Comparative assessment of aquifer susceptibilities to contaminant from dumpsites in different geological locations

Abstract

Geophysical and hydro-chemical investigations have been undertaken within the vicinity of Arapaja, Apete and Sabo dumpsites monolithically underlain by three different rock types namely; Migmatite Gneiss, Quartzite, and Granitic Gneiss respectively in a basement complex of Ibadan, southwestern Nigeria. This was with a view to assessing the pollution potential of the dumpsites on the soil and groundwater in the study locations. Electrical Resistivity methods involving Dipole-Dipole profiling and fifteen (15) Schlumberger Vertical Electrical Soundings (VES) were carried out across two orthogonal traverses established in each of the study locations. A maximum of three subsurface layers namely the topsoil, the weathered layer, and the fresh basement were identified from the geoelectric sections in the three locations. The weathered layer, which constitutes the major aquifer units in the areas and the overlying topsoils were suspected to have been impacted by leachates from the waste dumpsites in the three locations as revealed from characteristic relatively low resistivity values of these layers on the geoelectric sections and the 2D resistivity structures. The hydrochemical analysis of samples from wells in the three locations shows that majority of the analysed cations and anions in the three study locations were within the WHO permissible limits. However, there were indications that parameters such as Pb and Fe2+ in Arapaja and EC, TDS. K+, NO32−, Pb, and Fe2+ in both Apete and Sabo dumpsites were higher than the acceptable limits which revealed 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. It is further concluded that Arapaja which is underlain by the Migmatite Gneiss rock shows least impact of the infiltrating leachates from the dumpsite relative to other dumpsite locations in Apete and Sabo where the underlying rock types are quartzite and granite gneiss respectively. This can be attributed to the nature of the resulting weathered profile of these rock types which is mostly characterized by impermeable clayey materials in Arapaja and by permeable sandy materials in Apete and Sabo locations.