Application of Geoelectrical Resistivity Method to the Assessment of Groundwater Pollution: A case Study of Onibu-Eja Active Open Dumpsite, Osogbo, Southwestern Nigeria

Abstract

Electrical resistivity methods using the dipole – dipole array and Schlumberger Vertical Electrical Sounding (VES) techniques were conducted at the Onibu-Eja active open Dumpsite, Osogbo, Southwestern Nigeria in assessing groundwater pollution. Eight profiles and twenty four VES station measurements were carried out in the eastern and southern accessible area of the dumpsite. The VES data were quantitatively interpreted using the partial curve matching technique and 1-D inversions with WinResist software. The dipole-dipole data were inverted into 2-D resistivity images using the DIPPRO 4.0 software. Subsurface geologic layers delineated include the topsoil (20 to 998 Ωm and 0.4 to 1.0 m thickness), clay/weathered layer (63 to 333 Ωm and 1.2 to 7.6 m thickness), weathered basement (25 to 83 Ωm and 3.0 to 27.0 m thickness) and fractured/fresh basement (31 and 16213 Ωm). The results of the VES from the geosections suggest saturated weathered basement indicative of conductive material/leachate especially in traverse TR6 where resistivity less than 35 Ωm occurred. The aquifer has hydraulic conductivity (K) range 0.326 to 0.720 m/day indicating low conductivity. The transmissivity (T) values ranged between 1.47 m2/day and 17.40 m2/day, showing that the area has low to intermediate transmissivity capacity that can meet withdrawals for local water supply. The 2-D profiles distinctly delineated subsurface layers and contamination zones were also found within the aquifer units in the study area. These zones occurred at several traverses with resistivity values less than 31 Ωm and thickness variations from 5 m to 25 m. The leachate seeped to the bottom in vertical motion as seen at the eastern part of the dump site. This could be as a result of the relative permeability of the overburden, possible linear features and the downward sloping of the bedrock topography towards the dumpsite in that area. At the southern part of the dumpsite, the leachate is inferred to migrate laterally, which could imply that the neighboring rocks are relatively porous and permeable. This migration is considered slow because there was no trace of contamination 200 m away from the dumpsite.