Integrated geophysical investigation techniques for mapping cassava effluent leachate contamination plume, at a dumpsite in Ilero, southwestern Nigeria

Abstract

The subsurface illuminating strength of integrating Electrical Resistivity (ER) and Frequency Domain Electromagnetic (FDEM) geophysical investigation techniques to evaluate the level of subsurface leachate contamination plume generated by indiscriminate dumping of cassava product processing effluents and wastes around a dumpsite near cassava processing mill at Ilero southwestern Nigeria, has been demonstrated in this study. The lateral spread as well as depth extent of the cassava waste effluent leachate plume was determined from the subsurface resistivity and conductivity models generated from combined ER and FDEM geophysical surveys carried out through eight (8) Electrical Resistivity Tomography (ERT), eight (8) FDEM and (2) Vertical Electrical Sounding (VES) measurement profiles and stations purposively established across the dumpsite.Quality checked, inverted and depth matched ER and FDEM data generated 1D, 2D, and 3D resistivity and conductivity distribution in the form of subsurface logs, sections, constant depth maps and earth model employed to characterize the study area. Electrical resistivity result presents resistivity distribution that range in value between 21 Ωm to 213 Ωm from the topsoil to depths beyond 7.5 m. The FDEM sections and depth maps show conductivity distribution range between 4 mS/m – 286 mS/m across the dump site and provided information up to a depth of 60 m. The VES results generally delineate three (3) georesistivity layers, which include topsoil, weathered and fractured basement layers, occurring at depth between 0.4 – 10.0 m with characteristic resistivity values from 33.5 Ωm – 37.9 Ωm. The 3D resistivity and conductivity models generated by integrating the various results indicate anomalously low resistivity and relatively high conductivity (21 – 30 Ωm; 11 mS/m – 40 mS/m) zone which defines contamination plume in the central part of the cassava effluents and waste dumpsite.This study has through integrated electrical resistivity and frequency domain electromagnetic geophysical methods of investigation highlighted the subsurface contaminating potential of indiscriminating dumping of agricultural processing effluent and wastes, especially of cassava effluent, which has toxic cyanogenicglucosides constituent that is deleterious to the environment.