ASSESSMENT OF FACTORS CONTRIBUTING TO EUTROPHICATION OF ABA SAMUEL WATER RESERVOIR IN ADDIS ABABA, ETHIOPIA

Abstract

BACKGROUND: Aba Samuel water reservoir which drains water from Big and Small Akaki Rivers is found about 35 kms to the south of Addis Ababa. It is showing signs of eutrophication such as uncontrolled aquatic vegetation growth, smell, change in color, etc. The aim of this study was to assess factors contributing to eutrophication of Aba Samuel water reservoir in Addis Ababa. METHODS: This cross-sectional study was undertaken from January 30 to February 20 2003. Samples from the reservoir and two rivers were collected and transported to the laboratory for analysis in Addis Ababa Water and Sewerage Authority laboratory. Standard methods including stannous chloride for phosphate, Phonol-disulphonic acid for nitrate, Direct Nesslerization for ammonia were employed. RESULTS: The mean concentration of phosphate, nitrate and ammonia was 12.40 mg/l, 1.00mg/l and 56.98 mg/l respectively. Biological oxygen demand level was 13.00 mg/l at the inlet and 11.8 mg/l at the outlet site while level dissolved oxygen was found to decrease from 4.7 mg/l to 4.3 mg/l from the inler to the out let. Turbidity increased from 468 Nephlometric turbidity unit in the inlet to 577 Nephlometric turbidity unit at the outlet and Secchi disc visibility decreases in that order. Regarding to the relative contribution of nutrients of eutrophication Small Akaki River has 12.00 mg/l of phosphate and 1.2mg/of nitrate while Big Akaki River has 10.50 mg/l of phosphate and 1.00mg/l of nitrate respectively which was of similar magnitude but the concentration of ammonia is much higher in Small Akaki River (53.36 mg/l) than Big Akaki River (20 mg/l). CONCLUSION: The concentration of nutrients in the reservoir was much higher than the critical value required to trigger algal blooms or create overpopulation of aquatic vegetation. Hence, the ecological integrity of the reservoir is perturbed and fish species which were once a known products have been extinct from the reservoir.