Shale weathering: A lysimeter and modelling study for flow, transport, gas diffusion and reactivity assessment in the critical zone

Abstract

Shale weathering was characterized and quantified in a lysimeter and modelling study that jointly considered the flow, transport, gas diffusion and reactivity processes induced by exposure to the atmosphere of a recently excavated shale. In this 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 acidification associated with this oxidation of pyrite was efficiently buffered by calcite but a release of sulphates, cations, iron and trace metals (Pb, Ni, Zn, Co, Cu and As, mainly) was observed. Besides pyrite and calcite dissolution, iron (oxy-)hydroxide formed and proved to be a good phase for sorbing trace metals, whose content remained at low concentrations in the drainage water. Seasonal precipitation of gypsum was also identified, in connection with the summer desaturation of the shale’s shallow layers. The hydraulic, chemical and mineralogical observations made in the lysimeters were reproduced using HP1, a 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.