The effect of surge inundation on shallow groundwater salinity in the coastal low-lying poldered area of southwest Bangladesh-a 3D model investigation

Pervasive salinity in soil, surface water and groundwater is a significant challenge in many coastal poldered areas of Bangladesh. These areas are characterized by their vulnerability to cyclone-induced surge inundation due to the low-lying flat topography and their concave coastal profile. The surge water leads to the areas being inundated, which may result in elevated salinity levels in surface water and groundwater. High salinity can affect the availability of potable water and have a serious impact on agriculture, ecosystems, and the health of coastal communities. To effectively address the issue of groundwater salinity in these coastal areas, a comprehensive understanding of the contributing causes is required. A 3D model, HydroGeoSphere, was developed to examine the effect of surge inundations on surface and groundwater salinities. This model coupled surface and subsurface domains, using surge levels data from Chittagong station and a digital elevation model from USGS Earth Explorer. Evaporation and monsoon data were collected at the pond of the DAB site (Dacope, Khulna, Southwest coastal Bangladesh). The results show that even one year after the storm surges, salinity levels in the surface and near-surface areas remained elevated. The high salinity levels near the surface area may be primarily due to the surge water being trapped in depressions, as well as the effects of evaporation reducing the water content of the soil, leaving concentrated salt behind. Also, low-permeable sediments in the area may contribute to the persistent high salinity levels. The modelled groundwater salinity distributions showed good agreement with the measured groundwater salinity distributions derived from electrical conductivities obtained in 14 tube wells at various depths and locations along a cross-section.