Extent of the saltwater intrusion in coastal Minjur, India and implications for groundwater quality

Seawater intrusion poses a growing threat to groundwater quality in coastal regions under increasing water stress. This study investigates the spatial extent of salinization and associated hydrogeochemical processes in the Minjur area, a densely populated coastal area in Southern India. Transient electromagnetic (TEM) surveys were combined with borehole lithology and groundwater chemistry to characterize subsurface saline zones and evaluate water quality impacts. Results reveal extensive seawater intrusion in the eastern part of the study area, with very low resistivity values (< 5 Ohm·m) and elevated electrical conductivity and Cl- concentrations in groundwater. Despite the conductive conditions, TEM data successfully resolved coarse-grained, higher resistivity zones likely associated with fresh groundwater, demonstrating the method’s value for aquifer characterization in saline environments. Hydrogeochemical analysis indicates cation exchange, and sulfate reduction, in the seawater intrusion zone. Produced bisulfide (HS⁻) reduces trace metals-bearing iron (Fe) and manganese (Mn) oxides and leads to significant mobilization arsenic (As), lead (Pb), selenium (Se), nickel (Ni) and vanadium (V) in the seawater-freshwater mixing zone. These findings highlight the value of integrating geophysical and geochemical data to improve understanding of subsurface salinization processes and inform sustainable groundwater management in vulnerable coastal regions.