What is a drain field, and what not to do on it

The drain field is the part of a septic system you can’t see and the one that costs the most to replace. Most of what keeps it healthy comes down to what you do, and don’t do, on the patch of lawn above it.

What a drain field is

After wastewater settles in the septic tank, the liquid in the middle flows out to the drain field. The EPA describes it as a shallow, covered excavation in unsaturated soil. In most homes that means perforated pipes laid in trenches, often in gravel or plastic chambers, under the lawn.

Water seeps out of the pipes and filters down through the soil. That soil does most of the treatment: the EPA says it removes harmful bacteria, viruses and some nutrients before the water reaches groundwater. NC State Extension notes that a thin biological layer, the biomat, forms just under the trenches and helps with that work. For the full picture, see how a septic system works.

People also call it a leach field, a leach bed, a soil absorption field or a drainfield. They all mean the same thing.

What a drain field needs

To keep treating water, the soil needs three things:

• Air. The bacteria in the soil need oxygen, so the soil can’t be compacted or sealed over.
• Not too much water. A soil that is already soaked can’t take more. The EPA warns that a flooded drain field can push sewage to the surface or back into the house.
• No solids. Sludge and scum that escape a full tank clog the pores in the soil. That is why tank pumping protects the drain field.

What not to do on a drain field

The same rules apply to the reserve area, if your property has one. That is the spot set aside on the permit for a future replacement field, and Washington State’s health department advises keeping it just as protected.

What you can do on it

Grass is the best cover, according to Washington State’s health department. UF/IFAS Extension recommends shallow-rooted turfgrass and says the grass over the drain field should not be fertilized. Low flowers and ground covers work too.

Mowing is fine and is part of keeping grass healthy. Walking and light yard use are OK too. Keep play equipment light and avoid digging deeper than a few inches, since UF/IFAS notes the pipes can sit just a few inches below the surface in places.

If you are planning a pool, an addition or new landscaping, find the drain field first. Your county health department may have the system drawing on file, and a septic company can locate it.

Signs the drain field is struggling

• Soggy or spongy ground, or standing water, over the field when it hasn’t rained.
• Grass that is much greener and thicker over the field than elsewhere, in dry weather.
• Sewage odors outside.
• Slow drains and gurgling throughout the house, or backups.

Our guide on signs your septic tank is full explains how a full tank leads to these problems.

Can a treatment fix a drain field?

No. No additive, bacteria product or pod repairs a damaged drain field. Crushed pipes, roots, compacted soil, a clogged biomat or a field that sits in saturated ground need a septic professional’s diagnosis. Sometimes a soaked field can dry out and recover once the extra water is removed, as Washington State’s health department notes. Other times the field has to be repaired or replaced.

If sewage is surfacing in the yard, keep children and pets away from it, cut back on water use, and call a licensed septic company. Never open the septic tank and lean in, and never climb inside: the gases inside can kill a person very quickly.

How long a drain field lasts

A well-cared-for drain field lasts many years. The EPA’s septic FAQ says that after about 25 to 30 years the biomat can thicken enough to slow drainage, and suggests planning an upgrade before an emergency at that age. Neglect shortens that a lot.

Pumping the tank every 3 to 5 years, having it inspected at least every 3 years, and keeping water use spread out are the habits that protect the field. After a big storm, see our guide on septic systems after heavy rain.