Bioretention: Performances

Phosphorus Reduction

The performance results for Bioretention practices, located within TRCA's watershed originate from three primary sites:

• Kortright Centre
• Earth Rangers
• Seneca College
• Aurora Community Centre
• Central Parkway (Mississauga)
• Wychwood Subdivision (Brampton)
• IMAX Corporation, head office

The mean performance value recorded at the outlet for Bioretention practices' ability to remove Total Phosphorus (TP) was calculated based on 355 separate recordings between 2005-2007, 2011-2019 and 2021 amongst the seven sites previously mentioned.

As can be seen in the corresponding boxplot the mean performance removal efficiency of the bioretention practices monitored are not meeting the acceptable upper extent range of nutrients as of 0.03 mg/L (30 µg/L) (Environment Canada, 2004; OMOEE, 1994).

The median value of the 355 samples taken was 0.87 mg/L whereas the mean was 0.123 mg/L, with an 86% guideline exceedance. Given the age of most of these practices, more inspection, maintenance and necessary rehabilitation will be needed to ensure they are able to meet the federal and provincial governments' guideline requirement for stormwater quality.

TSS Reduction

The performance results for Bioretention practices, located within TRCA's watershed originate from three primary sites:

• Kortright Centre
• Earth Rangers
• Seneca College
• Central Parkway (Mississauga)
• Wychwood Subdivision (Brampton)
• IMAX Corporation, head office

The mean performance value recorded at the outlet for Bioretention practices' ability to remove Total Suspended Sediments (TSS) was was calculated based on 301 separate recordings between 2005-2007, 2011-2019 and 2021 amongst the six sites previously mentioned.

As can be seen in the corresponding boxplot the mean performance removal efficiency of the bioretention practices monitored are well below the suggested guideline of 30 mg/L (Canadian Water Quality Guideline (CWQG), or (background (assumed at <5 mg/L)+ 25 mg/L for short term (<24 hour) exposure) (CCME, 2002; (TRCA, 2021).

The median value of the 301 samples taken was 9.50 mg/L whereas the mean was 18.63 mg/L, with a 15% guideline exceedance.

External Performance Resources

• (USEPA, 2013) - Evaluation and Optimization of Bioretention Design for Nitrogen and Phosphorus Removal
  • USEPA conducted both field and laboratory testing on the performance of bioretention with augmented designs and filter media composition with aluminum hydroxide/oxide content, found normally within water treatment residuals. These additives added at 10-15% of the total filter media mix ad median removal efficiencies of 90-99% of orthophosphate and a second study found a bioretention design with WTR mixture in the filter media and a IWSZ optimized to remove phosphorus and nitrogen had a removal efficiency of 20% and effluent concentrations below 20µg/L (well below the MECP/CCME guideline in Ontario).

Comparison of an Enhanced dephosphorization bioretention cell (EBC) (above) vs. a traditional bioretention cell (TBC) (below). The EBC includes evenly spaced apart soil mixture layers, which includes 70-80% native soil found on site mixed with 20-30% of charcoal, oregani matter and iron, along with permeable layers of gravel pumice and zeolite, all of which help adsorb phosphates out of stormwater entering the system. This differs from the TBC design which generally includes just a gravel bed to aid in the facility's drainage ability (Ho and Lin, 2022).

• (Ho and Lin, 2022) - Pollutant Removal Efficiency of a Bioretention Cell with Enhanced Dephosphorization
  • Authors Ho and Lin, 2022 note that bioretention practices perform poorly in reducing phosphorus from influent stormwater when compared to their ability to remove ammonia and COD pollutants. The authors tested a new type of enhanced dephosphorization bioretention cell (EBC) which improves phosphorus removal performance. The difference between EBC and a traditional bioretention cell is that the lowest level of an EBC feature is comprised of a mixed fill material layer (permeable layers - PLs and soil mixed layers - SMLs) instead of a traditional gravel bed layer. The SMLs include active charcoal powder, organic matter and iron, evenly spaced apart, while the PLs include aggregates of gravel, pumice and zeolite. Over the two years that the same sized EBC feature was monitored in comparison to a standard bioretention cell they found that the EBC outperformed the traditional bioretention cell by removing 92% of total phosphorus to 52%. The average inflow concentration for both features from May 2019 - April 2021 was 0.76 mg/L, whereas the outflow concentration averages were 0.36 mg/L for the traditional bioretention cell and 0.06 mg/L for the EBC, respectively.

Toronto and Region Conservation Authority

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

Total phosphorus

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.

Total suspended solids

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

The natural ground material characteristic of or existing by virtue of geographic origin.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

A biological community, including humans and their natural environment.