Stormwater runoff quality in Malabe, Sri Lanka

Stormwater runoff is the primary nonpoint source that pollutes all water resources. Stormwater pollution at a sewage outfall is a mixture of different kinds and strengths of pollutants from different surfaces. It is essential to understand typical pollutants from each of several impervious surfaces of a specific suburbanized area in order to properly analyze and design water quality improvement systems. Two types of impervious surfaces, roads and pavement, of two catchments in northern and southern Malabe, a western suburb of Colombo, were studied to determine the physicochemical characteristics of their stormwater runoff pollutants. For each surface type from each catchment, the first flush was sampled using a sheetflow technique. Five pollution paramters, i.e., pH, turbidity, colour, electrical conductivity (EC), and nitrate content, were analyzed and compared with that of the rain water. The pavement surfaces showed higher values of turbidity, colour, and nitrate, while EC was higher for road surfaces. The turbidity and colour values were higher in the northern catchment than that in the southern one while EC values were opposite. The nitrate concentration values of pavements were consistently higher than that of the roads for both catchments, which were not much higher than that of rain water. The pH value was consistently neutral for both surface types while rain water was slightly acidic.