Scale‐ and Rate‐Dependent Solute Transport within an Unsaturated Sandy Monolith

Abstract

Solute transport in unsaturated soil is a scale‐ and flow rate‐dependent process, and high quality experimental studies are needed to introduce scaling relationships in the governing flow and transport models. Within this paper, we present an experimental study to characterize the solute transport for an unsaturated and undisturbed subsoil monolith sampled in a sandy aquifer. Leaching experiments were performed at different flow rates, and solute breakthrough at different positions in the monolith was measured by time domain reflectometry (TDR) probes. At TDR probe scale, the transport is assessed as being a convective‐dispersive process with mixing lengths <0.3 m. At the scale of the monolith, that is, the meso‐scale, transport is rather a stochastic‐convective process with mixing lengths >1 m. At the bottom of the monolith, the meso‐scale dispersivity is at least two times larger than the local‐scale dispersivity. In addition, this meso‐scale dispersivity can be successfully predicted using a stream tube model in combination with a local scale convective‐dispersive process. Important scale effects are also observed in the relationship between the square coefficient of variation of the local velocities and the dispersivity. In contrast to previous studies, we do not observe a significant difference in transport properties for different flow rates (from 1.05 to 54.16 cm d−1). We attribute this insignificance either to a balance between the decrease of the tortuosity and the increase of the coefficient of variation of the velocity with the flow rate or a non‐dependency of both terms on the flow rate.