GROUNDWATER VULNERABILITY ASSESSMENT USING HYDROGEOLOGIC AND GEOELECTRIC LAYER SUSCEPTIBILITY INDEXING AT IGBARA OKE, SOUTHWESTERN NIGERIA

Abstract

Aquifer protection is an essential ingredient for a sustainable use of groundwater resources and the protection of the dependent ecosystems. Several groundwater developments have been abandoned due to various reasons attributed to contamination/pollution from sources such as septic tanks, petroleum tanks, and refuse dump etc., after a huge investment on such, therefore a need for vulnerability assessment. Vulnerability assessment was carried out at Igbara Oke by applying the electrical resistivity method. The Schlumberger electrode arrays with maximum electrode separation (AB/2) of 65m in (41) different locations were engaged in data acquisition. The data were subsequently processed by partial curve matching andforward modelling computer algorithm using WinRESIST Version 1.0. Geoelectricparameters (layer resistivity and thickness) were determined from the interpreted data. The study area comprisesfour geoelectric layers (topsoil, lateritic layer, weathered/fractured layer and fresh basement). The geoelectric parametersof the overlying layers across the area were used to assess the vulnerability of the underlying aquifers to near-surface contaminantsby generating various vulnerability assessment mapsenabling theclassification of the study area into various vulnerability zones (low, moderate and high). Three models were compared by maps using geo-electrically derived models; longitudinal conductance, GOD (groundwater occurrence, overlying lithology and depth to the aquifer) and GLSI (geoelectric layer susceptibility indexing). The model parameters were also related empirically. The total longitudinal conductance map shows the north central part of the study area as a weakly protected (0.1-0.19) area while the northern and southern part have poor protective capacity (<0.1); this is in agreement with the GOD method which shows the northern part of the study area as less vulnerable (0-0.1) while the southern part has low/moderate (0.1-0.3) vulnerability to contamination. By empirical relationship, the longitudinal conductance shows 68% as poor protective capacity/very high vulnerability, 22% as weak protective capacity/high vulnerability and 10% as moderate protective capacity/moderate vulnerability. The GLSI shows 56% as moderate, 32% low and 12% as high vulnerability. The GOD shows 59% as low and 41% as negligible vulnerability. The GLSI shows the northern part as moderate vulnerability zone while the southern part has low vulnerability. The longitudinal conductance exaggerates the degree of susceptibility to contamination than the GOD and GLSI models. From the models, vulnerability to contamination can be considered higher at the southern part than the northern part and therefore, sources of contamination like septic tank, refuse dump should be located far from groundwater development in the area.