This work investigates the behaviour of Ra within a forest ecosystem (Montiers, Meuse). It is based on the quantification of several isotopic tracers (U- and Th- series disequilibria, isotopic ratios of 228Ra/226Ra and 87Sr/86Sr) in the different compartments of the water-soil-plant system.
The research goals were : 1) to study the mobility of Ra and its radioactive ascendants in the separated mineral fractions of a soil profile, 2) to evaluate the transfer of Ra to the water-plant system, 3) to characterise the transfer of Ra and its residence time onto the vegetation, 4) to realise an account of the biogeochemical cycle of Ra.
This work shows a strong redistribution of U, Th and Ra depending on their respective affinities with the different mineral fractions of the soil. Despite being predominantly concentrated in the clay fraction (<2 μm) of the soil, our findings indicate that trees (beeches) mostly extract Ra from the soil’s iron oxides. The subsequent transfer of this nuclide from the roots to the foliage is lesser than that of other alkaline-earth metals, leading to a vegetal residence time on the order of a few years (2.6 ± 1.6 years). Finally, the biogeochemical cycle of Ra is characterised by the degradation flux of fine-roots rather than that of leaves, by a negligible atmospheric input and by a strong weathering rate. Ra presents in the latter originates almost uniquely from litter degradation and does not pass through gravitational soil solutions.