Spatial assessment of submarine groundwater discharge influence on aquifer water quality in the coastal region of Chettikulam to Kolachel, southern India: using SMI and HFE-D techniques.

Submarine Groundwater Discharge (SGD) has a global impact, affecting coastal aquifers, the freshwater environment, and contributing to coastal development. The present study investigates the impact of Submarine Groundwater Discharge (SGD) on groundwater geochemistry along the coast from Chettikulam to Kolachel in Southern India, with an emphasis on regional changes pre and post monsoons in the years 2023-2024. A total of 80 groundwater samples (40 from both monsoons) were analyzed using hydrochemical plots such as Piper, Wilcox, Gibbs, and Hydrochemical Facies Evolution Diagrams (HFE-D), along with AquaChem software and spatial mapping techniques. Hydrogeochemical analysis reveals significant seawater intrusion during the pre-monsoon period, with 48% of groundwater samples categorized under the Mixed Ca-Mg-Cl facies and 30% under the Ca-Cl facies, leading to elevated salinity, total dissolved solids (TDS), chloride, and sulfate concentrations. In contrast, post-monsoon conditions demonstrate improved water quality due to monsoonal recharge, with 55% of samples in the mixed Ca-Mg-Cl facies but exhibiting reduced salinity. Water Quality Index (WQI) analysis shows an increase in 'Excellent' water samples from 22.5% pre-monsoon to 37.5% post-monsoon. The Wilcox diagram highlights a reduction in salinity hazards, improving groundwater suitability for irrigation. The Seawater Mixing Index (SMI) indicates the seawater intrusion in coastal zones, although monsoonal dilution mitigates its effects. Seasonal variation in hydrochemical facies reveals a decrease in the freshening phase from 60% pre-monsoon to 45% post-monsoon, alongside an expansion of the intrusion phase from 40 to 55%. The study underscores the need for ongoing groundwater monitoring and effective water management strategies to sustainable coastal aquifers impacted by Submarine Groundwater Discharge (SGD).