Watch Out for Changing Drainage Patterns

Over time, new driveways, excavation and cosmetic grading changes can lead to overloaded drainfields

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A few months ago, we wrote a column about the importance of paying attention to conditions outside the system installation and taking them into account during design and installation. At the end, we made a passing reference about paying attention to drainage conditions. Recently, we received the following from a reader:

“I have a property that has been in my family since the ’60s. ... It has always been a rental. In 2007, a serial distribution system was installed, as the original system was plagued with tree roots. The new system has never operated properly during heavy rains. It works fine otherwise. This year has presented exceptional rainfall, requiring the system to be pumped out many times. We are trying to locate the problem and make a permanent repair. There is much more information that has been gathered in an effort to reach a conclusion.”

This led to several exchanges back and forth. Gathering information involved digging up parts of the backyard to look at the septic tank outlet and the two seepage trenches. We asked why they did all that digging. In this particular area, we were told, it is not required to have inspection ports for the septic tank, dropboxes or seepage trenches. This is a situation where having access to parts of the system would have saved a lot of time and effort. We have always talked about this access as designing and installing for maintenance. It could also be called designing and installing for troubleshooting.


They discovered after excavating for tank access and at the outlet that tank water levels were elevated but no blockages were evident. It was also discovered that the trenches were filled with water. To us, it would appear that during heavy rain events the trenches are being filled with water and then backing up into the tank. What are some possible reasons for this?

In cases like this, we start by looking for the source of surface water flowing into the system and trenches during heavy rainfall events. One possibility is the roofs of the house or other buildings and where the gutters empty water into the yard. If they empty in the vicinity of the system, this can be rectified or minimized by directing flow from the downspouts away from the system. In some cases, this may involve actually burying the outlet and carrying water downslope past the system.

Another potential area is the driveway or adjacent streets. We have seen examples where road improvements have diverted water off the street and toward the drainfield area. Fixing this involves working with local government to route the water flow away from the onsite system.

We visited one site where the neighbor had rebuilt the driveway, laying down asphalt instead of gravel. It made for a great driveway, but resulted in dumping a lot more water downslope into the neighbor’s yard near their onsite system. The solution was to install a diversion to route runoff away from the system and into an area where it could infiltrate. Solutions like this are not always easy depending on neighborhood dynamics. It is also dependent on having someplace to put the water.

In the case of the rental property owners, building drains, driveway or road runoff did not appear to be the culprit. What other possible soil or site aspects could result in this situation? At another location we visited, we noted similar problems and were perplexed. We went back to the original design and the topographic survey of the lot and found something very interesting. On the initial topographic survey, there was a distinct drainageway that originated in an area upslope from the system.

The lot as viewed now showed no signs of the drainageway; it had been graded and filled during installation of the soil treatment trenches. Just because it was filled on the lower end, that doesn’t mean water will not move downslope, following the same path. This is exactly what was happening. During rains and spring runoff, water was flowing downslope in the soil into the trenches, causing them to fill and surface.


This led us to think about the soil condition in the area of the current drainfield. As we have stated many times, most system problems in drainfields can be traced to one of two things: a mistake in reading the soil condition in terms of seasonal or periodic saturation or permeability, or soil disturbance during installation (such as installing when the soil is too wet), resulting in reduced long-term acceptance rates.

Two common soil identification mistakes that can cause trenches to flood out during wet periods are: trenches installed into dense soil layers where the trenches act as a drain during heavy rainfall events or not recognizing evidence of periodic saturation in the soil. During times of heavy precipitation or spring runoff events, if trenches are installed into the more slowly permeable dense layer with a permeable layer above, the rock or other media-filled trenches act just like drain tile in the soil, filling with water. In this case, it flooded out the trenches. If the entire soil profile is saturated around the trenches, they simply cannot accept any more water, with backups and surfacing the result.

At the property in question, there was the double whammy of having a very slowly permeable layer below that is at or near saturation, with the water flowing through the more permeable surface horizons into the trenches. Identifying this condition by soil investigation was made difficult by the soils being very red in color due to high amounts of iron in the profile. If they don’t know the soils, this condition gives even very experienced site evaluators problems.


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