A 3D finite element model for seawater intrusion in coastal aquifers

Solving seawater intrusion in coastal aquifers in the context of a sharp interface approach is not usually, appropriate, in particular when the transition zone cannot be neglected due to the dispersion phenomenon. In this work, a 3D hexahedral finite element model based on the variable density flow approach is developed. The Galerkin technique is used for the spatial discretization and a weighted finite difference for time discretization. The variable density mathematical model is made of a set of nonlinear coupled PDEs of flow and solute transport to be solved for hydraulic head and mass fraction of the solute component. The Picard and Newton schemes are used to solve respectively the nonlinear flow, and transport equations. The Preconditioned Conjugate Gradients method is used for solving the symmetric flow finite element equations system, whereas the Preconditioned BiCGSTAB method is chosen for solving the non-symmetric solute transport equations system. Both solvers are based on the incomplete Factorisation method as a preconditioner. For transient simulations. the time stepping is dynamically adjusted depending on the convergence behaviour of the nonlinear iteration. Three benchmark examples are simulated to test and verify the model accuracy. The results compared well with those of some published analytical and numerical models. The model is also applied to a field case in Morocco to test its performance to solve field problems.