COMPARATIVE ASSESSMENT OF SUSCEPTIBILITY OF AQUIFERS TO CONTAMINANTS FROM DUMPSITES IN DIFFERENT GEOLOGICAL UNITS IN IBADAN, NIGERIA

Abstract

Comparative assessment of susceptibility of Aquifers to contaminant was undertaken within the vicinity of Arapaja, Apete and Sabo dumpsites monolithically underlain by Migmatite Gneiss, Quartzite, and Granitic Gneiss in a basement complex of Ibadan, southwestern Nigeria. Electrical Resistivity Surveys involving Dipole-Dipole profiling and fifteen (15) Schlumberger Vertical Electrical Soundings were carriedout across two orthogonal traverses. Samples from wells were taken from the three locations. A maximum of three subsurface layers namely; the topsoil,weathered layer, and fresh basement were identified from the geoelectric sections from the study area. The weathered layer (having layer resistivity ranging between 12 and 19 Ωm, 5 and 15 Ωm, 12 and 16 Ωm, for Arapaja, Apete, and Sabo respectively) which constitutes the major aquifer units in the areas, and the overlying topsoils (having layer resistivity ranging between 35 and 41 Ωm, 62 and 87 Ωm, 11 and 48 Ωm, for Arapaja, Apete, and Sabo respectively) are suspected to have been impacted by leachates from the waste dumpsites in the three locations as revealed from its characteristic relatively low resitivity values of these layers on the geoelectric sections and the 2D resistivity structures. The hydrochemical analysis of twelve (12)water samples from wells in the three locations shows that majority of the analysed cations and anions in the three study locations are within the WHO permissible limits. However, there are indications that parameters such as Pb2+ and Fe2+ in Arapaja and EC, TDS. K+, NO3 2-, Pb2+, and Fe2+ in both Apete and Sabo dumpsites are higher than the acceptable limits which reveal possible contamination impacts of infiltrating leachates from these dumpsites on the ambient groundwater in the three locations. This is in agreement with the pollution index rating of these parameters which indicates that they fall within the strongly polluted and seriously polluted classes of 4 and 5 respectively. It is further concluded that Arapaja which is underlain by the Migmatite Gneiss rock shows least impact of the infiltrating leachates relative to Apete and Sabo where the underlying rock types are quartzite and granite gneiss respectively. This findings can be attributed to the nature of the resulting weathered products of these different rock types characterized by impermeable clayey materials in Arapaja and permeable sandy materials in Apete and Sabo locations