Mapping the possible origin of anomalous saline water occurrence in Agbabu, Eastern Dahomey Basin, Nigeria: insights from geophysical and hydrochemical methods

Abstract

Many researchers have suspected saltwater occurrence in Agbabu in the eastern Dahomey Basin. However, there is a need to get an insight to the possible origin of saltwater in the area. The aim of this study is to determine the source of saltwater in the inland aquifers of Agbabu, southwestern Nigeria for proper management of scarcely available freshwater resource in the area. 23 vertical electrical sounding, 2D electrical resistivity tomography (ERT), aeromagnetic and hydrochemical data (11 subsurface water samples) analyses were utilized in this study. The results from the magnetic analysis revealed the inland extension of the Chain Fracture Zone (CFZ). This NE-SW trending fracture zone cuts across the entire eastern Dahomey Basin, including Agbabu and its environs. Four (4) geoelectric layers were delineated across Agbabu namely, the topsoil (4 - 2806 Ωm), clayey sand (3 - 1280 Ωm), clayey (3 - 161 Ωm) and bituminous sand (1 - 10390 Ωm) layers. Anomalously low resistivity (1 - 20 Ωm) zones suspected within the bituminous sand correspond to the saltwater intrusion zones. The bituminous sand unit harbouring freshwater exhibits characteristic resistivity values of 33 – 160 Ωm. The interface between the freshwater and saltwater intruded zones in the area was suspected at 10 m depth with resistivity values ranging between 21 Ωm and 33 Ωm. The results from the hydrochemical water analysis also confirmed the presence of saltwater intrusion in the area. The salinity extent chart which was generated from the VES results indicated the mobility of the saltwater into the groundwater system. The fracture connectivity between the complex aquifers of Agbabu and the Atlantic Ocean (CFZ), that was mapped from the aeromagnetic and electrical resistivity results, is suspected to be primarily controlling the localization of saltwater in the study area.