Influence of inland boundary conditions on coastal aquifer response to sea-level rise

Climate change-induced sea level rise (SLR) is widely perceived as one of the main reasons for increased saltwater intrusion (SWI) in coastal aquifers. However, past research using analytical and numerical models that predict the effects of SLR in coastal aquifers shows contrasting SWI responses depending on the choice of inland freshwater boundary conditions. While simulations employing a head-controlled (HC) freshwater boundary condition show considerable additional SWI, those that use a flux-controlled (FC) freshwater boundary condition show negligible additional SWI. Both confined and unconfined aquifers exhibit this contrasting behaviour; however, the difference is more pronounced in confined aquifers, which show no additional SWI under FC conditions. Past research has identified that FC systems limit additional SWI through a natural ‘head-lift’ effect wherein inland freshwater heads rise in proportion to SLR. The current understanding of the mechanism that explains the enhanced SWI response observed under HC conditions is the decrease in the hydraulic gradient between the two boundaries. However, decrease in the hydraulic gradient will induce a decline in the freshwater flux through the coastal aquifer. The hydrological ramifications of this boundary condition have not been sufficiently explored. Here we perform laboratory-scale physical experiments, and computational numerical simulations using the SEAWAT model to i) show that HC systems enhance SWI through a ‘flux-decline’ effect which reduces upstream freshwater boundary flux in response to SLR. Consequently, regional groundwater fluxes decrease, altering the aquifer system’s overall water balance. On the other hand, FC systems maintain the freshwater boundary fluxes and do not suffer from this effect. However, the head-lift effect in FC systems can lead to flooding in low-lying areas where the aquifer extent is constrained by topography. This study provides a comprehensive assessment of the mechanisms driving SWI and highlights the broader hydrological consequences of selecting different inland boundary conditions when evaluating the impacts of SLR.