Effects of sediments loads on water quality within the Nairobi river basins, Kenya

Abstract

This paper presents the findings of a study carried out in the years 1998-2005 within the Nairobi River basins on the effects and implications of sediment loads on water quality. The study was motivated by the worrying trends in water quality degradation within the basin to be enable indentify possible mitigation strategies for the river basin. Sediments from river water samples were obtained from the Ngong, Nairobi, and Mathare river sub-basins. The results indicated a seasonal variation/trend for suspended sediments in each basin, and a similar trend in water quality degradation. Annual suspended sediment load flux estimates for the Ngong, Nairobi, and Mathare rivers are 1700, 6300 and 3000 tonnes, respectively. A close relationship between certain water quality parameters, such as total dissolved solids (TDS), conductivity, turbidity and colour, to increased water quality degradation, on a seasonal basis was demonstrated. Land-use changes per basin, including agricultural, residential, industrial and urban, were used to identify the most dominant type of land-use activity and its impact on sediment loads and reduction in water quality. Water pollution and pollutant levels varied with season and distance away from the city of Nairobi in the three sub-basins. The streams were found to be less chemically polluted away from the city due to dilution effects and self purification during the wet season. The results indicated that sediment loads had a significant effect on the Nairobi River basin’s water in terms of water quality reduction. These appeared to significantly contribute to the pollution of the river and reduction in its water quality. This paper recommends the following water pollution control strategies, and hence reduce water quality reduction; removal of solid wastes from the river courses, protection of the river banks from construction activities, discourage people from dumping wastes into the river courses, relocation of the “Jua-Kali” garage and mechanics who operate near the river banks, as well as continuous monitoring to check on illegal dumping of wastes into the river as some of the Best Management Practices (BMPs) within the watershed, and the country in genera