Evaluation and risk Assessment of potentially toxic trace metals in water, sediment and fish from the City of Addis Ababa, Central Ethiopia

Because of fast urban expansion and increased industrial activities, large quantities of solid and liquid wastes contaminated by trace metals are released into the nearby water bodies situated in Addis Ababa. Greater Akaki River (GAR) and Little Akai River (LAR), which join at Aba Samuel Reservoir, are the main rivers draining the city. Eleven (11) water, 11 sediment and 18 fish samples were taken and tested for the level of trace metals content, distribution, pollutionlevel, bioaccumulation, possible sources of pollutants and associated ecological risk. Trace metals were tested using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). USEPA guideline values, geo-accumulation index, contamination factor, and pollution load index were used to evaluate contamination levels and eco-toxicity. The results indicated that the mean concentration of Cr, Fe, Mn and Ni in water samples and Cd, Pb, Mn, Ni and Zn insediment samples exceeded their respective background values. Besides, the levels of Cr, Fe, Mn, Ni and Pb in the muscle of Clarias gariepinus were above the permissible limits for human consumption. Furthermore, ecological risk assessments using sediment samples revealed the widespread pollution by Cd and Pband moderate pollution by Mn, Ni and Zn. A comparison with similar studies revealed that sediments from the study area had the highest average value of Pb. Generally, the study indicated that Akaki River catchment and Aba Samuel Reservoir were found to be moderately to strongly contaminated by heavy metals and thus posed high ecological risks.