A Job With Percs

There is no shortcut for a proper septic system. You need a complete site evaluation for optimal performance.

Interested in Onsite Systems?

Get Onsite Systems articles, news and videos right in your inbox! Sign up now.

Onsite Systems + Get Alerts


What do you think about perc tests? The county is now requiring a perc test before we put in a septic system. I have a lot of experience and I just dig around in the ground and find some good dirt and put in a system. We generally have good dirt around here, but there are some places with problems. I think messing around with a perc test would generally be a waste of time.



You didn’t mention what kind of systems you are installing and how you determine system sizing. You also didn’t say how long the systems you have installed are lasting.

In my opinion, the soils on a site must be carefully tested before any soil absorption system is put into the ground. Soil borings or excavations should be made to at least five feet to see if a trench system can be installed. Just digging a little way into the soil will not give enough information for sizing and designing a soil absorption system.

In an area where the soil treatment system is to be installed, the texture of the soil has to be evaluated. Is it a sandy loam or a clay loam? How much septic tank effluent will it handle?

If you are a soil scientist with training in soil structure and soil texture, you would be able to predict the percolation rate of a soil. If not, you will need to run percolation tests. Those tests will reveal the percolation rate and the likely soil texture. This information is used to figure out the size of the trench system.



The percolation test has been used for many years to determine if water would flow into a soil and to measure how fast the water would flow. It’s relatively easy to perform and provides information for sizing the drainfield.

I recommend a hand-augured hole six inches in diameter with the sidewalls scraped to remove compacted soil to a diameter of seven inches. The test hole should be drilled to the depth of the bottom of the proposed trench. Two inches of drainfield trench rock should be placed in the bottom of the percolation test hole to prevent soil disturbance when water is added to the hole.

In soils containing clay, the test hole should be pre-soaked for 24 hours unless the test is performed in the spring when the soil is wet. Some soil particles get larger when wet, and if the test is run when the soil is dry, the rate will be faster than when the drainfield trench is filled with effluent.

Water in the percolation test hole should not be deeper than six inches over the top of the rock at the bottom. Water should be added to bring the depth to six inches after each test reading. The percolation test should continue until two rates do not vary by more than 10 percent.



More details about the percolation test are explained in the bulletin How to Run a Percolation Test, from the Minnesota Extension Service at the University of Minnesota. The bulletin can be viewed online at www.extension.umn.edu.

Following is a table showing how soil texture relates to percolation rate:

After the soil absorption system is designed, its location on the property should be staked to prevent other construction activity nearby. The location of the water well should also be specified to separate it from the sewage system according to the local codes.

In some situations, the soil absorption system is designed for an area, and then the well is drilled too close. So a new spot for the drainfield has to be found. If only one area is suitable for the drainfield, the lot could not be developed unless extra land is available.



I respect your skills in being able to dig and analyze soils, but the onsite industry needs to design and install properly located systems. A complete site evaluation with soil borings and percolation tests is needed to get those results.


Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.