Chemical characteristics and reactive transport of soil salt ions in frozen soil during the freeze and thaw period

Abstract

Current researches on estimating soil salt movement during the freeze and thaw period are mainly based on total salt concentration, while ignoring the phase change of multi-component salts. In this study, the potential transport capability of soil water and salt ions in the frozen layer was investigated based on chemical characteristics and solute convection–diffusion theory. Field experiments were implemented to measure the change of total soil water, soil salt and its ion components. The soil water and salt content in liquid and solid phases were calculated by the FREZCHEM model, and then the movement of liquid soil water and soluble salt were simulated by the HYDRUS-1D model. The potential migration capability of soil water and salt components were then estimated by comparing the distribution profiles obtained by HYDRUS-1D and FREZCHEM. The results show that more than 74.5% of Cl, Na and SO4 were in liquid phase, which were less affected by precipitation-dissolution reaction and owned strong mitigation. The liquid soil water, soluble salt and salt ions in the frozen layer tended to decrease at the beginning of the freeze period and increase significantly at the end. The migration direction and quantity of various salt ions were different due to their concentration gradient and diffusion coefficient, causing the larger potential convection and dispersion quantity of Na+, Cl− and SO42- than those of Ca2+, Mg2+ and HCO3–. This study provides a new perspective for soil salt movement in frozen soil in agricultural areas with shallow groundwater table depth.