Evaluation of nutrient retention and sediment deposition in two urban stormwater retention ponds

Stormwater ponds have been implemented in many municipalities to control urban runoff and retain pollutants, such as nutrients and suspended solids. Two stormwater ponds in Toronto, Ontario were evaluated for their ability to retain nutrients and suspended solids and were also used to investigate mechanisms by which stormwater ponds remove nutrient pollutants, including the importance of deposition and internal loading. Over the entire study period, Hydro Pond East (HEP) retained 1415 mg of total suspended solids (TSS) and MAT retained 1127 mg of TSS. Both Hydro East Pond (HEP) and Mattamy Rouge (MAT) were net exporters of phosphorus (P) over the entire season, with 6.35 mol or 0.20 kg and 53.9 mol or 1.67 kg exported, respectively. HEP had net retention of 2672 mol or 37.4 kg of nitrogen (N) but MAT exported 264 mol or 3.7 kg of nitrogen over the entire study. This study has demonstrated that stormwater ponds have the ability to provide retention of nutrients and TSS, but their function may be enhanced as they may become exporters. However, the amount of nutrients exported was extremely low and may have been driven by the anomalously dry 2016 year in Toronto. Further research should be done on these same ponds to observe how they may perform under an anomalously wet year (e.g. 2017). There is a need for a future model to synthesize the data from literature on stormwater ponds to better understand their function to better help local water managers determine if these ponds are needed and how they may need to enhance their function.