CHARACTERIZATION OF THE COASTAL SALTWATER INTRUSION IN BADAGRY, LAGOS, SOUTH-WESTERN NIGERIA, USING GEOELECTRICAL SURVEY

Abstract

A geophysical technique was employed to investigate seawater intrusion into freshwater aquifers in the coastal environment of the Lagos, Badagry, and South-Western Nigeria. Electrical resistivity method employing the Schlumberger technique was used to acquire data for eighteen vertical electrical soundings, and five 2-D electrical resistivity imaging using the Wenner array to investigate the vertical extent of seawater intrusion. The Wenner electrode array, which utilizes current electrode half spacing from 1m to 600m, was used to acquire resistivity data in the proximity of the lagoon. Curve types such as the KQ, KQQ, and HKQ were generated for the investigated area also; 4-5 geoelectric layers were generated to a maximum depth of 68m. The subsurface lithology comprise of fine through medium grained sand to coarse sand intercalated in most cases with sandy clay and clayey sand. The resistivity of the intruded saline water was found to range between 7-32Ωm at a depth interval of 9.2 – 54.5m and the thickness of saline layers was found to be greater in areas close to the coast. This study shows that both the 2D electrical resistivity imaging and wenner sounding resistivity methods are efficient tools for investigating the saltwater-freshwater interface in coastal areas. Detailed survey should be carried out before citing boreholes due to variation in depth to deeper fresh aquifers. It is recommended that down-the-hole geophysical loggings should be carried out after drilling so that the screen can be installed at appropriate non-saline zone.