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.