Seasonal hydrogeochemical signatures of seawater intrusion and salinity stress in a coastal aquifer system of southern Tamil Nadu, India

This study examines seasonal changes in hydrogeochemistry and seawater intrusion in the coastal aquifer between Tharuvaikulam and Mookaiyur, Tamil Nadu, India, an area facing growing salinity and water quality issues. Groundwater samples were collected from 39 during the pre-monsoon and post-monsoon seasons of 2022. Major physicochemical parameters, including pH, electrical conductivity (EC), total dissolved solids (TDS), Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, and HCO₃⁻, were analysed to assess groundwater quality and hydrochemical evolution. Drinking water quality was evaluated using the Drinking Water Quality Index (DWQI), and irrigation suitability was assessed using selected irrigation indices. Chemometric tools, such as correlation matrices, Principal Component Analysis (PCA), and Hierarchical Cluster Analysis (HCA), were also applied to explore spatiotemporal trends and controlling factors. The results showed marked spatial and seasonal variations in salinity, with elevated EC, TDS, Na⁺, and Cl⁻ concentrations at several coastal locations. Post-monsoon recharge produced a slight improvement in DWQI, with excellent-quality samples increasing from 20.51% to 23.08% and good-quality samples from 48.72% to 51.28%, while poor-quality samples decreased from 30.77% to 25.64%. Hydrochemical evidence indicates that seawater mixing, evaporation, mineral dissolution, and ion exchange collectively influence groundwater chemistry. The findings identify persistent salinity stress in vulnerable coastal locations and provide useful information for groundwater monitoring, controlled abstraction, and sustainable management of the coastal aquifer.