By Jim Anderson, Ph.D., and David Gustafson, P.E. reprinted with permission

Take care to use the proper fill materials on top of your systems, and be sure to use quality topsoil to provide a seedbed for grass cover 

In previous articles, we have discussed the importance of locating an onsite system in an area where you can establish a good grass cover. This is especially important in cold regions, because the grass cover allows snow to accumulate and provide natural insulation against freezing.

It is also important to protect the area from any vehicle traffic such as the parking of cars or recreational vehicles, including four-wheelers and snowmobiles. This prevents compaction that could reduce oxygen exchange. It also prevents the penetration of frost to a depth that it will interfere with system operation. We have also discussed the need to locate the system so that it is not subject to surface water runoff from any sources, such as driveways, roofs and patios.

Quality cover

For typical systems with soil treatment trenches and gravity distribution, the cover over the top of the system consists of spreading the material excavated from the trenches. If this is what you are considering, then recognize that you need to prepare a proper seedbed to enable grass to grow.

It is not enough just to backfill the area, especially if the soil you excavate and spread leaves large clumps on the surface. The clumps indicate voids in the backfill over the system. That can result in excessive water infiltration into the system and create pathways when temperatures are cold for freezing to occur.

We cannot begin to count the times we have seen sites where the installer did an inadequate job of finishing the system, and as a result the system had problems. Often, you must apply additional topsoil over the system to provide the seedbed.

One thing to remember is that soil you excavated from the trenches has been replaced by rock or some other drainfield media. You need to consider this when determining the amount of additional topsoil you require for the seedbed. You can determine the additional topsoil you need by making a simple volume calculation. The lawn area covered by the system in square feet, multiplied by the depth of topsoil required in feet, gives you the cubic feet of soil required.

So, if the system has three 100-foot-long trenches, 8 feet on center, the area would be roughly 24 feet x 100 feet = 2,400 square feet. Now, if you want 6 inches of topsoil over the area, that would be 2,400 square feet x 0.5 feet = 1,200 cubic feet of topsoil. Dividing 1,200 cubic feet by 27 cubic feet per cubic yard gives you 45 cubic yards of soil needed.

Choosing your cover

We’re also asked what type of cover to use on a mound. For mounds, a sandy loam soil should be placed on top of the geotextile fabric over the rock to a depth of 12 inches in the center of the mound, and to a depth of 6 inches on the sides. The purpose of this sandy loam cap is to avoid undue soil compaction, so that the pore spaces are maintained, and soil air and moisture can move freely. You can then cover this with a loamy topsoil.

One important note about topsoil: It should be a mineral soil with about 4 to 5 percent organic matter, rather than excavated peat or organic soil. When applied as top dressing, peat will dry out and break down or decompose, leaving inadequate soil cover. Grass then will have a hard time establishing itself, and when the peat dries, the grass will not survive. The peat can also be blown away, reducing soil cover.

If you follow these examples, your customers will appreciate the finish of the system, and the system will perform better over the long term.