We propose a new concept that has the potential to mitigate seawater intrusion and increase the fresh groundwater storage of oceanic islands by creating a less permeable slice along the shoreline. We present a proof‐of‐concept study to examine its effectiveness through analytical and experimental studies. Analytical expressions for calculating the freshwater‐seawater interface location, water table elevation, fresh groundwater volume, and groundwater travel time are presented for both barrier and circular islands, which are found dependent on three different scenarios of interface locations. The analytical solution of the interface location in a barrier island is verified through sand‐tank experiments. Sensitivity analyses based on a simplified conceptual model of St. George Island in Florida, USA, indicate that the fresh groundwater volume monotonically increases with the decrease in the hydraulic conductivity of the coastal less permeable hydrogeologic unit. On the other hand, the increase of the coastal less permeable unit extent leads to an increased fresh groundwater volume. However, when the interface tip is on the aquifer bed of the coastal less permeable unit, a further increase of the less permeable unit extent only slightly increases the fresh groundwater volume, since the interface does not change any more and only the water table is elevated. We demonstrate here that the concept proposed has the potential in increasing the fresh groundwater storage of oceanic islands. Analytical expressions presented can improve our understanding of seawater intrusion in a dual‐unit oceanic island.

Previous studies on the impact of sea-level rise (SLR) on seawater intrusion (SWI) are mostly based on the assumption of a homogeneous coastal aquifer. In this study, we extend those studies by investigating SLR-induced SWI in a layered coastal aquifer using the analytical method developed by Strack and Ausk (2015). We provide analytical solutions for steady-state SWI in confined and unconfined coastal aquifers, where both constant-head and constant-flux inland boundary conditions are considered. The analysis based on a three-layer aquifer indicates that in general aquifer stratification affects either or both the initial location and response distance of the interface toe. Specifically, for flux-controlled unconfined coastal systems, the toe response distance driven by SLR is a linear function of the hydraulic conductivity of the top layer and independent of hydraulic conductivities of lower layers. Using an equivalent homogeneous hydraulic conductivity (derived based on the initial interface toe location before SLR) would result in overestimation or underestimation of the toe response distance, depending on the hydraulic conductivities and thicknesses of the layers. For flux-controlled confined layered coastal systems, by contrast, SLR can not cause variation of the steady-state interface toe location, which is consistent with previous findings for homogeneous coastal aquifers. The interface toe location in head-controlled layered coastal systems is only a function of relative hydraulic conductivities between the layers. Moreover, the effect of the layer thickness on the interface toe location and response distance in the head-controlled system exhibits a more complicated pattern than in the flux-controlled coastal system, as changing the layer thickness changes both the overall aquifer transmissivity and inland freshwater flux. The results obtained enhance the understanding of the impact of SLR on SWI, which could provide a first-order assessment tool for relevant practitioners.

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