Abstract

This paper presents the use of electrical resistivity survey technique in mapping the bedrock relief, and detection of the near surface groundwater regime around the new administrative block of the Federal Polytechnic, Ado-Ekiti, Ekiti State. Twelve vertical electrical soundings using the Schlumberger configuration were acquired. Also, groundwater samples were obtained at selected locations and subjected to both the chemical and microbial analyses accordingly. The resistivity dataset were processed using computer interaction interpretation of the sounding curves. A maximum of four subsurface geologic units were delineated from the VES-curves. These subsurface geoelectric layers correspond to loose sandy/clayey topsoil, sandy/gravel layer, clayey weathered basement forming the shallow unconfined aquifer in the area, and the underlying fresh bedrock of charnokite and migmatite gneiss. The resistivity values for each layer range from 29 to 514 Ωm (layer one), 87 to 115 Ωm (layer two), 10 to 45 Ωm (layer three), and 187 to 27863 Ωm (layer three) respectively for the these layers. The thickness of the regolith materials vary between 3.4 and 13.9 m. Also, the geophysical survey revealed bedrock depressions and low resistivity values of the weathered basement which may suggest possible groundwater contamination and/or accumulation likely associated with loose sandy topsoil, shallow water table and/or recharge from surface runoff into the subsurface aquifers. The water analysis of the water sample from the hand dug well depict the presence of high level of Coliforms (160 Coliforms per 10² ml) in the subsurface water, which points to the presence of high concentration of nitrogen compounds and pollution of animal origin, clearly indicates that the zone of water saturation is directly in contact with the surface runoff. Further, the total dissolved solids (TDS) is 684.83 mg/L. Generally, the chemical constituents of some of the major and minor elements fall considerably below the World Health Organization (WHO, 1963) permissible levels.

Key words: Geoelectric mapping, microbial analysis, coliform, total dissolved solids.