Development And Application Of The Numerical Models For Groundwater Flow And Contaminant Transport

The numerical models for groundwater flow and contaminant transport are essential for studying groundwater pollution migration,guiding groundwater resource management and pollution prevention.However,the applicability of groundwater models is a challenge due to the complexity of aquifer space and the need for surface water-groundwater coupling simulation.The development of groundwater numerical models with stronger spatial adaptability and higher coupling application is of great significance in promoting the management of groundwater,groundwater-surface water resources and environments.In this paper,according to the differences of the aquifer in horizontal and vertical spatial scales and the need for surface water-groundwater coupling simulation,two types of numerical models for groundwater flow and contaminant transport are developed:(1)Flow and contaminant transport models in saturated-unsaturated zone based on finite element and finite difference coupling(FE-FDM);(2)Flow and contaminant transport models in groundwater based on arbitrary polygon grids(APFVM).In this paper,the governing equations of flow and contaminant transport in the saturated-unsaturated zone are discretized by the finite element method in the horizontal direction and the finite difference method in the vertical direction.The numerical schemes are derived.The FE-FDM models for groundwater flow and contaminant transport are established based on the schemes.The accuracy of the proposed models is verified by comparing with the analytical solution and the simulation results of the traditional numerical model in typical examples.The proposed groundwater flow and contaminant transport modes are applied to the groundwater simulation research of landfills.The research results show that:(1)The results calculated by the proposed groundwater flow model are in good agreement with those of the analytical solution and common software in four typical examples,which shows that the proposed flow model can accurately simulate the flow process in the saturated-unsaturated zone;(2)The results calculated by the proposed solute transport model are according well to those of the analytical solution and common software in three typical examples,which shows that the model can accurately simulate the law of groundwater pollution migration;(3)The application research of the proposed models in a landfill site shows that the FE-FDM method for simulating flow and solute transport in the saturated-unsaturated zone can be effectively applied to practical cases and play crucial technical support for studying pollution transport law and pollution prevention and control;(4)The FE-FDM models combine the advantages of the finite element with strong boundary adaptability and simplicity of the finite difference,and improve the adaptability to the space of irregular aquifer system.In order to further improve the flexibility of discretization and ability of coupling application for groundwater models,the governing equations of groundwater flow and contaminant transport are discretized using the multipoint flux approximation method in the horizontal direction and the two-point approximation method in the vertical direction based on the finite volume method,Then,the numerical schemes are constructed.The APFVM models for flow and contaminant transport are established based on the schemes.The accuracy of the new models is verified by several typical examples.The new modes are applied to the groundwater simulation research in Pinggu.The research results show that:(1)The results simulated using the new flow model based on APFVM are in good agreement with those of the analytical solution and common software in five typical examples,which shows that the model can accurately simulate the process of groundwater flow;(2)The results calculated by the new contaminant transport model based on APFVM are consistent with those calculated by analytical solution and common software in five typical examples indicating that the new model can accurately calculate the law of contaminant transport in groundwater;(3)The application research of the new models in Pinggu Basin shows that the APFVM method for simulating flow and contaminant transport in groundwater can play a guiding role in the study of groundwater pollution migration law in Pinggu Basin indicating this method can be effectively applied to practical cases and can play an important technical support for the study of groundwater pollution migration law and pollution prevention and control;(4)The new models based on APFVM can use arbitrary polygon grid,which improves the discretization flexibility and nd coupling application ability for the groundwater models.In developing two groundwater models(FE-FDM and APFVM),the finite element and finite difference calculation methods are coupled together,and the arbitrary polygon subdivision calculation method is introduced,which improves the groundwater model’s spatial discretization flexibility and adaptability to the irregular aquifer system,and expands the groundwater calculation model and theory.The APFVM model based on the arbitrary polygon can use a sub-watershed as a grid unit,which lays a foundation for the coupling calculation and application of surface water and groundwater with a sub-watershed as a unified grid unit and is of great significance to the collaborative prevention and treatment of surface water and groundwater pollution.