ELECTRICAL RESISTIVITY SURVEY FOR PREDICTING POTENTIAL ZONES AND AQUIFER PROTECTIVE CAPACITY IN SAGAMU, OGUN STATE, NIGERIA

Abstract

In rapidly evolving peri-urban areas, producing aquifers must be evaluated with reference to natural protection afforded by overlying materials as well as identification of water-bearing formations. The groundwater potential zones and aquifer protective areas in the Apaken area of Sagamu, Ogun State, south-western Nigeria have been assessed in this study by the integrated ER approach. Natural electric field (NEF) measurements were supplemented by 12 vertical electrical sounding (VES) profiles with the Schlumberger array and six two-dimensional electrical resistivity survey (ERT) profiles with the Wenner array. VES data were partially curve matched and computer assisted iterated, while ERT data were inverted to form 2D resistivity sections. The aquifer occurrence and the overburden's protecting capacity have been interpreted based on geoelectric parameters and Dar-Zarrouk relationships. The interpreted geoelectric models suggest that there are 3 to 5 layers with the topsoil, lateritic layer, and the sandy clay and overburden layer of ferruginous, medium to coarse-grained sand, ranging from 1.2–24.0 m. The main aquiferous interval is ferruginous fine sand. According to the reported interpretation, the groundwater-yield classes are moderate-to-high at 50% of the VES locations and very low-to-low at 50% of the locations. The results of the longitudinal-conductance assessment suggest that 91.67% of the sites are poorly protected, while the remaining 8.33% are protected poorly. The sites of VES 4, 6 and 11 are all favourable for groundwater and have moderate protection, and were recommended for further drilling investigation. The results of the ERT and NEF are independent and indicative to good groundwater conditions along traverses 3 and 4. The results illustrate that groundwater productivity and aquifer protection are decision variables that are different but complementary. Groundwater-potential assessment alone is not as defensible as the integrated interpretation for borehole siting and groundwater-resource protection in Sagamu.