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Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor

Robinson, Clare E.

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.

Summary for Lay Audience

While essential to sustain life, an excess amount of phosphorus (P) can lead to harmful algal blooms and low oxygen conditions in surface water. While inputs of P to surface waters from agricultural areas are generally well quantified, P inputs from urban areas are less well understood. With more people living in urban areas, it important to understand how much P is released to surface waters from urban areas and where it is coming from. Understanding P inputs from different urban land uses such as residential, commercial, roads, as well as how they vary over the year is needed to better target reduction efforts. Different stormwater management devices are used across urban areas to control stormwater volumes and water quality. These include conventional approaches such as stormwater management ponds and more “green” approaches such as bioretention systems and swales. In addition to understanding how much P is released from different urban land use types, we also need to understand how much P is removed from stormwater using these different stormwater management devices.

This study aimed to address these knowledge gaps by conducting field monitoring in an urban area in London, Ontario. Samples of stormwater were collected during precipitation events as well as during dry conditions in all seasons within the study period. Results show the P concentrations in the stream leaving the watershed frequently exceeded concentrations known to cause algal blooms and oxygen depletion. Stormwater from an older low-density residential area was found to have higher SRP concentrations compared to other urban land uses. Construction activities on a commercial parking lot was also found to increase the concentrations and amount of P released downstream. Finally, while a wet stormwater management pond was found to be effective in removing P, the bioretention system and swales in the watershed did not remove P. Findings from this study may be used by managers and decision makers seeking to understand and reduce the amount of P released urban areas in similar climates.

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