Groundwater Assessment and Its Intrinsic Vulnerability Studies Using Aquifer Vulnerability Index and GOD Methods

Abstract

Groundwater assessment and aquifer/water bearing formation vulnerability studies were carried out in Ose and Owo Local Government areas of Ondo State, Southwestern Nigeria. The groundwater evaluation involved integrated electrical resistivity (vertical electrical sounding), very low frequency electromagnetic, and borehole logging. Aquifer vulnerability assessment was done using Aquifer vulnerability Index (AVI) and GOD approaches. Fifty two (52) vertical electrical soundings (VES) data were acquired with Schlumberger array using current electrode separation (AB/2) of 1 to 225 m. The acquired VES data were qualitatively interpreted to determine the geoelectric parameters (layer resistivity and thickness). The geoelectric sections revealed the lithological sequence comprising topsoil, weathered layer, partly weathered/fractured basement and fresh basement. The most occurring curve types identified are H and KH with % frequency of 30 and 26.9 respectively. The lineament density and interception maps show a low spatial variation as the lineaments are generally sparse in the study area especially in Ose local government area; while Owo area shows a low – moderate variation. The major water bearing units are confined/unconfined fracture basement and weathered layer composing of clay/sandy clay, clay sand and sand aquifers (found in the southern part of the study area with thickness generally above 20 m and could be up to 60 m). However, the fracture basement aquifer is widespread in Owo area with thickness that could up to 30 m. The depth to these water bearing geological formation is between 1.2 m and 15.9 m. The AVI characterized the study area into “extremely low – High vulnerability” with predominant very high vulnerability values. The GOD vulnerability model depicts that the study area is characterized by three vulnerability zones, which are low, moderate and high vulnerable zones. According to the model, about 5% of the area is highly vulnerable while about 45% is of moderate rating, and 50% low vulnerable rating. It is highly recommended that the least vulnerable zone should be the primary target for future groundwater development in the area in order to ensure continuous supply of safe and potable groundwater for human consumption; and more importantly, location of septic tanks, petroleum storage tanks, shallow subsurface piping utilities and other contaminant facilities should be confined to low vulnerable zones.