Phosphorus Retention in an Urban Stormwater Pond

Abstract

Stormwater ponds are a critical piece of stormwater infrastructure which were originally developed to reduce local floods associated with the increased impervious surface area of urban environments. They are now increasingly promoted as contaminant sinks preventing downstream export of excess nutrient loads or other contaminants. However, their ability to retain phosphorus (P) and the mechanisms through which retention is achieved are not fully understood. This study investigates the ability of a stormwater pond located in Richmond Hill, Ontario to retain different forms of phosphorus. To evaluate the pond’s ability to retain P, sediment cores were collected in different zones of the pond and analyzed for total and operationally defined sediment P pools. To establish a P mass balance for the pond, inlet and outlet water samples collected from 2020-2022 were analyzed for dissolved and particulate P concentrations. The sediment core analyses showed a transition from more reactive to more stable P pools as the sediment aged. The inlet and outlet mass balance results indicate that the pond retains 72% of the inflowing dissolved reactive P (DRP) on an annual basis. This net DRP retention is matched by an increase in the sediment calcium-bound P pool, suggesting that the precipitation of calcium phosphate phases is occurring in the sediments. Because calcium phosphates are effective and stable P sinks, their potential formation in the sediment warrants further study as a P retention mechanism in this urban pond. Overall, the pond retained 70% of the inflowing P, supporting its effectiveness at retaining nutrient P and highlighting the important role of stormwater ponds in stormwater and nutrient management.