Sewage discharges and nutrient levels in Marimba River, Zimbabwe

Abstract

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 phosphorus loads in the Marimba River

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

“…Nevertheless, there seems to be less improvement in the quality of the lake from 1996 to now. Lake Chivero is highly polluted with sewage effluent irrigation runoff and seepage, raw sewage or sewage effluent from sewage treatment plants that discharge their raw sewage or untreated effluent into rivers discharging into Lake Chivero (Marimba River- Nhapi and Tirivarombo, 2004;Mathuthu et al, 1997, Mukuvisi River-Kamudyariwa, 2000Zaranyika, 1997). Thus, although the lake is eutrophic with high photosynthetic activity, the incoming wastes exert an oxygen demand on the water which results in the depletion of oxygen hence the low results (Metcalf and Eddy Inc, 1995;Tebutt, 1998;Abowei, 2010).…”

“…Other uses include support for aquatic life and recreation (Nhapi and Tirivarombo, 2004). Cleveland dam used to provide drinking water source for the city of Harare until it became too small to supply increased demands of a growing population.…”

Hydrochemical assessment of water quality for irrigation: a case study of the Medjerda River in Tunisia

“…Thus, river water quality can be deteriorated by a heavy load of nutrients and contaminants coming from industrial activity discharge of wastewater, domestic sewage and agricultural practices. The aquatic ecosystem can thus be threatened by the presence of potentially toxic, mutagenic, or carcinogenic compounds from sewage discharges inducing various ecological impacts on aquatic life (Igbinosa and Okoh 2009;Nhapi and Tirivarombo 2004;Kanu and Achi 2011).…”

The Effects of Water Transfer from Swakoppoort and Omatako Dams on the Water Quality of Von Bach Dam, Namibia

“…In Lake Chivero, Zimbabwe, the increase in nutrient loads of total phosphorus was found to be due to the discharge of effluent from a wastewater treatment plant (Nhapi and Tirivarombo, 2004). The Manyame River supplies a significant concentration of total phosphorus to Lake Chivero due to the sewer overflow in Chitungwiza, and the waste-stabilisation pond effluent from Donnybrook and Ruwa sewage treatment works .…”

“…Human activities such as the disposal of municipal wastewater and crop farming in the catchment supply nutrients to lakes, and these activities are termed as external sources of nutrients (Nhapi and Tirivarombo, 2004). The internal source of nutrients, particularly total phosphorus, is the sediment found at the bottom of the lake.…”