Storm surge-driven salinization mechanisms in coastal aquifers: Insights from 2D and 3D modelling in southwestern Bangladesh

Coastal delta regions—particularly in Asia, where nearly half a billion people reside—are highly susceptible to tidal flooding and storm surges, both of which are being exacerbated by sea-level rise. In Bangladesh's Ganges–Brahmaputra–Meghna Delta, these events, particularly storm surges, can frequently cause saline inundation that threatens freshwater resources and community livelihoods. This study investigates the hydrodynamic and salinity responses of a low-lying coastal aquifer in Dacope Upazila, southwestern Bangladesh, using two-dimensional (2D) and three-dimensional (3D) numerical models developed with HydroGeoSphere. Field observations and additional hydrogeological data were integrated to examine groundwater and surface-water interactions under storm-surge, monsoon, and dry-season conditions. 3D simulations reveal interacting mechanisms that control both the persistence and spatial heterogeneity of storm-surge salinization: topographic retention of surge water in shrimp farms and ponds, lateral hydrologic connectivity through multi-directional flow pathways, and evaporation-. Comparison with 2D simulations shows that omitting lateral storage and cross-sectional flow leads to rapid surface drainage and systematic underestimation of near-surface salt accumulation and recovery timescales. The 3D framework therefore provides a more realistic basis for assessing salinization pathways and recovery timescales in low-lying coastal settings. While the model captures regional hydrodynamic and salinity behavior, limitations associated with smoothed topography at 30-m grid resolution, idealized tidal boundaries, and the one-year simulation period indicate that actual recovery may take longer than represented here. These findings provide process-based understanding of surge-driven salinization and offer guidance for identifying salinity-vulnerable zones, optimizing monitoring strategies, and evaluating drainage interventions to reduce storm-surge salinity exposure to critical freshwater resources in deltaic aquifers.