Water and Solute Fluxes in Soils Undergoing Freezing and Thawing

Abstract

Water flow and solute transport processes are more complex in frozen than in unfrozen soils. The objective of this study was to determine the effects of freezing and thawing on water flow and solute transport in field soils. Experiments were conducted at two field sites each containing soil with a distinct texture (clay loam and sandy loam). Bromide (Br⁻) was used as a tracer, and soil water content and tracer concentrations were measured at different depths and times. A mass balance method was proposed to estimate mean values of water and solute fluxes (apparent soil water and Br⁻ fluxes) in frozen, transition, and unfrozen zones during a freezing period and conduction and thawed zones during a thawing period. During freezing, soil water content significantly changed below and above the freezing front. Because of the presence of solute, relatively high liquid water content was found at soil temperature below zero. Soil water fluxes were mainly affected by the phase changes during freezing and thawing periods in the clay loam soil and by changes in the hydraulic properties related to liquid water content in the sandy soil. The contribution of water flux to the change of liquid water was much larger in the frozen zones than in the transition zones. During freezing, Br⁻ ions excluded from ice and the different directions of dispersion and convection fluxes resulted in increases and then decreases of the apparent Br⁻ concentration to mean measured Br concentration ratios. During thawing, the movement of liquid water and the superposition of convection and dispersion fluxes resulted in Br⁻ concentration peaks and higher variation of the ratios. The water flow process was less synchronous with the solute transport process in unfrozen soils than in frozen soils.