Simulating and characterising concurrent lateral–vertical saltwater intrusion in low-lying coastal aquifers under pumping and storm-surge forcing

Salinisation of low-lying coastal aquifers arises via two interacting pathways—pumping-induced lateral saltwater intrusion (LSWI) and storm-surge-induced vertical saltwater intrusion (VSWI)—yet most studies model them separately, risking mischaracterisation of overall salinisation and suboptimal management. To address this gap, this study develops a fully-coupled, density-dependent surface–subsurface model that simulates the concurrent evolution and interaction of both intrusion pathways. A conceptual coastal aquifer is subjected to ten scenarios combining five pumping rates (0–200 m 3 /d) with either no surge or a single storm surge event over a 50-year horizon. Landward salt-mass is integrated and decomposed into LSWI, VSWI, and cross-effect components using normalised metrics. Results show that pumping is the primary control on multi-decadal salinisation and is nonlinear: stepwise increases in abstraction induce inland migration and thickening of the seawater wedge with more-than-proportional salt accumulation. A storm surge introduces an early vertical salt pulse whose magnitude and persistence increase with pumping. Without abstraction, the pulse peaks within a few years and largely flushes within ∼ two decades; with abstraction, it peaks later, decays more slowly, and at high rates leaves residual salinity that persists to year 50. Normalised analyses confirm that pumping amplifies surge impacts. Targeted comparison analyses under alternate hydraulic conductivity and recurrent-surge conditions indicate that the interaction framework remains qualitatively robust, although its expression depends on aquifer permeability and interruption of recovery by repeated flooding. Practically, the results argue for limiting overall pumping, temporarily curtailing abstraction during/after surges, and re-evaluating management interventions to avoid trapping surge-derived salinity in low-lying coastal aquifers.