Zvikomborero Hoko

Assessment of the major water and nutrient flows in the Chivero catchment area, Zimbabwe

Abstract The management of water resources is best done on an integrated catchment basis; taking into account the impact of pollution on available water quantity and quality. A study was done from June 2000 to March 2001 in Harare, Zimbabwe, to establish major water and nutrient flows in the Chivero subcatchment area. Lake Chivero is part of the major water supply source for Harare and is located downstream. Discharges from sewage treatment works, urban and rural agriculture, and industries, have caused a severe stress on water quality of the eutrophic lake. The study quantified major water flows through gauging stations on rivers, current metering, and hydrological modelling. Flows for raw water abstractions, sewage and water treatment works were obtained from continuous metering and pumping readings. Water samples were collected monthly for sewage, rivers and Lake Chivero, and analysed according to standard methods. Water and nutrient balances were developed. The results showed that urban water demand would exceed available treatment capacity by the year 2003. Sewage effluent presently is the major source of nutrients in the rivers. The absence of adequate flushing of the lake gives rise to accumulation of the nutrients in the lake. It was concluded that the current situation is not sustainable. It was recommended that the next water supply source be found outside the catchment so that its water can help flush the lake, and that sewage be treated to higher standards (without any pasture irrigation) and discharged directly into the rivers for recycling. The other alternative is to recycle the nutrients in controlled urban agriculture, thereby reducing fertiliser runoff. Stricter regulations and regular monitoring was recommended to control industrial pollution.