THE ROLE OF GEOPHYSICS IN THE INVESTIGATION OF WASTE DISPOSAL SITE IN IKARE-AKOKO AREA, SOUTHWESTERN NIGERIA

Abstract

Water pollution is serious problem in the global world. Increasing population, urbanization, environmental degradation, changing agricultural sectors, etc., are some of the factors that are determining the future water demands. The impact of solid waste disposal at Alapata area along Secretariat Road, Ikare Akoko on the quality of the surrounding surface and groundwater sources was investigated. The study area is underlain by migmatite-gneiss complex with the predominant rock type being granite gneiss, which is concealed in many places. In this study, integrated geophysical techniques involving very low frequency (VLF) electromagnetic survey and the electrical resistivity sounding method have been helpful in mapping areas of contaminated groundwater. The VLF-EM profiling was carried out at 5 m interval along seven traverses and interpretation of the data along these traverses indicate a resistivity low over the contaminant plume which were used to pick point fourteen VES stations, which correlates with the electrical resistivity data. The VES stations were occupied using Schlumberger electrode array configuration, with half-current electrode separation (AB/2) varying from 0.25 m to 100 m so that the basement can be penetrated via the overburden. The resistivity sounding curves obtained from the study area vary from 3-layer earth model to 4-layer earth model (H, KH, QH, and HA types). The H-type is the most dominant curve type in the study with percentage occurrence of 64.29%.

The results revealed that the study area is underlain by four geoelectric layers; the topsoil, weathered layer, fractured layer and the fresh bedrock which are characteristics of the basement complex terrain. The resistivity of the topsoil ranges from 194.1 Ωm to 1528.4 Ωm while the thickness varies from 0.6 m to 2.8m. The weathered layer resistivity values range from 28.8 Ωm to 647.8 Ωm while the thickness varies from 3.2 m to 9.0 m. Information obtained from the existing hand dug wells in the study area show that the total depth varies from 2.9 m to 8.3 m while the depth to water table varies greatly from 0.1 m to 7.0 m during the dry season and 2.3 m to 6.0 m during the rainy season depending mainly on the recharge from precipitation. The weathered layer in the study area is underlain by fractured basement of varying thickness of 1.3 m to 13.1 m and layer resistivity values ranging from 47.5 Ωm to 717.4 Ωm. The presence of fractures in the underlying rock therefore enhances the pollution of surface and groundwater in the vicinity of the dumpsite. The overburden thickness i.e. depth to basement varies between 3.2 m to 13.1 m with a mean of 5.45 m. There is an increase in the resistivity of the water bearing layer with increasing distance from the dumpsite while the anomalously low resistivities indicate areas with contaminated groundwater. The fresh bedrock has infinitely high resistivity values between 3322.1 Ωm to 15079.3 Ωm. The bedrock has a permeable section at the top due to the existence of a fractured zone caused by weathering, which contains the portion of the contamination plume. With these results, the area under investigation is not suitable for the location of a landfill site.

                On this note, since the dumpsite constitutes a potential hazard to the people living in the area who depend on hand-dug shallow wells for their source of drinking water while the quality aspect is often neglected, the local authorities are hereby urged to discontinue dumping of refuse in this area. Finally, the water abstracted from the area must be treated appropriately to mitigate hazards to the inhabitants.

 Therefore, integrated use of Very Low Frequency (VLF) electromagnetic survey and the Electrical Resistivity sounding method has led to a far better understanding of the site in terms of determining the effects of refuse dumping on the quality of groundwater in the vicinity than could have been achieved using any other site investigation methods.