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ABSTRACT
Integrated Geophysical and Physico-chemical studies of Soil and Water was carried out to map the leachate migraion and determine the extent of subsurface contamination at Otofure Municipal Waste Dump and its environs.
One (1) Vertical Electrical Soundings, Twelve (12) 2-D Electrical Resistivity imaging were carried out in the study area to delineate the plume and probable trend of migration. 2-D parallel lines were independently inverted and then collated to 3-D data sets which were also inverted using a full 3-D inversion code. Four (4) Very Low Frequency Electromagnetic (VLF-EM) profiles of 250m each with 10m interline spacing were taken across the waste dump and Water samples from 12 boreholes around the North, South, East and West of the dump were collected for physical and chemical analysis while soil samples were also analysed for heavy chemical contents at varying depths up to 3m.
The inverted 1-D resistivity section was able to delineate four subsurface geoelectric units and the depth to aquifer was estimated at 66.2m within the dump. The inverted 2-D Resistivity images around the dump showed that there was horizontal leachate migration into the surroundings up to 26m into the subsurface. Conductive zones were also identified by the VLF-EM which showed conduction up to 40m into the subsurface. The Physical and chemical analysis of the water samples showed pH values of 6.8 – 8.5. The conductivity, resistivity and total dissolved solids were all within the acceptable limit. The chemical analysis showed that Pb, Zn, Cu Cd and Cr where all below the permissible World Health Organization (WHO) and Federal Environmental Protection agency (FEPA) values. The contamination and pollution index (C/PI) of the analysed heavy metals in soil showed that only Iron (Fe) was in very severe contamination to the pollution ranges with high risk degree while other analysed heavy metals were in the contamination ranges with low risk degree as indicated by the C/PI. The overall assessment showed that the aquifer has not been contaminated, however the natural clay which acts as seal has been broken and the aquifer could be contaminated within the nearest future. The continuous practice of open waste disposal pose a threat to the shallow aquifer depth in Benin city and Proper Landfill system should be encouraged. Government should designate areas that may not pose danger to the underground water system in urban areas. Time lapse studies should be done so as to ascertain the estimated time it will take the vertical migrating plume to reach the aquifer.
