Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor

Robinson, Clare

Abstract

Bioretention systems are a low impact development system that can remove pollutants such as phosphorus (P) from urban stormwater. P retention in bioretention systems is complicated in cold climate regions due to factors such as inputs of road de-icing salts. This study evaluates the impact of prolonged and periodic salt inputs on P retention by conducting column experiments using three different bioretention media with and without an amendment added. Non-amended columns showed net P release, whereas amended columns showed net P retention. While some non-amended columns showed prolonged salt exposure increases P release, the largest P release for all columns occurred during the freshening period following the switch from high to low salt influent. High porewater pH (> 9) observed during the freshening period may be causing the high P release. This study provides new insights needed to improve year-round P retention in bioretention systems installed in cold climates.

Summary for Lay Audience

As rainwater is collected on the streets of urban areas it picks up many pollutants including phosphorus and transports them to streams, rivers, and lakes. While phosphorus is essential for plant and animal growth, it can also lead to toxic algal blooms. This can cause serious public health, economic and environmental problems. Bioretention systems are a low impact development engineering approach that promotes the infiltration of stormwater into the ground. As the stormwater infiltrates, physical and chemical processes can remove phosphorus from the water, thereby improving 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 promote phosphorus retention. To address this, water treatment residuals (WTRs), a by-product of drinking water treatment plants, are a promising material that can be added to bioretention media to improve phosphorus retention. The behaviour of phosphorus within the system is further complicated by the use of de-icing roads salts used during the winter months in cold climate regions. The impact of de-icing road salts on phosphorus retention in bioretention media with and without WTRs is unclear.

In this study, laboratory columns with bioretention media (with and without an aluminum WTR added) were exposed to artificial stormwater with high and low salt concentrations. Columns without WTR were found to release phosphorus, whereas columns with WTR were found to remove phosphorus. The highest release of phosphorus occurred when the columns switched from receiving water with high salt concentrations to no salt. The pH also increased during this switch from high salt input to no salt input indicating that pH may be a major control on the behaviour of phosphorus in bioretention media exposed to de-icing road salts. This study provides important new insights into the impact of de-icing salts on the ability of bioretention systems to remove phosphorus as needed to improve the year-round performance of these systems in cold climate environments.

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