Groundwater vulnerability to Amphan and other recent cyclones in a tropical mega-delta

Delta and Coastal habitats are most susceptible to sea level rise and climate change impacts like extreme climatic events (ECs), thereby putting a huge population at survival risk, primarily through access to clean water. ECs are known to pollute coastal surface water by seawater ingression, however, their impacts on groundwater are less documented. Here, through in-situ multi-depth piezometry, hydrogeochemistry, and stable isotope, and numerical simulations, we demonstrate the impact of recent ECs (n=11, May 2017-2020), including tropical atmospheric depressions (ADs, n=7) and cyclones (TCs, n=4, including category 5 cyclone Amphan ) on shallow and deep groundwater at one of the world’s most climate-vulnerable sites, the Ganges River delta. Field observations and simulation results suggest that ECs create a strong impact on the hydro-and chemo-dynamics of the groundwater, across depths, through induced surface-groundwater interactions that are modulated by EC landfall distance and atmospheric pressure gradients. Terrestrial groundwater flux and EC-induced enhanced ingress of seawater develop disequilibrium hydrodynamics, leading to depth-dependent lateral mixing in aquifers>100 m deep. Such observations indicate that climate change, through ECs, could pose a previously unexplored threat to potable water supplies for millions of groundwater-dependent inhabitants in coastal areas globally.