INTEGRATED GEOPHYSICAL INVESTIGATION OF GROUNDWATER POTENTIAL AND VULNERABILITY AROUND CASSAVA PROCESSING MILL AT IJU, SOUTHWESTERN NIGERIA

Abstract

A tripartite approach involving combined electromagnetic, electrical methods and well water analysis for geophysical assessment of groundwater potential and vulnerability of aquifer units around a major Cassava Processing Mill at Iju, Southwestern Nigeria was undertaken. The E.M results showed relatively low magnitude of subsurface conductivity (0.1 – 7.05 mS/m) along the traverses while the results of the Dipole-dipole profiling showed three to four geoelectric layers comprising, the topsoil, sandy, lateritic and the weathered layers. The VES results showed that the aquifer layers thickness is in the range of 0.9 to 24.9 m, while resistivity values are between 22 and 385 Ohm-m, diagnostic of clayey weathered materials to fractured bedrock. Considering criteria that include aquifer resistivity (R), thickness of aquifer (H), bedrock relief (BR) and Hydraulic conductivity of the aquifers (HC), the concept of Analytical Hierarchy Process (AHP) was used to generate normalized weights (0.51, 0.29, 0.06 and 0.14) for the criteria respectively, these were used to compute the groundwater potential indexes (GPI) in the range of 0.42 to 0.86 for the area. Exploring the efficacy of GIS software, the GPI values were processed to generate groundwater potentiality map that zoned the area into very low, low, moderate and high groundwater potential zones. Similarly, the determined weights for the vulnerability conditioning factors: 0.54, 0.30, 0.08 and 0.08 for Longitudinal conductance (L), Aquifer type (G), Aquifer overlying material (O) and Depth to aquifer (D), was evaluated with the same AHP approach. The vulnerability index values based on the AHP weightage are in the range 0.64 to 0.86. The values were processed with the same GIS software to generate the groundwater vulnerability prediction map that zoned the area into very low, low, moderate and highly vulnerable zones. Areas characterized by very low - low groundwater potential makes up 80% of the entire area, hence, the groundwater prospect in most of the study area is presumed to be unpromising. The physicochemical analysis of water samples showed that conductivity, PH, total hardness, total dissolved solid, Nitrates and Sulphates are all below the WHO maximum permissible limits, validating the results of the groundwater vulnerability studies which showed that the groundwater in the study area has not been impacted by pollutants.