Infiltration

Infiltration of stormwater into underlying soils is one the priority level 1. mechanisms to achieve MOECC 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)
• Dry ponds (surface)
• Bioretention (surface)
• Bioswales (surface)

Constraints

Infiltration practices are not recommended:

1. over swelling clays or unstable sub-soils,
2. over contaminated soils or sites with high risk of contamination from onsite activities,
3. in flood prone areas where the wastewater system is sensitive to groundwater conditions causing sewer backups, and where LID BMPs have been found to be ineffective,
4. in areas where the ecology and natural hydrology are dependent upon surface water,
5. in areas where increased infiltration will result in elevated groundwater levels, which can be demonstrated to damage critical utilities or private property, or
6. where prohibitions and/or restrictions exist per approved Source Protection Plans (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 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, and
• Results from STEP studies over 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

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.

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.

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.