Evaluation of the Extent of Leachate Contamination of the Subsurface Structures, Using Resistivity Method, In Parts of Minna, Niger State, Nigeria.

This research was undertaken to determine the extent of leachate contamination of underground sediments in Eastern bypass of Minna in Northern Nigeria, Niger State. Abem Terrameter (SAS 1000) was employed to carry out the survey using Schlumberger Vertical Electrical Sounding (VES) configuration for a maximum electrode spread of 200m. Thirty six VES stations were occupied, on an area of about 20000m2 , six on each of the six gridded profiles, spaced 40m apart from each other. Four of the profiles were positioned on the dumpsite while the last two, which served as the control site, were not. RESIT computer software programme was used for the inversion of the derived field data while Suffer 8 was engaged for the Iso-resistivity contour map at 5m, 10m, 12m, and 20m depths. The results from the dump site showed three subsurface layers which are contaminated topsoil and weathered basement, fractured basement and fresh basement. At the control site, four geoelectric and geologic sections were identified; topsoil, weathered basement, fractured basement and fresh basement. Contamination of the first layer and weathered basement was due to the effect of refused dump which percolated down to about 11m, lowering the resistivity to between 9.5Ωm and 39.9Ωm. The apparent resistivity value of the topsoil was higher at the control site which ranged between 150.4 Ωm and 170.5 Ωm without any sign of leachate. Also the results from Iso-resistivity contour maps at 5m and 10m also indicated low resistivity at the dump site and high resistivity at the control site as depicted by the contour lines. It was only at about 12m depth that the contour maps have the same trend on both sides indicating that the effect of leachate has ceased at that depth. The nature of resistivity curves obtained from the study area indicated that resistivity increase continues in the substratum.