Timescale of radium and alkaline earth cations ascent in the trees: Use of isotopic (228Ra/226Ra) ratio

François Chabaux (1) , Peter Stille (2) , Jonathan Prunier (3, 2) , Sophie Gangloff (2) , Damien Lemarchand (2) , Gilles Morvan (2) , Justine Négrel (2) , Eric Pelt (3, 2) , Marie-Claire Pierret (3, 2) , Sophie Rihs (1, 4, 5) , Anne-Désirée Schmitt (2) , Michèle Trémolières (2) , Daniel Viville (2)

1 ITES - Institut Terre Environnement Strasbourg
2 LHyGeS - Laboratoire d'Hydrologie et de Géochimie de Strasbourg
3 UMR ECOFOG - Ecologie des forêts de Guyane
4 EOST - Ecole et Observatoire des Sciences de la Terre
5 UNISTRA - Université de Strasbourg

Due to its short-lived progeny, including gaseous radon isotopes that can be inhaled and its significant transfer into vegetation, radium poses a substantial concern in radiation risk assessment. Although the transfer of Ra from soil to plants has been previously documented, the distribution and speciation of radium within the plant tissues, particularly within trees, remain unclear. The temporal dimension of Ra transfer is also poorly known. Therefore, the transfer of radium and others alkaline earth cations in beech trees was investigated through the analysis of fine (<2 mm) roots and leaves corresponding to tree organs with a life cycle limited to one season. This study was conducted at the experimental Montiers (INRAE-Andra site, France) and Strengbach watershed (OHGE site, France) beech-forested site. The activity of 226Ra and the isotopic ratio (228Ra/226Ra) were measured in roots and leaves using gamma spectrometry. Additionally, the concentration of alkaline earth cations (Mg, Ca, Sr, and Ba) was measured, to better constraint the mechanism of alkaline earth cation transfer through the xylem. The retention of these cations within the trees increases with the mass of the element, following the order Mg < Ca < Sr ≈ Ba < Ra, i.e.: light alkaline earth cations are more efficiently transferred from roots to leaves than heavy one. Our results suggest that these cations maintain their hydration shell during their ascent in stems and a low pH ion-exchange process rather than a complexation process with carboxylic group occurs. The decay of the (228Ra/226Ra) isotopic ratio over time during transfer within the trees allows us to quantify the slow Ra translocation from roots to the leaves, ranging from 1 to more than 16 years. This duration increases with the age of the trees. Hence, radioactive decay of Ra provides particularly valuable timescale information that is usually difficult to assess for Ca or other alkaline earth cations.

Mots clés: Translocation time; Radium isotope; Alkaline earth cations; Leaves; Roots; Beech

Domaines: Sciences de l'environnement

François Chabaux, Peter Stille, Jonathan Prunier, Sophie Gangloff, Damien Lemarchand, et al.. Timescale of radium and alkaline earth cations ascent in the trees: Use of isotopic (228Ra/226Ra) ratio. Applied Geochemistry, 2024, 165, pp.105938. ⟨10.1016/j.apgeochem.2024.105938⟩. ⟨irsn-04570938⟩