Integrated Geophysical and Hydrochemical Investigation of Pollution Associated with the Ilara-Mokin Dumpsite, South-western Nigeria.
Bayode Sunday, Adeniyi Kemisola Easther

Geophysical and hydrochemical investigations have been used to map pollution in the area around a dumpsite in Ilara-Mokin, southwestern Nigeria. Three (3) Very Low Frequency Electromagnetic (VLF-EM) profiling, dipoledipole 2-D electrical imaging and fourteen (14) Vertical Electrical Soundings (VES) were carried out in the study area. Four (4) water samples from hand dug wells and two (2) surface water samples were collected around the dumpsite for physical and chemical analyses. The inverted 2-D resistivity structures and the geoelectric sections delineated four subsurface geoelectric units. These are the topsoil, weathered layer, partly weathered/fractured basement and the fresh basement bedrock. Within the premises of the dumpsite, the topsoil, the weathered layer, and the partly weathered/fractured basement are characterized by relatively low layer resistivity values of 6 – 50 ohm-m suspected to be due to conductive leachate from the dumpsite. The conductive zones were also identified by the VLF-EM 2-D subsurface image. The physical and chemical characteristics of the sampled waters include: electrical conductivity of 850 – 1000 μmho/m; pH value of 6.2 – 7.3; total hardness values of 29 – 47 mg/l; sulphate concentration of 80 – 90 mg/l and nitrate concentration of non detectable – 10 mg/l. The relatively high electrical conductivity may be indicative of surface and groundwater pollution. The polluted zone has a depth extent of about 2.5 – > 15 m and lateral extent of about 140 m west and up to 50 m south of the dumpsite.

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