Migration Characteristics And Numerical Simulation Of Heavy Metal Cd In Layered Soils

With the development of industry and agriculture,the way heavy metals enter the environment more and more,heavy metal pollution is becoming more and more serious,and soil as an important medium in the environment,its heavy metal pollution has also become a problem that can not be ignored.As a typical heavy metal pollutant,Cd is not easily degraded after entering the environment,and the decomposition cycle is long,it has very high toxicity,and it is easily absorbed by the crop,and then enriched in the object,which seriously affects the crop yield and quality.Finally,it enters the human body throμgh the food chain and then accumulates in the human body and endanger the human body health.The soil in nature is mostly stratified structure,including clay layer and sand layer,etc,and different soil configurations have different effects on the movement of heavy metals in soil.In this paper,heavy metal Cd is taken as the research object.Through a large number of numerical simulation and indoor soil column outflow experiments,the transport of the heavy metal in the clay layer and sand layer soil is studied in order to provide scientific basis for the pollution risk assessment and restoration of heavy metals in soil and groundwater.The main contents of this study include three aspects:The first is to simulate the migration process of water in twelve different texture soils by using Hydrus-1D software to predict the effect of time step and space step on the stability of the numerical solution of the soil moisture equation（Richard equation）.The two is to simulate the migration process of heavy metal Cd²⁺in the clay layer（loam-clay-loam）and sand layer（loam-sand-loam）based on different upper boundary conditions and choose different solute transport models to further understand its migration characteristics and analyze the possible pollution risk of groundwater.The three is to study the movement of non reactive solute Cl^-and reactive solute Cd²⁺in the sand layer soil through the experiment of indoor soil column outflow,and the time and space changes of water infiltration and solute transport are simulated with Hydrus-1D software,and the related parameters are obtained.Through the study of this paper,the following conclusions are drawn:（1）When studying the influence of space and time dispersion on the numerical simulation of soil infiltration,the initial time step length is at 0.072-72min and the space step should be between 0.2-10.0cm for sandy soil.For the loamy soil,the range of space step is also between 0.2-10.0cm.And for clay soil,no matter how the initial time step and the space step take values,the simulation results will not converge or appear numerical oscillation.In the process of solving the Richards equation of soil moisture movement,the stability of numerical solution of soil moisture infiltration process with different textures is also different,the stability of the numerical solution of sandy soil is the best,the loamy soil is the second,and the clay soil is the worst.（2）When heavy metal Cd was transported in the soil with 200 cm thickness in the clay and sand layer.The migration equations selected one-site sorption model（OSM）,two-site sorption model（TSM）and two-region model（TRM）respectively.Taking 500,400 and 300 mm as the upper boundary condition,the lower boundary condition is free drainage.The results showed that the amount of infiltration（upper boundary）had little effect on the migration of Cd in soil.（3）For the clay layer soil,when selecting TSM,the migration distance of Cd in soil is the shortest,about 40 cm,mainly in loam soil layer.Next is the OSM,the migration distance of Cd is about 70 cm.Finally,for TRM,the migration distance of Cd is about100 cm.For the sand layer soil,when TSM is selected,the migration distance of Cd in the soil is the shortest,about 100 cm,mainly in loam soil and loamy sand soil layer.The second is TRM,there will be a small amount of Cd to the bottom of the soil migration simulation period.The last one is OSM,on the third day of Cd can be migrated to the bottom of the soil.（4）When different models were used to simulate the migration of Cd in soil,the retardation effect of the interface between the clay layer and the sand layer was also different.When the OSM is selected,the stratification of Cd in the loam-silty clay surface is obvious.The silty clay has obvious blocking effect on the migration of Cd.For the sandy layer soil,when the TSM was selected,the interface between the loamy sand and the loam also had a certain retardation effect on the migration of heavy metal Cd.（5）Indoor soil column experiments show that the loam-sand interface has a certain water resistance,mainly because of the existence of capillary barrier at the interface,reducing the soil water infiltration rate,and then increasing the soil moisture content in the upper layer.Because solute moves with water,the solute also has a certain retardation at the interface of loam-sand.In the process of experiment,the bottom loam soil is more moisture than the middle sand layer.It is speculated that there is a finger flow phenomenon in the middle sand soil layer.