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Aquatic ecosystems are susceptible to pollution with heavy metals arising from anthropogenic and natural sources. In this study the pH, organic matter content, and concentrations of selected heavy metals (Cd, Pb, Cu, Cr, Co) in the sediments of Gilgel Gibe I Hydroelectric Reservoir and its tributaries were investigated. Sediment samples were taken by grab sampling from 5 sample sites. Microwave digestion was used for the extraction of the samples prior to quantitative determination of the target heavy metals by Flame Atomic Absorption Spectroscopy (FAAS).The obtained concentrations of the heavy metals were varied from 0.80 ± 0.46 - 10.4 ± 0.68 mg/kg (Cd), 0.80 ± 0.46 - 10.4 ± 0.68 mg/kg (Pb), 17.26 ± 1.94 - 28.38 ± 0.30 mg/kg ( Cr);0.55 ± 1.38 - 9.74 ± 0.45 mg/kg (for Cu); and 7.53±0.51 - 18.50±1.11 g/kg (Co). One-way ANOVA (p < 0.05) indicated that the concentrations the studied heavy metals were significantly different among the sediment samples collected from the dam reservoir and its tributaries. The dam reservoir sediment accumulate higher concentration of Cd (i.e., 10.4±0.68 mg/kg) and this value is above the sever effect level of National Oceanic and Atmospheric Administration (NOAA) and Canadian interim sediment quality guideline. Nada Kala contribute highest amount of metal accumulation compared with other tributaries. The degree of metal pollution was analyzed in terms of contamination factor and Cd has showed moderate contamination of reservoir sediment. Generally, the obtained findings demonstrated that the studied sediments have high concentrations of target heavy metals. Speciation study is recommended to determine the desorption tendency of the heavy metals towards water column as well as their risk on aquatic ecosystems.
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