Detection and Delineation of Lateral and Vertical Extent to Fresh-Saline Water Boundary: Recent Development in Badagary Axis of the Dahomey Basin, Nigeria

Abstract

This research is aimed at detecting evidence of saline water intrusion in the aquifers, delineate the depth to and lateral extent of saline water incursion into the Badagary area of Dahomey Basin, South western Nigeria. This was done by combining information derived from water quality analysis, geophysical well logs, lithologic logs and geophysical field data. Groundwater samples were collected from shallow wells and abandoned deep wells close to the vertical electrical sounding (VES) points. Variations in resistivity of the subsurface downwards were measured from six (6) different points using the VES technique along six traverses that covers about 1 kilometer long. VES data were interpreted using the RES 2D inversion tool Result from water quality test revealed saline water incursion into the aquifers with values of Na+ concentration found to be as low as 76 mg/l in Badagary well 2, a shallow well and as high as 162 mg/l in Badagary well 4, an abandoned deep well. Cl− values range from as low as 257 mg/l in Badagary well 6 and as high as 321 mg/l in well 3, an average of 287.83 mg/l. Na+ and Cl− ions contributing about 70% of the concentration of the cations and the anions. The results of geophysical study revealed that the Badagary axis of Dahomey Basin exhibit a multi-aquifer system having three levels of aquifers as follows: shallow unconfined aquifer unit which exist around 30–40 meters deep at the hinterland, intermediate semi-confined and confined aquifer at about 40–120 meters deep and deeper confined aquifer unit occurring at about 40–150 meters deep or at the coast at about 120–270 meters deep. The aquifers are thick at greater depths towards the coast but thins out at shallower depths around the hinterland except in certain cases where thick sequence of Coastal Plain Sands confines the aquifers at the hinterland. Results from geophysical field data interpretation and lithologic logs indicates saline water incursion especially in areas that borders the Atlantic Ocean. To date, saline water incursion has extended up to about 2,265 meters from the sea shore and about 138 meters deep in boreholes.