Sometimes when troubleshooting an onsite system, the answers do not seem to come and there are no immediately visible signs of something that would indicate the cause of the problem. In at least some of these situations, there were practices or actions that were taken during installation that are not visible or easily identified.

Soil compaction and smearing before or during installation are actions that can fall into this category. There are several people who could have caused compaction or smearing issues besides the installer. They include other subcontractors, the road contractor, and cement or other delivery trucks during construction or remodeling. The homeowner may be a major culprit if they are not aware of the importance of maintaining the drainfield area in as natural a condition as possible.

Any amount of soil compaction in the final dispersal area can reduce soil permeability to air and water, reducing the amount of effluent that can be accepted. Compaction is usually caused by excavating when the soil is too wet or by unwanted traffic back and forth across the area during construction activities at the residence or by the installer moving or storing materials for installation. Remember the simple field test to determine if the soil is too wet: Take a handful of soil and try to roll it into a wire between your hands. If a wire 1/8 inch in diameter can be formed, the soil moisture content is higher than the plastic limit and installation should not proceed until the soil has dried out.

Smearing occurs on the sidewalls of trenches during excavation when the soil moisture content exceeds the plastic limit. Another way smearing occurs during installation is if the infiltrative surface is left open and unprotected during a precipitation event. Soil structure is broken down at the surface and the cracks and pores are effectively sealed, which reduces the infiltration capacity. The solution at the time of installation is to allow the soil to dry and then rework the bottom and sides of the trenches to expose a new surface.

Since you likely were not present at time of installation, these problems are hard to identify. In some situations, you will see evidence of compaction around the area such as poor vegetative growth or a dramatic change in vegetation type. Plants need oxygen and water too, and compaction limits the availability of water and nutrients for them to grow. A potential source of information is the homeowner; see if they have construction photos or recall aspects of the installation.

I have heard installers say, “But we only drove over the surface and since we are excavating trenches we will avoid any compaction problems.” Remember that compaction affects the soil to depth, reducing its capacity to conduct water. Heavy equipment and multiple passes over the area can affect soil capacity to depths of 2-3 feet or more. So if your trenches are installed 2-3 feet deep as well, they are well within the affected zone.

Unfortunately it takes a long time to mitigate the effects of compaction. Research has shown reduced infiltration capacity as long as 10 years after the compaction event. Some positive results have been shown using deep chisel plowing, but success depends on the depth of the compaction and the trenches.

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About the author Jim Anderson is connected with the University of Minnesota onsite wastewater treatment program and is an emeritus professor in the university’s Department of Soil, Water and Climate. Send him questions about septic system maintenance and operation by sending an email to kim.peterson@colepublishing.com.

This article is part of a series on troubleshooting the drainfield and yard:

• Troubleshooting the Drainfield and Yard
• More Tips for Drainfield Troubleshooting
• Drainfield Troubleshooting: Identifying Invisible Installation Issues
• Drainfield Troubleshooting: Trench Concerns
• Troubleshooting Tips: Tanks and Risers