Retention of $^{10}$Be, $^{137}$Cs and $^{210}$Pb$_{xs}$ in soils: Impact of physico-chemical characteristics

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Inglés. The $^{10}$Be, $^{137}$Cs and $^{210}$Pb$_{xs}$ radionuclide fallout has been used for the last several decades to quantify various soil and geomorphological processes and their rates, on different time scales. However, a basic assumption of the studies relying on these radionuclides is that they have a strong affinity for soil particles and that their mobility in soil solution and losses through leaching can be neglected. Another area of the scientific literature deals with the radionuclide mobility in soils as solute. In that context, the objective of this work is to determine the pedological conditions under which this hypothesis of poor solute mobility of radionuclides is valid. To this end, meteoric $^{10}$Be, $^{137}$Cs and $^{210}$Pb$_{xs}$ concentrations were measured in six soil profiles representative of 5 soil types contrasted in terms of physico-chemical properties: an Andosol and a Luvisol under pasture, a Ferralsol and a Leptosol under forest and a Podzol both under forest and cultivation. The main soil properties (soil pH, organic carbon (OC) content, particle size distribution and specific extractions) were measured. The <2 µm fraction of the samples was extracted to measure radionuclide activities and undertake mineralogical analysis. Mass balance calculations were made to estimate the potential isotope losses from the soil profiles. Results show that meteoric $^{10}$Be is significantly leached from soils whose pHw is lower than 5, regardless of the <2 µm particle proportion and Fe oxides content. Significant $^{137}$Cs losses through leaching can generally beneglected except in sandy soils whose pHw is lower than 4.5 (Podzol). No significant $^{210}$Pb$_{xs}$ losses were evidenced.For the three radionuclides considered, the major part of their budget is associated with the<2 μmfraction. However, concerning the Andosol, the proportion of radionuclide budget associated with the<2 μmfraction represents<40%. With regards to the forested Podzol, two thirds of the $^{210}$Pb$_{xs}$ budget is associatedwith the litter. Well-crystallized Fe oxides, illite and interlayered clay minerals as well as allophane, imogoliteand other Al-phases in the Andosol and kaolinite in highly weathered acidic soils (Ferralsol) were found toefficiently retain $^{10}$Be. Finally, litter degradation and the content of large particulate organic matter were shownto control $^{210}$Pb$_{xs}$ concentrations. As expected, our results highlight strong contrasts in the retention of theconsidered isotopes according to soil physico-chemical properties. Accordingly, their mobility and losses throughsolute transport should be considered when using them for quantifying solid transport and future mass transportmodels must be improved, in particular through the addition of a solute transfer term. Otherwise, soil redistributionmight be strongly overestimated for Podzols, Ferralsols and also most probably for other acidic tropicalsoil types (Nitisols, Acrisols, Plinthisols).