New evidence for the dominance of vapour diffusion during the re-saturation of compacted bentonite

The established codes for modelling the re-saturation of compacted bentonite are based on the assumption that the hydraulic processes can be described as a two-phase flow of liquid water and air. Some models additionally take vapour flow into account. Recent investigations suggest, though, that compacted bentonite in contact with liquid water does not re-saturate via the liquid water phase but exclusively by evaporation in the pore space and subsequent vapour diffusion. The present paper adds further evidence to support this hypothesis. certain conditions a balance equation for the water vapour in the bentonite can be transformed into the well-known Fickian approach that has been empirically derived from observations of uptake experiments with liquid water. This transformed balance equation allows to interpret the so-called “diffusion coefficient” as a product of several simple physical parameters. An exemplary set of parameters for compacted MX-80 bentonite yields a value that lies within the narrow band of empirically determined “diffusion coefficients”.