Groundwater Assessment and Aquifer Vulnerability Studies of Emure Ile, Southwestern Nigeria

Abstract

Hydrogeological studies and groundwater flow of Emure Ile located in the crystalline basement area of Emure, near Owo, Southwestern Nigeria, has been done. The local geology of the area is predominantly granite-gneiss and migmatite. Hydrogeological measurements were determined from twenty eight existing non-flowing wells across the area. The measurements were used to compute the thickness of the water column/vadose zone, static water level, and hydraulic head of the wells across the area. The hydraulic head measurements were used to develop the groundwater flow model for the area. A total of twenty depth sounding data were acquired using Schlumberger array and presented as sounding curves. The groundwater flow model map shows that groundwater flow is towards the west – east direction representing the central part of the town. The thickness of the water columns/vadose water (average of 2.6 m) and total depth of the wells (average of 5.4 m) are generally low and shallow. The depth to groundwater (water table) ranges from 2.6 m to 4.7 m, with a modal range of 1 – 3 m suggesting a thin vadose zone. Based on DRASTIC index rating, the vadose zone thickness generally fall within high vulnerability rating, aptly suggesting that the aquifer in the area is significantly vulnerable to contaminants deriving from anthropogenic sources. This result corroborates the deduction from the overburden protective map. The interpreted sounding curves revealed three distinct geoelectric layers overlying the resistive basement, the topsoil, the weathered layer and the partially weathered/fractured basement. The unconfined weathered layer is the main aquifer unit in the area, with resistivity ranging from 7 to 101 Ohm-m suggestive of sandy-clay; and thickness ranging from 2.0 to 84.9 but generally less than 20 m. Therefore combining all the results, the best groundwater development areas with less vulnerability to contamination are found in northwest - southwest parts which constitute for about 30% of the area. However, the groundwater potential of the area is generally low.