How to Size a Soil Treatment System

Testing to predict how water will move through the soil with a morphology or percolation test helps determine a soil treatment area's size

How to Size a Soil Treatment System

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Across the U.S., there are two common approaches to determine how large a soil treatment area, or STA, should be: soil morphology evaluations, and/or percolation or a similar hydraulic test. 

The general trend and recommendation has been to move to soil morphology since it provides a more complete picture of what is going on in undisturbed soil. In disturbed soils, hydraulic testing (as done in a percolation test) can provide insight into the level of damage to the natural soil structure.

Both methods, if performed properly, can predict how water will move through the soil. A summary of each, along with applications, is discussed below.   

1. Soil morphology - The texture and structure of the soil are used to predict how water will move through the soil and how the soil will treat wastewater. Soil observations must use tools that preserve soil integrity so undisturbed soil colors, textures and structure can be observed. The purpose of this observation is to determine the texture and structure of the soil along with the depth to periodically saturated soils, bedrock, standing water and any other soil characteristics important to the proper functioning of the STA. Several soil observations are commonly conducted across the proposed area to adequately assess soil variations. Several soil observations should be performed for system sizing. The minimum depth of the soil observations should be to the saturated layer, bedrock or required separation below the proposed depth of the system. The septic system professional assigns a "loading rate" in gallons per day per square foot for that particular site location. All of the soil layers should be evaluated to determine the most limited zone in the soil profile needed for treatment and dispersal. It is critical that the person performing the soil evaluation be properly trained.

2. Percolation test - The test helps to determine the permeability and suitability of the soils on a property for the purposes of onsite sewage treatment. The percolation test can be a required or used as an additional tool in the site evaluation process. Soil observations must first be done, as they are used to locate a suitable area and to identify the limiting condition of bedrock or soil saturation. Perc testing is one method to determine loading rates instead of or in addition to evaluating soil structure and consistence. It is recommended that soil morphology including texture and structure always be considered in system sizing. Perc tests are also useful in providing a better understanding of site impacts. When soil disturbance is suspected, running perc tests across the proposed areas will determine the extent and severity of the disturbance on soil water movement (soil loading rate). One of the biggest criticisms of perc tests is that they are not always conducted properly; but if performed properly, a percolation test is a valuable tool.

If the soil texture is similar over the selected site, two to three percolation tests should be run across the area with additional tests added for sites with significant variability. The percolation test measures only the rate of the drop of water in a test hole of a specific diameter, and it does not measure the rate of movement of water through the soil. However, the relative values obtained by the perc test will give some index of the ability of soil to transmit water. For consistent results, it is critical that the hole be at the correct depth, prepared properly and soaked appropriately. For a detailed percolation procedure please see: The ABCs of Soil Perc Testing Success. The percolation rate can then be used along with soil description to choose the loading rate.

Very slow permeability indicates that a soil is relatively high in fine material such as silt and clay and thus may need extreme care during the installation of the soil treatment system, whereas soil that has a fast percolation rate is typically sandy. Also, if the soil percolates much slower than expected based on the texture and structure of the soil, it can be an indication of a site that has been cut, filled or compacted.

In summary, a soil observation based on soil morphology must always be performed. A percolation test or similar hydraulic test is sometimes required or recommended to confirm water movement. A hydraulic test is recommended on sites where there are concerns about soil impact from humans including compaction and areas that have been filled or cut.

About the author: Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to


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