Groundwater quality index as a hydrochemical tool for monitoring saltwater intrusion into coastal freshwater aquifer of Eastern Dahomey Basin, Southwestern Nigeria

Saltwater intrusion into coastal freshwater aquifers is a threat to groundwater quality globally. This study aims to determine the extent of saltwater intrusion into the coastal freshwater aquifer of the Eastern Dahomey Basin (EDB), Nigeria. Groundwater chemistry was sampled and analysed for ionic ratios and interpreted using a hydrochemical facie evolution diagram (HFE-D), the saltwater mixing index (SMI) and the Groundwater quality index for saltwater intrusion (GQIswi). High EC and TDS and the concentration of dissolved ions showed increased salinity as a result of seawater intrusion in wells located around communities in Seme, Lekki, Eleko, Okun-Ajah, Ode-Mahin and Igbokoda. Correlation of ions in the wet season also suggests higher salinities which originate partly from industrial and municipal effluents especially from wells which are close to river channels, while dry season groundwater shows the dominant influence of seawater intrusion. HFE-D revealed that mixed groundwater of Na + Ca–HCO₃, Na–Cl and Ca–Cl dominate the area due to gravity-driven flow leading to groundwater freshening inland from the coastline towards the northern part of the basin. The groundwater quality index from SMI and GQIswi shows areas within 3 km from the coastline that are more sensitive to saltwater intrusion based on abstraction rate and depth of the wells. The present study provides information of value to planners and policy-makers for the sustainable management and protection of coastal groundwater resources in the Eastern Dahomey Basin.