Percolation Test Thunder Bay / Ontario (2025 Guide)

What is a Percolation Test?

A percolation test is a controlled field test used to evaluate how quickly water infiltrates into the soil. By measuring the rate at which water level drops in a prepared test hole, designers can quantify the soil's ability to accept and treat wastewater from a septic system.

The main purpose of a perc test is to protect groundwater, surface water, and nearby properties. If the soil drains too slowly, effluent may surface or back up. If it drains too quickly, there may not be enough contact time in the soil to properly treat wastewater. The test helps strike the right balance between absorption capacity and treatment performance.

In Ontario, the OBC requires a soil evaluation whenever a new or replacement on-site sewage system is proposed. A percolation test is a key part of this evaluation and is typically combined with soil profiling, test pits, and observation of groundwater conditions.

Perc test results feed directly into the engineered design of the leaching bed or Eljen GSF field. The measured infiltration rate determines:

• The size of the leach field or Eljen bed.
• The spacing and length of trenches or modules.
• Whether the system can be built in-ground or must be raised.
• Whether conventional construction is appropriate or if advanced treatment is recommended.

What a Perc Test Evaluates

• Soil type: Sand, silt, clay, or mixed glacial till.
• Water table: Observed or interpreted seasonal high-water levels.
• Soil permeability: How readily water moves through the soil matrix.
• Drainage capacity: Overall ability of the site to accept effluent over time.
• Seasonal moisture: How wet the soil becomes during spring melt or heavy rain.
• Depth to bedrock: Distance from ground surface to solid rock or refusal.

How Percolation Tests Are Performed

While details can vary by designer and municipality, most percolation tests in Thunder Bay and Ontario follow a similar sequence. The goal is to obtain repeatable, conservative results that reflect how the soil behaves under wet conditions.

Step 1 — Site Selection & Soil Profile

The first step is to identify where on the property a septic system is likely to be located. Test holes are typically placed within the proposed leaching area or Eljen field, in undisturbed soil that has not been previously filled, excavated, or compacted.

In Thunder Bay and surrounding communities, soil profiles often include layers of heavy clay, glacial till, and occasional pockets of sand or gravel. A soil profile is established using test pits or hand augers to understand:

• Soil texture and layering.
• Presence of mottling or staining that indicates seasonal high water.
• Depth to bedrock or refusal.
• Any evidence of seepage or groundwater movement.

Step 2 — Test Hole Preparation

Test holes are excavated to a depth that represents the proposed elevation of the dispersal trench or Eljen modules. The diameter and depth of each hole are controlled so results can be interpreted against OBC guidelines.

The bottom and sides of the hole are gently scarified to remove any smeared surfaces left by digging and to expose natural soil structure. A small layer of clean gravel may be placed at the bottom to protect against scouring during filling and testing.

The hole is then pre-soaked by filling it with water and allowing the level to drop. This simulates soil that has been exposed to sustained moisture, which is especially important in clayey Thunder Bay soils that swell when wet.

Step 3 — Saturation & Measurement

After pre-soaking, the test hole is filled to a specific depth, and the rate at which the water level drops is measured over timed intervals. Depending on the soil type, measurements may be taken every 10–30 minutes over a period of several hours.

The data is converted into an infiltration rate (often expressed in minutes per centimetre or minutes per inch of drop). Slower rates indicate poorer drainage and typically require larger or more advanced systems. Faster rates indicate well-draining soils but must still be checked against minimum residence-time requirements to ensure adequate treatment.

Step 4 — Reporting & Engineering

Once field measurements are complete, results are documented in a report that includes soil logs, percolation rates, groundwater observations, and site sketches. These results are then used by an engineer or OBC-licensed designer to size and configure the leaching bed or Eljen system.

The engineer will use the most conservative (slowest) results when designing the system to ensure that it performs reliably during wet seasons and over many years of use.

Percolation Test Requirements in Ontario

In Ontario, on-site sewage systems with a design capacity of 10,000 litres per day or less are covered under Part 8 of the Ontario Building Code. The Code requires that soil conditions be suitable for receiving and treating effluent before a permit can be issued for a new or replacement system.

A percolation test, combined with soil profiling and groundwater evaluation, is the standard method for demonstrating that a site meets these requirements. Many municipalities and health units have additional guidelines or standardized forms that must be followed.

In practice, a perc test is typically required when:

• A new home or cottage will be built on a property without municipal sewer service.
• An existing septic system is being replaced or substantially upgraded.
• A property is being severed or redeveloped, and septic suitability must be confirmed.
• A change in use is proposed that increases daily sewage flow (more bedrooms, added units, etc.).

Documentation generally includes a site plan, soil logs, percolation test data, and an engineered or OBC-licensed design. Portions of the work (such as data interpretation and final design) must be completed by qualified professionals.

Typical 2025 cost ranges for percolation testing and soil reports in Ontario are:

• Basic percolation testing and field observations: $500–$1,500+.
• Comprehensive soil reports with multiple test holes and detailed logging: $1,000–$2,500+.
• Full engineered design package for the septic system: $1,500–$4,000+, depending on site complexity.

Thunder Bay Soil Conditions & Their Impact

Thunder Bay and Northwestern Ontario have a very different geotechnical profile compared to many parts of Southern Ontario. Instead of deep, uniform sands, it is common to encounter a mix of heavy clays, glacial till, shallow bedrock, and variable groundwater conditions over relatively short distances.

