Infiltration

Infiltration of stormwater into underlying soils is one mechanism to achieving MECP runoff volume control targets. In many areas of Ontario, where municipal supplies of potable water are drawn from aquifers, it's also an important mechanism for groundwater recharge.

Infiltration is promoted through a number of LID BMPs:

• Infiltration chambers (underground)
• Infiltration trenches (underground)
• Exfiltration systems (underground)
• Bioretention (surface)
• Bioswales (surface)
• Soil cells (surface)
• Permeable pavements (surface)
• Dry ponds or infiltration basins (surface)

Constraints

Infiltration practices are not recommended:

1. over-swelling clays or unstable subsoils,
2. contaminated soils or sites with high risk of contamination from on-site activities (i.e. pollution hot spots),
3. in areas of known or implied karst topography,
4. on unstable or steep slopes (15 to 20%),
5. to treat construction site runoff,
6. to treat combined sewer overflows,
7. in flood prone areas where the wastewater system is sensitive to groundwater conditions causing sewer backups,
8. in areas where the ecology and natural hydrology are dependent upon surface water,
9. in areas where increased infiltration will result in elevated groundwater levels that will damage critical utilities or private property, or
10. where prohibitions and/or restrictions exist per approved Drinking Water Source Protection Plans (e.g. should not be used to treat pavement runoff within 2 year time of travel wellhead protection areas; see MOECC guidance).

Infiltration facilities should be designed with drainage modifications where:

1. the underlying bedrock is within 1 m below the bottom of the proposed BMP, or
2. the seasonal high groundwater level (i.e. water table) comes within 1 m below the bottom of the proposed BMP.

Designing with constraints

Infiltration can still be optimized over 'tight' soils with infiltration rates ≤ 15 mm/hr:

• See underdrains for bioretention and tree pits, trenches and soil cells, and
• Results from STEP studies of infiltration practices on low permeability soils

Where infiltration is impossible, LID design alternatives exist which can still capture and retain stormwater:

• Stormwater planters
• Rainwater harvesting
• Green roofs
• Blue roofs

Planning

Hydrogeology

The Conservation Authorities Geoscience Group has prepared a guidance document which attempts to standardize the hydrogeological study requirements for development applications made to Conservation Authorities. It is a helpful guide for all low impact development applications which include infiltration. It is provided with the caveat that not all sections or content are necessary or appropriate for every case. Pre-consultation with your Conservation Authority and municipality is strongly recommended in all cases.

Hydrogeological Assessment Submissions Conservation Authority Guidelines for Development Applications

Beginning a BMP design?

See Design infiltration rates

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See also

• Groundwater

Layer of rock or soil that holds or transmits water.

Increases in groundwater storage by natural conditions or by human activity. See also artificial recharge

The inflow of surface water to a groundwater reservoir or aquifer.

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.

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

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.

Land owned by private individuals or companies.

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.

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

Sustainable Technologies Evaluation Program

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

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.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.