Sewage discharges and nutrient levels in Marimba River, Zimbabwe

Population distribution, land-use, industrial activity, urban agricultural activities, and pollution control strategies in Harare, Zimbabwe, have all changed significantly since independence in 1980. The effects of these changes on water quality and the consequences on human and ecological health are of major economic and social importance. This study looked at pollution in the Marimba River, one of the major inflow rivers into the Lake Chivero, Harare city's main water supply source. The river was characterised using standard physical and chemical methods to assess water quality from June 2000 to December 2001. Monthly sampling results indicated high pollution levels, especially nutrients. Nitrogen values averaged 13.5 +/- 2.0 mg l(-1) as N, and phosphorus 2.6 +/- 0.6 mg l(-1) as P at the river's discharge point into Lake Chivero. The high nutrient values, exceeding acceptable limits, are indicative of industrial, agricultural, and sewage discharges upstream. The gross river pollution levels are detrimental to the downstream Lake Chivero. It was concluded that although there are other significant incidental sources of nutrient pollution in Marimba River, sewage effluent is by far the major polluter. It was established that pond overflow from Crowborough Sewage Treatment Works is the major source of sewage inflows and treating this to tertiary standard would result in 46% and 35% reductions respectively for total nitrogen and total phosphors loads in the Marimba River.