Transition Zone Width in Ground Wateron Ocean Atolls

Abstract

An analytical solution for the thickness of the transition zone between fresh and seawater in aquifers on ocean islands is presented. The interface is treated as a mixing layer similar to a laminar boundary layer between fluids moving at different velocities. The method requires assumptions of homogeneous isotropic dispersion, uniform hydraulic conductivity and steady flow; all water withdrawal is assumed to occur at the water table. The slope of the sharp interface is used in the solution for the mixing layer and the sharp interface is determined using Dupuit‐Forchheimer assumptions of horizontal flow. The method presented efficiently estimates the transition zone thickness, thus providing information not available from sharp interface models. The method is illustrated by application to the freshwater lens on an island of the Tarawa Atoll in the Pacific Ocean, and is used to show the effects of different pumping arrangements on the width of the mixing layer.