GROUNDWATER VULNERABLITY ANALYSES USING VERTICAL ELECTRICAL SOUNDING AND 2D ELECTRICAL RESISTIVITY TOMOGRAPHY IN PAPALANTO SOUTH-WEST NIGERIA

Abstract

Vertical Electrical Sounding (VES) and 2-D Electrical Resistivity Tomography (ERT) surveys using Schlumberger and Wenner arrays were respectively adopted for groundwater vulnerability studies in Papalanto. The field equipment used for the study is mainly AGI Super Sting Earth Resistivity meter with displayed apparent resistivity as digitally computed from the electrical principle of Ohm’s law. The depth to the aquifer varies from 8.5m to 100m while the longitudinal unit conductance (S) and hence, the protective capacity (Pc) values are generally less than 1.0 Siemens (Pc ˂ 1.0 Siemens) except in few locations around VESPAP2, VESPAP4, VESPAP10, VESPAP12, VESPAP13, VESPAP20 alongside VESPAP21 and VESPAP22; they are classified as low and are characteristics of depositional successions of overburden layers with no significant impermeable clay/shale overlying rock in the study areas. However, the aforementioned investigated locations where the protective capacity values were greater than 1.0 (Pc ˂ 1.0 Siemens); implying that these locations have considerable layers of Clay separating the subsurface aquiferous zones. In addition to high transmissivity and low protective capacity values identified in the study area, the aquifers are relatively close to the surface (˂100m) and thus susceptible to contamination over large areas and can infiltrate the subsurface with unprecedented impacts on the groundwater system. ERT and VES results indicated a polluted depth of over 24m beneath the subsurface which coincides with the upper section of the second aquifer and serves as an indication for a possible impairment of the first groundwater harness by majority of the inhabitants through shallow wells which might be detrimental to the overall health of inhabitants when consumed without treatment.