Baseline evaluation of freshwater–saltwater interface in coastal aquifers of Badagry, south-western Nigeria

This study focused on the evaluation of the baseline condition of saline water–groundwater interface in the coastal aquifers along Badagry, south-western Nigeria. Geologically, Badagry lies within the coastal sands and recent alluvium of the Dahomey Basin. Two-dimensional electrical resistivity tomography (2D-ERT) data along 15 survey lines were acquired in the study area, adopting the Wenner electrode arrays system with minimum spacing of 10 m. Ninety-one water samples were also collected, and their physical parameters were measured using hand-held devices. The (2D-ERT) data were processed and interpreted with the aid of DiProWin software. Sandy topmost layer, freshwater sand, brackish water sand and saline water sand were delineated from the study. Brackish water sand and freshwater sand layers were dominant in areas with high proximity to the Atlantic Ocean located in the south, which were not observed in the northern part of the mapped area. The study established the freshwater–saltwater interface at a depth of 10 m and above, in areas around the coast, while the interface occurs at depth greater than 50 m in areas 3–5 km from the coastal area. Observations from the pH and the TDS show that 87.3% of the samples are slightly acid, while less than 12.7% of them are slightly alkaline, while the TDS vary from 8 to 520 mg/l. The EC of the samples varies from 13 to 1140 µS/cm. All water samples are fresh groundwater, which confirms the occurrence of freshwater aquifer even in areas closer to the Atlantic Ocean.