometry analyses conducted on the samples showing the highest Pu concentrations. In the future, the current investigation will be extended to samples of the LUCAS bank collected at both erosion and accumulation sites in order to calculate the cumulative soil redistribution rates since the 1960s in cultivated land across Europe.