Quality Assessment Of Rain And Storm Water Runoff For Nairobi City Industrial And Sub-Urban Areas

Abstract

Nairobi like most cities in the world is faced with water shortages because all the surface water sources have been tapped and the ground water overexploited, yet the water demand continues to rise as the population grows. The city must therefore seek alternative means of water supply. One of the promising sources is rainwater harvesting, which has successfully been adopted to supply water in many other cities. However, there is a concern about the quality of the rainwater falling through a heavily industrialized city atmosphere and flowing over polluted grounds. There is need to determine the quality of rainwater and the resulting storm water so as to make a decision on the best application or treatment of the water. The purpose of the study was therefore to determine the physical and chemical properties of rain and storm water runoff in sub-urban and industrial settings in Nairobi. Two sites were identified namely Upper Kabete Campus (heavily vegetated agricultural suburb), and Jomo Kenyatta International Airport (heavily industrialized area of the city) to assess the water quality of rainwater received and storm water runoff exiting to drains. Water samples were collected directly from falling rain and also from runoff water at the sites for laboratory analysis. The samples were analyzed for water quality parameters namely pH, alkalinity, hardness, total dissolved solids, chlorides, calcium, nitrates, iron. The results from the two sites were compared statistically. It was found that the quality of rain water does not differ significantly in physiochemical parameters at 0.05 significant levels between the sub-urban and industrial setting. The falling rainwater was only slightly above the WHO requirements and required only modest treatment while the storm water was significantly above the WHO limits and either need treatment or may be used for non-potable application. Results of the study are useful in addressing challenges of water quality partly by encouraging use of rain and storm water for non-portable uses and preserving the limited treated water for essential household uses