Project Abstract

Abstract
—An integrated approach involving Electrical Resistivity Tomography (ERT), Vertical Electrical Sounding (VES), and Very Low FrequencyElectromagnetic (VLF-EM) methods has been used to map the concentration and pathway of leachates plumes beneath Laka dump site in OgbomosoSouthwestern Nigeria. In-situ analysis, specifically, Electrical Conductivity (EC) and Total Dissolved Solids (TDS) were also carried out on water samplescollected from hand dug wells, bore holes and stream around the area. The objectives of the study were to describe the physical/hydrogeological condi-tions in relation to the extent of the zone of contaminated groundwater in the aquifer.The subsurface ERT image obtained on the dump site indi-cated that the leachate plumes were probably near-surface at the western end and eastward at depth between 1.3m and 4.0m at horizontal distancebetween 40m and 44m. This is as corroborated by two high conductivity peaks on the conductivity map obtained from the VLF-EM measurements.Whereas, low resistivity values observed on the ERT image obtained at the control site were more gradational and not associated with the leachateplumes but rather indicate clayey material. The apparent resistivity curves obtained from the interpretation of VES data revealed 4-layer HA-type earthmodels for dump and control site comprising top soil, clayey intermediate conductive layer, clayey sand saprolite zone and fresh bedrock. The results ofin-situ measurements indicated the electrical conductivity ranged from 0.52 mS/m to 0.94 mS/m for water samples from borehole; 0.90 mS/m to 4.67mS/m for water samples from hand dug wells and highest in the surface water along the dump site with a value of 5.06 mS/m. The total dissolved solidranged from 1240 ppm to 3350 ppm for water samples from hand dug wells; 980 ppm to 1380 ppm for water samples from bore holes and highest in thesurface water along the dump site with a value of 4320 ppm. Thus noninvasive geophysical methods such as ERT, VES and VLF provide quick, efficient,and cost-effective methods for detecting, monitoring, and characterizing leachate migration patterns in dump sites and to a larger scale, the land fill sites.The identified aquifer was sandy clay which is overlain by clayey soil that prevents or inhibits downward migration of the contaminant plumes to deepgroundwater. Though no priori information is currently available about spatial migration of the leachate, the results obtained could provide a basis forestimating future rates of movement by systematically repeating the survey at fixed time intervals.

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