Can Seawater Intrusion Modeling be Simplified?

Modeling seawater intrusion has evolved from a tool for understanding to a water management need. Yet, it remains a challenge. Inherent difficulties of solute transport modeling (e.g., assessment of dispersion coefficients; three-dimensional complexity of aquifer geometries and heterogeneity in hydraulic parameters) are combined with the concentration dependence of density. This dependence causes flow and transport to become coupled and the resulting problem non-linear. Here we explore approaches for overcoming the above difficulties. We start by modifying the current paradigm for seawater intrusion (Henry problem) in the hope of increasing realism and gaining insights into the factors controlling seawater penetration, width of the mixing zone and influx of seawater. Second, we modify the problem by incorporating heterogeneity in hydraulic conductivity. We find that heterogeneity causes the toe to recede while increasing both the width and slope of the mixing zone, but these effects are small in cases of moderate heterogeneity. Third, we further modify the problem to represent seawater intrusion in laterally sloping confined aquifers. The lateral slope leads to a dramatic increase in seawater penetration, caused by the development of sub-horizontal seawater convection cells. Since the main component of flow is horizontal, one would expect vertical flux to be of subleading order and thus negligible. Unfortunately, vertical fluxes produce mixing mechanisms that are lost in the simplified 2D areal model. These mechanisms can be approximated by an additional term in the dispersion tensor that arises from vertical integration of the original 3D governing equations. The resulting 2D model approximates the 3D dynamics with satisfactory results.