Near-surface solute redistribution during evaporation

We present results from solute transport experiments in an evaporating composite porous medium consisting of a cylindrical inner core with coarse sand that was surrounded by a mantle with fine sand. Small volumes of dye and salt tracer were applied at the surface of the fine material of the evaporating column. The pressure head at the bottom boundary was kept constant using a hanging water table ensuring liquid phase continuity to top surface in both fine and coarse material, whereby the latter was hydraulically less conductive at these pressure conditions. Contrary to the expectation that solute accumulation at an evaporating surface is proportional to local cumulative evaporation, high concentration spots developed at the surface of the coarse material, for which IR surface temperature measurements did not indicate higher evaporation fluxes. 3D unsaturated flow and transport simulations and a second tracer experiment monitored with magnetic resonance imaging (MRI) demonstrated that preferential upward water flux in the fine sand deeper in the column and near-surface lateral water flow from the fine into the coarse sand in combination with a downward diffusive flux are responsible for the local solute accumulation. We propose that at the wet regions of a soil surface, solute accumulation is largely decoupled from local evaporation fluxes and strongly governed by relative differences of the hydraulic conductivities. The possible formation of high solute concentration spots at the surface of coarser regions usually representing preferential flow pathways during strong precipitation may have an accelerating effect on the leaching of solutes.