Geoelectrical assessment of polluted zone by sewage effluent in University of Ibadan campus southwestern Nigeria.

Sewage effluent was discharged through leaking sewer into the agricultural training farms, University of Ibadan which is used for cultivation of vegetables and cereals, thus constituting threat to human health. This study examined the reliability of the electrical resistivity technique in delineating sewage polluted zones and possible pathways into the groundwater network. The area investigated is underlain by banded gneisses. Ten resistivity Wenner-imaging lines (N-S) were established and inverted section generated using RES2DINV. Thirty VES stations were occupied using Schlumberger array, the current electrode spacing (AB/2) expanded from 1 to 75 m and data analysed using WinResist. 2-D inverted section depicted the top soil and clay unit. Resistivity in the top soil (36-70 Ω-m) showed a decrease of 57 to 78% while the clayey unit (6-18 Ω-m) showed a reduction of 63 to 88% relative to the control units. Plunge in resistivity within the top soil and clay units suggests the presence of pollutants. VES-derived geoelectric section recorded three varying sequences, with a fall in resistivity in the top soil (22-58 Ω-m) and the clay unit (6-15 Ω-m) compared with the reference layers having 233 and 34 Ω-m, respectively; the fractured sections in some parts of the bedrock could serve as a conduit for the seepage of the pollutants into the groundwater system. Drop in resistivity suggests the presence of pollutants within the top soil and clay units. These techniques have helped in mapping out unnatural zones around the farm.