Topographic controls on seawater intrusion in island aquifers

Abstract:

Islands are extensively distributed in global oceans and considered as one of the most vulnerable places in the world. Based on the Ghijben-Herzberg and hillslope-storage Boussinesq equations, we derived analytical solutions of steady-state seawater intrusion for radial islands, with a focus on the freshwater-seawater interface and its corresponding watertable elevation. These analytical solutions, verified against laboratory experiments, were employed to investigate the effects of topography on seawater intrusion in island aquifers. Three different shapes of island aquifer were compared: convergent, rectangular and divergent. The results show that the extent of seawater intrusion is the most serious in divergent aquifers while the lightest in convergent aquifers. In contrast, the watertable elevation is the lowest in divergent aquifers while the highest in convergent aquifers. Moreover, the effects of topography on the freshwater-seawater interface and watertable elevation relate to the aquifer width and distance from the circle centre to the no-flow boundary. Both a larger aquifer width and distance from the circle centre to the no-flow boundary can weaken topography effects and hence lead to a smaller deviation of seawater intrusion between three shapes. Furthermore, the combined effects of topography and unsaturated flow on seawater intrusion were investigated. It is found that unsaturated effects are more significant for shallow divergent aquifers with a large capillary fringe and recharged by a small recharge.