Integrated geophysical characterization of municipal solid waste disposal sites in the Kumasi Metropolis

Full wave spectral time-domain induced polarization, magnetic susceptibility and ground base magnetic datasets were acquired to map and characterize both engineered (Dompoase Landfill) and unengineered (Ohwim and Kwame Nkrumah University of Science and Technology municipal solid waste disposal sites) located in the Kumasi Metropolis. These geophysical datasets were selectively applied and integrated to help in full characterization of the waste disposal sites in terms of waste thickness, pollution plume mapping and the geological model development of the sites. In addition, a total of six boreholes were drilled around the sites to help in correlating the observed geophysical signatures with the waste thickness and the lithostratigraphic sequence in order to help in interpretation and characterization of the sites. The study was carried out with the aim of determining the risk posed by the waste deposit to the quality of soil and the groundwater system which is now becoming the main means of potable water supply in the metropolis due to the erratic supply supply and the inability of the Ghana Water Company (GWC) to supply water especially to new developing settlements in the metropolis. The intrinsic Cole-Cole IP parameters such as chargeability, resistivity and relaxation time as well as the normalized chargeability distributions, together with the magnetic results, aided in a full characterization of the wastes (vertical and lateral distribution), associated pollution plumes and for developing the geological model of the sites. The KNUST site was characterized using Cole-Cole parameters namely resistivity and chargeability as well as normalised chargeability and magnetic datasets. In particular, a clear contrast in resistivity and polarization effect between the saprolite layer and the granite bedrock, the main lithological units of the area aided in the development of a geological model of the site. Furthermore, it was found that the KNUST waste deposit is characterized by a low-chargeability and low-resistivity signature, and that the low-resistivity area spreads out from the waste deposit into the permeable saprolite layer, indicating the presence of a leachate plume which was mapped to be in the range of 5 to 30 m thick. A mapped fracture zone within the granitic bedrock linked to the pollution plume with the potential of aiding in leachate percolation of groundwater by serving as a conduit for infiltration of the leachate was also mapped. Similarly, the Ohwim Waste Disposal Site was mapped with the magnetic, magnetic susceptibility and full wave spectral time-domain induced polarization. These methods helped in determining the waste thickness and the extent of the pollution plume as well as the geological model of the site. The waste recorded high v magnetic anomaly, low resistivity and high normalized chargeability. The geological model of the site was developed by using the strong chargeability signature it produced. Furthermore, the Dompoase Landfill survey which was planned mainly to determine and monitor possible pollution plume around the waste, produced some intriguing result. The magnetic and the magnetic susceptibility results aided in the full mapping of the lateral extent of the waste as well as the channel of the released effluent from the treatment sewage pond, showing the high ferromagnetic iron content of the leachate. The strong IP effect in terms of chargeability and normalized chargeability as well as the low resistivity signature associated with the plume helped in mapping the plume around the catchment area of the waste. The controlled nature of the waste disposal at the Dompoase landfill makes it a possible resource for the production of natural gas. The research provides a cost effective means of monitoring and characterizing municipal solid waste site in the Kumasi Metropolis. It provides a full outline of the solid waste disposal sites investigated and also provides the information needed for assessing the future impact of the waste on the water quality in the area, and the need for designing risk-mitigation actions in these sites. The impact of waste disposal sites on the quality of the environment as shown by this work calls for a review of national policy in the waste site management with focus on reduction of the impact of the waste on the ecosystem.

Source: KNUST