Operational Monitoring of Urban Stormwater Management Facilities and Receiving Subwatersheds in Richmond Hill, Ontario

Untreated stormwater discharge has been found to have major impacts on urban watercourses. Despite the acknowledgement of the importance of adopting broader-scale approaches to managing stormwater, there is limited information describing the subwatershed benefits of implementing numerous best management practices (BMPs). Operational monitoring of stormwater management facilities (SWMFs) and streams in three subwatersheds undergoing urbanization was undertaken over a five-year period to measure potential changes in suspended solids concentrations, hydrology and temperature. SWMFs at construction sites had elevated total suspended solids (TSS) discharges relative to facilities in stable catchments, but reduced TSS discharges in comparison to uncontrolled construction sites. The discharge from single facilities was found to have the potential to increase downstream watercourse TSS concentrations in headwater areas, but increases were often not detectable further downstream. TSS discharges from SWMFs in stable catchments were similar to published ranges, and were a useful tool to identify facilities that were not functioning as designed. Based upon the current subwatershed imperviousness levels, water quality impacts from cumulative SWMF discharges appear to be less likely to be significant than some other factors, such as the changes in the subwatershed hydrological response. No increase in peakflow or duration of midbankfull flow for the 25-mm and 2-year storm event and no trend in baseflow or water temperature were detectable over the study period, despite the increase in imperviousness due to watershed urbanization. The findings from this operational monitoring study suggest that implementation of BMPs may help to mitigate urban impacts at the subwatershed level.