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Addis Ababa City’s river ecosystem is under extreme pressure as a result of inappropriate practices of dumping domestic and industrial wastes; thus, threatening its ability to maintain basic ecological, social and economic functions. Little Akaki River which drains through Addis Ababa City receives inorganic and organic pollutants from various anthropogenic sources. Most of inorganic pollutants such as toxic heavy metals released into the river are eventually adsorbed and settle in the sediment. The objective of this study was to evaluate the enrichment levels, pollution load and ecological risks of selected heavy metals (Zn, Cr, Cd and Pb) using various indices. The mean concentrations of heavy metals in Little Akaki River sediment were: Zn (78.96 ± 0.021–235.2 ± 0.001 mg/kg); Cr (2.19 ± 0.014–440.8 ± 0.003 mg/kg); Cd (2.09 ± 0.001–4.16 ± 0.0001 mg/kg) and Pb (30.92 ± 0.018–596.4 ± 0.066 mg/kg). Enrichment factor values indicated that sediments were moderate to significantly enriched with Zn and Cr; moderate to very highly enriched with Pb, and very highly enriched in all sampled sites with Cd. Geo-accumulation index and contamination factor values indicated that the sediments were moderate to very highly contaminated with toxic Cd and Pb. The decreasing order of pollution load index (PLI) in downstream was: (S9) > (S4) > (S8) > (S3) > (S6) > (S10) > (S5) > (S2) > (S7) > (S1). PLI and hierarchical cluster analysis revealed that the highest pollution load occurred in the lower course of the river (S9) which may be due to metals inputs from anthropogenic sources. The ecological risk (RI = 350.62) suggested that the contaminated Little Akaki River sediment can pose considerable ecological risks of pollution. The concentrations of Zn, Cr, Cd and Pb in Little Akaki River sediment surpassed eco-toxicological guideline limits of USEPA (threshold effect concentration) and CCME (Interim Sediment Quality Guidelines). Thus, the contaminated sediments can pose adverse biological effects on sediment dwelling organisms.

Background: The Addis Ababa City’s river ecosystem is under extreme pressure as a result of inappropriate practices of dumping domestic and industrial wastes; thus, threatening its ability to maintain basic ecological, social and economic functions. Little Akaki River which drains through Addis Ababa City receives inorganic and organic pollutants from various anthropogenic sources. Most of inorganic pollutants such as toxic heavy metals relased into the river are eventually adsorbed and settled in the sediment. The objective of this study was to evaluate the enrichment levels, pollution load and ecological risks of selected heavy metals ( Zn, Cr, Cd and Pb) using various indices .Results: The mean concentrations of heavy metals in Little Akaki River sediment were: Zn (78.96±0.021 - 235.2 ±0.001mg / kg); Cr (2.19±0.014 - 440.8±0.003 mg / kg); Cd (2.09±0.001-4.16 ±0.0001mg / kg) and Pb (30.92±0.018 -596.4±0.066 mg / kg). Enrichment factor values indicated that sediments were moderate to significant enrichment with Zn and Cr; moderate to very high enrichment with Pb, and very high enrichment in all sampled sites with Cd. Geo-accumulation index and contamination factor values indicated that the sediments were moderate to very high contamination with toxic Cd and Pb. The decreasing order of pollution load index (PLI) in downstream was: (S9) > (S4) > (S8) > (S3)> (S6) > (S10) > (S5) > (S2)> (S7) > (S1). PLI and hierarchal cluster analysis revealed that highest pollution load occurred in the lower course of the river (S9) which may be due to metals inputs from anthropogenic sources; hence, its quality was deteriorated showing that the site is polluted. The ecological risk (RI =350.62) suggested that the contaminated Little Akaki River (LAR) sediment can pose considerable ecological risks of pollution.Conclusions: The concentrations of Zn, Cr, Cd and Pb in Little Akaki River sediment surpassed eco-toxicological guideline limits of USEPA (threshold effect concentration) and CCME (Interim Sediment Quality Guidelines). Thus, the contaminated sediments can occasionally pose adverse biological effects on sediment dwelling organisms. Thus, measures must be taken to regulate discharge of untreated wastes into river and surrounding environment.

Background: Little Akaki River (LAR) passes through Addis Ababa City, receives inorganic and organic pollutants from various sources. The objective of this study was to investigate the pollution level of LAR by selected heavy metals and evaluate sediment quality using contamination indices.Methods: sediment samples were collected from 10 stations along LAR, processed, digested and heavy metal content was analyzed using ICP-OES. Enrichment factor (EF), geo-accumulation index (Igeo), contamination factor (CF), pollution load index (PLI), ecological risk index (RI) were determined. Comparison was made with standard sediment qualities (SQGs) to evaluate ecological and toxicological implication.Results: the mean concentrations of heavy metals in LAR sediment were: Zn (78.96-235.2 mg / kg); Cr (2.19-440.8 mg / kg); Cd (2.09-4.16 mg / kg) and Pb (30.92-596.4 mg / kg). EF values indicated that LAR sediments were moderate to significant enrichment with Zn and Cr; moderate to very high enrichment with Pb, and very high enrichment in all sampled sites with Cd. Igeo and CF values indicated that the sediments were moderate to very high contamination with toxic Cd and Pb.PLI and hierarchal cluster analysis revealed that highest pollution load occurred at sampling site (S9), in lower course of the river mainly due to anthropogenic metals inputs from industrial wastes, municipal wastewater treatment plant and agrochemical wastes; hence, its quality was deteriorated and depicted polluted site. The decreasing order of PLI in downstream was: (S9) > (S4) > (S8) > (S3)> (S6) > (S10) > (S5) > (S2)> (S7) > (S1). Pearson correlation indicated that Zn and Cd were generated from common sources of pollution. The ecological risk (RI =350.62) suggested that the contaminated LAR sediment can pose considerable ecological risks of pollution.Conclusions: The concentrations of Zn, Cr, Cd and Pb in LAR sediment were surpassed sediment quality guidelines (USEPA) and eco-toxicological guideline limit values of USEPA (TEC) and CMME (ISQGs). Thus, the contaminated sediments can occasionally pose adverse biological effects on sediment dwelling organisms and impairs the quality of river water. Thus, monitoring and addressing sediment contamination becomes necessary to sustain beneficial uses of river water for various development purposes.

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