WRRCTR No.139 Numerical Simulation of a Thick Freshwater Lens: Pearl Harbor Groundwater Model

Abstract

The freshwater aquifer in the Pearl Harbor area on O'ahu, Hawai'i is the most important water resource of the island and constitutes a large proportion of its freshwater supply. The aquifer has a freshwater lens up to 304.8 m (1000 ft) thick, floating on top of a saline water zone. Mechanisms of groundwater movement are extremely complex because the upper boundary is confined near the coast and phreatic inland, while the lower boundary is nowhere confined. In this study, regional groundwater movement due to various pumping schemes is mathematically simulated by a set of partial differential equations. These equations are then solved numerically with a finite difference approximation. The location of the freshwater and saltwater interface, which constitutes the lower boundary of the system model, was estimated by Hubbert's formula. Consideration was also given to features concerning geohydrologic boundaries of the Pearl Harbor aquifer. It was concluded that a mathematical model can be used as a tool in the management of groundwater development in a thick Ghyben-Herzberg aquifer. Improvement of system simulation may be achieved by adding a mass transport element to study hydrodynamic dispersion within a transition zone where fluid density changes gradually.