Experiment And Numerical Simulation Analysis Of Solute Transfer In Unsaturated Soil In Cold Area

The exploration of soil solute migration in cold regions plays an important role in increasing soil utilization in permafrost regions,rationally developing and utilizing the environment in permafrost regions,controlling soil pollution problems,solving soil salinization in cold regions,and promoting agricultural development in permafrost regions.In this paper,TDR and one-dimensional soil column instruments are used to simulate indoor conditions such as different salt concentrations,different types of ions,different porosities,and different temperatures to find the law of solute redistribution in unsaturated soils under unidirectional freezing.Finally,numerical simulation was carried out based on the experimental data,and the specific situation of soil solute migration during freezing was obtained.According to the results of experiments and numerical simulations:?1?Concentration is the main factor that affects conductivity,temperature is a secondary factor that affects conductivity,and temperature and concentration are two variables that have no cross-effect.At the same time,when the four solutions have the same temperature and the same concentration,the conductivity isz_{Na Cl}>z_{K Cl}>z_{Mg Cl2}>z_{Ca Cl2}.?2?Under normal temperature and low temperature conditions,the same soil sample,temperature and moisture content are controlled,the larger the soil density,the smaller the permeability coefficient.At low temperature,when the soil sample,temperature and density are the same,the higher the soil moisture content,the smaller the permeability coefficient,but when the moisture content reaches a certain value,the permeability coefficient will not decrease as the moisture content increases.?3?In the solute migration test of unsaturated soils under one-way freezing,the factors that affect the solute migration process are the density of the soil,the initial moisture content,the type of solute,the freezing temperature,the particle size gradation,and the solute concentration.Increasing soil density and initial moisture content will both reduce the efficiency of soil solute migration,and increasing the content of coarse particles in the soil will effectively reduce the efficiency of soil solute migration.The decrease of freezing temperature and the increase of soil solute concentration can improve the efficiency of solute migration.In addition,the stability of the four solutes in the soil is Ca Cl₂>Mg Cl₂>KCl>Na Cl.?4?The reduction of freezing temperature will increase the rate of soil temperature reduction and the rate of moisture freezing.According to the conservation of mass,the moisture content of unfrozen water will continue to decrease.It can be seen that the moisture is moving upward,and the amount of migration is Q_-30?>Q_-20?>Q_-10?.In addition,according to the conservation of mass and the location of the frozen front,it can be obtained that the amount of solute migration in the soil isC_-30?>C_-20?>C_-10?.?5?When the water content in the soil increases,the cooling rate of the soil decreases,and the difference between the maximum and minimum temperature of the soil in each period becomes smaller.At the same time,the rate of solute migration has decreased.And the increase of the water content will cause a slight numerical oscillation at the position of the maximum solute concentration in the redistribution of the solute.However,the amount of water migration in the soil will increase,and the rate of change in water per unit time will also increase.