Effect of Alongshore Aquifer Property Changes on the Steady‐State Extent of Seawater in Coastal Aquifers: Analytical Derivation and Numerical Validation

Abstract The spatial variability in coastal aquifer hydraulic properties significantly affects the extent of seawater and its movement. Yet, previous analytical solutions have almost exclusively evaluated only homogeneous and stratified aquifers. This study derived analytical solutions for the steady‐state, sharp‐interface seawater extent in two‐dimensional coastal aquifers (in the horizontal plane) comprising two alongshore regions of differing hydraulic conductivities ( K ), based on Laplace's and Poisson's equations. Numerical simulations were performed to validate the analytical solutions. The derived analytical solutions demonstrate that an alongshore change in K has a negligible effect on the steady‐state seawater extent under a constant and uniform head inland boundary, because the head is constrained uniformly alongshore, causing no groundwater to flow across the two regions. Any spatial change in head (e.g., the groundwater mound caused by distributed recharge) would reintroduce sensitivity. Otherwise, a constant‐flux inland boundary induces an alongshore variation of the seawater extent when an abrupt change in K is considered. Fresh groundwater flows from the lower‐ K to the higher‐ K zone, producing an over‐representation of freshwater discharge to the sea in the higher‐ K zone. The alongshore distance from the K transition boundary where the seawater extent is affected is approximately two to two‐and‐a‐half times the distance from the shoreline to the inland boundary, based on a defined threshold for negligible influence. The analytical solutions provide a convenient method for rapidly assessing the steady‐state seawater extent in aquifers with an alongshore change in K , with potential applications in seawater intrusion prevention and coastal groundwater management.