Adumasun lies on a Precambrian basement complex of Southwestern Nigeria. Some kilometers away from the study area especially at Igangan is characterized by outcrops of crystalline basement rocks. Inadequate municipal water supply from State Water Corporation, coupled with hydrogeologically difficult nature of the terrain, individuals and corporate bodies indiscriminately sink tube wells and boreholes within the unconsolidated overburden materials, with glaring lack of concerns for the vulnerability status of aquifers, and possible environmental risk. Vertical Electrical Sounding method was used to map Adumasun area, Oniye, Oyo State with a view to assessing the groundwater prospect, focused on the thickness of the unconsolidated materials overlying the crystalline bedrock. The resistivity parameters of the geoelectric topmost layer across the area were also used to assess the vulnerability of the underlying aquifers to near-surface contaminants. The thickness of the unconsolidated overburden varies from 3.1m to 20.1m, where about 60% falls within the 10m-14.9m brackets. This shows that unconsolidated materials are not thick and hence averagely low groundwater prospect. 80% of the topmost geoelectric layer in the study area has resistivity mostly within the range of 1-100Wm. Resistivity values within these brackets tend to indicate silt or clay sequence, which can constitute effective protective geologic barriers for the underlying aquifers. This suggests that aquifers within the unconsolidated overburden at Adumasun are mostly capped by impervious/semi-pervious materials, geologically protecting the underlying aquifers from near-surface contaminants.