Some of the most common conditions that influence perc tests include:

• Heavy clay soils: These can swell when wet and drain very slowly, leading to conservative design rates.
• Glacial till: A mixture of clay, silt, sand, and rock fragments that may have inconsistent drainage behaviour.
• Shallow bedrock: Limits available depth for in-ground systems and affects vertical separation to groundwater.
• High water table areas: Locations such as Pass Lake, Shuniah, Oliver Paipoonge, and Slate River often experience elevated groundwater levels.
• Seasonal saturation: Spring melt and heavy rainfall can raise water levels and slow infiltration rates.
• Freeze–thaw cycles: Repeated freezing and thawing can impact soil structure and drainage pathways.

Because of these challenges, engineered systems such as Eljen GSF advanced treatment systems and raised beds are more common in Northwestern Ontario than in many Southern Ontario municipalities. Perc tests help determine when these systems are warranted and how they should be configured.

What Happens if Your Property Fails a Percolation Test?

A common misconception is that failing a percolation test means a property cannot have a septic system. In reality, a "fail" usually means that a conventional in-ground field is not appropriate in its simplest form, and that an engineered solution is required.

Options that may be considered when perc rates are poor or the water table is high include:

• Eljen GSF advanced treatment systems: Provide high levels of treatment in a compact footprint.
• Raised beds or mounded systems: Build the dispersal area above existing grade to achieve required separation to groundwater or rock.
• Imported sand and engineered fill: Replace unsuitable native soil with approved, free-draining material.
• Shallow buried trench systems: Use specially designed shallow trenches to work with available soil depth.
• Targeted soil replacement: Remove and replace specific layers that are impeding drainage.
• Alternative locations on the property: Relocate the system to a portion of the lot with better soils or topography.

In most cases, experienced engineers and designers can develop a compliant solution even on difficult sites. The key is to treat a failed perc test as information, not a verdict. The test results guide the type of system chosen and the level of engineering required.

Typical Cost of Percolation Testing in Ontario

Costs for percolation testing and related services vary based on location, access, and the number of test points required. For planning purposes in 2025, homeowners in Ontario can expect approximate ranges such as:

• Standard percolation test: $500–$1,500+ for a basic field program with a limited number of test holes.
• Deep test holes or machine-dug test pits: $1,000–$2,500+, particularly where excavators are required.
• Soil analysis and laboratory review (if needed): $300–$800+, depending on the scope of testing.
• Full report and engineered septic design: commonly $1,500–$4,000+, depending on complexity and permitting requirements.

In Northern Ontario, factors such as travel distance, mobilization of excavation equipment, and deeper test holes through clay or till can push costs toward the higher end of these ranges. However, these costs are often small compared to the overall septic system investment and are essential for a compliant, long-lasting installation.

How Percolation Test Results Affect Septic System Cost

Perc test results are one of the main drivers of septic system cost because they determine the type and size of system that can be approved.

• Faster-draining soils: When soils are sandy or well-draining, a more compact traditional system may be possible. This usually means smaller leaching areas, less imported sand, and lower overall construction cost.
• Slower-draining soils: Heavy clays and dense tills require larger leaching areas or advanced systems like Eljen GSF to achieve proper treatment. This increases excavation, materials, and engineering effort.
• High water table: Elevated groundwater levels often call for raised beds or mounded systems with significant imported sand. This is common in low-lying areas and near lakes or rivers.
• Bedrock: Where shallow bedrock is encountered, systems may need to be completely above grade with engineered fill and advanced treatment, which adds both complexity and cost.

The percolation test, combined with soil profiling, allows engineers to develop a realistic cost range before construction begins, reducing the risk of surprises once excavation starts.

Percolation Test FAQ

How long does a percolation test take?

Most fieldwork can be completed in half a day to a full day, depending on soil conditions and the number of test holes. Additional time is required afterward for calculations, documentation, and engineering review.

When is the best time of year to perform a test?

Perc tests are often performed during periods when soils are near their wetter seasonal conditions, such as spring or early summer. However, tests can be completed at other times of year as long as conditions are documented and results interpreted appropriately.

Can a property pass in winter?

Winter testing is more challenging due to frozen ground, snow cover, and equipment access. In some cases, engineers may rely more heavily on soil profiling and historical groundwater data rather than traditional winter percolation measurements. Local authorities may have specific requirements for winter testing.

How deep are test holes?

Test holes are usually dug to the depth where the bottom of the leaching bed or Eljen modules are expected to be installed. This can range from less than a metre to more than 1.5 metres below existing grade, depending on soil conditions and design.

What's the difference between a soil test and a perc test?

A soil test is a broader term that may include grain-size analysis, classification, or laboratory testing. A percolation test is a specific field procedure focused on how quickly water infiltrates into the in-place soil. In septic design, both types of information may be used together to understand site suitability.

Do I need engineering if my soil is poor?

Yes. When soils are marginal or poor, engineering becomes even more important. An experienced engineer or OBC-licensed designer can often develop a safe, code-compliant solution using advanced treatment, raised beds, or imported fill, even when raw percolation rates are unfavourable.

How do I prepare for a perc test?

Property owners can help by providing site plans, marking property boundaries, identifying any known wells or watercourses, and ensuring reasonable access for equipment. Clearing excessive brush or snow (where safe to do so) can also make fieldwork more efficient.

Related Guides

For a deeper understanding of how percolation testing fits into septic system planning and cost in Ontario, consider the following resources:

• Septic System Cost Ontario (2025 Guide)
• Septic Installation in Thunder Bay – Local Design & Soil Considerations
• Eljen GSF System – Ontario Homeowner's Guide to Advanced Septic Solutions
• Septic Tank Installation Cost Guide – Ontario 2025

Need Help Scheduling a Percolation Test in Thunder Bay?

Coordinating percolation testing, soil evaluation, and septic design can feel complex, especially on challenging Northern Ontario properties. Working with experienced professionals ensures that testing is performed correctly and that results lead to a durable, code-compliant septic system.