Origin and evolutionary processes of deep groundwater salinity in southwestern coastal region of the Ganges-Brahmaputra-Meghna Delta, Bangladesh

Abstract

Study RegionSouthwest coastal region of Bangladesh.Study FocusDeep groundwater in the southern Ganges-Brahmaputra-Meghna delta, the coastal region of Bangladesh, is partially salty whereas the drivers behind this salinity are not well understood. In this study, we explored the origin and evolutionary processes of the salinization in deep groundwater by analyzing geochemistry and environmental isotopes, paleogeography, and sedimentation processes.New Hydrological Insights for the RegionThe deep groundwater in the paleo-channel area exhibited higher salinity and lighter stable isotopes of water while that in the paleo-interfluve area lower salinity and heavier stable isotopes. Based on the Cl−/Br− molar ratios, the seawater and the coastal rainwater were surmised to be the ultimate source of the observed deep groundwater salinity. Evolution of the deep groundwater salinity can be explained by the paleogeography and associated paleo- and recent processes. The brackish estuarine water formed in the paleo-channels during the postglacial transgression was possibly intruded into the fresh deep groundwater due to the density contrast. The brackish deep groundwater in the paleo-channel area was subsequently mixed with the fresh groundwater in the paleo-interfluve area through lateral flow, driven by both anthropogenic (municipal pumping) and natural processes. These new insights about the occurrence and distribution of the fresh and the brackish deep groundwater can facilitate the management of potable water supply in the Ganges-Brahmaputra-Meghna delta or other similar regions.