Controls on groundwater and surface water salinity in coastal Bangladesh

Salinity in surface water and groundwater is a pervasive issue along coastal Bangladesh, a low-lying megadelta where around 35 million people live. A large amount of this land has been reclaimed using a network of low-lying polders. The area is particularly susceptible to flooding from tropical cyclones. Cyclone induced storm surges coupled with the low-lying reclaimed land can breach polder embankments and cause extensive flooding, resulting in excess salinity in soil and surface water. Salinity in drinking water is known to cause adverse effects on human health. It is, therefore, important to identify the controls surface water and groundwater salinity in these coastal areas. A fully coupled surface-subsurface model of a coastal polder by using HydroGeo- Sphere is developed to investigate the impact of storm surge events on groundwater salinity. The hydrological parameters were calibrated from the fieldwork at a field site in the Dacope Upazila, in the southwest coastal region of Bangladesh. The results suggest that sudden salt fluxes in the pond are likely to build up salinity in the underlying sediment. A set of scenarios were considered: a cyclone induced storm surge during both the monsoon and dry seasons, and both with and without remediation. The results show that surge events caused a rise in salinity in drinking water and near-surface groundwater. However, rapid remediation after a surge event could help mitigate the severity of the impact on drinking water. This provides suggestions for water resources management planning. The 2D cross-section model was extended to the 3D model to improve the understanding of the salinity process. Climate change scenarios were then used to evaluate the effects of episodic cyclone surges on shallow groundwater salinity. This study suggests that more frequent cyclones would worsen not only salinity in near-surface groundwater but lateral saltwater intrusion at the shallow or deep aquifers.Open Acces