Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Collaborative Specialization

Environment and Sustainability

Supervisor

Robinson, Clare E.

Abstract

High nutrient loading can degrade surface water quality worldwide. Bioretention systems are low impact development stormwater management features designed to remove pollutants, including phosphorus (P), from urban stormwater runoff. In this study, two field-scale bioretention systems installed in London, Ontario, Canada were monitored to develop detailed understanding of P behaviour and the hydro-biogeochemical mechanisms that govern overall P retention. Net retention of total P and dissolved organic P, and net release of soluble reactive P (SRP) and total dissolved P were observed. Prolonged input of road de-icing salts (NaCl) in winter and early spring may result in high P release from the bioretention systems in spring. Porewater samples revealed the distribution of SRP within the bioretention systems to be highly heterogeneous and without a monotonic decrease with depth as commonly assumed in literature, highlighting complex temporal and spatial behaviour of P and controlling biogeochemical processes within field-scale bioretention systems.

Summary for Lay Audience

Rainwater from urban areas can become polluted by nutrients (nitrogen and phosphorus) because of human activities such a lawn fertilization and animal waste. While some nitrogen and phosphorus is needed for plant and animal growth, high levels in lakes and rivers can create toxic algal blooms which cause serious public health, economic and environmental problems. Bioretention systems, sometimes called rain gardens, are designed to clean and control rainwater runoff from urban areas to protect streams, rivers, and lakes. They allow water to soak into the ground and use natural methods such as filtration to improve the water quality. However, these systems do not always perform as designed and can release high levels of phosphorus under certain conditions. It is still unclear what controls the behavior of phosphorus within bioretention systems and what conditions result in phosphorus release instead of phosphorus retention.

For this study, water samples were analyzed from the input and output of two bioretention systems in London, Ontario, Canada over an entire year. The output concentrations of the different chemical forms of phosphorus were considerably higher in spring compared to the rest of the year. This is an important finding because high phosphorus inputs to surface waters in spring can result in large toxic algal blooms in summer. Experiments in the lab were used to show that high road salt use in winter and early spring may lead to high phosphorus release from bioretention systems during late spring.

This study also investigated the processes within bioretention systems that may influence phosphorus behaviour. Porewater samples collected from within the bioretention system showed that the distribution of phosphorus was highly heterogeneous. Therefore, the processes governing phosphorus removal are complex and, in contrast to what is often thought, phosphorus removal may not increase with depth. It is possible that several processes occur simultaneously, making it challenging to predict the behaviour of phosphorus in bioretention systems. More detailed field analysis should be performed to better understand the biogeochemical processes governing phosphorus removal so that the design of bioretention systems can be improved and the quality of lakes and rivers can be protected.

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