Soil Perc Testing 101

Stop! Is that site suitable for an onsite system? Learn how to perform a proper soil percolation test.
Soil Perc Testing 101
Dig four holes that are 6 to 8 inches in diameter to the depth of the proposed absorption field (standard system is 18 to 30 inches).

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Soil naturally treats and degrades organic matter, destroys pathogens, and acts as a filter to remove contaminants from water. Because the soil plays such an important role it’s vital to ensure the selected site can properly handle the water from the distribution laterals. 

First, the basics. The ideal slope for a soil absorption area is between 2 and 15 percent. In some areas a slope of 15 to 30 percent is provisionally acceptable. Soil texture, depth to seasonally high water table, depth to bedrock and depth to restrictive clay layers should all be considered when determining lateral location. 

These soil characteristics help you determine if the soil and the selected site are suitable for effluent absorption and dispersal. A soil morphology report — if available — is by far the best way to determine if the soil is suitable. But if a soil morphology report isn’t available a common practice in many states is to do a soil percolation test. 

For soils to effectively treat effluent, percolation rates must be between 10 and 60 minutes per inch of percolation. You need at least 20 to 21 hours to do a standard percolation test. This creates a worst-case scenario in the soil. If the selected area still percolates in the listed range (10 to 60 minutes per inch of percolation) then the site will work well for effluent treatment. 

Follow these steps to perform a soil percolation test that will determine if the selected area is acceptable for a soil adsorption field. 

1. Dig four holes that are 6 to 8 inches in diameter to the depth of the proposed absorption field (standard system is 18 to 30 inches). Dig three holes around the proposed absorption site periphery and one hole in the middle of the site.

  • Test hole preparation — bore or dig each 6- to 8-inch-diameter hole to a depth of the bottom of the proposed soil absorption system.
  • Scratch/scarify hole sides and bottom to remove smearing and present natural soil surfaces for water penetration.
  • Remove all loose material from the hole.
  • Place 2 inches of washed gravel in bottom of hole to prevent scouring. 

2. Presoak by adding water to holes.

  • Fill holes to a minimum of 12 inches above the bottom with clear water — do not scour sides.
  • Maintain water level for a minimum of four hours for the presoak.
  • Add water as needed to maintain 12 inches of water in the hole.
  • Measure the water level drop at 30 min intervals until the water depth is stabilized at 12 inches. 

This allows the soil to become saturated. Saturation is when air space between the soil particles becomes filled with water. 

3. Once the presoaking has occurred, allow the soil to swell. Swelling is the intrusion of water into the individual soil particles. This is a slow process and must be done for 16 to 24 hours to get proper swelling unless the initial filling of the hole seeps away in less than 10 minutes. 

4. At the end of the 16 to 24 hours to allow for soil swelling, adjust the water level in the hole to 8 inches above the bottom of the hole. 

5. Measure drop-in water level at 10-minute intervals to the nearest 1/8 inch. 

6. Add water as necessary between measurements to maintain 8-inch depth, do not exceed the 8-inch depth. If 8 inches of water seeps away in less than 10 minutes, record the time required for level to drop 1 inch from the 8-inch reference point. 

7. Continue to test until three consecutive measurements vary less than 10 percent. 

8. Calculate percolation rate, which is time interval divided by drop-in water level to obtain percolation rate in minutes per inch. 

It helps to have a table drawn ahead of time for writing down information for each hole.

If the drop is 5/16 inch in 10 minutes you would determine perc rate as follows:

10 divided by 5/16, or written as a decimal 10 divided by .31 = 32.26

The slowest percolation rate of the four holes is used for absorption system design. If the slowest rate varies more than 20 minutes per inch from others, a detailed soil morphology evaluation is required.

About the Author

Bob Broz is water quality specialist with the University of Missouri Extension. He teaches classes on soil percolation for onsite installers and developed a class for real estate professionals about understanding onsite systems. He has developed a class for homeowners on the care and maintenance of onsite systems. Readers are welcome to submit questions or article suggestions to Bob. Write to


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