Groundwater Vulnerability Assessment around Open Dumpsites in a Nigerian Basement Complex Terrain Using a Modified DRASTIC-L Model

Abstract

The risk of aquifer contamination is increasing in areas underlain by Basement Complex terrain due to continuous waste disposal. The primary aim of this study is to evaluate the intrinsic and specific vulnerability of groundwater resources to contamination from leachate originating from open dumpsites within a Nigerian Basement Complex terrain by applying a Modified DRASTIC-L model. This study develops a composite aquifer vulnerability map for selected dumpsites using a modified version of the DRASTIC model, referred to as DRASTIC-L. The study area is situated in Itaogbolu, within the Akure North Local Government Area of Ondo State, Nigeria. Unlike the traditional DRASTIC framework, which evaluates seven hydrogeological parameters; depth to groundwater, net recharge, aquifer media, soil media, impact of the vadose zone, and hydraulic conductivity. The DRASTIC-L model incorporates land use as an additional factor. Vulnerability mapping was carried out in ArcGIS using inputs from Vertical Electrical Soundings (VES), remote sensing, and hydrogeological surveys. The VES campaign comprised 56 depth soundings with electrode separations up to 65 m, while remotely sensed parameters were derived from LANDSAT 7 ETM+ imagery and a Digital Elevation Model (DEM). The DRASTIC-L Vulnerability Index (DVI) was calculated by assigning ratings (1–10) and weights (1–5) to each parameter. The study classified the area into five vulnerability zones: very high, high, medium, low, and very low. Findings indicate that several dumpsites are located in high-vulnerability zones, posing significant risks to groundwater quality. Model validation using water sample analysis confirmed an accuracy of 76%, highlighting the importance of improved waste management practices for groundwater protection. Given these findings, the implementation of proper waste and water management policies in the study area is essential.