|Monday, May 31|
How much water can bioretention retain, and where does the water go?
* Sylvie Spraakman, University of Toronto, Canada
Jean-Luc Martel, Canada
Jennifer Drake, Canada
Green stormwater infrastructure (GSI) is designed to retain the 90th percentile rainfall event, yet it is often evaluated using water balance results at an annual scale, which is skewed towards larger rainfall events. As a result, the contribution of evapotranspiration has been largely ignored, contributing to an overall lack of interest in the role that vegetation plays in GSI. This study hypothesizes that evapotranspiration forms a larger portion of the water balance for events for which GSI was designed to retain. The focus of this study is on rainfall events within the rainfall statistics for the local area in which the GSI was installed. The research questions are: (1) does the GSI retain the required rainfall volume and (2) what portion of the GSI water balance is evapotranspiration, when considering the design retention amounts? The study site is a bioretention cell located in a parking lot in Vaughan, Ontario, monitored in 2018 and 2019. The hydrology of the bioretention cell was measured using inlet and outlet flow monitoring and a lysimeter for evapotranspiration monitoring. Results from continuous monitoring show that the average retention was 16 mm, and that for 80% of events, evapotranspiration was 25% of the inflow. Evapotranspiration is a significant portion of the water balance when considering event sizes within the range that GSI has been designed for, and therefore the contribution of vegetation to GSI should be considered in research and design.
TWO-FLUID MODELING OF SEDIMENT TRANSPORT IN A VORTEX-TYPE STORMWATER RETENTION POND
* Mina Ahadi, University of Saskatchewan, Canada
Donald J. Bergstrom, Canada
"Designing and optimizing the stormwater ponds for removal purposes without understanding the flow and sediment dynamics is not possible. In spite of the advances in multiphase Computational Fluid Dynamics (CFD) modeling approaches, in the stormwater retention pond CFD modeling, it is common practice to ignore the sediment phase, solve the conservation equations only for the liquid phase, and then to use an advection-diffusion equation for a tracer material. Stormwater retention ponds deal with high sediment loads. Solving an advection-diffusion equation is incapable of bringing in important physics of sediment transport. Exploring sediment removal requires an authentic two-phase model to simulate the flow and sediment dynamics. A three-dimensional multiphase (flow and sediment) numerical model previously developed and validated in a turbulent open channel flow by the authors is applied to the complex case of an innovative vortex-type stormwater retention pond. The flow pattern in the vortex-type stormwater retention pond was recently characterized experimentally and computationally by the authors. In this study, the sediment and flow dynamics are studied using the two-fluid method. The two-fluid models solve the transport equations for both phases of water and sediment. These models are capable of fully bringing in the physics of sediment transport. The two-fluid models are developing, and their potential is not completely omitted in water resources engineering problems, yet. In this study, the two-fluid model is tested for the first time in a new vortex-type stormwater retention pond. Predictions of the liquid and particle velocity fields based on the two-fluid model framework are presented for two different inflow sediment concentrations. This study provided useful information for the design of stormwater retention ponds as well as highlighting the effect of particles on the flow. "
Combined field and laboratory evaluation of the performance of multiple bioretention systems in retaining phosphorus in urban stormwater
* Clare Robinson, University of Western Ontario, United States
Brennan Donado, Canada
Yi Liu, Canada
Nick Mocan, Canada
Jaeleah Goor, Canada
Amanda Pinto, Canada
"Bioretention systems are an increasingly popular low impact development stormwater management approach in Canada for attenuating stormwater runoff and improving water quality. Prior field studies have shown highly variable performance of these systems for reducing phosphorus concentrations and loads. As these field studies generally treat bioretention systems as a black box conducting only input-output water quantity and quality monitoring, the conditions (hydrological, geochemical) within bioretention systems that contribute to phosphorus retention and release remain unclear. Furthermore, studies that do evaluate the influence of different bioretention soil media compositions on phosphorus retention generally conduct only laboratory columns experiments and it is unknown how applicable these findings are for real field conditions. These knowledge gaps need to be addressed to optimize the performance of bioretention systems with respect to water quality improvements including phosphorus reduction. For this study we monitored six bioretention systems of different ages and with different soil media compositions to identify and compare the mechanisms governing phosphorus retention and release in the systems. Porewater samples were collected from the bioretention systems and analyzed for phosphorus and other chemical species related to phosphorus mobility to investigate the geochemical evolution of the stormwater as it infiltrates through the systems. Sediment cores were also analyzed to identify the major mechanisms controlling partitioning of phosphorus between the infiltrating water and soil media. In addition, laboratory column experiments were conducted to compare the behavior of the bioretention soil media in retaining phosphorus between field-scale and laboratory-scale systems. Results show widely varying performance of the bioretention systems with respect to phosphorus retention, although the performance of each individual systems was consistent over multiple precipitation events. For systems that showed a net release of phosphorus, phosphorus was derived from internal soil phosphorus stores including phosphorus adsorbed onto metal (Al- and Fe-) oxides. The porewater pH was found to be a major control on phosphorus release from the soil media with the bioretention system with porewater pH > 8 found to have the highest porewater phosphorus concentrations (>3 mg/L). Results for column experiments compared reasonably well with field-scale results with respect to P retention and release but important differences were observed highlighting the need for further understanding of the impact of field environmental factors on the performance of bioretention systems. This study provides important new insights into factors that govern phosphorus retention in bioretention systems as needed to improve the performance of these systems including their overall system design and the composition of the engineered soil media used. "
Factors affecting seasonal performance of field bioretention systems in retaining phosphorus in urban stormwater
* Jaeleah Goor, Western University, Canada
Julia Cantelon, Canada
Chris, Smart, Canada
Clare Robinson, Canada
"Bioretention systems are popular low impact development stormwater management features designed to remove pollutants, including phosphorus (P), from urban stormwater runoff. While the performance of bioretention systems in retaining P has been well studied, it remains unclear how P retention varies seasonally in field-scale systems installed in cold climates, as well as the influence of cold climate factors including high road de-icing salt (sodium chloride) inputs on P retention. This study aimed to evaluate the seasonal trends in the retention of different forms of P within bioretention systems and provide insight into the mechanisms that control the mobility of P within systems subjected to cold climate factors. Two large field-scale bioretention systems installed in London, Ontario in 2017 were monitored for 12 months during their initial operational period. Influent and effluent water samples were collected during 24 precipitation events and analyzed for different forms of P as well as chloride (Cl). Concentrations were combined with influent and effluent water volumes to calculate loading and determine net P retention or release. Overall, a net retention of total P and dissolved organic P, and a net release of soluble reactive P and total dissolved P mass were observed. Reduced hydrological performance and increased effluent P concentrations resulted in high P release from the bioretention systems in early to mid-Spring (March and April), with most release occurring during a few individual large precipitation events. In addition, porewater samples were collected during precipitation events and dry periods using an extensive array of MacroRhizon samplers installed throughout the systems. The distribution of P, Cl, and other relevant dissolved constituents in the porewater over time support the increased release of P from the bioretention media in spring, likely due to prolonged high salt loading. Finally, laboratory-scale column experiments were performed on the soil media installed in the field-scale bioretention systems to isolate the effect of high salt loading on P release. Column experiments combined with field data indicate that prolonged high salt loads through winter and spring may have contributed to elevated spring P, mostly in the form of soluble reactive P, release from the field-scale bioretention systems. Findings from this study are needed to better understand the performance of bioretention systems with respect to P retention for urban stormwater management in cold climates. Results have implications for further investigations of the impact of road salt on P mobility in bioretention systems and more broadly in roadside soils and groundwater systems."