Effects of sewage and industrial effluent on the concentration of Zn, Cu, Pb and Cd in water and sediments along Waterfalls stream and lower Mukuvisi River in Harare, Zimbabwe

Abstract

The disposal of sewage and industrial effluent is a problem confronting municipalities in most developing countries. A study was conducted to determine the effects of the disposal of sewage and industrial effluent into Mukuvisi River and Waterfalls stream, a tributary of Mukuvisi River in Harare, Zimbabwe. Water and sediment samples were collected over two seasons (October 2003 to November 2004). Sampling sites were located before and after Firle Sewage Treatment Works (FSTW) along Mukuvisi River and before and after Prospect Industrial Area (PIA) along the Waterfalls stream. The water and sediment samples were analysed for pH, and total Zn, Cu, Pb and Cd. The FSTW had no effect on water and sediment pH in Mukuvisi River, which ranged 6.8–7.0 and 5.0–5.6, respectively. The heavy metal concentration upstream of the sewage processing plant was higher than down stream implying that the effluent disposed into the river had lower metal concentrations compared to the river water. Metal concentrations in water and sediment samples along the Waterfalls stream showed an increase just after the PIA, and were more significant in sediment samples. It was concluded that the metal pollution of the two streams was due to industrial pollution rather than sewage effluent disposal. Accumulation of heavy metals in streams is better monitored using sediments where they accumulate rather than water. Continued dumping of industrial effluent into Mukuvisi River and its tributaries will cause further damage to the ecosystem and the food chain.