Question:

I have been in the septic business (in Alabama) for 25 years. I hold a master level II license. I installed a mound a year ago and in about eight months a couple of wet spots appeared. The area is low and holds water in spots for a day or so after a rain.

I installed chambers in the mound system. The dirt rose up into the chambers. I uncovered all 400 feet and all elevations were OK. The designing engineer did a perc test on the mound after the chambers were removed. His perc test holes filled with soil.

We left the mound open for a while and the mound has not dried out. Also, this morning I dug seven 5-foot observation holes and asked the engineer to look at them and see if he could suggest something.

Each hole was fairly consistent. About 8 inches down, the soil was dry as you would expect. From that point to the original soil it was wet enough to make a mud-ball. The original soil was damp as if you pulled a piece of plastic off the ground and had the dampness from the condensation. I was there a couple hours to see if any groundwater came in and none did. I did hit an old gravel field line that had water in it. It was about 3 feet below the original ground.

Answer:

First of all, let’s not refer to the system you have described as a “mound’’ sewage treatment system, even though it may physically appear to be mounded. It is not a sewage treatment mound, but it is a seepage bed, which has been improperly placed on fill soil.

A soil in its natural location has a characteristic the soil scientists call “structure.” Structure develops in soil over many, many years. There are void spaces, there are worm and root channels, and there are soil aggregates, etc., in natural soil. The coarser the soil texture, the larger the void spaces to conduct liquid. That is the reason sandy loam soil has a faster percolation rate than clay loam soil.

Disturbed soil

When natural soil is moved to another location, those “channels” that have formed naturally allowing water to move downward are severely compacted and essentially destroyed. This will happen no matter how careful one may be in placing the fill soil. Also, it is likely equipment will be used to spread the fill soil, and the weight of the equipment also contributes to the compaction.

In Minnesota, we conducted percolation tests on sandy loam soil, which had been placed as fill. A developer wanted to use fill area for a soil absorption system. As I recall, the sandy loam soil, in its natural location, had a percolation rate a little faster than five minutes per inch. The fill was carefully placed, but to spread it out over an area, equipment had to be used. The fill soil was allowed to settle for one year.

The percolation rate of the fill was tested one year later. The percolation rate of the sandy loam soil placed as fill was found to be slower than 120 minutes per inch. The conclusion was the sandy loam soil had been severely compacted and the soil structure had been destroyed. The fill soil was considered unsuitable for any soil absorption system.

As a result of those tests on sandy loam fill soil, the Code for Individual Sewage Treatment Systems in Minnesota will not allow any soil absorption system to be placed on fill soil.

If the original soil at the location where you placed the fill had a percolation rate faster than 120 minutes per inch and did not show any evidence of seasonally saturated conditions to a depth of at least 2 feet, then a sewage treatment mound could have been designed and installed.

However, you stated, “The area is low and holds water in spots for a day or so after a rain.” You also stated, “I did hit an old gravel field line that had water in it. It was about 3 feet below the original ground.” This information indicates the area has a seasonal high water table and had been tile-drained in the past. An area with such soil and drainage conditions is not a suitable location for any soil absorption system.

Find a better site

The engineer who designed the system should have located a suitable site with natural soil, such as described earlier. Then the engineer should have designed a proper sewage treatment mound using clean sand, just as is specified in the Minnesota State Onsite Sewage Treatment Code.

There are specific procedures for proper sewage treatment mound construction, including careful preparation of the original soil surface, the use of “clean” sand over the original soil, the use of equipment with low earth pressure, absolutely no heavy equipment on the original soil or any part of the mound, placement of a rock bed layer over the clean sand, and the use of a pressure distribution system over the rock layer to evenly distribute each dose over the entire rock bed. The entire sewage treatment mound, including the side and end dikes, is covered with 6 to 12 inches of topsoil upon which grass vegetation is established and maintained.

I would hope the State of Alabama has the specifications for proper sewage treatment mound construction in their Rules and Regulations for Onsite Sewage Treatment Systems. I would also hope they do not allow the placement of seepage beds or other types of soil absorption systems on fill soils.

Poor system choice

Based on the information you sent, I suspect the failure of this raised bed system is because of the fill soil upon which the chambers were placed. The fill soil became totally saturated when effluent was applied and lost all of its structural stability. You explained that after the chambers were removed and some drying took place, the fill soil was still saturated deeper than 8 inches and the underlying soil was just damp. This indicates the fill soil is not draining and the liquid is not moving downward into the underlying natural soil.

In my opinion, this existing fill soil cannot be used for any type of soil absorption system. Adding more liquid to the fill soil will likely cause the same result you experienced.

From what I understand based on your description, there are two apparent problems: the location was not suitable because of low elevation and drainage problems, and the chambers were placed on fill soil that became compacted and had no internal structure.