Groundwater vulnerability assessment was carried out at Igbara Oke Southwestern Nigeria, with a view to classify the area into vulnerability zones, by applying the electrical resistivity method, using Schlumberger electrode arrays with maximum electrode separation (AB/2) of 65 m in (41) different locations for data acquisition. Geoelectric parameters (layer resistivity and thickness) were determined from the interpreted data. The study area comprises four geoelectric layers (topsoil, lateritic layer, weathered/ fractured layer and fresh basement). The geoelectric parameters of the overlying layers across the area were used to assess the vulnerability of the underlying aquifers to near-surface contaminants with the aid of vulnerability maps generated. Three models were compared by maps using geo-electrically derived models; longitudinal conductance, GOD (groundwater occurrence, overlying lithology and depth to the aquifer) and GLSI (geoelectric layer susceptibility indexing). The total longitudinal conductance map shows the north central part of the study area as a weakly protected (0.1-0.19) area, while the northern and southern parts have poor protective capacity (<0.1); this is in agreement with the GOD method which shows the northern part of the study area as less vulnerable (0-0.1) while the southern part has low/ moderate (0.1-0.3) vulnerability to contamination. The longitudinal conductance exaggerates the degree of susceptibility to contamination than the GOD and GLSI models. From the models, vulnerability to contamination can be considered higher at the southern part than the northern part and therefore, sources of contamination like septic tank, refuse dump should be cited far from groundwater development area.

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ABSTRACT:Integrated surface geophysical methods were used in investigating causes of pavement instability along a portion of Akure-Owo expressway, southwestern, Nigeria. The methods comprise of ground magnetic profiling, Very Low Frequency Electromagnetic (VLF-EM) profiling and geoelectric sounding. The magnetic profile shows a drop below 31500nT at distance 100m and above 32500nT at distances 140 and 310m, these drops in value could correspond to lithological contacts or bedrock depressions. The combined plots of raw real and filtered real VLF-EM data show conductive zones at distances 70m, 230-240m and 350-360m, which could be indicative of fractured zones or conductive clay materials. The 2-D geoelectric section shows bedrock depressions at distance 80-160m (stable segment) and 240-360m (unstable segment), while bedrock fractures was delineated beneath VES points 1, 3, 5 and 10, corresponding to distances 0, 80m, 160m and 400m respectively. The topsoil resistivity varies from 83 to 865 ohm-m, while resistivity in the weathered layer materials ranges from 182-1139ohm-m along the stable segment and 27-262ohm-m along the unstable segment. The low resistivity values observed beneath the unstable segment are typical of expansive clay. Thus the instability of the road pavement along the studied portion of the road is probably precipitated by the presence of near surface bedrock depressions, occupied by low resistivity weathered materials, typical of expansive clay and sandy clay, adjudged unsuitable construction materials.

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