Troubleshooting: Traffic Over the Drainfield

Be sure to remind homeowners that all traffic should be kept off the system to ensure long-term operation and performance.
Troubleshooting: Traffic Over the Drainfield
A drainfield located in a horse paddock included a broken distribution box and damaged distribution lines. (Photos by Dave Gustafson)

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One of the recurring points that I have made during troubleshooting discussions is the importance of communication with the homeowner regarding proper care of the system once a problem has been fixed. 

For instance, I recently saw an elevated mound system surrounded by a fence and inside the fence were two horses. It was spring and the snow was melting, so the soil around and over the top of mound had deep hoof marks and the grass was almost non-existent. 

This type of traffic over a system can cause a number of serious problems for an onsite system, and if you see it while troubleshooting a problem, the system might need to be replaced. I have seen these types of situations quite often in areas across the country on farms with horses. 

The mound system I saw — like all mound systems — should have had about a foot of cover over the rock and the piping. During wet periods with constant traffic by large animals, the distribution and supply piping are subject to damage and breaking. In mound systems, equal distribution through the pressure distribution system is a key element to proper operation. If the distribution system is broken or interrupted, the effluent will collect where the line is broken and surface. 

Other systems that require less than a foot of cover include advanced systems such as drip dispersal systems and conventional gravity-fed systems. The piping in drip systems can be totally destroyed by sharp animal hooves. Since all systems installed today are placed as shallow as possible to take advantage of the best soil conditions for treatment and acceptance, all systems are at risk to damage of piping, distribution or drop boxes. 

Smearing and soil compaction to 1-foot deep or more is another reason to stress the importance of traffic over a drainfield. This will impact the flow rate of water and oxygen through soil in and around the system. If soil permeability is reduced it will not accept the amount of effluent applied. Reducing the oxygen flow around the system will result in a double whammy with increased biomat development, which further reduced the flow rate.

Regardless of the type of system, be sure to remind homeowners that animal traffic should be kept off the system to ensure long-term operation and performance. 

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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