Drain Field

How Drain Field Works

The drainfield is a network of perforated pipes (or "laterals") laid in gravel-filled trenches or beds. After solids settle in the septic tank, the liquid wastewater (or effluent) is discharged, either by gravity or pressure, to an absorption field, also known as a drainfield or leachfield (see diagram of septic system/drainfield layout). NOTE: In most gravity systems the wastewater first flows into a distribution box (d-box) or tee, which then disburses the effluent equally among the trenches in the drainfield, which is where the final treatment takes place.

Diagram of a drainfield

Effluent trickles out of the pipes, through the gravel layer, and into the soil where further treatment occurs. The soil filters the wastewater as it passes (or "percolates) through the pore spaces and the soil microbes treat it before it eventually enters the groundwater. These processes work best where the soil is somewhat dry, permeable, and contains plenty of oxygen for several feet below the drainfield.

The drainfield is generally located in a stretch of lawn in the back or side yard of a property. The size and type of drainfield depends on the estimated daily wastewater flow and local soil conditions.
The Soil

The soil below the drainfield provides the final treatment and disposal of the septic tank effluent. After the wastewater has passed into the soil, organisms in the soil treat the effluent before it percolates downward and outward, eventually entering ground or surface water. The type of soil also impacts the effectiveness of the drainfield; for instance, clay soils may be too tight to allow much wastewater to pass through and gravelly soil may be too coarse to provide much treatment.

Replacement (Reserve) Area

Every new home or building served by a septic system is required to have a designated replacement or reserve area. This is a designated area suitable for a new drainfield and must be treated in the same manner as your existing drainfield. (A reserve area should have been designated as part of the permit process for any sewage system installed since 1980.)

Once a septic system has failed, it is too late to solve the problem by pumping your tank. A new drainfield will have to be installed at a different location. This is why it is important to know where the replacement area is located and how to protect it (see "Drainfield Do's and Don'ts" for replacement area care). For additional information, see Landscaping Your Drainfield or Locating Your Drainfield.

Drainfield Do's and Don'ts


Do These Things

    Know where your drainfield and replacement area are located. When you know where it is located, it is easier to protect. For more information, see Locate Your Drainfield.
    Keep heavy equipment off your drainfield. Cars and heavy equipment should not park or drive over the drainfield; doing so can crack pipes. Create a barrier if accessible to cars, livestock, or heavy equipment.
    Keep water usage to a minimum. Drainfields do not have an unlimited capacity. When there is more water than it can absorb, the system is unable to drain and filter effluent before it reaches groundwater.
    Divert water away from the drainfield. Water runoff from roofs and drainage ditches can saturate the soil. Drainfields are most efficient when the soil beneath the drainfield is not saturated.
    Keep trees and shrubs at least 30 feet away from the drainfield. (NOTE: Some soil conditions may require that plantings be kept an even greater distance from the drainfield.) Trees and shrubs generally have extensive root systems that seek out and grow into wet areas, such as drainfields. This can lead to clogged and damaged drain lines. For more information, see Landscape Your Drainfield.
    Plant only grass or shallow-rooted plants over the drainfield. This will prevent soil erosion.
    Protect your replacement area. It may be the only area with acceptable soil conditions in case you need to replace, repair, or add on to the drainfield. All of the above suggestions apply to the replacement area as well.

Don't Do These Things

    Don't build over your drainfield. This includes patios, carports, and other structures. You may damage the drainfield.
    Don't pave over the drainfield. Drainfields need air to function properly. Oxygen is needed by bacteria to break down and treat sewage.
    Don't dig in your drainfield. Damage to the pipes can occur.
    Keep large animals and livestock off the drainfield. Soil compaction prevents oxygen from getting into the soil and prevents water from flowing away from the drainfield.
    Don't use landscaping plastic over the drainfield. Air is necessary for the drainfield to function efficiently.
    Don't plant a vegetable garden over a drainfield. You risk the possibility of food contamination.
    Don't install an irrigation system in the drainfield. Neither should the irrigation system drain toward the drainfield.

Is Your System Failing... Warning Signs

If you notice any of the following signs of a potential failure or if you suspect your septic system may be having problems, contact a qualified septic professional for further diagnostics. Should your septic fail, contact Thurston County Environmental Health at 360-786-5490.

    Odors, surfacing sewage, or wet spots in the drainfield area.
    Plumbing or septic tank backups (often a black liquid with a disagreeable odor).
    Slow draining fixtures.
    Gurgling sounds in the plumbing system.
    If you have a well and tests show the presence of coliform (bacteria) or nitrates, your drainfield may be failing.
    Standing liquid over the drainfield, even during dry weather. This may indicate an excessive amount of effluent is moving up through the soil, instead of downward.