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.

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This study, building on the comprehensive discharge potential theory presented by Strack and Ausk (2015) https://doi.org/10.1002/2015WR016887, discovered that the effects of layer arrangements on steady state seawater intrusion and groundwater discharge in stratified confined coastal aquifers can be approximated via transmissivity centroid elevation (TCE), defined as the summation of the product of the transmissivity and elevation (above the aquifer base) of each layer divided by the total aquifer transmissivity. Specifically, a higher TCE in aquifers with high transmissivity layers at high elevations results in a more landward interface‐toe position in both flux‐ and head‐controlled coastal aquifer systems, as well as a higher freshwater discharge rate in head‐controlled systems. Furthermore, the toe position in head‐controlled stratified systems is only a function of the TCE and independent of the total transmissivity. Therefore, we can estimate the upper limit of seawater intrusion, that is, the furthest inland toe position, in stratified aquifers by letting the TCE equal to the elevation of the aquifer top. Another important implication of our results is that the interface toe position in coastal aquifers containing a preferential flow layer is controlled by the elevation of the preferential flow layer. Additionally, effective parameters and homogenization of the interface flow in stratified aquifers were developed to approximate the toe position and discharge rate. The results developed in this study provide significant advances in understanding the effect of aquifer stratification on groundwater flow and seawater intrusion in coastal aquifers.

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The freshwater-seawater interface is one of the most important regions in coastal aquifer systems, delineating the subsurface into zones with distinct fluid density and biogeochemical properties. Heterogeneity in hydraulic conductivity is inherent to geological formations, resulting in distinct interface profiles. Currently available analytical solutions are limited to homogeneous and stratified aquifers, and numerical simulations of variable-density flow and transport are the only option to characterize the interface in a two-or three-dimensional heterogeneous aquifer. This study presents a novel semianalytical method to fast delineate the steady seawater-freshwater interface in a two-dimensional, confined, heterogeneous aquifer with a constant flux inland boundary and indistinct seepage face at the vertical coastal boundary. The heterogeneous domain is conceptualized as a series of thin columns of stratified aquifers with the horizontal flow in each of them. The proposed approach is based on the concept of local transmissivity parameters and shows high accuracy in the interface delineation. Leveraging the ability to delineate the interface rapidly along with the Monte Carlo approach, we numerically investigate the stochastic behavior of the interface profile. The uncertainty in seawater intrusion is heavily influenced by the degree of heterogeneity and weakly influenced by the scale of heterogeneity. To homogenize the aquifer, we found that geometric mean generally underestimates the seawater intrusion and cannot be used as an effective parameter. We also showed that the near-coast, near-top region of the aquifer is the most influential region and should be characterized at a higher resolution to reduce uncertainties in estimating seawater intrusion.