Published December 2007

The Grass is Always Greener ...

By Roger E. Machmeier, Ph.D. (page 108)

... Over the septic system drainfield if you effectively utilize shallow trenches, says the Answer Man.

I started installing shallow trenches with no more than a foot of dirt over the drainfield rock. The trenches are working fine, but now some of my customers are complaining about green strips of grass on their lawn. We had a pretty dry summer here. What should I tell my customers about their systems?

First of all, let me say that I always enjoyed the writings of Erma Bombeck. One of her books was The Grass is Always Greener over the Septic Tank. But dear Erma was a little wrong since, in reality, the grass is always greener over the drainfield. But that probably wouldn’t have been as catchy a title for her book.

I am pleased you are installing trenches with a small amount of earth cover. And those trenches are doing what they should to treat sewage tank effluent. I presume you have placed geotextile fabric over the top of the trench rock. This prevents the soil cover from filtering into the rock. I also presume you are using sequential distribution so the liquid level is at the top of the rock in the trenches.

Capillary action is now taking place in the soil above the rock layer in the trench. The attraction of the dry soil to water raises the liquid upward to the grass roots. Grass loves water and the grass is also getting some nitrogen from the effluent. The water and the nutrient nitrogen make the grass grow nice and green.

When we moved to our lake home many years ago, the onsite sewage treatment system needed some additional trenches. The trenches were installed with no more than 12 inches of soil cover. We did not sprinkle our lawn. So when the lawn grass turned brown from lack of rainfall, the green strips were very obvious. At first I thought only the water was helping the grass grow.

When city sewer became available, we were forced to connect and I reluctantly had to abandon our onsite system, which was working just fine. The trench area continued to show green during dry weather for several years.

LESS SYSTEM AREA REQUIRED

There was obviously no moisture from the abandoned trenches, but the green color where the trenches had been located convinced me nitrogen was still present in the soil. The use of the nitrogen by the grass is certainly more environmentally friendly than letting it leach downward to the water table.

Grass uses water in the soil to grow. The blades of grass transpire during the growth process and release water vapor. This liquid moves upward into the atmosphere. So there is less liquid moving downward to the water table from the onsite system.

Another benefit of shallow trenches is that less bottom and sidewall area is required for the system. Trenches with a large amount of soil cover have all of the liquid moving downward. Thus, more trench area is required to be in contact with the soil.

When there is sufficient rainfall to keep the entire lawn green, the extra green color over the drainfield trenches won’t be so obvious. But when the season is dry, the grass over the trenches will remain green.

I hesitate to suggest sprinkling the lawn to keep it consistently green, but that is what a lot of homeowners do. Many lawns have underground sprinkler systems installed. Some lawn sprinkler systems are programmed to apply water every so many days. The sprinkling takes place whether or not rain has fallen on the lawn.

If there is more water in the soil than the grass can use, the excess water must percolate downward. The extra water reduces the capillary attraction of the soil for the liquid effluent in the drainfield trenches. So the onsite system becomes less efficient. Too much water can be a problem for an onsite sewage system.

MESSAGE TO HOMEOWNERS

What should you tell your customers? First of all I would tell them that you have installed an efficient and an environmentally friendly sewage treatment system for them. Tell them they will always have nice green grass over their drainfield trenches. And the green strips will be most apparent when there is not enough rainfall to keep the entire lawn green. Tell them how the grass uses the water and nitrogen to grow lush and green. Tell them their trenches use less bottom and sidewall area than deep trenches. (You can also tell them that you have followed my suggestions and they can write to me with their concerns, if they have any after your explanation.)

If your customers want to sprinkle their lawns, caution them about using too much water. Point out that the extra water from the sprinklers will interfere with the water coming out of the drainfield trenches into the soil.

I am pleased you have started to install trenches shallow enough to promote the upward movement of moisture. I wish this were a standard practice in more regions of our country. I have received reports of trenches in Southwestern states being installed with five feet of cover. (And these are called shallow trenches!) I suspect the reason is that the sewer pipe from the basement flows into a deep septic tank. Then the effluent flows to even deeper drainfield trench lines.

The onsite sewage treatment systems in typically dry states would be much more efficient with a soil cover of only 6 to 12 inches over the top of the trench rock. The moisture would be used to grow grass and be released to the environment. Less and less liquid would percolate downward. A dosing station would be needed to keep the trenches shallow.

Keep up the good work and continue to install those shallow trenches. Tell your installer friends about them, too.