Water quality

Overview

Improvements in the quality of stormwater runoff is one of the primary benefits of low impact development strategies. Although some chemical changes occur inside the practices, much of the pollution reduction of low impact development strategies comes from runoff reduction. i.e. diverting excess water through infiltration or evapotranspiration.

Nutrients

Plants (including algae) require three macro nutrients to grow: Nitrogen, potassium, and phosphorus. See Nutrients page for more information about nutrients. Of these three, phosphorus is the often the "growth-limiting" nutrient (i.e., the local environment may have an abundance of nitrogen and potassium, but algae won't develop unless phosphorus is available too). See Phosphorus page for more information about phosphorus. See Additives page for guidance on filter media additives to enhance retention of phosphorus and other types of pollutants in Bioretention, Bioswale, Stormwater Planter and Stormwater Tree Trench facilities.

Under the CCME Water Quality Guidelines for the Protection of Aquatic Life. Freshwater – Long-term Suspended Sediment Concentration: “Maximum increase of 25 mg/L from background levels for any short-term exposure (e.g., 24-h period).” Assumes background level of 5 mg/L and objective of 30 mg/L total (CCME 2002)

Under the Provincial Water Quality Objectives set out by the Ministry of Environment and Energy in 1994, Total phosphorus effluent leaving a SWM facility should be below a concentration level of 30 μg/L. (0.03 mg/L) (MOEE 1994)

STEP Database Summary Statistics, TSS and TP concentrations, 2022.

STEP stormwater practice effluent quality monitoring results

The box plot figures to the right show stormwater effluent quality monitoring results from STEP monitoring projects conducted over a 25-year time period (between 1997 and 2021) at sites within Greater Toronto Area municipalities.

Median concentrations of Total Suspended Solids (TSS) in untreated stormwater and Oil and Grit Separator (OGS) outlet locations were found to exceed the Canadian Water Quality Guideline of 30 mg/L. Median TSS concentrations at stormwater detention pond and chamber system outlet locations were found to be below 30 mg/L, but exceeded the guideline value in 36% of the 231 monitored storm events. Median TSS concentrations at bioretention, permeable pavement and infiltration trench outlet locations were found to be all below 30 mg/L. TSS concentrations in effluent from bioretention was found to exceed 30 mg/L in only 15% of the 301 monitored storm events, and in only 12% of the 296 monitored events for permeable pavements.

Median concentrations of Total Phosphorus (TP) in untreated stormwater and all stormwater treatment facility type outlet locations were found to exceed the Ontario Provincial Water Quality Objective of 0.03 mg/L. Untreated stormwater was found to exceed 0.03 mg/L in between 96 to 98% of the 545 monitored storm events (Asphalt Reference and SWM Inlet data sets). Stormwater detention pond or chamber system effluent was found to exceed 0.03 mg/L during 91% of the 211 monitored events. Effluent from bioretention outlet locations was found to exceeded 0.03 mg/L during 86% of the 355 monitored events. Effluent from permeable pavement outlet locations exceeded 0.03 mg/L during 62% of the 300 monitored storm events.

It is notable that green roof and infiltration trench results are from only one site each, and based on a relatively small number of storm events (24 to 25).

Click on the links below for LID BMP-specific treatment performance sections for more figures and information on water quality treatment performance.

• Bioretention Performance
• Permeable Pavements Performance

Heavy metals

The heavy metals noted as particularly harmful to aquatic ecosystems are:

• Chromium(Cr),
• Copper (Cu),
• Lead (Pb), and
• Zinc (Zn)

Laboratory experiments on bioretention media have demonstrated that the organic matter significantly improves retention of lead (Pb), copper (Cu), and zinc (Zn) . This research also found that extended detention of the stormwater in the cell did not improve this water quality benefit.

Cores of media were extracted from five 10 year old bioretention cells in Queensland, Auz and tested for a suite of heavy metals: “Although trace amounts of several heavy metals (most prominently Mn and Zn) were found in most of the basins, all heavy metal levels found in the soil were either below detectable limits, or within acceptable limits based on legislated health-based investigation levels.”

External Links

• Environment Canada on Phosphorus and Excess Algal Growth
• Lake Simcoe phosphorus reduction strategy
• Canadian BMP Water Quality Database

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

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.

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.

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.

Stormwater Management

Sustainable Technologies Evaluation Program

Total suspended solids

Total phosphorus

The total amount of particulate matter that is suspended in the water column.

Systems designed to remove trash, debris and some amount of sediment, oil and grease from stormwater runoff based on the principles of sedimentation for the grit and phase separation for the oil.

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

A body of water smaller than a lake, often artificially formed.

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

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

A thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

A stormwater design features that provides for the gradual release of a volume of water in order to increase settling of pollutants and protect downstream channels from frequent storm events.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

A bioretention BMP featuring an impermeable liner and underdrain that prevents infiltration of runoff into the underlying native soil; provides sedimentation and filtration of urban runoff as it passes through the mulch layer, engineered filter media and vegetation root zone.

A bioretention cell that features an impermeable liner that collects and treats stormwater through sedimentation and filtration only (i.e., no infiltration).

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