Modeling of 2D Density-Dependent Flow and Transport in the Subsurface

Abstract

A 2-dimensional finite-element model for density-dependent flow and transport through saturated-unsaturated porous media has been developed. The combined flow and transport model can handle a wide range of real-world problems, including the simulations of flow alone, contaminant transport alone, and combined flow and transport. The conventional finite-element methods and a hybrid Lagrangian-Eulerian finite-element method were incorporated in the transport module. Saltwater intrusion problems and instability caused by denser water on the top were investigated in this paper. Because the fundamental mechanism causing saltwater intrusion most likely is caused by density-induced convection and dispersion, the developed model was used to assess the interplay between density-driven flow and the subsurface media through which the saltwater intrusion occurs. The mathematical formulation of the model is comprised of fluid flow and solute transport equations, coupled by fluid density. In the specific case of saltwater intrusion and unstable brine transport problems, this set of governing equations is nonlinear and requires iterative methods to solve them simultaneously. Three case studies, which include a wide spectrum of physical conditions, show the verification and effectiveness of the model by comparing previously published solutions from other researchers with the simulation results of the present model. Two demonstrated problems examine the model capabilities to handle saltwater intrusion problems through unsaturated-saturated porous media and density-dependent flow and transport under unstable conditions.