The Groundwater Potential and Vulnerability Assessment from Integrated Geophysical Methods, Borehole Logging, and Hydrogeological Measurement

Abstract

Geophysical investigations and hydrogeological measurements were carried out in Akoko area of Ondo State, Nigeria which are characterized by rugged hills of Precambrian Basement Complex of Southwestern Nigeria comprising mainly of the Migmatite-Gneiss-Quartzite Complex of Archean-Proterozoic age. The study was aimed at evaluating the groundwater potential of the area and assess the vulnerability of the aquifers/water bearing units within the phreatic zone to contamination especially from surface activities/anthropogenetic sources. Seventy soundings were carried out along fifteen traverses where the VLF-EM measurement was also conducted. The hydrogeological measurement involved determination of static water level/vadose zone thickness from 138 wells. Also four borehole were drilled after thorough geophysical investigations in order to constrain the interpretation of the geophysical investigation. Findings show that the major water bearing units/aquifer in the area are the weathered layer (clayey in nature) and fractured basement as delinated on the geoelectric sections, borehole sections, and very low frequency electromagnetic. The combination of these aquifer units are common in Ikare and Akungba parts of the study area. The visible depth of groundwater exploitation in the area is between 10 – 40 m. The static water level is generally less than 5 m. The map of the overburden shows that Akoko area is generally made up of thin overburden thickness except at Irun, Ogbagi, Arigidi, and Oke Agbe that showed moderate overburden thickness (10 m – 20 m). The static water level measured in the study area ranged from
0.75 m – 16 m, with a modal range of 1.1 – 3.0 m as they constituted for about 78 % of the area. This generally showed that the static water level was shallow across the study area, thus suggesting that the surficial aquifers/water bearing units would be highly vulnerable to anthropogenetic/surface activities which are the major sources of groundwater contamination. Clay or silt, characterized by low resistivity (1 – 100 Ω-m) and very low permeability, and impervious or semi-impervious topsoil materials (laterite and lateritic sand), characterized by high resistivity values (>400 Ω-m) constitutedthe protective layers that cappedthe vadose zones in the area. The topsoil can provide protection to a reasonable extent, but limited due to its thin thickness across the area.