Vulnerability assessment of an aquifer in the basement complex terrain of Nigeria using ‘LAHBUD’ model

We integrated hydrogeological and hydrogeophysical parameters to evaluate aquifer vulnerability to contamination in a basement complex terrain in Nigeria. Aeromagnetic and Shuttle Radar Topography Mission (SRTM) data were used to generate the composite lineament density map. Lithology information was obtained from the existing geological map, while land-use and land-use changes were derived from the Landsat 8 imagery. We also acquired 216 vertical electrical soundings (VES) to estimate the geoelectric parameters including depth to aquifer, hydraulic conductivity, and bedrock relief maps. The information obtained from geoelectric parameters, lithology, lineament density, and land use were used as the parameters in assessing the aquifer vulnerability using multi-criteria decision analysis (MCDA) in the context of the analytical hierarchy process (AHP). The assigned weights for the various aquifer vulnerability indices are 0.4569, 0.2482, 0.1469, 0.0775, 0.0402 and 0.0303 respectively for lithology (L), depth to aquifer (A), hydraulic conductivity (H), bedrock relief (B), land-use (U) and lineament density (D). These values were normalized and used in establishing a consistency ratio of 6.94%, which is lower than 10% (recommended limit). The MCDA was used to develop the aquifer vulnerability (‘LAHBUD’) model which was used to classify the study area into very low, low, moderate, high, and very high aquifer vulnerability zones. Geochemical parameters obtained from water samples were used to validate the ‘LAHBUD’ model, and gave 81% agreement in predicting the risk level of possible contaminant influence with the conceptual model, and thus confirm the reliability of the model adopted for this study.