Suitability of storm water runoff for water supply in fast urbanizing cities: The case of Mbezi River catchment in Dar es Salaam City, Tanzania

Abstract

Quality aspects of stormwater are often less considered in conventional stormwater management practices, a condition that jeopardize the usability potential of stormwater. Aiming at advocating storm water quality management practices in fast developing cities, this study evaluated the quality of storm water runoff in the rapidly urbanizing Mbezi River catchment in Dar es Salaam city, as delineated from digital elevation model using Arc-GIS tools. Runoff from four rain events of the 2016 wet season, with rain depths ranging from 7.1 to 11.8 mm was sampled from nine building roofs and six ground runoff sites located in areas with impervious surfaces ranging from 5 to 68%. Findings suggest that having acceptable levels of heavy metals (Zn, Cu, Pb), nutrients (N, P), and faecal coliforms, roof runoff can be used to supplement portable water supply. The concentration of total suspended solids (883 mg/L), Lead (0.18 mg/L), and faecal coliforms (4.3x105CFU/100 mL) in ground runoff exceeded the recommended standards specified in Tanzanian and international water quality guidelines. A positive correlation was observed between the degree of catchment imperviousness and concentration of nutrients in ground runoff. The findings indicate further that simple storm water quality management options like retention and sedimentation can be used to reclaim the usefulness of ground runoff especially for non-portable water uses. More options for treatment of ground runoff to enhance its potentials to supplement water supply are widely discussed.

Keywords: Green infrastructure, Water supply, Environmental protection, Flood control, Runoff routing, Rainwater harvesting