Taking a Deep Dive Into Shallow Systems

Finding the sweet spot for drainfield depth will ensure optimal treatment and a long life for your next installation

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Lately we have received comments and questions about the advisability of hauling in additional soil fill over the top of soil treatment areas as part of landscaping projects. If you have read our columns in the past, you know one of our most-quoted design and installation principles is KISS, or Keep It Shallow S——-.  You can fill in the last word of the acronym.

The answer to the questions about whether additional cover can be applied is: “It depends.” It depends on the general landscape shape in the area, the original soil characteristics including texture, structure and permeability, current depth of the system, the media used in the treatment area (rock, chambers, etc.), and the depth to limiting soil layers like water tables, bedrock, slowly permeable or dense soil layers. 

We thought it would be a good time to revisit some of the considerations for keeping systems shallow, and whether adding cover is a good idea.

One key to siting the soil treatment part of the system is to identify a location where water will not concentrate. Any additional surface or subsurface water can interfere with the ability of the soil to accept effluent. In the most extreme cases, the soil treatment area can be flooded or hydraulically overwhelmed. 

Watch drainage patterns

Since systems are designed to accept several hundred gallons of water per day, any water added into the system through runoff is a large negative. It will be important to ensure the re-landscaping effort does not change the current drainage patterns to direct more water to the area of the system. 

In general, the most permeable soil layers are located nearer the surface. This is due to increased levels of organic matter and better developed soil structure in horizons closer to the surface. Permeability relates to both movement of water and air. Higher permeability means a better long-term acceptance rate (LTAR), which results in the having a smaller-sized area and a better environment for the aerobic organisms in soil we rely on to provide treatment of effluent.

As a system is installed deeper, the LTAR is reduced, requiring a larger system. There is less oxygen exchange because it is more difficult to move air between the atmosphere and the soil around and under the sewage treatment trenches. It’s kind of a double whammy where soil structure is less desirable and the ability of the soil to accept effluent is less, but with less oxygen available, the biomat will tend to be thicker and more resistant. This further reduces the ability of the soil to accept effluent.

One other key to proper acceptance and treatment of septic tank effluent is the separation distance from the infiltrative surface to limiting soil condition. This can be depth to watert able, depth to bedrock or dense, slowly permeable soil layers. The 3-foot recommended separation distance from these conditions to be adequate depends on having enough oxygen available to keep the biomat from becoming too thick. 

If the system has already been installed deep, additional cover over the top will reduce further oxygen exchange. Even though the permeability characteristics of the soil may be good, the exchange may not be adequate to provide necessary oxygen. The result is lowered treatment efficiency and lower acceptance which may result in system failure.

The 4-foot rule 

For these reasons, our general guidelines for system installation is to have a maximum excavation depth of 4 feet. Of course, there is nothing magic about 4 feet, but it is based on the permeability issues above plus deeper excavations will bring the system closer to one of those limiting soil conditions that may further impact the ability of system to accept effluent. 

Given the fact landscaping around the system often occurs after system installation, the total depth of cover should never exceed 7 feet. This would be an absolute maximum and we never like to see this much cover. Keeping total cover to 4 feet or less is more desirable.

We are often asked how much of the effluent acceptance is through evapotranspiration versus downward through the soil. Evidence that evapotranspiration is a factor is indicated by the greener, lush-growing grass over the top of a system where the trenches are installed shallow. 

A few studies have found that where systems are installed 4 feet or less in depth, 20 to 40 % of effluent evaporates. Of course, we design for all of it to move downward, but the deeper the system the less evapotranspiration, putting more pressure on the system to move all the effluent through the infiltrative surface.

One final issue to keep in mind: It is one thing to add cover to a system where the trench media is rock versus chambers or other media. We have seen numerous examples where the depth of soil cover has exceeded the strength of the other media, causing premature failure due to deformation and soil subsequently filling in the trenches. If you are adding cover after the fact, it is important to know the maximum allowable cover for the media installed and the actual depth of excavation. 

For us, the bottom line is that applying additional cover over a system after the fact should be approached with caution. Before proceeding, research original soil and landscape characteristics, along with information about system media, to ensure the landscaping does not impact system performance. 


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