Assessing Hydrological and Sediment Factors for Ecotoxicological Risk of the Orashi River, Rivers State, Nigeria

Rivers in the Niger Delta deliver vital ecological and economic benefits but face increasing pressures from human activities that disrupt hydrology and harm sediment quality. This research assessed the hydrological features, sediment condition, and ecological risks of the Orashi River in Rivers State, Nigeria. Hydrological data were gathered from upstream, midstream, and downstream sections, while surface sediments (0–10 cm) were collected in triplicate for analysis of physicochemical properties and heavy metals through standard methods. Sediment pH, electrical conductivity, organic matter, total organic carbon, and cation exchange capacity were measured, alongside concentrations of Fe, Pb, Cd, Cr, Ni, Zn, and Cu using Atomic Absorption Spectrophotometry. Environmental risks were evaluated via contamination factor (CF), geo‑accumulation index (Igeo), potential ecological risk index (PERI), and ecotoxicological risk quotients. River discharge ranges from 38.6 to 214.7 m³/sec, with flow velocities between 0.21 and 0.84 m/sec, indicating a high sediment transport capacity. Sediment pH (5.5–6.8) and organic matter (1.32–4.76%) showed moderate variation across sites. Elevated levels of Cd (1.27 mg/kg) and Pb (46.2 mg/kg) surpassed sediment quality standards. Cadmium indicated significant contamination (CF = 7.1; Igeo = 2.9) and posed a very high ecological risk (Er = 213), predominantly influencing the overall PERI of 249.9. Nickel exhibited the highest ecotoxicological risk quotient (0.81), implying potential long-term stress to benthic life. Metal concentrations were consistently higher downstream, reflecting increased human influence. Overall, sediments of the Orashi River demonstrate notable contamination, with cadmium and lead representing major ecological hazards. On-going monitoring and improved catchment management are essential to safeguard aquatic ecosystems.