Trace metal distributions in the sediments of the Little Akaki River, Addis Ababa, Ethiopia.
The levels and distribution of trace metals (Cr, Mn, Co, Ni, Cu, Zn, Cd and Pb) at eleven water and sediment stations on the Little Akaki River (LAR) in Addis Ababa, Ethiopia, were determined. The binding forms of the metals in various geochemical fractions of the sediments were also quantified. The molar ratio of the sum of the simultaneously extractable metals (∑SEM) and acid-volatile sulphide (AVS)-as a measure for predicting metal-induced toxicity-was estimated. LAR trace levels in water for Cu, Zn, and, particularly Mn were, in most instances, higher than the recommended guidelines for healthy aquatic ecosystems. Total trace metal (TTM) contents in the LAR sediments at certain stations exceeded "threshold effect concentrations" and even "probable effect concentrations", especially in the cases of Zn, Cu, Ni, Pb, and at all stations for Mn. This became more apparent after applying "normalizations" to the relatively lower TTM adsorption capacities of coarse-grained, organic-poor sediments. Sequential extraction of the sediments showed that trace metals generally have a higher affinity for Fe-Mn oxide and organic matter/sulphidic fractions, followed by the residual fraction. Mn was relatively strongly bound to the exchangeable, carbonate bound fractions, whereas a large proportion of Cr was found in the residual fraction. The Σ[SEM]/[AVS] ratio pointed to potential metal-induced toxicity of sediments collected from seven out of the eleven stations. The results indicate that trace metal pollution pose risks to the health of ecosystems, and to human communities that use the river for a range of different purposes.
Department of Water Science and Engineering, UNESCO-IHE, Institute for Water Education, P.O. Box 3015, 2601, Delft, The Netherlands. alexander.legesse@gmail.com.|Department of Chemistry, College of Natural and Computational Sciences, University of Gondar, P.O. Box 196, Gondar, Ethiopia. alexander.legesse@gmail.com.
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Akele
Department of Water Science and Engineering, UNESCO-IHE, Institute for Water Education, P.O. Box 3015, 2601, Delft, The Netherlands.
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Kelderman
Ministry of the Environment and Parks, 2938-11 Street NE, Calgary, AB, T2E 7L7, Canada.
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Koning
Department of Water Science and Engineering, UNESCO-IHE, Institute for Water Education, P.O. Box 3015, 2601, Delft, The Netherlands.|Aquatic Ecology and Water Quality Management, University of Wageningen, P.O. Box 47, 6700, Wageningen, The Netherlands.
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Irvine
D000327:Adsorption; D017753:Ecosystem; D004784:Environmental Monitoring; D005002:Ethiopia; D019015:Geologic Sediments / Q000737:chemistry; D000075322:Heavy Metal Poisoning; D019216:Metals, Heavy / Q000032:analysis*; D011041:Poisoning; D045483:Rivers / Q000737:chemistry*; D013440:Sulfides / Q000032:analysis; D014874:Water Pollutants, Chemical / Q000032:analysis*
D019216:Metals, Heavy; D013440:Sulfides; D014874:Water Pollutants, Chemical
Acid-volatile sulphide; Binding forms; Ethiopia; Sequential extraction; Trace metals