Septic System Drain Fields: Where the Grass is Greener

Septic system drain fields are the place where, if the grass is greener, you might have a problem.

The most important aspects of drain fields are the permeability of the soil and the system design. Trenches should not be overworked, or they will fail.

The regulatory agency usually locates and sizes septic system drain fields. The regulatory agency, or sometimes a soil scientist, will test the ground with a percolation test, commonly called a “perc” test.

The perc test determines the absorption rate of the soil. The regulatory agency will also review other factors, such as system capacity, the ground slope and the depth of bedrock, before establishing the size and location of septic system drain fields (also called leach fields, disposal fields, or the soil absorption system).

Generally speaking, the larger a septic system drainfield, the more capacity it has. A larger system provides a measure of safety against errors as well as a margin for future increased use (a house built with a family of four that, when sold, will hold a family of six will require a bigger system than originally thought, for example.)

In conventional gravity septic systems, solid piping leads from the septic tank to the distribution box. From the distribution box, the effluent runs via gravity into the drain field.

The drain field in a conventional system is a construction of trenches with piping. The drainfield trenches, which usually are not longer than 100 feet, should run in a straight line. The trenches are placed in land that “perced” properly, as defined by the regulatory agent.

The drain field should not be constructed in areas known for standing water, nor should it should be near drinking water wells, streams, lakes, property lines, roads, water-carrying ditches, etc.

The septic installer will either dig the trenches by hand (back breaking work) or use a machine such as a trencher or backhoe. During the digging process, the trench depth must be kept in mind, so a yardstick or ruler is handy.

Trenches are generally about 18 inches wide, with a flat bottom. Piping in septic system drain fields should be placed one foot or more below the surface, but again, this is often determined by the local regulatory agency. Some localities may require depths of three feet or greater.

The trenches and piping are generally laid out in parallel lines, something like the tines of a fork, with the distribution box sitting at the head of the design.

When everything is in place, the septic installer will fill the trenches with gravel, up to about six inches from the top. Over this, the installer will place a layer of tarpaper or other fabric barrier. This keeps the soil out of the gravel. The fabric will be covered with soil, which is mounded. The earth eventually will compact and the mounds will not be visible.

Septic system drain fields provide both disposal and treatment of the septic tank effluent. The drainfield is where the main purification process takes place, a result of biological action. The liquid eventually evaporates, is used by plant life, or flows into the groundwater as a renewed resource.

If the grass is greener over the drainfield, it is not necessarily a sign of problems, but it does indicate that the effluent is rising to the top instead of seeping into the ground
where it belongs.


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