Hydrogeochemical characterization of groundwater and critical assessment of its quality in a coastal basin

In the twenty-first century, the ramifications of human civilization on natural resources are immense, which has led to water quantity, quality and environmental problems across the globe. Groundwater is a vital resource for ensuring water, food and environmental securities. This study focuses on in-depth hydrogeochemical characterization, and assessment of seawater intrusion and groundwater suitability in a coastal alluvial basin of eastern India. The groundwater-quality data of pre-monsoon season (April) for the 2012-2021 period were used in this study. The results of different analyses revealed that the comparative order of ionic concentrations in groundwater is: Na+ > Ca2+ > Mg2+ > K+ > Fe2+, and HCO3- > Cl- > SO42- > NO3- > F-. The Piper and Chadha's Diagrams revealed that the major groundwater types are: 'Ca-Mg-HCO3', 'Mixed Ca-Mg-Cl', 'Na-K-Cl-SO4', 'Ca-Mg-Cl-SO4' and 'Mixed Ca-Na-HCO3'. Based on the USSL Diagrams, groundwater from the irrigation viewpoint is characterized as 'C2-S1' type, followed by 'C1-S1', 'C3-S1' and 'C3-S2' types, which suggest that groundwater should be utilized carefully for irrigation in the study area. The Gibb's Diagrams indicated that mainly 'Rock-Water Interaction Dominance (Weathering)' is responsible for controlling groundwater chemistry. The HFE Diagrams suggested that groundwater in similar to 22% of the observation wells is influenced by the 'Reverse Cation-Exchange (Intrusion Phase)' process. The 'Seawater Mixing Index' values of > 1 in the unconfined aquifer (similar to 84% of observation wells) and the confined aquifer (similar to 69% of observation wells) indicate that the freshwater-seawater mixing phenomenon considerably influences the groundwater. Furthermore, the Entropy Groundwater Quality Index for unconfined and confined aquifers indicated that the groundwater in 4-18% of the study area is of 'Good' to 'Excellent' quality, followed by 'Medium' quality (36-58% of the area), 'Poor' quality (17-36% of the area), and 'Extremely Poor' quality (6-24% of the area). On the other hand, the Irrigation Groundwater Quality Index of unconfined and confined aquifers suggested that the groundwater is suitable for irrigation ('Good' to 'Excellent' quality) in 95-97% of the study area.