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Theses

Gradients chimiques dans les systèmes hydrologiques : origine, dynamique et conséquences sur les réactions chimiques

Abstract : Subsurface chemical reactions play a crucial role in the dynamic of the critical zone. Geochemical reaction kinetics are generally estimated from experiments in homogeneous media. However, the natural environment is characterized by multi-scale chemical gradients, which could have major consequences on the reactions efficiency. This thesis therefore focuses on the origin, dynamics and consequences of chemical gradients on reactions. First, we developed a multidisciplinary approach to understand the mechanisms behind a strong geochemical gradient observed along a river at a topographic break in an instrumented catchment in Guadeloupe (SNO OBSERA, OZCAR ). Geophysical, hydrological and geochemical measurements are coupled with a hydrogeological model , which represents deep circulations and the weathering of rocks. Our study shows that the depth of the hydrogeological circulation loops and the weathering front controls the river chemistry. This mechanism generates active weathering zones localized at the intersection of the descending flows and the weathering front. The export zone is located in the areas of convergence of upward flows. Secondly, we carry out a theoretical and numerical study on the effect of the temporal evolution of chemical gradients by diffusion on the effective kinetics of a non-linear reaction. We demonstrate that the spatio-temporal dynamics of a chemical gradient leads to effective kinetic laws up to several orders of magnitude greater or less than the kinetics determined in a homogeneous medium, depending on the order of the reaction. An analytical theory of effective kinetics is validated against numerical simulations. This work offers new perspectives to observe and understand the origin of subsurface chemical gradients, and to model their effects on the reactive dynamics of the critical zone, by providing new concepts of reactive transport.
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Submitted on : Tuesday, June 21, 2022 - 2:48:31 PM
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Charlotte Le Traon. Gradients chimiques dans les systèmes hydrologiques : origine, dynamique et conséquences sur les réactions chimiques. Sciences de la Terre. Université Rennes 1, 2021. Français. ⟨NNT : 2021REN1B022⟩. ⟨tel-03700863⟩

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