Application of geochemical and geophysical approach to environmental impact assessment: a case study of Emirin active open dumpsite, Ado-Ekiti Southwestern Nigeria

Abstract

An integrated physico-chemical, geochemical and geophysical investigation was carried-out in and around Emirin waste dumpsite Ado-Ekiti southwestern Nigeria with the view to assess the extent of possible soil and groundwater pollution arising from leachate plume from the dumpsite. Seven (7) water samples were collected which consist of three (3) hand dug wells and four (4) from different river locations beside the dumpsite. Ten (10) soil samples were collected at B horizon in and away from the dumpsite at a random interval. Ten (10) vertical electrical resistivity (VES) stations were established using schlumberger configuration with half-current electrode spacing that ranged from 1 to 150 m while dipole–dipole profiling was carried-out along four (4) profiles. The physico–chemical result shows that the total dissolved solid (TDS) of the three well samples ranges between 785 and 21,600 mg/L. While the TDS for the river samples was between 2030 and 2050 mg/L, both the upper and lower limit of the TDS were above the Nigerian Standard for Drinking Water Quality (NSDWQ 2007) and World Health Organization (WHO 2008) maximum permissible standard. The electrical conductivity (EC) of both well and river samples ranges from 1200 to 3650 μ/cm and 34,200 to 34,350 μ/cm respectively was also above the maximum permissible standard. The elemental concentration of Pb, Fe, Cd and K in the soil samples located within the dumpsite are higher than those of the control point which indicate pollution. The VES interpreted result delineated three subsurface layers comprising, the topsoil, weathered layer and fresh basement. While the dipole–dipole results shows that the topsoil and weathered layer has been virtually merged with characteristic low resistivity of <5 Ωm, which is attributed to leachate infiltration in and around the zones of the dumpsite. The extent of leachate migration was estimated to be >25 m. The leachate seeped to the bottom in vertical motion as seen in the dumpsite. 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. 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. It could be concluded that the soil and water in the vicinity of the investigated dumpsite has been polluted.