Heavy Metal Geochemistry and Ecological Risk of Stream Sediments in the Orashi River, Niger Delta, Nigeria

Stream sediments play a crucial role in regulating the distribution, retention, and ecological effects of heavy metals in fluvial environments. This study evaluated the geochemical characteristics and ecological risks of heavy metals in surface sediments of the Orashi River, Rivers State, Niger Delta, Nigeria. Surface sediment samples were collected from six spatially distributed sites along the river during the dry season, with triplicate sampling at each location. Sediment physicochemical properties were determined using standard analytical methods, while concentrations of Fe, Mn, Pb, Cd, Cr, Ni, Zn, and Cu were quantified following mixed-acid digestion using Atomic Absorption Spectrophotometry. Sediment quality and ecological risk were assessed using contamination factor, geo-accumulation index, enrichment factor, and potential ecological risk index models. Sediment pH ranged from 6.2 ± 0.2 to 6.9 ± 0.2, indicating slightly acidic to near-neutral conditions, while organic matter content increased downstream from 2.74 ± 0.68% to 4.18 ± 0.90%. Mean metal concentrations exhibited a clear downstream enrichment trend, with iron dominating the sediment matrix (15,840 ± 2,760 to 20,180 ± 3,720 mg/kg). Cadmium concentrations (0.72 ± 0.24 to 1.94 ± 0.55 mg/kg) recorded the highest contamination and enrichment levels among the analysed metals. Contamination factor and geo-accumulation index classified cadmium as moderately to strongly contaminated, while lead and zinc showed moderate contamination. Ecological risk assessment identified cadmium as the principal contributor to ecological risk, with potential ecological risk index values increasing from 85.4 upstream to 217.4 downstream, indicating low to moderate ecological risk. The findings demonstrate progressive sediment enrichment driven largely by anthropogenic inputs along the Orashi River, with cadmium posing the greatest ecological concern. Continuous sediment monitoring and targeted management strategies are recommended to minimise long-term ecological impacts within the river system.