Comparative evaluation of index-based techniques for seawater intrusion vulnerability analysis

Groundwater contamination is becoming a major concern worldwide, especially in the coastal regions, due to the threat of seawater intrusion. Keeping in mind growing freshwater contamination problems, especially in eastern India, the current study was conducted to assess the hydrogeological and geochemical characteristics and critically analyze the aquifer vulnerability to seawater intrusion in a coastal alluvial ‘leaky confined aquifer’ of West Bengal and Odisha. The seasonal (Pre-Monsoon and Post-Monsoon) groundwater-level and quality (EC, Clˉ and HCO3ˉ) data of the ‘leaky confined aquifer’ in 2021 and well logs data were used in this study. Two overlay-and-index-based methods were applied, namely, ‘Original GALDIT’ and ‘Modified GALDIT’ (GALDIT-AHP) methods. The five GALDIT model parameters/themes considered are: ‘Aquifer Hydraulic Conductivity (A)’, ‘Groundwater Elevation (L)’, ‘Distance from the Coastline (D)’, ‘Extent of Seawater Intrusion (I)’, and ‘Aquifer Thickness (T)’. The weights of the themes and their features were modified using the Analytic Hierarchy Process (AHP) method, and the raster layers of themes were integrated in ArcGIS v10.8.2 using the ‘Weighted Overlay’ tool. Results reveal that the ‘Original GALDIT’ method delineated 50–64% lesser, 47–61% higher and 3–4% higher areas under ‘Low’, ‘Moderate’ and ‘High’ vulnerability classes, respectively, compared to the three corresponding EC classes. Results further indicate that the ‘Modified GALDIT’ (GALDIT-AHP) method predicted 32–45% lesser, 14–24% higher and 17–21% higher areas under ‘Low’, ‘Moderate’ and ‘High’ vulnerability classes, respectively, than the EC classes. The spatial validation method and correlation analysis results suggest that the ‘GALDIT-AHP’ (r=0.712–0.742) method performed much better than the ‘Original GALDIT’ (r=0.518–0.589) method in delineating aquifer vulnerable zones. Therefore, the combined application of geospatial technologies (Remote Sensing/GIS) and multi-criteria decision making techniques (like AHP) provides a reliable approach for seawater intrusion vulnerability assessment. The outcomes of this study will aid in formulating efficient plans for the sustainable management of groundwater resources.