Assessing aquifer vulnerability to seawater intrusion and impact of land use changes using GALDIT model in the southwest coastal region of Bangladesh

Groundwater, the main drinking water source for coastal populations, is increasingly threatened by seawater intrusion in southwestern coastal districts of Bangladesh due to low elevation, sea proximity, cyclones, rising sea levels, coastal flooding and excessive shrimp farming. The aim of this study is to assess groundwater vulnerability in Bangladesh's southwestern coastal districts and evaluate the impact of land use changes on seawater intrusion. This study used the GALDIT method to map the vulnerability of southwest coastal aquifers to seawater intrusion, classifying the area into low (0.5 %), moderate (58.66 %), and high (40.84 %) vulnerability zones. The GALDIT vulnerability map shows that Shyamnagar, Kaliganj, and Satkhira Sadar in Satkhira district are highly vulnerable to seawater intrusion, while the other areas are moderately vulnerable. In Khulna district, most of the Upazilas are moderately vulnerable. In Bagerhat district, most of the Upazilas are classified as moderately vulnerable, while Sarankhola, Mongla and Morrelganj are identified as highly vulnerable. The most extensive irrigation development was observed in Satkhira district, particularly in Satkhira Sadar, Kolaroa, Assasuni, and Tala upazilas, classified as moderately vulnerable. The Dumuria upazila of Khulna district is highly vulnerable due to overexploitation of water for irrigation purposes. In Bagerhat district, where irrigation has recently expanded in Fakirhat and Mollahat, both are in the intermediate susceptibility zone. The LULC analysis shows a decline in agricultural and vegetative areas, alongside a rise in built-up regions and shrimp farming, primarily indicating the increasing of salinity levels in the study area. The dominance of brackish water shrimp farming across the study area suggests shallow groundwater aquifers may be at risk of seawater intrusion.