Heavy Metal Geochemistry and Ecological Risk of Stream Sediments in the Orashi River, Niger Delta, Nigeria
Ekpe John E.
Department of Physics, Alex Ekwueme Federal University, Ndufu-Alike, Nigeria.
Ede Nnamdi E.
Department of Biochemistry, Federal University, Gashua, Nigeria.
Okpoji Awajiiroijana U. *
Department of Pure and Industrial Chemistry, University of Port Harcourt, Choba, Nigeria.
Anumaka Collins C.
Department of Geological Sciences, Nnamdi Azikiwe University, Awka, Nigeria.
Awa Emmanuel O.
Institute of Ecology, People’s Friendship University of Russia, RUDN, Russia.
Akpan Nsima A.
Department of Chemical Science, Ritman University, Ikot Ekpene, Nigeria.
Otokwala Okino N.
Department of Physics and Electronics, University of Port Harcourt, Choba, Nigeria.
Osuagwu Eze L.
Department of Fisheries and Marine Technology, Imo State Polytechnic, Omuma, Nigeria.
Obi Justina N.
Department of Chemistry, Federal University of Technology, Owerri, Nigeria.
Warder Amaminor B.
Department of Biology, Ignatius Ajuru University of Education, Port Harcourt, Nigeria.
*Author to whom correspondence should be addressed.
Abstract
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.
Keywords: Stream sediments, heavy metals, ecological risk, contamination indices, Orashi River