Integrated geophysical and physicochemical assessment of Olushosun sanitary landfill site, southwest Nigeria

Abstract

An integrated surface geophysical and physicochemical study involving 2-D electrical resistivity imaging (terrain conductivity measurement using EM34-3) complimented with measurement of some physical parameters was conducted at Olushosun sanitary landfill site in Lagos metropolis, southwestern Nigeria, with the aim of investigating the lateral extent and depth of the possible subsurface leachate contamination plumes (electrically conductive anomalies) within the area. Fourteen 2-D resistivity imaging lines were investigated with a maximum spread length of 249 m per line. The result of the resistivity imaging delineated the contaminant plume as low-resistivity zones (0.24–36 Ωm) to a maximum depth of 59 m. The electrical resistivity tomography (ERT) lines were projected to produce stacked block models of the site which show southeast flow pattern of the leachate and, possibly, the groundwater flow direction. Eleven electromagnetic (EM) profiles were established with a maximum spread length of 150 m per profile using both the vertical and horizontal dipole configurations to measure terrain conductivity of the study area; 10, 20, and 40 m coil separations were deployed for the measurement. Qualitative interpretation of the EM34-3 data reveals high conductivity range of values (30–264 mmho/m) within the dumpsite as compared to conductivity values ranging between 4 and 26 mmho/m for the control site. It delineated the vertical extent of the contaminated zones up to a maximum depth of about 30 m (horizontal dipole configuration) and a maximum depth of about 60 m (vertical dipole configuration). Physicochemical analysis of the water samples taken from wells and boreholes within the precinct of the dumpsite reveal an elevation in concentrations of total dissolved solid (TDS) (range of 513–2,000 mg/l) and electrical conductivity (EC) (range of 1,019–3,999 μS/m) in wells 4, 7, 10, 11, 15, 16, and 19 with the values obtained moderately above the WHO standards. The pH obtained from water samples indicates high acidic content (5.34–6.85). These possibly indicate contamination of the groundwater as a result of solid waste leachate accumulation, thus complimenting the geophysical data. Leachate flow direction was generated from the increasing concentration of TDS and EC in southeast direction which agrees with similar flow pattern deduced from ERT results.