Infiltration Testing (LID)

Why reviewers want measured rates

Reviewers want a measured infiltration rate because LID features only work if the ground accepts water at the rate the design assumes, and layering, compaction or a shallow water table can cut that rate sharply.

LID features only work if the ground below them accepts water at the rate the design assumes. Soil maps and textbook values are a starting point, but layering, compaction and a shallow water table can cut the real rate sharply. The LID Stormwater Management Guide published by the Sustainable Technologies Evaluation Program (a partnership of Toronto and Region, Credit Valley and Lake Simcoe Region conservation authorities) bases design rates on on-site testing.

Conservation authorities also tie testing to water balance reviews. The Lake Simcoe Region Conservation Authority requires seasonal high groundwater levels and in-situ infiltration rates whenever LID features are proposed to mitigate a water balance deficit.

Test methods

Infiltration testing uses four main methods, the borehole permeameter, the test pit percolation test, the double ring infiltrometer and grain size analysis, and the LID Stormwater Management Guide gives on-site permeameter and percolation tests the most weight.

The guide treats permeameter and percolation tests on site as stronger evidence than double ring tests or grain size analysis, which is reflected in the safety factor.

From measured rate to design rate

The design infiltration rate is the measured rate divided by a safety correction factor between 2 and 3, with factors closer to 3 for larger catchments, weaker test methods, clayey soil, changing layers below the base or nearby septic systems and foundations.

The measured rate is divided by a safety correction factor between 2 and 3. Factors closer to 3 apply when:

• the catchment draining to the feature is larger than 100 m²;
• the rate comes from a double ring test or grain size analysis;
• the soil is clayey;
• geology, texture or density changes within 1.5 m below the feature's base; or
• septic systems or building foundations are close by.

The field day, and what comes before it

Before testing, the site is mapped: surficial geology, watercourses and wetlands, slopes and drainage, wells and septic systems, and the preliminary grading plan with areas of cut and fill. Test locations then follow the proposed features.

1. Test pits or boreholes at each proposed feature, logged by the hydrogeologist.
2. Tests at the base elevation and through the profile below it.
3. Monitoring wells, where needed, to establish the seasonal high water table.
4. Report with measured rates, design rates and the separation to groundwater at each location.

Do the LID features need a ministry permission?

LID features sometimes need a ministry permission: away from a single lot used for residential purposes, storm water management works generally need an Environmental Compliance Approval or an EASR registration unless another exemption applies.

Sometimes, and it affects how the testing is reported. The ministry's sewage and storm water permissions page treats storm water green infrastructure and low impact development (rain gardens, permeable pavement, green roofs, rainwater harvesting and similar works) as storm water management works. Such works may not need an environmental permission when they are on one lot used for residential purposes, with all buildings residential. Otherwise, storm water management works generally need an Environmental Compliance Approval or an EASR registration for storm water management works, unless another exemption applies.

Where an approval is needed, the infiltration rates, the separation to groundwater and the test locations become part of the design record that supports it, so the testing report should state them feature by feature rather than as a site average.

Timing

Infiltration testing timing is set by frozen ground, which rules out meaningful tests until thaw, and by any requirement for seasonal high groundwater data, so the tests belong before the stormwater design is finalized.

The testing itself is usually a short field program once access and locates are arranged, followed by the report. Two things set the calendar: frozen ground, which rules out meaningful testing until thaw, and any requirement for seasonal high groundwater data, which means monitoring wells read over the period the reviewing agency specifies. Schedule testing before the stormwater design is finalized, so features can be moved if a location fails.

Infiltration testing cost

The cost of infiltration testing depends mostly on the number of feature locations and whether boreholes already exist, with deep bases or seasonal groundwater monitoring pushing it higher.

Reusing existing boreholes, or combining the work with geotechnical drilling, keeps the cost down. Many feature locations, deep bases, or a need for seasonal groundwater monitoring push it up.

What to have ready

Before the tests are booked, have the site plan with LID feature locations and base elevations, the grading plan, earlier borehole logs and the water balance or stormwater targets the features must meet.

• The site plan with proposed LID feature locations and base elevations
• The grading plan, if available
• Borehole logs from earlier investigations
• The water balance or stormwater targets the features must meet

What goes wrong

Infiltration tests go wrong most often when they are run in fill or topsoil, at a base elevation the grading plan has since changed, without groundwater data, or only once across a large feature.

• Testing in fill or topsoil instead of the native soil the feature will sit on.
• Testing at the wrong depth after the grading plan changed the base elevation.
• No groundwater data, so the reviewer cannot confirm the feature stays above the water table.
• Single test for a large feature, missing variability across its footprint.

Worked cost examples

Each example runs the estimator's own calculation on a realistic set of answers.

• A few soakaways on a small site, existing boreholes$2,500 – $4,000

  Test locations: 2 to 4 · Boreholes: Use existing boreholes

  Testing in boreholes already drilled for the geotechnical work, at the planned base elevations. The cheapest case.

• Subdivision with five to eight LID features, new boreholes$6,000 – $9,500

  Test locations: 5 to 8 · Boreholes: New boreholes needed

  Both answers multiply: more test locations, each needing its own borehole or test pit at the right depth.

• Large site with nine or more features, new boreholes$9,000 – $14,500

  Test locations: 9 or more · Boreholes: New boreholes needed

  The top of the range. Seasonal groundwater monitoring under the features, if required, adds to this.

What the reviewer checks

A reviewer of an infiltration testing report checks that each test sits in native soil at the feature base, that the method and safety factor are justified, and that each base stays above the seasonal high water table.

• Tests in native soil at each feature's base elevation, not in fill or topsoil
• Conditions within 1.5 m below the base, where a tighter layer would control drainage
• The test method, and why it suits the soil and depth
• The safety correction factor chosen, with the reasons for it
• Separation between each feature's base and the seasonal high water table
• Test locations that match the current site and grading plans
• Logs for every test pit or borehole

Infiltration testing or water balance?

Infiltration testing measures how fast the ground takes water at each LID feature, while a water balance assessment calculates the annual infiltration deficit for the whole site that the features must make up.

Where this study is commonly required

Location pages with local requirements, groundwater data and the agencies involved, by region: