Monitoring of Dissolved Oxygen and Biochemical Oxygen Demand in the Osun River, South West Nigeria

The Osun River holds ecological, cultural, and socioeconomic significance for communities across Osun State, yet rapid urbanisation, agricultural intensification, and solid waste disposal are contributing to deteriorating water quality that undermines these values. This study monitors dissolved oxygen and biochemical oxygen demand at eight sampling stations along a sixty-kilometre stretch of the Osun River in Osun State, South West Nigeria, over a twelve-month period covering wet and dry seasons. Dissolved oxygen is measured in the field using a calibrated polarographic meter, while biochemical oxygen demand is determined using the standard five-day incubation method at twenty degrees Celsius. Additional parameters measured include temperature, pH, conductivity, and total suspended solids to provide contextual water quality information. The study applies spatial and temporal statistical analysis, including repeated measures analysis of variance, to identify significant trends in oxygen depletion across stations and seasons. Mapping of point-source discharges including abattoir effluents, laundry sites, and market drainage adjacent to each sampling station is conducted to establish cause-effect relationships. Results are expected to show dissolved oxygen levels below the four milligrams per litre threshold associated with fish mortality at downstream stations proximate to urban discharge zones. Findings will guide evidence-based water resources management decisions by the Osun State Water Corporation. Keywords: dissolved oxygen, biochemical oxygen demand, Osun River, water quality monitoring, South West Nigeria