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.

Drainage via infiltration to underlying native subgrade soil can be achieved through a number of LID BMP types:

• Infiltration chambers (underground)
• Infiltration trenches (underground)
• Exfiltration systems (underground)
• Bioretention (surface)
• Bioswales (surface)
• Stormwater Tree Trenches (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%),
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).

Designing within 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

Infiltration facilities should be designed with 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.

Target groundwater separation

In many areas of Ontario the target separation between the base of an exfiltrating LID practice and the water table is 1.0 m. This is to mitigate risks due to short periods of groundwater mounding and potentially unobserved seasonal fluctuations. In areas where a 1.0 m separation cannot be provided, or where conditions dictate that an even greater separation may be warranted, additional discussion and/or analysis specific to the physical characteristics of the site and the proposed design should be completed. The design practitioner is advised to consult with approval agencies to understand their requirements and/or expectations prior to undertaking work, and to complete an appropriate level of analysis to support their conclusion. The requirement for additional investigation and/or documentation supporting a proposed design may be reduced in areas where ≥ 1.0 m separation is anticipated.

Retention of an unsaturated zone beneath the practice :

• Minimizes the potential for functional impacts associated with reduced percolation rates,
• Maintains the physical and biochemical water quality treatment benefits provided within the vadose zone.

Groundwater mounding

Groundwater mounding describes the localised raising of the water table beneath infiltration practices. It may be of concern if it affects nearby structures including building foundations. When you wish to model the extent of groundwater mounding beneath an infiltration facility, use this Tool that incorporates Hantush's derivation (1967).

Download the Hantush groundwater mounding tool

Note that this is a minor adaptation (metric units and formatting) from the original tool, written and hosted by the USGS.

Preventing groundwater interaction

Many LID systems rely upon reuse, or evaporation and transpiration instead of infiltration to the ground. If the site cannot support any infiltration, consider

• Rainwater harvesting,
• Stormwater planters,
• Green roofs, or
• Blue roofs.

Oak Ridges Moraine Groundwater Program

Depth to groundwater raster and contours from the ORMGP

https://oakridgeswater.ca/

The Oak Ridges Moraine focused program stretches from the Credit and Nottawasaga River watersheds in the west to the Trent River in the east and reaches from the shores of Lake Ontario northwards to beyond Lake Simcoe and the Kawartha Lakes. Within the publicly accessible maps section there is a depth to water table map layer.

Other sources of groundwater data

• Ontario groundwater
• Provincial groundwater monitoring network map

---

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 local conservation authority and municipality is strongly recommended in all cases.

Hydrogeological Assessment Submissions Conservation Authority Guidelines for Development Applications

Beginning an infiltration BMP design?

See Design infiltration rates

---

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.

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.

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.

Sustainable Technologies Evaluation Program

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.

Abiotic transfer of water vapour to the atmosphere.

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

Penetration of water through the ground 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.