Numerical Simulation Of Nitrate Transport Characteristics During The Freezing-thawing Process In Vadose Zone

With the development and utilization of groundwater resources in China,the problem of groundwater pollution is gradually highlighted.Recently,Today,the ecological and environmental impacts of groundwater pollution and the safety of drinking water are one of the hot issues of environmental concern.The vadose zone is a necessary area for surface contaminants to enter the saturated groundwater.Seasonal freezing-thawing changes the distribution of water and control the migration of salt in the vertical direction.Nitrate,is a common contaminating component of soil groundwater,due to its wide range of sources and chemical stability,can be long-term occur in the underground rock and soil media,so the groundwater environment is wildly contaminated for a long time by nitrate.In this paper,the freezing process of the unsaturated zone was simulated by laboratory experiment,and a one-dimensional water-hot salt coupling model was established by using HYDRUS-1D.Then,Combining laboratory experiment and the model to study the change characteristics of the moisture,heat and solute in the freezing process.The research paper draws the following conclusion:During the freezing process,the moisture produced significant redistribution characteristics in the vertical section: in the case of one-way freezing,the frozen frontal is moved from the top of the column to the lower part,and the upper part of the soil column is frozen and the liquid water content becomes smaller,Capillary negative pressure decreases and capillary force increases.it results that the water in the lower part moves to the upper part.In the case of two-way freezing,the frozen frontal movement moves from the ends of the soil column to the middle,so during the freezing process,the middle water of the soil column moves toward both ends.The freezing temperature affects the liquid water content and the redistribution of potassium nitrate in liquid water content and ice-water.The temperature control of the non-frozen water content changes,the amount of unfrozen water affected potassium nitrate concentration.When the temperature drops to zero,the liquid water becomes solid ice,and the decrease of the unfrozen water content will cause the migration of potassium nitrate to the liquid water,resulting in the increase of the concentration of potassium nitrate,the unfavorable water content and the concentration of potassium nitrate.The simulation result shows that the higher the initial water content,the slower the freezing rate,the smaller the freezing depth in the same time,the greater the concentration of potassium nitrate after freezing.The daily change rate of temperature has an effect on the salt-rejecting rate of Potassium nitrate,but the concentration of potassium nitrate after freezing is not effected.when near surface is contaminated,freeze-thawing action will accelerates the downward movement of solute and expands the contamination range of potassium nitrate.Sensitivity analysis of the three input model parameters showed the order of magnitude is: porosity>longitudinal thermal dispersivity >longitudinal solute dispersivity.