Synergistic amplification and threshold vulnerability of coastal aquifer barriers under sea-level rise and freshwater...

Coastal aquifers are increasingly threatened by saltwater intrusion (SWI) driven by climate change-induced sea-level rise (SLR) and anthropogenic freshwater decline (FWD). Although physical barriers such as cutoff walls (COWs) and subsurface dams (SSDs) are widely used to mitigate SWI, their performance under hydrological stresses such as SLR and FWD remains underexplored. In this study, four widely investigated field-scale conceptual aquifers are used to systematically evaluate the resilience and vulnerability of COWs and SSDs to these hydrological stresses under contrasting inland boundary conditions. Numerical simulations with the variable-density SEAWAT model show that inland boundary conditions strongly control barrier performance. Barriers remain largely resilient under flux-controlled (FC) conditions but become vulnerable under head-controlled (HC) conditions. Under HC conditions across a range of stress levels, COWs provide consistent mitigation benefits. However, SSDs exhibit distinct threshold behavior: they remain effective up to a critical stress level, beyond which their performance rapidly deteriorates. Model simulations reveal synergistic amplification under the combined effects of SLR and FWD, wherein SWI magnitudes far exceed the sum of their individual impacts. Furthermore, vulnerability maps are developed to delineate resilient and vulnerable regimes of barrier performance across a range of hydrological stress scenarios. These findings highlight the boundary-condition-dependent, threshold-driven, and synergistic amplification of barrier vulnerability, underscoring the need to explicitly account for combined climatic and anthropogenic stresses in the design of coastal groundwater protection strategies.