Critical Organic Loading and Microbial Contamination in an Urban West African River: A Water Quality Assessment of River Ona, Nigeria

The present study evaluated the physicochemical and microbial water quality of River Ona, Ibadan, Nigeria, at three sampling stations representing the upstream (Loc 1), midstream (Loc 2), and downstream (Loc 3) reaches of the river's residential corridor. These stations were selected to capture the gradient of domestic and organic pollution loading along the catchment. Rapid, unmanaged urbanisation and the near-absence of sewage infrastructure along many urban rivers in West Africa make such baseline assessments necessary for evidence-based catchment management (Babuji et al., 2023; Nawaz et al., 2023). The monitored parameters included pH, electrical conductivity (EC), total dissolved solids (TDS), total suspended solids (TSS), turbidity, dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), and microbial indicators; all values were compared with World Health Organization (WHO, 2017), Nigerian Standard for Drinking Water Quality (NSDWQ, 2015), and United States Environmental Protection Agency (U.S. EPA, 2018) guidelines. Dissolved oxygen was critically low (1.23–2.00 mg/L), below the minimum of 5 mg/L; BOD (4.67–6.00 mg/L) and COD (400.00–973.33 mg/L) exceeded their respective limits of 5 mg/L and 20 mg/L; and turbidity (7.33–8.67 NTU) exceeded the ≤5 NTU guideline. Water Quality Index (WQI) scores, computed using the weighted arithmetic mean method, ranged from 469.24 to 1034.18, placing all three stations in the 'unsuitable for drinking' category. Microbial analysis indicated station-specific rather than uniform contamination: Escherichia coli occurred across all three stations, Klebsiella spp. occurred at Loc 1 and Loc 3, Klebsiella pneumoniae was detected exclusively at Loc 2, Bacillus spp. occurred at all three stations, and Staphylococcus spp. occurred at Loc 1 and Loc 3. Together, these findings indicate overlapping faecal and environmental contamination pathways. River Ona, in its present state, therefore presents organic-pollution and microbiological risks to communities that depend on it for domestic and recreational purposes, although a formal quantitative human health risk model was beyond the scope of this preliminary assessment. The results emphasise the need for improved sewage management, wastewater treatment infrastructure, and routine physicochemical and microbiological monitoring to protect public health and the ecological integrity of River Ona.