How To Fix a Failing Drainfield

Unless you’re an amateur soil scientist, here’s what to do if you’re up against a failing drainfield.
How To Fix a Failing Drainfield
An example of redoximorphic features (mottles) that indicate saturated soil conditions.

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In the past few months, we’ve discussed troubleshooting overall onsite system problems. Now let’s take a closer look at troubleshooting a failing drainfield. 

Drainfield, or soil treatment area, problems occur when soil is not accepting as much effluent as the house is producing. The first step should be to speak directly with the homeowner to see if lower water usage to reduce flows fixes the problem. If that doesn’t work, the soil itself may be the problem, which means you’ll need to find a soil scientist to do some detective work. 

Unless you’re an amateur soil scientist, here’s what to do if you’re up against a failing drainfield. 

Take another look

First, look at the soil from several different perspectives. An adequate soil description includes digging at least one, and perhaps multiple, soil pits at approximately the same elevation that the soil treatment trench or bed was installed. Soil borings can also be made around the perimeter of the system to look for any changes in soil conditions, which would also indicate a need for additional soil pits. 

Evaluate and compare soil texture, structure and consistency to the site evaluation information that was provided during system design. This information can be used to enter your state’s sizing table for design to back calculate the area needed to accept the estimated daily sewage flow. 

If the soil size appears accurate in relation to absorption area size, then the problem lies elsewhere. If the absorption area is smaller than design values indicate, you’ll need to increase the treatment area if that is feasible or move it to an entirely different location on the lot and put in a new set of trenches. 

True colors

You should also evaluate soil color for redoximorphic features. Redoximorphic features are specific types of soil mottles (more than one color present) that indicate if the soil is periodically saturated at certain depths even if it is completely dry the day you examine the soil. The separation distance of soil colors cannot exceed state regulations. The separation distance is from the depth in the soil the features are identified to the infiltrative surface of the soil treatment and dispersal unit; in other words, the bottom of the trench. For instance, if state requirements mandate that you have redoximorphic features at a 4-foot depth from the surface and a 3-foot separation distance, then a trench could be excavated no more than 1 foot into the soil without being too close. 

A periodic saturated condition interferes with the movement of water away from the system, which increases biomat thickness and further reduces effluent flow through the soil. Treatment capability is also reduced as a result of a lack of oxygen. 

If the color indicates saturation, a new treatment unit will need to be installed at an elevation with proper separation distance. This also applies to any other limiting layer identified in the soil investigation such as bedrock or dense soil conditions. 

Pack it all in

There is one more soil-related issue that bears investigation: Was the soil was compacted and smeared during system installation? This is much more difficult to identify and probably requires a drainfield excavation to look for evidence of compaction. If the soil was compacted during installation, then the soil infiltration rate would be significantly reduced, creating an undersized system. 

Here, the homeowner might have photos of the site during installation that could help identify whether this is the problem. Compaction problems during wet periods increases in heavier textured soils, so it’s important to know what time of year the system was installed. 

Lastly, if there is visual evidence of compaction around the system during inspection, the best solution is to install a new soil treatment unit. 

If the problem is not with the soil, then you’ll have to backtrack to homeowner use and maintenance. The effluent may have a higher than desirable amount of organic material, which creates a thicker, more resistant biomat, and ultimately slows flow into and through the soil. 

If this is the case, then the predominant fixes will involve several things we will explore in the next several months, including resting the area for a period of time, using chemical agents to unclog the soil, and adding oxygen into the system.

For more information on soil characteristics and classification, visit

About the Author

Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians. Send him questions about septic system maintenance and operation by email to


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