Solute Segregation during Soil Freezing and Solute and Heat Transport inPermafrost

摘要

The primary objectives of the research were to develop a better understanding ofsalt segregation processes during freezing of saturated soils and of the resulting salt concentration profiles in annually frozen soil and permafrost. Laboratory experiments on the freezing of saline sand columns showed that substantial salt redistribution occurred with downward freezing but not with upward freezing. Convection by salt fingering in the partially frozen region above and below the phase boundary was found to play a major role in the flow and redistribution of salts during freezing. While there was never any evidence for the existence of a salt-enriched layer in the unfrozen region adjacent to the phase boundary, the theory for an effective distribution coefficient was useful in describing salt redistribution during freezing of these saline sands. An analytical solution was developed for the Stefan problem for a freezing saline soil which used an apparent heat capacity formulation in a multiple moving boundary problem. Solutions were obtained for the thermal regime and phase boundary position with temperature-dependent thermal properties. For this problem, a numerical (finite element) model was also developed which was capable of handling a wide range of boundary conditions and initial conditions. The primary problem for developing a model for salt redistribution during soil freezing is that of coupling of heat and salt flow that occurs in the region near the phase boundary (freezing front), through the freezing front, and in the unfrozen region adjacent to the freezing front.