Reframing Deltaic Salinisation: Why Offshore Controls are Primary Drivers and Anthropogenic Factors are Accelerants

Coastal salinisation is frequently attributed to contemporary anthropogenic and climatic drivers, such as upstream freshwater withdrawal, land-use change, and sea-level rise. However, these explanations often overlook the fundamental role of offshore bathymetry and continental shelf geometry in regulating tidal dynamics and saltwater retention. We argue that these deeper geological and oceanographic controls govern where salinisation is spatially persistent, structurally organised, and resistant to reversal.

Across many large deltas, offshore geometry exerts first-order control on inland flushing efficiency. While wide, gently sloping shelves typically support large tidal ranges, the narrow and steep shelves—such as those found in the western Bengal Delta—generate tidal ranges approximately  1m lower than in the eastern delta. This reduction in tidal energy supports the development of dense, intricate tidal creek networks, while the accompanying weaker vertical mixing promotes the persistence of saline water. These networks distribute saline water laterally across the landscape and facilitate the formation of persistent, density-driven salinity wedges in underlying shallow aquifers. The lateral prevalence of these high-density wedges, coupled with relict salinity from the geological past, renders groundwater salinity a ubiquitous feature of the coastal region.

We illustrate this structural vulnerability using the Bengal Delta, where a pronounced east–west hydro-salinity divide is dictated by the "Swatch of No Ground"- a 1-km deep Pleistocene submarine canyon. This NNE-SSW deep-water feature on the narrow western shelf fundamentally influences creek-induced salinity patterns. While the eastern delta remains comparatively fresh due to higher-magnitude tidal ranges that promote the mixing and flushing of fluvial and saline water, the south-western delta exhibits persistent salinisation despite similar climatic forcing.

Leveraging a two-decade spatio-temporal dataset from 54 stations, we reveal a sharp asymmetry in salinisation rates: the Western Estuarine System is experiencing rapid increases averaging 111 ± 28 µS/cm yr^-1. To explain this variability, we introduced the Offshore Controlled Estuarine and Aquifer Nexus (OCEAN) Salinisation Framework. Our findings indicate that while declining river discharge and polderisation are critical accelerating factors, they operate within a system already structurally predisposed to salinity. Notably, similar anthropogenic forcing in the eastern delta has not produced comparable salinity responses, reinforcing the primacy of underlying structural controls.

Recognising that contemporary drivers function primarily as accelerants of existing vulnerabilities, rather than root causes, is essential for realistic adaptation planning and water security strategies. Interventions targeting only recent drivers risk underestimating the persistence of salinity in systems where offshore-controlled geometry imposes long-term constraints on freshwater recovery. This reframing has global relevance for the management and health-oriented water security of low-lying coastal deltas.