Infiltration

Infiltration of stormwater into underlying soils is one the priority level 1. mechanisms to achieve MOECC Runoff Volume Control targets.

Infiltration is promoted through a number of LID BMPs:

• Infiltration chambers and trenches (underground)
• Infiltration basins (surface)
• Bioretention cells (surface)
• Bioswales (surface)

Constraints

Infiltration practices are not recommended:

1. Where the bedrock is within 1 m below the bottom of the proposed BMP,
2. Where the seasonal high groundwater level comes within 1 m below the bottom of the proposed BMP,
   Or in areas where increased infiltration will result in elevated groundwater levels, which can be demonstrated to damage critical utilities or private property,
3. Over swelling clays or unstable sub-soils,
4. Over contaminated soils or sites with high risk of contamination from onsite activities,
5. 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,
6. In areas where the ecology and natural hydrology are dependent upon surface water,
7. Where prohibitions and/or restrictions exist per approved Source Protection Plans (see MOECC guidance).

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

Partially infiltrating bioretention

Without infiltration, many LID design alternatives exist which can still capture and retain stormwater:

• Bioretention planters
• Rainwater harvesting
• Green roofs

Planning

Soil Infiltration Testing

Infiltration tests must be undertaken at the location, depth and with a head of water that replicates the proposed design.

Four Step Process

Soil infiltration testing is a four-step process to obtain the necessary information for stormwater management planning and design. The four steps include:

1. Background Evaluation

• Based on available published and site specific data;
• Includes consideration of proposed development plan;
• Used to identify potential BMP types, locations and soil test locations;
• Done prior to field work; and
• On-site soil tests may be done to identify/screen potential BMP locations.

2. Test Pit or Soil Boring Observations

• Includes multiple testing locations;
• Provides an understanding of sub-surface conditions; and
• Identifies limiting conditions (e.g., aquitard, bedrock or water table elevations).

3. Infiltration Testing

• Must be conducted on-site;
• Various testing methods are available; and
• Different testing methods for screening versus verification purposes.

4. Design Considerations

• Determination of a suitable infiltration rate for design calculations; and
• Consideration of desired BMP drawdown time.

Test Pit or Soil Boring Observations

Test pits or soil borings provide information regarding the soil horizons and overall soil conditions both horizontally and vertically in that portion of the site. Multiple observations can be made across a site at a relatively low cost and in a short time period.

Test pit excavations or soil borings should extend to a depth of between 2.5 - 5 m below ground surface or until bedrock or fully saturated conditions are encountered. It is important that the tests provide information related to conditions at least 1.5 m below the proposed bottom elevation of the infiltration BMP. Test pit trenches should be benched at 1 m depth intervals for access and infiltration testing.

At each test location, the following conditions should be noted and described:

• Soil horizons (upper and lower boundary);
• Soil texture and colour for each horizon;
• Color patterns (mottling) and observed depth;
• Depth to water table (if encountered);
• Depth to bedrock (if encountered);
• Observations of pores or roots (size, depth);
• Estimated type and percent coarse fragments;
• Hardpan or other limiting layers; and
• Strike and dip of soil horizons.

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Groundwater

The bottom of the infiltration BMP must ≥ 1 m vertically separated from the seasonally high water table, and underlying bedrock. This reduces the risk of contaminating groundwater and helps to ensure reliable infiltration rates throughout the year.

Suggested safety factors to be applied when designing infiltration facilities

Catchment area	Consequence of failure
	No damage or inconvenience	Minor damage or inconvenience to external structures (e.g. ponding in parking lot)	Significant damage to buildings or infrastructure (e.g. flooding damage)
<100 m2	1.5	2	10
100 - 1000 m2	1.5	3	10

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.

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.

Land owned by private individuals or companies.

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

Penetration of water through the ground surface.

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

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.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

The water below the surface, and typically below the groundwater table.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.