What Is a Percolation Test?

A percolation test is a crucial step in assessing how suitable your soil is for installing a septic system. This test measures the soil’s ability to absorb water, which a septic system needs to function properly.

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Understanding what a percolation test involves and the implications of its results can help you ensure your land is ready for effective wastewater management.

What Is a Septic Percolation Test?

A septic percolation test, or soil percolation test, involves digging a series of holes in the ground and filling them with water to see how fast the water level drops. The test measures the rate at which soil can absorb water, a critical factor for designing a septic system’s leach or drain field.

The Purpose of a Percolation Test

The primary purpose of a percolation test is to determine whether the soil can adequately absorb and treat the effluent from a septic tank. The percolation rate helps in designing a septic system that will effectively manage household wastewater, preventing system failures and environmental contamination. County health departments often require percolation tests to approve septic system permits.

How Much Does a Percolation Test Cost?

The cost can vary depending on several factors, including the location, the property's size and the complexity of the test. On average, HomeGuide says homeowners can expect to pay between $200 and $1,500 (CAD 275 to CAD 2,065) for a hand-dug percolation test. If an excavator is needed, expect to pay between $500 and $3,000 (CAD 688 and CAD 4,129). This fee typically includes digging the test holes, conducting the test and preparing a report with the findings.

What Is a Good Percolation Test Result?

A good percolation test result indicates that the soil absorbs water at an appropriate rate for a septic system. The ideal percolation rate is usually between 1 and 30 minutes per inch. This means that the water level in the test holes should drop by 1 inch within this time frame. If the rate is too fast (less than 1 minute per inch) or too slow (more than 30 minutes per inch), the soil may not be suitable for a standard septic system.

What Happens if You Fail the Test?

A failed test is when the soil hasn't absorbed the water at the required rate. This can happen for several reasons, such as having too much clay in the soil or a high water table (where the groundwater level is relatively close to the surface of the soil). Failing the test doesn’t necessarily mean you can't install a septic system; it may just require alternative solutions. Here are a few options:

• Soil modification:

  Sometimes, improving soil drainage by adding sand or gravel can help.

• Alternative septic systems:

  In some cases, engineered systems like mound systems or aerobic treatment units can be installed.

• Relocating the leach field:

  Moving the septic system to another part of the property with better soil conditions might be an option.

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Conducting a Percolation Test

The process of conducting a percolation test involves the following three steps:

1. Digging Test Holes

Typically, the test requires several holes, each about 6 to 12 inches in diameter and 12 to 24 inches deep. These holes are spread across the area where the septic system will be installed.

2. Filling the Holes

After digging, the holes are filled with water to saturate the soil. This presoaking helps mimic the long-term conditions the soil will experience once the septic system is in use.

3. Measuring the Percolation Rate

Once the soil is saturated, the test begins by refilling the holes with water. The technician records the rate (in inches per hour) at which the water level drops. This percolation rate indicates how quickly the soil can absorb water, which is crucial for designing the leach field.

Factors Affecting Percolation Test Results

Several factors can influence the outcome of a percolation test, including:

• Soil composition:

  Sandy soil absorbs water quickly, whereas clay soil absorbs it slowly.

• Water table level:

  A high water table can impede absorption, leading to poor percolation test results.

• Seasonal variations:

  Soil moisture levels can vary with seasons, affecting test results.