Geophysical and Hydrochemical Investigation of a Municipal Dumpsite in Ibadan, Southwest Nigeria

Abstract

Geophysical and hydrochemical investigations have been undertaken within a reclaimed site of municipal dumpsite in Ibadan, southwestern Nigeria with a view of assessing the impact of effluent from the ancient dumpsite on the soil and groundwater sytem. The study area is underlain by precambrian Basement Complex rocks mainly granite gneiss. The geophysical investigation involved electrical resistivity methods using dipole-dipole profiling and Schlumberger Vertical Electrical Sounding (VES), while the hydrochemical investigation involved physical, chemical and microbial analyses of water samples within the reclaimed land and active dumpsite. A total of 175 sampling points were occupied using dipole-dipole profiling, while 29 Schlumberger electrical soundings were carried out. In addition, water samples from available nine wells in the area whose static water levels range between 1.2 m and 7.4 m were analysed. The results from electrical surveys show that the study area is underlain by a maximum of four subsurface layers namely the topsoil, the weathered layer, the partly weathered/fractured basement and the fresh basement whose resistivities values range from 41-495 ohm-m, 13-643 ohm-m, 86-720 ohm-m and 2800 ohm-m and above. Their thickness of the overburden units varies from 0.7- 49.5 m. The partly weathered/fractured basement constitute the main aquifer. The resistivity distribution of the topsoil and weathered layer indicates that parts of these layers have been infiltrated by plumes from the reclaimed land and active dumpsite, especially in areas characterised by low resistivity (<30 Ohm-m). In most cases, the suspected leachates are held within the clayey overburden and are prevented from infiltrating the aquifer by local barriers. There are indications that the leachate migration is topographically controlled. The hydrochemical analysis of samples from the wells show that the concentration of the analysed anions (Cl^-, S0₄^2- and N0₃^-) and cations (Na⁺,  Ca²⁺, Mg²⁺ and Fe⁺) are within the World Health Organisation (2004) and Standard Organisation of Nigeria (2007) permissible limits. This indicates that the aquifer system in the area might be free from contamination. However, there is possiblity of future impacts on wells in the area from downwards  migration of the effluents from active dumpsite and other anthropogenic activities relating to human impacts on existing geo-environmental systems.

Keywords: effluent, contaminantion, hydrochemical analysis, leachate, aquifer