Observation and Modeling of Hysteresis in Solute Diffusion in Unsaturated Soils.

Hysteresis in solute diffusion refers to the unequal solute diffusivity at the same water content or matric potential when a soil is subjected to drying or wetting process. Hysteresis of other soil properties such as conductivity, permeability, and gas diffusion has been extensively examined; but very limited information is available for hysteresis of solute diffusivity. This research used three soils, a sand, a sandy clay loam and a clay soil, to examine the hysteresis phenomena in solute diffusion. The 2-mm sieved soil samples were packed into cylindrical acrylic Tempe cells to the predetermined bulk densities. To obtain the drying limb, the diffusion cells were first saturated with 0.01M potassium bromide (KBr) solution and placed inside a pressure chamber. After applying the prefixed matric potential, the corresponding volumetric water contents of the cells at equilibrium were recorded. The cells were then transferred to a vapor saturated humidity chamber to equilibrate for another 7 days before the diffusion experiment. The diffusivity of bromide was then measured through the anion-exchange membrane method. The wetting limb was achieved by injecting known amount of 0.01 M KBr solution into the packed cell from the bottom port through a hanging column, which was conducted in a vacuum chamber to accelerate the KBr uptake. Hysteresis of solute diffusivity versus water content was observed in all three test soils. Our preliminary modeling results show that the drying limb diffusivities of the test soils can be well described by an empirical model with constants estimated by particle-size distribution and bulk density; while the wetting limbs are better described by a conceptual model that assumes solute diffusive pathway is comprised by the serial arrangement of pore water and film water.