AQUIFER VULNERABILITY STUDY IN THE VICINITY OF SOME SELECTED DUMPSITES IN ITAOGBOLU, SOUTHWESTERN NIGERIA

Abstract

The aquifer vulnerability to contamination risk may increase daily due to continuous dumping of waste in an area within a typical Basement Complex terrain of Itaogbolu in Ondo State, southwestern Nigeria. The aim of the research is to develop a composite aquifer vulnerability map around some selected dumpsites within the area using a modified ‘DRASTIC’ (DRASTIC-L) model. DRASTIC model accounts for seven aquifer parameters namely, depth to water table, net recharge, aquifer media, soil media, impact of vadose zone and hydraulic conductivity. However, the modified DRASTIC (DRASTIC-L) incorporates land use as additional input parameter and the analysis was performed using ArcGIS software to prepare a vulnerability map of the study area. Data from Vertical Electrical Soundings (VES), remote sensing and hydrogeology were used for the eight parameters. The VES involved fifty six (56) depth soundings using Schlumberger array, with half maximum current electrode separation (AB/2) of 65 m. The remotely sensed parameters were extracted using LANDSAT 7 Enhanced Thematic Mapper Plus (EMT+) and Digital Elevation Model (DEM). The DRASTIC-L Vulnerability Index (DVI) was calculated as the sum of product of ratings and weights assigned to each of the parameter on the scale of 1 to 10 and 1 to 5 respectively. Using the produced vulnerability index, the area was classified into five classes; very high, high, medium, low and very low vulnerability zones. The research revealed that, part of the dumpsites falls within the high vulnerable zone; hence the groundwater within the area may be under the threat of contamination from the leachate products of the dumpsites. Using the chemical analysis of water samples within the classified zones to validate the model, the modified DRASTIC-L shows a reliable accuracy of 76%.