Assessment of the impact of sea-level rise on steady-state seawater intrusion in a layered coastal aquifer

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.