Reducing Stormwater Pollution - Performance of Retention Ponds and Street Sweeping

Doctoral thesis, 2003

Stormwater, in this study defined as surface runoff in urban areas, is known to transport large quantities of toxic substances to receiving waters. Therefore different practices to treat stormwater have been proposed. In Sweden retention ponds have become a common treatment method during the past decade. The main objective of the study is to investigate the performance of stormwater ponds and street sweeping. Performance of retention ponds was investigated by field measurements of water and sediments, and through simulation of hydrodynamics and removal processes. This showed that ponds are an efficient way of reducing pollutant loads from stormwater and that both water and sediment were highly polluted. A clear correlation between pollutant concentrations in the water and in sediments was found, which indicates that pond sediments can be used for characterising pollutant loads from urban catchments. In order to distinguish between different parameters affecting removal efficiency in ponds, simulations using a numerical model were done. This demonstrated that surface load is the most important parameter governing removal efficiency, however hydraulic efficiency, i.e. how efficient the volume of the pond is utilised, has for engineering purposes a crucial significance, since the space available for construction normally is limited. Further it was shown that the most important factor explaining changes in removal efficiencies between winter and summer seasons is the change of hydrology during the winter. Wind was found to be an important parameter governing oxygenation of ponds. In order to investigate the efficiency of street sweeping a street was swept and the amount of removed sediment and heavy metals were measured. This showed that street sweeping could be an efficient control measure of pollutants. The amount of sediments on the street decreased during the sweeping period and approached a steady state condition. The largest amounts of metals were found in the sandy fractions of the sediment, however the highest concentrations were found in the finest grain sizes and the concentrations were proportional to the inverse of the particle diameter. Copper was found to be the critical pollutant for stormwater and sediments from ponds and streets.