IMPACT OF A WASTE DISPOSAL SITE ON GROUNDWATER QUALITY (A CASE STUDY OF OKEODO REFUSE DUMPSITE, IWO OSUN STATE SOUTHWESTERN, NIGERIA)

Groundwater contamination occurs when urban waste materials, mainly domestic garbage, are disposed of without the appropriate measures imposing a high risk to the underground water resources. Groundwater pollution happens mostly due to percolation of pluvial water and the infiltration of contaminants through the soil. Due to the dangerous effect of the pollution on underground water, this work was carried out. A study of the impact of waste disposal on the groundwater resources around a refuse dumpsite at Oke-odo, Iwo, Osun state, Nigeria was carried out using Very Low Frequency Electromagnetic method (VLF-EM) and Vertical Electrical Sounding (VES) of the Schlumberger array.The VLF-EM survey comprised seven profiles with four which run from East to West with 10m inter-station spacing and another three from south to the north to cover the entire study area. A total of two resistivity soundings were also established; one each at the extreme western and eastern region of the waste disposal site. Hydrochemical analyses were conducted at the peak of the raining season and also during the dry season on five water samples from five hand-dug wells situated at the western and the eastern ends of the study area. These hand-dug wells have depths ranging from 3 to 5m. Water samples were tested for the concentration level of NO3-, PO43-, As, Fe, Cu, Pb, SO42-, Zn, and the pH levels. The results of the VLF-EM and VES revealed the presence of contaminant plumes which are detected as conductive anomalies mainly of dissolved salts from decayed organic matters. The geoelectric section generated from the VES data revealed that the polluted region has resistivity values as low as 12.0Ωm and 40.5Ωm, and very shallow depths 3.0m and 4.0m to the anomalous source (contaminant plume). Results of the hydrochemical analyses conducted were compared with the standard guidelines of the World Health Organization (WHO) to ascertain their quality and it was shown that the quality of water in the study area falls below standard.