EVALUATION OF THE EFFECT OF ROCK TYPES ON AQUIFER VULNERABILITY AT BOLORUNDURO, ONDO STATE, NIGERIA

Abstract

Geophysical survey involving electrical resistivity method of investigation and hydrogeologic measurements were carried out at Bolorunduro in Ondo State Nigeria with the aim of evaluating the susceptibility of sub-surface geologic structures to infiltrating contaminant in the area. Fortyeight (48) Schlumberger vertical electrical soundings (VES) were conducted, employing current electrode (AB) spacing of 65 m. Wells depth and static water levels of twenty-two (22) accessible wells in the study area were measured. The sounding delineated four geoelectric layers: topsoil, weathered layer, partly weathered /fractured basement and fresh basement. Based on the geoelectric parameters of the top soil, the apparent resistivity of the granitic rock ranges from 32 ohm-m to 1175 ohm-m, ranges from 22 ohm-m to 1186 ohm-m for migmatite-gneiss and ranges from 52 ohm-m to 731 ohm-m for quartzite. The top soil resistivity values (1-100) ohm-m, decipher clay/silt and resistivity values (>400 ohm-m), suggests laterite/lateritic sand. These range of resistivity values implies impervious / semi impervious materials and are considered less vulnerable. Resistivity values in the range of 101 ohm-m to 400 ohm-m implies pervious material and are considered vulnerable to environmental risk. The measured vadose zone thickness in the granite ranges from 0.63 m to 7.95 m with an average thickness of 4.07 m. The measured thickness of wells on migmatite-gneiss, retailing its vadose zone ranges from 1.31 m to 4.87 m with an average of 3.17 m and range from 4.27 m to 5.31 m with an average thickness of 4.79 m for wells on quartzite. The vadose zone thickness in the three geologic setting zones is generally thin, suggesting that the aquifers in the area are vulnerable to contaminants infiltrating from anthropogenic activities. The longitudinal unit conductance of the granitic rock ranges from 0.010584 mhos to 0.16503 mhos while that of the migmatite gneiss zones varies from 0.013457 mhos to 0.22303 mhos. In the quartzite zones, it ranges from 0.002191 mhos to 0.147121 mhos. The resistivity of top soil, vadose zone thickness and longitudinal conductance results were used to evaluate the aquifer vulnerability of the study area which varies from 3.5 to 10 where higher values implies relative greater vulnerability. These values are classified into 3 classes: high vulnerability, moderate and low vulnerability. The high vulnerability zones cover 44% of the study area, 37% represents area of moderate vulnerability while the remaining 19% represents zone of low vulnerability. The decomposed materials overlying quartzite is depicted as the most vulnerable in the area, followed by those overlying granite and migmatite-gneiss. This implies that the absence and/or presence of thin clayey formation in the quartzite region exposes the groundwater to infiltrating contaminants while clayey formation of significant thickness shields aquifers in migmatite-gneiss and granite from being infiltrated by contaminants.