Retention of Be-10, Cs-137 and Pb-210(xs) in soils : impact of physico-chemical characteristics

The Be-10, Cs-137 and Pb-210(xs) radionuclide fallout has been used for the last several decades to quantify various soil and geomorphological processes 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 Be-10, Cs-137 and Pb-210(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 mu m fraction was extracted to measure radionuclide activities and undertake mineralogical analysis. Results show that meteoric Be-10 is significantly leached from soils whose pH(w) is lower than 5, regardless of the < 2 mu m particle proportion and Fe oxides content. Significant Cs-137 losses through leaching can generally be neglected except in sandy soils whose pH(w) is lower than 4.5 (Podzol). No significant Pb-210(xs) losses were evidenced. For the three radionuclides considered, the major part of their budget is associated with the < 2 mu m fraction. However, concerning the Andosol, the proportion of radionuclide budget associated with the < 2 mu m fraction represents < 40%. With regards to the forested Podzol, two thirds of the Pb-210(xs) budget is associated with the litter. Well-crystallized Fe oxides, illite and interlayered clay minerals as well as allophane, imogolite and other Al-phases in the Andosol and kaolinite in highly weathered acidic soils (Ferralsol) were found to efficiently retain Be-10. Finally, litter degradation and the content of large particulate organic matter were shown to control Pb-210(xs) concentrations. As expected, our results highlight strong contrasts in the retention of the considered isotopes according to soil physico-chemical properties. Accordingly, their mobility and losses through solute transport should be considered when using them for quantifying solid transport and future mass transport models must be improved, in particular through the addition of a solute transfer term. Otherwise, soil redistribution might be strongly overestimated for Podzols, Ferralsols and also most probably for other acidic tropical soil types (Nitisols, Acrisols, Plinthisols).