ASSESSMENT OF THE WATER QUALITY STATUS OF SASUMUA WATERSHED, KENYA

Abstract

This study focuses on Sasumua watershed (107km2) of the Upper Tana basin and source to 20% of Nairobi’s water supply where intensification of human activity has resulted in increased pollutional load to Sasumua reservoir with implications on water treatment costs for Nairobi Water and Sewerage Company (CNWSC). The objective of the study was to determine the physico-chemical and bacteriological characteristics of the water entering Sasumua reservoir and to assess the relative importance of the various sources of contamination. Water samples were collected at distinct land use boundaries, at reservoir entry/exit points, and at the surface of the reservoir during both dry and wet seasons. These were analysed to determine total suspended solids, total dissolved solids, turbidity, dissolved oxygen, faecal coliforms, nutrients, heavy metals and pesticides. Analysis was done as per the standard method of analysis and evaluation based on World Health Organisation (WHO) standards. For both dry and wet seasons most parameters were within WHO standards except Ming’utio River which showed exaggerated levels of potassium, iron, lead, manganese, pH and turbidity. For the wet season both turbidity and pH values were above WHO standards for most samples analysed. No pesticides were detected but samples showed signs of contamination with human waste indicating unsuitability for domestic use without treatment. Turbidity and pH were the major issues of concern because of their bearing on water treatment costs. The study contributes towards understanding the water quality status of the contributing rivers and reservoir and can be used by planners to devise ecologically-sound watershed management plans, or by policy makers to evaluate alternative land management options that can abate pollution of water bodies.