Hydrogeophysical Mapping and Seawater Intrusion Assessment in Southern Tamil Nadu, India: Application of Vertical Electrical Sounding and Dar-Zarrouk Parameters

This study presents a comprehensive hydrogeophysical investigation of coastal aquifers in Southern Tamil Nadu, employing Vertical Electrical Sounding (VES) with the Wenner configuration across 25 profiles to delineate subsurface lithology, assess aquifer potential, and detect seawater intrusion zones. Apparent resistivity values varied significantly, ranging from 0.009 Ω·m in VES 19, indicating highly conductive saline or clay-rich layers, to 205 Ω·m in VES 4, associated with dry sand or consolidated formations. The innovative application of Dar-Zarrouk parameters, longitudinal conductance, transverse resistance, and average resistivity, enabled a robust classification of aquifer conditions, with VES 5 identified as the zone of highest aquifer potential. At the same time, eleven sites showed poor prospects due to saline or clay influences. Curve-type analysis showed H-type (32%) and Q-type (20%) profiles were dominant, corresponding to fractured and alluvial aquifers, respectively. The integration of three-dimensional subsurface visualization using the Groundwater Modeling System (GMS) tool enabled precise visualization of freshwater zones at intermediate depths (5–25 m) and of advancing seawater intrusion in low-lying coastal areas, underscoring dynamic interactions between saline and freshwater. Transitions in resistivity patterns confirmed the inland movement of saline water in low-lying coastal areas. Borehole lithology data supported the VES interpretations, with high-resistivity zones corresponding to coarse sand and gravel, and low-resistivity zones matching clay or brackish intervals. The integration of geophysical parameters with field validation enabled accurate delineation of freshwater-bearing zones and saline-affected areas. These findings are essential for guiding groundwater development and resource management in salinity-prone coastal regions. • Integrated VES, Dar-Zarrouk parameters, and 3D GMS visualization to map aquifer zones. • Identified freshwater pockets (98-120 Ω·m) at 5-25 m depth and seawater intrusion zones (<6.3 Ω·m). • Classified aquifer types, with H-type fractured and Q-type alluvial profiles dominating. • Borehole lithology validation strengthened resistivity-lithology-salinity correlations. • Provided actionable insights for salinity management and sustainable groundwater use.