Stormwater Ponds for Pollution Reduction

Stormwater transports particulate-bound and soluble pollution from urban areas to receiving waters during rain events. Measurements have shown that open detention ponds, originally used for flood control, effectively reduce stormwater pollutant load and have therefore been increasingly used for stormwater treatment. To fill a research gap, accurate long-term measurements have been carried out to determine the pollutant removal efficiency in existing stormwater ponds in Sweden. Physical and, to some extent, chemical processes during single storm events have been studied.<p /> All studied stormwater ponds were equipped, at the inlet and the outlet, with continuous flow meters and automatic samplers. During storm events, suspended solids, heavy metals and nutrients were analysed. Results show that pollutant removal rates vary, during single storm events, from negative values up to almost 100%. Cumulative pollutographs, for inflow and outflow pollutants, were used for calculation of the removal efficiency. The outflow curve shows an almost straight line, where the gradient is a load coefficient useful for calculation of annual pollutant loads from the pond to the receiving waters. The pollutant removal efficiency varies for different ponds, due to different specific pond areas (pond area / impervious catchment area). For the heaviest loaded pond (40 m²/ha) the removal efficiencies for suspended solids and heavy metals were 70% and 30-50% respectively, and for a less loaded pond (240 m²/ha) 84% and 75-88% respectively. It could be concluded that a further increase in specific pond area above 250 m²/ha only marginally increased the pollutant removal efficiency. Modelling and measurements of internal flow pattern in two different ponds have also been carried out. The results show that flow modelling in a stormwater pond should be performed with a three-dimensional model. In addition, the pond geometry has to be properly designed to avoid dead zones or recirculation zones that decrease the effective pond volume and consequently decrease the residence time in the pond, which is unfavourable for the pollutant removal.<p /> Conclusions from this project are that the pollutant removal efficiency should be determined from measurements of several successive storm events and that there is an optimal size for a stormwater pond of around 250 m²/ha