This interdisciplinary research project will focus on a better understanding of processes controlling water and Rare Earth Elements (REEs) exchanges in the critical zone (CZ). Given the intricate complexity of plant transpiration and limits on the use of stable (H-O) isotopes in complex systems, we propose an experimental validation of the use of REEs coupled with stable isotopes, as a proxy of water mixing and exchange within the CZ. One of the main pillars of this project consists in the combination of quantitative (hydrology) and qualitative (geochemistry) approach to estimate the bio-geo-chemical processes contributing to the temporal dynamics of water exchanges between the regolith and the plants. The main objective of this research particularly relies on a rigorous and detailed study of the REEs fractionation processes in a slate regolith–plant continuum. This will allow identifying the sources of REEs absorbed by the trees and the bio-geo-chemical processes that promote their release from soil, their uptake by plants and their fate from leaves. The project will be decoupled in progressive steps: (1) REEs fractionation in the regolith/water interface; (2) soil/water/roots interactions with a focus on the REEs uptake; (3) REEs transportations through the plant towards aerial parts; (4) REEs fate from the plant’s leaves by integrating atmospheric dust contribution and linking with stemflow and throughfall. Monitoring of O-H stable isotopic composition in different water sources and finding correlation between this isotopic signature and REEs patterns will lead us to implement and validate at least one hydrological model.