Hydrogeologic and geochemical investigations in a coastal basin of West Bengal

A majority of saltwater intrusion into freshwater aquifers occurs in coastal regions. The interactions between groundwater, surface water, and estuarine water are complicated and highly dynamic. A decrease in the groundwater level below the mean sea level results in the reversal of the hydraulic gradient thereby causing the seawater to move inland. Seawater intrusion is triggered by the excessive pumping of coastal groundwater and sea-level rise. The inland motion of seawater into the coastal aquifer is an important reason for the deterioration of the coastal groundwater resources. Geochemical criteria like chloride and TDS concentration, chloride/bicarbonate ratio, and isotope studies have been used by researchers across the world to identify the origin, pathways, rates, and current status of salinization in order to properly understand the interplay between transport and chemistry in the seawater-freshwater mixing zones prevalent in coastal aquifers. This study deals with a comprehensive hydrogeologic and geochemical investigation to identify seawater intrusion into a coastal groundwater basin of West Bengal, Eastern India. The major cations (Ca 2+ , Mg 2+ , and Na + ) and anions (HCO 3 ˉ, Clˉ, and SO 4 2ˉ ) dissolved in the coastal groundwater were analyzed in this study. The relative abundance of major cations and anions was analyzed using Pie and Radial diagrams. Piper and Chadha’s diagrams were used to find the dominant groundwater types and their geochemistry in the study area. USSL diagrams were used to classify groundwater for analyzing its suitability for agricultural uses. In addition, Gibbs and HFE-D diagrams were also analyzed to identify major geochemical processes influencing existing groundwater chemistry. Finally, the seawater mixing index (SMI) was developed to identify the aquifers affected by seawater-freshwater mixing as well as to explore the current status of seawater intrusion in the study area.