Hydrogeophysical exploration and groundwater quality assessment of Obada Oko, Ogun State, Nigeria

Abstract

Water is one of the prime necessities of life for the survival of man and a host of other living things. Access to potable water supply over  the years has been difficult in developing countries leading to high rates of water borne diseases, hence the need to assess the  hydrogeological potential of Obada-Oko in Ewekoro Local Government Area of Ogun State. The study investigated aquifer distribution  and locations of possible sites where boreholes and hand-dug wells could be drilled for potable and uncontaminated groundwater  supply. The study area falls within a transition zone between sedimentary and basement complex rocks of southwestern Nigeria. Vertical Electrical Sounding (VES) and 2-Dimensional Electrical Resistivity Tomography (2D ERT) surveys were carried out using Schlumberger and  Wenner arrays respectively. Resistivity data were acquired using Campus Tigre resistivity meter at twelve VES points and six 2D traverses  of lengths varying from 100 to 150 m. The resistivity data obtained from 2D ERT were inverted using RES2DINV software while IPI2WIN  and WinResist software were used for VES data interpretation. A maximum of five geoelectric layers namely: topsoil, clayey soil, clayey  sand, sandy soil, weathered/fractured and fresh basement were delineated beneath the traverses. The weathered and fractured layers  had resistivity varying from 20 to 690 Ωm and thickness varying from 12 to 27 m. The 2D ERT models showed three layers: top layer,  weathered layer and fresh basement. The topsoil thickness varies from 1 to 10 m while fractured/fresh basement layer thickness vary  between 6 and 13 m. The maximum depth of penetration of 2D ERT and VES were 13 m and 31 m respectively. Productive hand-dug wells/boreholes could be sited on traverses 2, 3 and 6; VES points 6 and 12 were suspected to have high groundwater potential. Though  the longitudinal conductance values revealed protective capacity rating to be mostly within poor category, which made the aquifer  system highly vulnerable to contamination. Physical and chemical results of analyzed water samples fell within permissible limit of World  Health Organization (WHO) and Nigerian Standard for Drinking Water Quality (NSDWQ) limit for drinking purposes. Piper trilinear diagram interpretation showed calcium and chloride to be the dominant cation and anion respectively. Groundwater potential tends to  be high in five locations in the study area with suitable aquiferous media for groundwater extraction.