Vijesh V. K.¹*, Rakhi U. R.², Suresh Kumar¹, Madhukar Singh³ and Guru Prasad Mohapatra¹

¹Central Ground Water Board, CHQ, Faridabad- 121001 (HR), India

²Central Ground Water Board, Kerala Region, Trivandrum- 695004 (KL), India

³Central Ground Water Board, State Unit Office, New Delhi-110066(DL), India

(*Corresponding Author, E-mail: vijeshvkgeo@gmail.com)

Abstract

This study evaluates the spatial and seasonal dynamics of seawater ingress in the coastal phreatic aquifer systems of Kollam District, Kerala, using an integrated hydrochemical and index-based approach. A total of 21 groundwater samples were collected during the pre- and post-monsoon seasons of 2022 and analysed for major ions and physicochemical parameters. The Groundwater Quality Index for Seawater Mixing (GQISWI) was employed to identify salinization levels by integrating hydrochemical facies and chloride-derived mixing ratios. The Hill–Piper diagrams of the analysed water samples from the

study area showed a shift, with samples transitioning from Na-Cl dominated facies during the pre-monsoon to Ca–HCO₃ type in the post-monsoon,

indicating freshening of groundwater due to monsoon recharge. Similarly, the USSL diagram displayed a shift in several post-monsoon samples toward

lower salinity zones, reflecting the dilution effect of rainfall. An inverse relation between depth to the water table and electrical conductivity was observed across coastal transects, with higher electrical conductivity observed in the shallow coastal zones and a decrease in conductivity with increasing depth towards the east. Despite the overall improvement in water quality during the post-monsoon season, a few localized zones continued to exhibit elevated salinity levels—likely attributable to limited aquifer thickness, proximity to the coastline/saline water bodies. The study demonstrates that monsoonal recharge plays a vital role in reversing salinity levels and provides a replicable framework for seawater ingress assessment and aquifer vulnerability mapping in coastal regions.

Keywords: Kollam District, Seawater Ingress, Seasonal Salinity Dynamics, Coastal Phreatic Aquifer System, Hydrochemical Facies, Ground Water Quality Index