ASSESSING THE ENVIRONMENTAL IMPACT OF AN ABANDONED REFUSE DUMP SITE IN IBADAN, SOUTHWESTERN NIGERIA, USING GEOELECTRICAL IMAGING AND TOPOGRAPHICAL MODELING

We carried out geoelectrical imaging at an abandoned landfill/refuse dump site in Ibadan, southwestern Nigeria, with the aim of accessing the environmental impact of the refuse dump site. The imaging was also expected to reveal the heterogeneous material compositions of the dump site. The dump site is owned and maintained by Ibadan Solid Waste Management Authority and is bordered to the north and south by buildings, and to the west by a mini-market. Eight multi-electrode electrical resistivity imaging profiles were measured at the site. Four of the traverses were conducted directly on the dump site whereas two other traverses were measured towards the lower side to assess possible ingress of the leachate. The two other lines were measured about 300 m from the site to serve as control. We collected elevation data using global positioning system (GPS). The resistivity data were inverted using the least-square optimization method, whereas the GPS data were analyzed with ArcGIS using Spatial Analyst tools to generate terrain models and evaluate possible surface water-flow directions. The resistivity data inversion delineates regions of low resistivity (less than 20 Ohm-m) believed to be leachate derived from decomposed waste. Non-degraded solid refuse occurs as isolated regions of relatively high resistivity (greater than 20 ohm-m). The highest resistivity regions (greater than 100 ohm-m) are interpreted as regolith derived from chemical weathering of the crystalline bedrock. Resistivity-derived thickness of the leachate zone is consistent with the thickness derived from the elevation data. It could be inferred that there is high concentration of leachate towards the lower elevation hence the adjoining stream is prone to pollution. Results are consistent with previous geochemical studies which show that groundwater and surface water in the vicinity of the site are polluted. The integrative approach used in this study is shown to be effective in imaging pollution plumes around refuse dump sites. The approach presented can be useful to assess opportunity for remediation measures in situations where the leachate has reached the groundwater system. We conclude with recommendations to prevent and/or reduce further environmental hazard from the refuse dump site.