GIS-based integrated water quality assessment and pollution hotspot mapping of the Oyun River–Reservoir system, Nigeria

Abstract

This study assessed the water quality of the Oyun River–Reservoir system, Nigeria, using an integrated Canadian Council of Ministers of the Environment Water Quality Index and Geographic Information System (GIS)-based approach (WQI–GIS). Twenty-five geo-referenced stations were sampled across eight surveys (2022–2023) and analysed against Nigerian regulatory standards. The results revealed marked spatial variability, with the river exhibiting consistently higher contamination than the reservoir. Elevated phosphate levels (3.12–19.40 mg l⁻¹) and biochemical oxygen demand (2.80–7.40 mg l⁻¹) were observed, while the dissolved oxygen (0.05–1.90 mg l⁻¹) was critically low. Heavy metals, particularly Cd and Pb, exceeded regulatory limits. Water quality index scores classified most river sites as ‘poor’ to ‘very poor’ (22.3–47.35), whereas reservoir sites ranged from ‘marginal’ to ‘good’. GIS-based hotspot analysis identified severe pollution clusters in midstream river sections and upstream reservoir zones. Significant correlations between land-use and water quality parameters were observed; for instance, between built-up areas and Cu (r = +0.68) and between forested areas and biochemical oxygen demand (r = −0.80), highlighting the buffering role of forests and vegetation. In summary, the WQI–GIS approach identified pollution gradients and management priorities, highlighting the need for strengthened catchment control, improved sanitation and stricter land-use regulations to protect the Oyun freshwater system.