GEOPHYSICAL AND HYDROCHEMICAL INVESTIGATION OF THE AREA AROUND A MUNICIPAL DUMPSITE IN IBADAN, SOUTHWEST, NIGERIA

Abstract

Geophysical and hydrochemical investigations have been undertaken within the vicinity of the Jembewon dumpsite Ibadan, southwestern Nigeria with a view of assessing the contaminant impact of the dumpsite on the soil and groundwater. 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. A total of 175 station points were occupied using dipole-dipole profiling while 29 Schlumberger VES were carried out. Also water samples from available nine wells in the area whose static water levels range between 1.2 and 7.4 m,were analysed. Typical curve types obtained are A, H, KH, HA, HK, KHA and HKH. The results show that the 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 and thicknes values values ranges between 41-495 Ωm, 13-643 Ωm, 86-720 Ωm and 2800 Ωm to infinity respectively and thickness ranging from (0.7-1.6 m) and (3.2-49.5 m), respectively. The partly weathered/fractured basement constitute the major aquifer. The resistivity distribution of the topsoil and weathered layer indicates that parts of these layers have been infiltrated by leachate plume especially in areas characterised by low resistivity (<30 Ωm). In most cases, the suspected leachate plumes are held within the weathered layer and are prevented from infiltrating the aquifer. There are indications that the leachate migration are topographically controlled, but more importantly, the disposition of the leachate saturation was mainly controlled by the presence of suspected natural barriers within the weathered layer whose geometries are considered to be responsible for the different forms and shapes assumed by the suspected leachates. 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+) that are indicative of pollution are within the WHO (2004) and SON (2007) permissible limits of (250, 100 and 10 mg/L) respectively for the anions and (200, 75, 150 and 0.3 mg/L) respectively for the cations. This indicates that the aquifer system from which the available wells are tapping is free from contamination. It is, therefore, concluded that the scenerios presented by the geophysical interpretations and the hydrochemical analysis can be attributed to the complexity of the geology of the area which informs the safety of the available wells of the area from the direct impact of the dumpsites. However, it is highly likely that other wells or future wells penetrating areas where the suspected leachates are held may not be free from contamination.