Multi-criteria assessment of groundwater potential of shallow aquifers in the vicinity of Osun state university, Ikire campus and environs, southwestern Nigeria

Abstract

In order to investigate the groundwater potentials of shallow water-bearing layers within the vicinity of the Osun State University Campus at Ikire, fifteen vertical electrical sounding (VES) resistivity data were acquired at random. The groundwater potential of the shallow water-bearing layers was inferred from the resistivity data based on three (3) existing criteria namely aquifer resistivity, transmissivity and reflective coefficient. At each VES station, the maximum electrode spreading employed for the resistivity data acquisition was between 200 m and 240 m, with the interpretation of the data carried out using partial curve-matching and computer iterative software. Geoelectric parameters obtained were used to compute the longitudinal conductance, transverse resistance, hydraulic conductivity, transmissivity and reflective coefficient using empirical relationships. The computed parameters were compared with existing classification tables to deduce the groundwater propensity in the shallow water-bearing layers of the study area. Four to five geoelectric layers corresponding to the HQ, HA and HKA geoelectric curve types were observed having lateritic materials as topsoil, sandy clay/clayey sand, weathered/fractured basement and fresh basement. In most cases, the water-bearing layer is mostly of clayey material with high porosity and low permeability and resistivity values that varies from 14.6 Ωm in VES 1 to 65.6 Ωm in VES 6. A few consist of sandy clay also with low resistivity values 76.9 Ωm in VES 7 to 107.1 Ωm in VES 4. The transmitivity values vary from 0.239 m2/day in VES 2 to 2.522 m2/day in VES 5. This same behavior was observed with reflective coefficient values which range from 0.690 in VES 6 to 0.994 in VES 1. The conclusions from the classification tables correlate in most of the VES stations. This study shows that the groundwater potential of the shallow water-bearing layers in the study area is generally poor within 0 to 80 m depth due to the low transmissivity of the clay materials at the shallow depths. Effective groundwater development in the study area must therefore integrate geophysical methods and remote sensing techniques to investigate depths beyond 100 m.