Deep fossil freshwater in coastal Bangladesh

Aquifer overexploitation and saltwater intrusion threaten freshwater resources in coastal regions worldwide ^1,2 . In Bangladesh, arsenic contamination and droughts further impact shallow freshwater availability in parts of the country ^3,4 . In the coastal zone, much of the shallow groundwater is saline while availability of deeper fresh groundwater is poorly understood. Here, we utilize deep-sensing magnetotelluric soundings to image the contrast in electrical resistivity between resistive fresh and conductive saline groundwater along a 120 km profile adjacent to the Pusur River in the Ganges-Brahmaputra Delta, Bangladesh. Our data reveal two distinct deep freshwater bodies: a seaward-dipping wedge extending to 600 m depth in the northern transect, and a smaller fresh-to-brackish zone located near the coastline. We propose that these aquifers are remnants of preserved Pleistocene groundwater, supported by 14C ages up to 30,000 years ⁵ . An intervening high-salinity zone between the two freshwater bodies likely formed by Ganges paleovalley incision during the last glacial maximum, followed by marine transgression and deposition of conductive muddy sediments. Our results provide a framework for location and distribution of fossil groundwater reserves in Bangladesh’s riverine coastline, offering a potential resource for this water-stressed region. The interplay between past sea-level cycles, sedimentation, and hydrogeological processes may also control the distribution of deep fresh groundwater at other deltas.