Anthropogenic activities constitute a major threat to groundwater resources if not monitored and controlled and thus may hamper the optimum harness and utilization of the scarce resources. Most groundwater resources evaluation focuses on delineation of the water bearing unit and less attention is centered on the quality and vulnerability of the resources. It is on the latter that the focus of this paper is based.
Field measurements using electrical geophysical methods was adopted for the study and sixty five vertical electrical sounding (VES) data using schlumberger array method were acquired . The computer assisted VES data interpretation enables the delineation of the subsurface into three to four geoelectric layers; the topsoil, the lateritic or weathered layer, the fractured basement and the resistive bedrock. In the topsoil, resistivity values range from 20 ohm meters – 580 ohm meters with the layer thickness varying from 0.4 m – 2.0m. The second layer which is presumably the weathered zone, has resistivity in the range 21 – 648 ohm meters with the thickness ranging from 0.3 m to 36.6 m. The fractured bedrock has resistivity in the range 36 ohm- m – 796 ohm meters. Geoelctric layers with relatively low resistivity values are considered zone of saturation which are the potential target for groundwater resources.
Assessment of the vulnerability potential of the aquifer especially the topmost aquifer was based on the geo-electric parameter evaluated from the resistivity data. Aquifer overlain by topmost layer with low resistivity (< 100 ohm-m) has been interpreted to imply clayey formations with good protective capability and less vulnerable. This occupies most of the central part and the southwestern part and constitutes about 65% of the study area. Aquifers within the zones overlain by geoelectrically pervious materials (resistivity of 101 to 300 ohm meters) interpreted to be sand around the northern part and some portions of the southern part of the study area is about 35%. This is considered vulnerable to infiltrating surface contaminants.
The study has made possible the characterization of the vulnerability potential of the aquifer in the study area. The area is mostly covered by materials of thin protective capacity top layer with pockets of weak protective topsoil (sandy). This makes the groundwater resources in the area vulnerable to surface anthropogenic source pollution. However precautionary measures to protecting the resources must be put in place by the local municipal authority for adequate monitoring of waste disposal by individuals and factories in the area.
Key words: Groundwater, Electrical sounding, Resistivity, Aquifer, vulnerability.