Evaluation of Water Quality Indices of the Osun River in Osun State Before and After the Annual Osun-Osogbo Festival

Religious and cultural festivals involving large gatherings near natural water bodies can result in temporary but measurable deterioration of water quality. This study evaluates the water quality index of the Osun River in Osun State, South West Nigeria, before, during, and after the annual Osun-Osogbo Festival, which attracts thousands of worshippers to the river banks. Water samples were collected from six designated stations two weeks before the festival, on the peak celebration day, and two weeks post-festival. Parameters analysed include pH, turbidity, dissolved oxygen, biochemical oxygen demand, total coliform count, nitrate, phosphate, and selected heavy metals. Water Quality Index scores were computed using the weighted arithmetic method. Results indicate a significant decline in water quality index scores during the festival period, driven primarily by increases in faecal coliform counts, turbidity, and biochemical oxygen demand, consistent with elevated human activity and ritual offerings introduced into the water. Recovery to near-baseline conditions was observed within approximately ten days post-festival. The study recommends the installation of sanitation facilities at festival grounds, the designation of clearly defined sacred versus unrestricted zones, and post-festival decontamination protocols to protect both the ecological integrity of the river and the health of communities relying on it as a water source. Keywords: Osun River, water quality index, Osun-Osogbo, festival, Osun State