Experimental study on simultaneous heat-water-salt migration of bare soil subjected to evaporation

Evaporation in shallow soil has a great influence on the properties of surface soil. Therefore, under evaporation conditions, research on the rule and mechanism of heat-water-salt migration in surface soil has great significance for engineering geology and environmental science. In this study, with full consideration of the initial water content, salt content, atmospheric temperature, and groundwater supply, a series of indoor evaporation tests were performed on shallow saline soil to observe the heat-water-salt migration process. In addition, before and after the evaporation test, the soil microstructure at different depths was observed and analysed to explore the influence of heat-water-salt migration on the soil. The results show that during the evaporation process, salt carried by water migrates in the soil column, and the water-vapour phase transition causes salt accumulation, which also leads to the electrical conductivity (EC) profile showing a V-shape, and the salt content in the middle of the soil column is lower than that in the end. In addition, high atmospheric temperature significantly reduces the surface water content but alleviates salt accumulation. Under the condition of groundwater supplementation, the salt accumulation in the soil column surface is rapid. By observing the microstructure, it can be seen that a flocculent structure exists in the soil before evaporation. After evaporation, the surface microstructure changes significantly, and pores and salt crystals can be observed. The results provide a reference for understanding heat water-salt transport in arid areas.