Effect of land use type and stormwater control measures on non-point source phosphorus concentrations and loads in a cold climate urban subwatershed
Abstract
Non-point source phosphorus (P) exports from urban areas including contributions from different urban land use types and seasonal variations are not well understood, especially for cold climates. Additionally, the effectiveness of different stormwater control measures (SCMs) in reducing P concentrations and loads are poorly quantified with assessments rarely considering total watershed P loads. To address these gaps, detailed field monitoring of total P (TP), soluble reactive P (SRP), total suspended solids (TSS), and water quantity was conducted over a 19-month period in a mixed urban watershed in London, ON. Monitoring sites were located at watershed outlets, at the outlets of different urban land use types, and at the inlet and outlet of four SCMs (wet pond, bioretention system, two swales). Samples were collected during rain, rain-on-snow, snowmelt, and baseflow conditions. TP concentrations at the upper and total watershed outlets were above the threshold for eutrophic conditions over 90% of the time, with particulate P (PP) found to be the dominant form of P, especially in summer. SRP concentrations were significantly higher in runoff from an older low-density residential area compared to other land uses areas, but TP concentrations were not significantly different. Construction activities on a commercial parking lot led to higher TP and SRP concentrations and loads during summer compared to other seasons. Finally, while reductions of TP, SRP, and TSS concentrations and loads were observed across the wet pond, the bioretention system generally acted as a source of SRP and the swales had minimal impact on TP and SRP concentrations and loads. The findings from this study may be used to inform design and implementation of strategies to reduce non-point source P exports from cold climate urban areas.