Numerical Simulation Of Saturated-Unsaturated Solute Transport In A Chromium Contaminated Site

In recent years,with the development of society,more and more harmful substances are produced by human activities,the soil and groundwater conditions are deteriorating,and the environmental quality is in urgent need of improvement,especially heavy metal pollution,which seriously endangers the safety of human and ecological environment.The vadose zone is regarded as the upper boundary of phreatic water and confined water system.It exchanges water and solute with phreatic water and confined water system,forming a coupling system of saturatedunsaturated water and solute transport.Due to the difficulty of data acquisition and the limited amount of simulation calculation,the existing research mostly uses one-dimensional equation to describe the water movement in vadose zone,but only uses three-dimensional simulation for saturated groundwater.The recharge flow of saturated groundwater is calculated by the vadose zone model,which is substituted into the saturated zone equation to calculate the new groundwater level and update the lower boundary of the unsaturated zone.This method is not conducive to the calculation feedback between models,and is prone to calculation error.Therefore,it is very important to establish a coupling model of unsaturated saturated system for groundwater protection engineering.The present situation of heavy metal pollution in soil and groundwater in China was summarized;This paper reviews the research progress of numerical simulation of unsaturated zone and saturated zone,and establishes the research purpose and content of this paper.In order to accurately depict the pollutant migration law in the unsaturated saturated system of the actual site and reveal the parameter sensitivity of the variable saturation reaction solute transport model,a three-dimensional unsaturated saturated water flow and solute transport model of a chromium contaminated site was established by using FEFLOW software.The variation of pollution plume distribution in unsaturated zone caused by groundwater level variation with the increase of recharge is analyzed;The effects of retardation coefficient and reaction constant on solute transport were discussed.The main research results are as follows(1)In order to solve the problem of poor interaction between unsaturated zone system and saturated zone system,based on the principle of variable saturation,this paper uses FEFLOW software to establish unsaturated saturated flow and solute transport model.The model does not need to presuppose the water table and delineate the unsaturated zone and saturated zone.The software calculates the initial water level,boundary conditions and hydrogeological parameters.The unsaturated saturated flow model is closer to the unsaturated zone,the saturated zone and the groundwater flow between them.(2)By adjusting the parameters,the initial flow field in the study area was obtained,and then the solute transport was simulated.This model simulates the horizontal and vertical migration of hexavalent chromium in unsaturated zone and saturated zone respectively,and simulates the change of pollutant concentration and distribution in unsaturated zone and saturated zone due to the fluctuation of groundwater level when the recharge increases.The related research expands the research scope of groundwater numerical simulation and enriches the means of contaminated site evaluation.(3)Finally,the effects of retardation coefficient and reaction constant on solute transport are discussed.The results show that the reaction constant has the strongest effect on solute transport,which provides some reference value for future pollution control.