Integrated electrical resistivity and isotopic approach for delineating groundwater potential zones in a coastal urbanizing aquifer

Understanding recharge processes and surface–groundwater interactions in rapidly urbanizing coastal regions is essential for sustaining freshwater availability under growing climate and anthropogenic pressures. The objective of this study was to evaluate surface water–groundwater interactions and delineate groundwater potential zones (GWPZ) in coastal regions of Chengalpattu district, Tamil Nadu, using an integrated approach of water stable isotope analysis (δ¹⁸O, δ²H), analytical hierarchy process (AHP) and electrical resistivity survey (ERS). For the isotopic analysis, we collected precipitation, surface water, and groundwater samples for the pre-monsoon, monsoon, and post-monsoon seasons. The ERS was conducted to identify the aquifer geometry, fractures, and lineament for identifying the GWPZ. Seasonal variations in isotopic compositions were observed, with a depletion in isotopic compositions during the monsoon season in wells and lake waters, indicating a surface and groundwater interaction, and an isotopic enrichment during the pre-monsoon and post-monsoon seasons due to evaporation. Based on the δ18O–δ²H plot, two sources of groundwater recharge were discerned, (i) direct recharge from precipitation; (ii) recharge from the lakes. Using a conservative isotope (δ18O) mixing model, the contribution of lake to the groundwater was estimated. The average contribution from lake water to groundwater was found to be nearly 25 %. ERS using vertical electrical sounding identified true resistivity ranging from 13.10 to 59.49 Ω·m with an average resistivity value of 20.68 Ω·m. The integrated isotopic–geophysical approach provides an effective framework for understanding recharge dynamics and managing groundwater in coastal semi-urban environments.