Geospatial modelling of seawater intrusion risk in coastal aquifer systems of Southern Malabar Coast, Kerala using GALDIT index and hydrogeochemical analysis

Seawater intrusion (SWI) poses a severe hazard to coastal aquifers, which need detailed vulnerability evaluations to preserve groundwater supplies. This study identifies and maps SWI prone zones in the coastal region of Thiruvananthapuram district, Kerala, located along the Southern Malabar Coast by merging three methodologies: the GALDIT index, Groundwater Quality Index for Seawater Intrusion (GQI SWI ), and Hydrogeochemical Facies Diagram (HFE-D). The GALDIT index, incorporating six critical parameters Groundwater occurrence (G), Aquifer hydraulic conductivity (A), Level of groundwater above sea level (L), Distance from the shore (D), Impact of existing seawater intrusion (I) and Aquifer thickness (T), classified 35.31% of the study area as highly vulnerable highlighting regions such as Varkala and Edava as less vulnerable, while areas near Thiruvananthapuram Airport and Vizhinjam Fishing Harbour were identified as highly susceptible. The GQI SWI analysis revealed pre-monsoon values ranging from 62.51 to 85.07, with 70% of samples falling within the mixed water quality category (50–75), demonstrating significant vulnerability in coastal locations such as Vizhinjam Fishing Harbour. Post-monsoon improvements showed that 57.5% of samples achieved freshwater classification (> 75), reflecting the effectiveness of monsoon recharge. The HFE-D indicated that 52.5% of pre-monsoon samples exhibited Na + -Cl − facies, reducing to 7.5% post-monsoon, resulting in an 86% reduction and confirming seasonal salinity dynamics. The synergistic integration of GALDIT for SWI vulnerability zone delineation, GQI SWI for salinization severity quantification, and HFE-D for hydrochemical evolution provides a comprehensive multidimensional framework for coastal aquifer management, directly supporting the implementation of SDG 6 and SDG 12 in vulnerable coastal regions.