Groundwater Vulnerability Mapping and Quality Assessment around Coastal Environment of Ilaje Local Government Area, Southwestern Nigeria

Abstract

Aquifer vulnerability study assists in the implementation of groundwater management strategies to prevent degradation of groundwater quality. The study was focused on the determination of groundwater quality and vulnerability potential of material overlying the aquifer units in Ilaje area of Ondo State, southwestern Nigeria using geo-electrical method, aquifer vulnerability index (AVI), GOD index, and Dar Zarrouk longitudinal conductance. This was complemented by physicochemical analysis of 25 water samples, randomly taken from boreholes within depths range of 1 to 50 m. All the tested samples have values within the World Health Organization (WH) except nitrate which shows relatively high values higher than maximum contaminant level of 10 mg/l recommended by WHO in southern part. The range of nitrate concentration (0.25 – 15.4 mg/l) was as a result of high anthropogenic discharge of domestic and municipal waste into drainages especially during the wet season. Also, some degree of contamination by lead, cyanide, arsenic, ammonia, and mineral oil from petroleum exploitation by major oil companies was also observed in the samples. The average calculated water quality index is 121 which falls within poor/unfit water for drinking due to high level of contamination. The GOD vulnerability and AVI maps show that the aquifer units in the area are vulnerable by 60% and 55%, respectively. The longitudinal conductance index indicates that 53.3% of the area have poor/weak protective capacity. All the vulnerability maps and indices used corroborate very well as they show little variations. Therefore, appropriate water treatment should be conducted on the water before drinking, and also government should enact laws that would discourage indiscriminate dumping of refuse, waste water etc., enhanced containment for storage of wastes and chemicals on vulnerable soils. Also, the activities of the oil companies should be regulated to reduce the heavy metal/mineral oil contamination.