Mechanism And Modeling Of Coupled Water-Heat-Solute Movement In Unidirectional Freezing Soils

Frozen soils is widely distributed on this planet, while much of them both at seashore and inland areas, such as north and northwest parts of China, has suffered salinization for long, and severely limits local agriculture production. Meanwhile, since permafrost is becoming more active as global temperature rises, and also because of intensified human activities in cold region recently, frozen soils are facing unprecedented environmental pollutions. As a result, for the sake of protection and sustainable development of cold region, it's critical to understand distributions and movements of water and solute in cold region.Based on previous studies, mechanism of water and solute movements in freezing soils in field scale were discussed in this study. Meanwhile, coupled movements of water, heat and solute in unidirectional freezing soils were also studied by field tests as well as mathematic modeling.Results showed water-ice interface in freezing soils were thermally non-equilibrium, though slow water movement in frozen part makes its water-ice interface closer to thermally equilibrium status than at freezing front. Freezing front is much like an evaporation interface, because its low hydraulic potential would induce water in unfrozen moving towards it. However, water movement in the whole soil profile is non-continuous, since much of the water would freeze near the freezing front without moving into frozen part.Solute existed in frozen soils with various statuses, which could be classified into isolated (which was dissolved in brine pocket and films around isolated particles) and open (which was dissolved in continuous films). Isolated solute would move towards warmer areas and transfer into open status at the same time, while the solute in open status would redistribute to form a stable profile in frozen soils.A mathematic model was also developed to simulate coupled movement of water, heat and solute in freezing soils. The frozen part, unfrozen part and freezing front were described differently in this model. Water-ice interface were assumed to be thermally non-equilibrium, and different solute move in different mechanisms.According to results of field tests in Inner Mongolia and model simulation, both soil water and solute content would continuously decrease before freezing, and would reach their lowest points at freezing front; they would then increase dramatically for a short time; and water content would stay stable while solute content would decrease slowly thereafter.The modeling results also showed that water, heat and solute movement would influence each other. Slow freezing rate would result in less water and solute increment in frozen part. Though higher initial water and solute content would decrease freezing rate to some extent, it would result in more water and solute increment in frozen part.