Possible factors for increasing water salinity in an embanked coastal island in the southwest Bengal Delta of Bangladesh.

Increasing water salinity in coastal areas is a concern for the coastal environment. Increased salinity is affecting water quality, freshwater availability, and water-related ecosystems in the southwest coastal region of the Bengal Delta. The study used a synergies and robust approach to assess the possible factors for increasing water salinity in an embanked coastal island in the southwest Bengal Delta. The hydrochemical analysis revealed that surface and groundwater are enriching with Na+ and Cl- concentration, also controlling by seawater through the ion exchange mechanism (Ca2+ and Mg2+ replacing by Na+, and Cl-), mixing stage of water solution (freshwater-saltwater), and anthropogenic salt contamination by human activities (e.g., saltwater shrimp cultivation and excessive irrigation). Piper diagram showed that river water is occupying at the mixing stage of solution, where pond and groundwater are occupying at seawater (saline) zone. The water quality index showed that surface water is not fresh anymore and unsuitable for drinking purposes. The isotope analyses exposed the presence of strong precipitation variance in the study area. Cluster observation analysis showed a strong correlation between Na+, Cl-, and TDS (similarity is 97% to 99%). The remote sensing application illustrated that high salinity zones are in the northern part, and groundwater salinity is higher (7.5 to 8 ppt) in the northern part of polder 32. The salinity of both groundwater and surface water showed a positive correlation with land surface temperature and potential evapotranspiration. The study exposed four responding factors for increasing groundwater salinity in this region, which are - regional surface geological settings, hydrological settings, hydraulic head gradient, and human activities. A conceptual model illustrated the presence of lateral recharge of saltwater from the surrounding tidal rivers to the groundwater.