Joachim Tremosa

A HPx model that jointly considered the flow, transport, gas diffusion and reactivity processes induced by exposure to the atmosphere of a recently excavated shale was used for the interpretation of a weathering experiment in a lysimeter. The lysimeter was filled with a mechanically disaggregated shale presenting preferential pathways for water and a hydraulic conductivity at saturation of 100 cm/day. The water content and the seasonal saturation and desaturation cycles were identified as the main driving mechanisms of shale alteration. The water content determined the diffusion of gaseous oxygen in the shale’s unsaturated porosity, which gave rise to a zonation of the oxidation of pyrite, contained at 1 wt% in the shale. The oxidation of pyrite induces a release of sulphates, cations, iron and trace metals (Pb, Ni, Zn, Co, Cu and As). The acidification due to pyrite oxidation is buffered by calcite dissolution. While the formation of iron (oxy-)hydroxide is efficient for sorbing trace metals, whose content remained at low concentrations in the drainage water. Seasonal precipitation of gypsum during the summer desaturation at the top of the lysimeter was also identified. The hydraulic, chemical and mineralogical observations made in the lysimeters were reproduced using HPx reactive transport code under unsaturated conditions. It was possible to account for the gas diffusion where O2 availability controlled the reactivity with the shale, depending on the meteorological conditions and the drainage at the base of the lysimeter.

In the present study, we report lithium concentrations and Li isotopic compositions for different samples within a granitic watershed (Margeride, France). We investigate unweathered bedrock and samples displaying different stages of weathering in order to characterize Li isotopic fractionation. This was achieved by coupling lithium isotope geochemistry and reactive transport modeling during granite weathering at the scale of this watershed. The following manuscript reports methodology and the preliminary data.

In this study, a new formulation for the thermo‐osmotic permeability of natural pore solutions containing monovalent and divalent cations is proposed. The mathematical formulation proposed here is based on the theoretical framework supporting thermo‐osmosis which relies on water structure alteration in the pore space of surface‐charged materials caused by solid‐fluid electrochemical interactions. The ionic content balancing the surface charge of clay minerals causes a disruption in the hydrogen bond network when more structured water is present at the clay surface. Analytical expressions based on our heuristic model are proposed and compared to the available data for NaCl solutions. It is shown that the introduction of divalent cations reduces the thermo‐osmotic permeability by one third compared to the monovalent case. The analytical expressions provided here can be used to advantage for safety calculations in deep underground nuclear waste repositories.