Researchers identify two deep ‘paleowater’ sources in Sundarbans

Scientists have identified two previously unknown deep freshwater reserves beneath coastal Bangladesh, a finding researchers say could help ease pressure on water supplies in areas affected by salinity.

The study, conducted along the Pusur River in the southwestern Ganges-Brahmaputra Delta (GBD), used deep-sensing magnetotelluric (MT) technology to map differences in underground electrical resistivity. The results revealed two major bodies of deep freshwater, known as R1 and R2, buried hundreds of metres below the surface, scientists say in their research paper “Buried deep freshwater reserves beneath salinity-stresse.

According to the researchers, the larger of the two, R1, lies to the north of the survey line and extends about 40 km, reaching depths of roughly 800 metres. The second, R2, is a shallower and smaller zone located within the central Sundarbans mangrove forest, stretching a similar horizontal distance down to about 250 metres.

Scientists describe the reserves as “paleowater” – ancient freshwater that infiltrated the ground thousands of years ago when sea levels were lower during the Last Glacial Maximum. Over time, layers of fine clay and compacted soil formed above the aquifers, creating a natural seal that has protected the trapped freshwater from seawater mixing.

The newly mapped deep reserves differ from typical coastal groundwater because they formed under ancient environmental conditions. They recharge extremely slowly, meaning they behave more like a finite geological resource than a renewable one.

Much of Bangladesh’s coastal groundwater is already affected by saltwater intrusion, worsened by sea-level rise, shrimp farming and seasonal shortages of surface water. In many inland areas, shallow aquifers also contain arsenic, further limiting safe supplies.

The study also found that the two aquifers are separated by a 20 km-wide highly saline zone, known as C1. This conductive gap, situated in the northern Sundarbans, corresponds to an old Ganges River valley that was carved during the last ice age. When sea levels rose again, the valley filled with marine sediments and saltwater, forming a natural barrier between the two freshwater bodies.

Researchers say the findings offer a clearer picture of how freshwater and saline water are arranged beneath coastal Bangladesh. They add that the MT model provides a framework for identifying suitable areas for future deep groundwater extraction while helping water managers set sustainable pumping limits.

The discovery may also be relevant to other coastal deltas worldwide where ancient sea-level cycles and river erosion shaped similar deep groundwater systems.