Nutrient loads associated with different sediment sizes in urban stormwater and surface pollutants

A significant amount of pollutants in urban stormwater runoff is transported as sediment-bound contaminants. It is important to have a clear understanding of the amount of pollutants attached to the different sediment sizes so that treatment facilities can be designed to effectively target the removal of the most polluted sediment sizes. This paper presents results from a study carried out to determine nutrient loads associated with different particle size ranges for dry surface pollutants and stormwater samples collected from an urban road surface. The results indicate that although more that half of the surface pollutant is coarser than 300 mum (microns), less than 15% of the total phosphorus (TP) and total nitrogen (TN) are attached to particle sizes greater than 300 mum. The pollutant loads in the different particle size ranges from different stormwater samples show a larger variability for TN than for TP. The dissolved component for TN ranges between 20 and 50% compared to 20-30% for TP. Practically all the particulate TP and TN in stormwater samples are attached to sediments between 1 and 150 mum. This suggests that to effectively remove TP and TN, pollutant treatment facilities must be able to remove particulates down to 11 mum.