ASSESSMENT OF HEAVY METAL CONTAMINATION AND SELF-CLEANSING CAPACITY OF THE OSUN RIVER, SOUTHWESTERN NIGERIA

Abstract

The Osun River, a vital water resource in Southwestern Nigeria and a cultural landmark, faces increasing pollution pressure from urbanization and industrial activities. This study assesses its current water quality and self cleansing capacity by analyzing spatial variations in physicochemical parameters and heavy metal concentrations. Water samples were collected from five locations along the river's course during the dry season and analyzed using standard methods, including Atomic Absorption Spectrophotometry (AAS). Results indicated that heavy metals typically associated with intensive mining (Cd, Cr, Co, Pb, Cu, Ni) were below detectable limits. However, iron (Fe) concentrations (0.38 - 2.82 mg/L) consistently exceeded the World Health Organization (0.3 mg/L) and Nigerian (0.3 mg/L) standards across all but the most downstream site. A clear downstream trend of decreasing turbidity, iron, and zinc concentrations was observed, demonstrating the river's natural self-purification capacity. Despite this, high turbidity levels (up to 53.51 NTU) in urban stretches indicate significant organic and particulate pollution. The study concludes that while the Osun River retains a measurable self-cleansing ability, it is contaminated with Fe and suspended solids, necessitating improved wastewater management and continuous monitoring to safeguard public and ecosystem health in line with SDG 6 targets.