Investigation of transient freshwater storage in island aquifers

Abstract

Fresh groundwater in an island aquifer is an extremely important resource that is highly vulnerable to variations in natural weather cycles and climate change effects. On small islands, precipitation creates subsurface freshwater lenses that float on top of coalesced saltwater that has intruded from the surrounding seawater. The volume and shape of these highly vulnerable freshwater lenses depend on the size and shape of the island, underlying geology, and the rate and duration of groundwater recharge. This study focuses on the transient changes in freshwater storage volume within these lenses. We completed a sand tank experiment to visualize an evolving freshwater lens that undergoes a dry- and -wet recharge cycle resulting in forming a shrinking and an expanding lens. The physical experiments provided a dataset to develop a robust numerical model that can rigorously simulate different types of evolving transient freshwater lenses. Our laboratory data along with past literature information have indicated that the shrinking of a freshwater lens occurred at a slower rate relative to an expanding freshwater lens. This aspect of a rather rapid freshwater volume recovery was investigated further using the numerical model by imposing cyclic recharge patterns to study the impacts of shortened recharge periods on freshwater storage in island aquifers. The results show that a sustainable dynamic equilibrium condition can be achieved within a few cycles of periodic recharge. Unsustainable conditions were only encountered at extremely short recharge periods. The model was then extended to simulate a freshwater lens in Dauphin Island, Alabama, to assess changes in freshwater storage under realistic wet and dry recharge cycles. The experimental data together with the modeling results presented in this study provide a better understanding of transient changes in freshwater storage patterns in small islands.