Impacts of alum residues from Morton Jaffray Water Works on water quality and fish, Harare, Zimbabwe

Abstract

Metal pollution of freshwater due to human activities is a major problem confronting most urban centres in developing countries. This study determined the extent to which aluminium in the residues from Morton Jaffray Water Works in Harare were affecting the water quality of Manyame River and Lake Manyame. The study also measured aluminium bioaccumulation in Nile Tilapia (Oreochromis niloticus) which is of importance to the commercial fisheries industry in Zimbabwe. Depth integrated water, and sediment grab samples and adult fish were collected per site in January and March, 2010. A total of six sites were selected on the Manyame River and in Lake Manyame. The levels of Total Aluminium (Al) were determined in sediments, water and fish tissues (liver, kidney, gill and muscle). Total solids, total dissolved solids, conductivity, pH, dissolved oxygen and temperature were also determined in water and residues. The texture of the sediments was also assessed. Aluminium concentration in water ranged from 2.19mg/L to 68.93mg/L during both sampling campaigns surpassing permissible maximum concentration limits of 0.087 to 0.75mg/L suggested by the Environmental Protection Agency and African Union. The site upstream of the discharge point of the residues always had the lowest levels though it was higher than acceptable levels indicated above, thus suggesting the existence of other sources of aluminium in the catchment besides Morton Jaffray Water Works. However, there was a 10-fold and 100-fold increase in levels of aluminium in water and sediments, respectively, at the site 100m downstream of the discharge point on the Manyame River. Mean aluminium concentrations in water and sediments at this site averaged 68.93±61.74mg/L and 38.18±21.54mg/L in water and 103.79±55.96mg/L and 131.84±16.48mg/L in sediments in sampling campaigns 1 and 2, respectively. These levels were significantly higher than levels obtained from all the other sites during both sampling campaigns (ANOVA: p<0.05). This also indicated that the contribution of aluminium pollution from the water works residues was higher than all the other sources in the catchment. Aluminium levels were generally in the order of; sediments>fish>water. Bioaccumulation occurred in the fish and the order of bioconcentration was; kidney>liver>gill>muscle. The amounts of aluminium in the fish tissues investigated were significantly higher (maximum=2.92mg/g) than was reported in other studies reviewed (maximum=0.18mg/g). Thus, the water treatment plant residues are greatly increasing the concentrations of aluminium in the water system downstream of the plant thus creating a great risk of aluminium toxicity for fish. Treatment of the residues before discharge, substitution of alum with other coagulants, and re-use of the residues in buffer strips, agricultural lands and in sewage works should be considered.