Effects of interactions between soil particles and electrolytes on saturated hydraulic conductivity

Interactions between soil particles can strongly affect water movement in soil. In this study, a model to predict soil saturated hydraulic conductivity based on interactions between soil particles was proposed. Firstly, a model of ion-soil particle interactions at the soil/water interface was established taking specific ion effects into account. Based on the ion-soil interaction model, the theoretical relationship between the sodium adsorption ratio and the surface potential was derived, and the surface potentials were calculated at different sodium adsorption ratios and electrolyte concentrations. Secondly, a theoretical expression of midpoint potential between two adjacent soil particles in a mixed 1:1 + 2:1 electrolyte solution (e.g., NaCl + CaCl2) was derived. The midpoint potential was estimated based on the calculated surface potential. Thirdly, the interaction pressure (repulsive pressure) between soil particles was quantified based on the obtained midpoint potential. Finally, an empirical model of the relationship between measured saturated hydraulic conductivity and maximum net repulsive pressure was proposed. The surface potential, electrostatic repulsive pressure and net pressure between soil particles in the presence of Na+ is higher than that of Ca2+ because the effective charge of the former (1.18) is smaller than that of the latter (1.98). Although the soil saturated hydraulic conductivities were substantially different in Na+ and Ca2+ solutions, they can be described as a function of maximum repulsive pressure between soil particles. Employing this empirical relationship, saturated hydraulic conductivities might be predicted in other soils at different sodium adsorption ratios. This work implies that the sodium adsorption ratio and electrolyte concentration control saturated hydraulic conductivity through affecting the electrostatic repulsive pressure between soil particles. Highlights A model of ion-soil particle interactions was established considering specific ion effects. The theoretical relationship between sodium adsorption ratio and surface potential was derived. The interaction pressure between soil particles in mixed electrolytes was quantified. An empirical model of the relationship between saturated hydraulic conductivity and net repulsive pressure was proposed.