Drainfield Troubleshooting: Trench Concerns

Drainfield Troubleshooting: Trench Concerns

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Drainfield trenches should be installed level and on the contour. If surfacing occurs at the end of one or more trenches, they should be checked to see that they are level. A trench sloping away from a dropbox or the distribution box may result in effluent surfacing rather than going on to the next trench. Depth to the top of the drainfield media should be measured to determine if the trench is level and on the contour. Solutions may require replacing the trenches or, depending on slope and location, adding fill while staying within code requirements.

If effluent is surfacing in the drainfield area but not in one well-defined location, then once all the trenches have been identified and located, measure their length and width. Based on estimated daily sewage flow and soil sizing, determine whether the amount of trench installed should handle the daily sewage flow. If the size appears right, there are other potential problems to be identified or ruled out.

One problem that is usually easily identified is that the installation of some underground utility ran through or over the drainfield area. In the old days, this used to be primarily telephone lines or electrical lines run along lot lines and then brought in to serve the residence. These days, they can be the television cable or lines from satellite internet. Culprits could be the utility company personnel or, in the case of the latter examples, the homeowners themselves digging in the lines after acquiring service. This is an item to add to your checklist of things to discuss with the homeowner. Hopefully, the damage can be repaired by redoing the piping, moving the utility line and backfilling.

When the final landscape grading over and around the system was completed there may be too little soil cover over the top of the drainfield media, either at the ends of the trenches or where effluent is transferred to the next trench in sequence. There are a few places in the country that still allow use of earthen dam with pipe stepdowns, or perhaps the system you are looking at was “grandfathered” during a code change. These configurations are used to avoid installing dropboxes. In my opinion, this is a bad practice because one of the advantages of dropboxes is to provide access points in the system to evaluate performance and provide system management.

The problem arises because the invert of the pipe must be installed at an elevation equal to the top of the trench above so effluent completely ponds in the trench before flowing on to the lower trench. Often not enough cover (6 to 8 inches) is provided over these areas during final grading. Or if the installer has not done elevations correctly, each location provides the opportunity for effluent to surface. I have seen cases where there are several wet spots in the yard due to this problem. Bottom line is using dropboxes is a much better idea and they can be retrofitted to solve the problem.


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.



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