Saltwater Intrusion and Recirculation of Seawater in Isotropic and Anisotropic Coastal Aquifers

Saltwater intrusion and the recirculation of seawater that occur at the seacoast in coastal aquifers are investigated for vertically isotropic and anisotropic aquifers as dependent functions of two independent dimensionless ratios, i.e.,the ratio of the freshwater inflow relative to the vertical density-driven buoyancy flux (az) and the ratio of the vertical hydraulic conductivity relative to the horizontal hydraulic conductivity (rK). Values of az are varied over a range from 0.10 to 10.0 for three cross sections in which rK=0.01, 0.10, and 1.00. The maximum values of saltwater intrusion and recirculation occur at the smallest values of az=0.10 and rK=0.01, and the minimum values occur at the largest values of az=10.0 and rK=1.00. Numerical experiments are performed first for the uncoupled condition, in which the flow and transport equations are solved for a constant-density flow field, and then for the coupled condition, in which the flow and transport equations are solved for a variable-density flow field. For all three hydraulic conductivity ratios, saltwater intrusion is significantly greater and the recirculation of seawater is significantly less for the coupled condition than for the uncoupled condition at the smaller values of az. The results of this investigation clearly demonstrate that saltwater intrusion and recirculation in coastal aquifers increase at the smaller values of az as the degree of vertical anisotropy increases, and that variable-density numerical codes should be used to obtain accurate solutions for saltwater intrusion and recirculation in both isotropic and vertically anisotropic coastal aquifers in which the freshwater inflow is small relative to the vertical density-driven buoyancy flux.