Topographic Influences on the Development of Freshwater Lenses in Coastal and Island Aquifers in Lower and Middle Latitude Climatic Zones

Freshwater lenses overlying saltwater commonly occur in coastal areas. Frequently, the thickest parts of lenses with the highest quality water occur in close proximity to the shore under hills, beach ridges, and dunes rather than beneath areas of lower topographic relief even though these may be farther from the ocean. The seasonal precipitation pattern of the middle and lower latitudes increases these variations in lens quality. The higher topography protects the lens from losses due to evapotranspiration. In addition, the longer flow paths followed by the infiltrating precipitation dampen the "flashiness" of recharge event, thus minimizing water table fluctuations and the resultant disturbance at the transition zone. The two dimensional, density-dependent, fluid flow and mass transport mathematical model SUTRA-GUI is used to model the effects of varying recharge and evapotranspiration rates in aquifers with different permeabilities and dispersivities. The model is an effective tool for predicting lens configurations. The model can also be used to assess the sustainable yield of the lens by treating withdrawal as an increase in evapotranspiration. The implications of these findings regarding the impacts of land use activities that could result in the lowering of the topography are discussed.