Seawater intrusion in river delta systems. Inter-annual dynamics and drivers of salinity variations in the southern Mekong Delta, Vietnam

Amidst intensifying impacts of climate change and anthropogenic pressure, seawater intrusion (SWI) emerges as a growing threat for delta systems worldwide, compromising freshwater resources and ecosystem stability. The Mekong Delta (MD) and its southernmost province Ca Mau are at the frontline of climate change impacts and anthropogenic modifications of the hydrological regime. To assess the inter-annual variability of river SWI, factors contributing to changes of the hydraulic pressure gradient between freshwater and seawater are analyzed. By utilizing long-term hydro-meteorological data from 2000 to 2022, the collective impact of catchment-scale and local-scale factors on salinity is evaluated. While salinity gradually increases by more than 10 g/L during the dry season, significant inter-annual differences of up to 15 g/L in peak salinity occur, regularly reaching and exceeding seawater salinity levels. Upstream freshwater discharge patterns were identified as major control for coastal water levels and resulting salinity levels. While climate variability governs natural upstream discharge fluctuations, dam constructions and operational responses to low-rainfall years lead to unfavorable conditions along the Mekong River, shifting and prolonging the low discharge period in the delta. This intensifies hydrological drought conditions and triggers extreme SWI events. Land subsidence of up to 2 cm per year, derived from water level records, is driving relative sea level rise and increasing the MD’s vulnerability to future SWI. Mitigation strategies for SWI are examined from new perspectives, highlighting the benefits of increasing early dry-season discharge through rethought upstream reservoir operations and water storage concepts, as well as adapted local sluice gate management.