The expansive development in Osogbo, the Osun State (Figure 1) capital in Nigeria, occasioned population rise in the state capital and  beyond.  The  increased  population and urbanization had resulted in the increased production of Municipal Solid Wastes (MSW). The inability to properly  manage  these wastes generated poses a great concern. Waste disposal and management remain two of the major challenges in poor underdeveloped and developing cities, where improperly disposed wastes could put surface and ground water at risk of contamination. Landfills/dumpsites serve as the ultimate recipient of municipal solid wastes. It is a common waste management practice employed by many nations in many parts of the world (Jhamnani and Singh, 2009). Groundwater contamination from landfills often results from leaking "leachate”, water that has percolated through waste and accumulated various ions in solution (Bhuiyan et al., 2011). The handling of solid wastes (S.W.) calls for immediate attention and adoption of the best practicable environmental management/approach, towards preserving the environment. In achieving a sustainable solid waste management strategies (SSWMS), all effective  practicable  management  processes  in  that regard must be functional, as wastes are be ingenerated. They include solid wastes characterization, collection, transportation and treatment.

Dumpsite-related geophysical surveys especially the two dimensional geoelectrical imaging have frequently been used in the subsurface pollution studies (Carpenter et al., 1990; Bernstone and Dahil, 1997; Samsudin et al., 2006; Irunkwor and Abanjo, 2022). The electrical resistivity method maps the distribution of resistivity of subsurface materials and it is also used in delineating areas affected by the leached substances (Offodile, 2002).

Due to its efficiency and effectiveness in producing images of the subsurface, the resistivity-imaging method has now become more popular in electrical exploration. The 2-D geoelectrical resistivity imaging actually measures  the  apparent   resistivity   of   the  subsurface, which can be inverted to develop a model of the subsurface structure and stratigraphy in terms of its electrical properties (Dahlin and Zhou, 2002; Samsudin et al., 2006; Meju, 2000).

This paper discusses the results of VES and 2-D resistivity imaging which were conducted to identify and delineate the extent of leachate plume generation at Osogbo central dumpsite. The resistivity image provides general information on subsurface geology and condition of buried waste and contaminated soil, below the lines of traverse.

Location and geology of the study area

The study area is an abandoned dumpsite located in Onibu-Eja of Ido-Osun area, at the outskirt of Osogbo in Osun State (southwesten Nigeria), within Latitudes 07° 53' 33.09” North and 07° 53' 34.14” North and Longitudes 04° 29' 09.80” East and 04° 29' 48.83" East (Figure 2 and Plates 1 to 3). The project site situates along Osogbo-Iwo road. It was active until about 2018, before it was relocated. The site covers an area of 79 200 m². The study area has an undulating topography as a result of the  dumping   of   refuse  over  a  period  of  time; hence, artificial hills were created. There are also natural uplands/hills.

The study area falls within the southwestern Basement Complex of Nigeria, which had been described in detail in various publications of the Geological Survey of Nigeria (GSN), and by various authors, such as Oyawoye (1972), Ajibade (1972), McCury (1976), Rahaman (1976), Rahaman (1998) and Oyelami et al. (2013).

The area falls within Ilesha Schist Belt (Figures 3 and 4). The main lithotypes are represented by amphibolites, pegmatite and quartzite. Quartzites outcrop as a massive ridge, well visible in the southern part of the area, with only a few exposures occurring as boulders. Pegmatites are the most widespread.

Thirty-three (33) VES were conducted across various stations in the study area (Figure 5) using ABEM Terrameter SAS 300. The measurement technique adopted was Schlumberger configuration with a maximum spread length of 100 m. The depth range of the measurement was customarily increased by an outward displacement of the current electrodes. The typical field procedure was to transmit current (I) through the electrode (C₁ and C₂), while the resulting ground voltage (V) was measured with the potential electrode  (P₁  and  P₂)  (Figure  6).  Details  about  the  survey  and interpretation method can be found in published papers by Keller and Frischknecht (1966), Griffiths et al. (1990), Griffiths and  Barker (1993) and Loke and Barker (1996). The computation of the apparent   resistivity   (ρ_a)  was  achieved  using  the  values  of  the apparent resistance (Ra) and geometric factor (Gs) for Schlumberger array:

Gs is the geometric factor for Schlumberger array mainly affected by electrode arrangement and Ra is the field apparent resistance measured from the equipment.

Equation (1) can also be written as:

where AB is the distance separating the two current electrodes while MN is the separating distance between the potential electrodes                    

The vertical electric sounding (VES) was deployed, prior to ERT investigation. Four ERT traverses were conducted, using Dipole-Dipole Array (Figure 6) to investigate a total of one hundred and fifty-four (154) data points, which produced 2-Dimensional Imaging of the survey (Figures 11 to 15). The apparent resistivity (ρ_a) was recorded in ohm meter at each dipole separation. In Dipole-Dipole array, the values obtained were plotted on a depth section along intersecting 45°, beneath the centre of the dipoles. At the end of each session, the entire array was moved by a distance equal to one dipole separation and the process was repeated.

To interpret the 2-D resistivity data, a 2-D model for the subsurface consisting of a large number of rectangular blocks was used (Figure 7). The inversion software, Kigam Dipro for Windows version 4.01 (2001), was  used  to  determine  the  resistivity  of  the block so that the calculated apparent resistivity values agree with the measured apparent resistivity values from the field survey. The apparent resistivity of dipole-dipole array is given by:

ρ_a = πRna(n+1)(n+2)                                                                      (4)

or

ρ_{a =} πn(n+1)(n+2)a ?V/I                                                                    (5)

where n= expansion factor, a = electrode spacing, R = resistance measured, I is the current, and ?V is the potential difference. The theoretical depth of investigation is approximately 0.2AB. The dipole-dipole array gives good quality information of the subsurface in both vertical and horizontal directions (Ward, 1990; Zume et al., 2006). Increase in separation between the dipoles (C₁-C₂ and P₁- P₂) (Figure 7) increases depth of investigation. The inter-electrode spacing (a) was equal to 10 m, whereas the expansion factor (n) was equal to 4 m. The total length of the transverse profile  was 100 m for traverse 1 (Figures 11 and 12), traverse 3 (Figure 14) and traverse 4 (Figure 15), respectively, while traverse 2 (Figure 13) was equal to 150 m.

Vertical electrical sounding (VES) and 2D imaging have been successfully used to map the contaminant plume and characterize the subsurface of the study area, in terms of distribution of resistivity values of the solid waste materials and soils beneath  the  site.  The areas infested by the leachate could be inferred from the VES data as well as the 2D inversion sections. The weathered and fractured basement rocks, which constituted the major aquifer units in the area, were overlain by thin clayey formations, with low resistivity values at the west end, and were suspected to be contaminant plume. The 2D Inversion also delineated contamination plumes as low resistivity zones with resistivity values ranging between 24 and 67 ?m at depths of 10 to 20 m. Given these low depths of the leachate plume, it was suggested that sinking shallow water wells around the dumpsite should be avoided.

Industrial wastes contain large number of heavy metal toxicants that contaminate our diets and water. Framing appropriate laws and strict implementation by regulatory bodies, must be encouraged. Hence, it becomes obligatory for the state government to designate the site and environs as hazardous area, and adopt a sustainable environmental management system, which should include environmental impact assessment (EIA), and also ensure that the decommissioning of all unsanitary landfills must go through a  sustainable  integrated  scientific process of remediation, reclamation and restoration, in line with national standards, (NESREA Training Manual, 2014).

The authors have not declared any conflict of interests.