Dynamics of fresh and saline groundwater interaction in coastal aquifers: a case study from the east coast, India

Abstract

Coastal regions are often vulnerable to seawater intrusion caused by sea-level rise driven by climate change. Therefore, it is essential to map coastal aquifers and groundwater vulnerability zones along with seawater intrusion risks for effective groundwater management. Three methods, namely the Analytical Hierarchy Process (AHP), Entropy, the AHP-Entropy model framework, and other hydro-geochemical models, have been employed to identify groundwater vulnerability zones. The results were validated and compared with intrinsic vulnerability models such as Entropy-DRASTIC and modified GALDIT (M-GALDIT). Additional hydro-geochemical approaches, including the Entropy Water Quality index, GQISWI, and HFE-D, were also utilized to assess groundwater quality. According to the Entropy-DRASTIC model, approximately 27% of the study area shows medium vulnerability, followed by about 24.31% with high vulnerability and around 24.31% with very high vulnerability. The eastern, northwestern, and southern parts are more vulnerable than other regions due to seawater mixing into the freshwater aquifers. Additionally, the seawater intrusion model, known as the M-GALDIT model, indicates that roughly 22.54% of the area is moderately vulnerable to seawater intrusion. In comparison, about 17.43% and 9.73% of the region are relatively high and very highly vulnerable, respectively. The GQISWI values in the region range from 0 to 96.96, suggesting most groundwater samples are classified as mixed water types. Factors influencing groundwater salinization include faults and fractures, population distribution, ionic exchange reactions, climatic phenomena, and seawater intrusion. The groundwater vulnerability models have been validated using the ROC curve, which shows satisfactory results, with an AUC value of approximately 0.69 for the AHP-Entropy framework integrated method.