Urbanization-driven freshwater pollution in the Ona River Basin, Southwestern Nigeria

This study investigates freshwater pollution and its relationship with urbanization dynamics in the Ona River Basin, Southwestern Nigeria. A total of 60 water samples were collected across four hydrological seasons and analysed for physicochemical parameters, heavy metals, and microplastics. Results indicate slightly alkaline conditions, with notable spatial variability in total dissolved solids (TDS) and total suspended solids (TSS), reflecting the influence of urban runoff. Concentrations of heavy metals, particularly lead (Pb) and cadmium (Cd), exceeded World Health Organization (WHO) drinking water limits, indicating potential ecological and public health risks. Microplastics (19–72 particles/L), dominated by fibres, were identified using stereomicroscopic visual analysis. Pearson correlation analysis revealed statistically significant relationships (p < 0.05) between microplastic abundance and urbanization indicators, including built volume (r = 0.805) and population density (r = 0.715). Seasonal patterns showed increased transport of fragments and films during high-flow conditions. This study provides new insights by integrating microplastic pollution with urban morphological metrics in a tropical watershed, advancing understanding of urbanization-driven freshwater degradation in West Africa..