Simulation of shale weathering by seasonal O2 diffusion in a variably saturated soil

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.