How to take care of a septic drain field

People think of the septic tank as “the system”, but the tank only separates and stores solids. The treating happens in the drain field: a shallow, covered excavation in unsaturated soil, where liquid from the tank is released through pipes and filters down through the ground¹. As it moves through the soil, harmful bacteria, viruses and nutrients are removed¹.

Ohio State University Extension calls the soil “the most important portion of any septic system”⁷. It is also the part you can damage without noticing, because everything that hurts it happens at the surface: weight, water and roots.

Keep weight off it

Weight compacts soil, and compacted soil is less able to absorb and treat wastewater⁷.

• Don’t park or drive on it². Washington State’s health department extends that to heavy equipment and livestock, and to the tank and the replacement area as well as the field⁴.
• Don’t build on it. No decks, patios or swimming pools that cover the field or block access to the tank⁵. EPA says not to build any structure there without checking with a local designer or permitting authority³.
• Don’t pave or cover it. No concrete or asphalt³, and no plastic sheeting⁴.
• Be most careful when the ground is wet. Ohio State notes that working on wet soil compacts and smears it, which reduces its ability to absorb and treat wastewater⁷.

What to plant

Plant	Verdict	Why
Grass	Best choice	Washington State calls grass the best cover
Native grasses and ground covers	Good	EPA calls them the most suitable plantings
Other shallow-rooted plants	Fine	New York’s advice is grass and other shallow-rooted plants
Vegetable gardens	Not recommended	EPA advises against them on the field
Shrubs	Not recommended	EPA advises against them on the field
Trees	Keep well away	Roots grow into the pipes and clog the system

Sources: EPA, Washington State Department of Health, New York State Department of Health.

EPA says to plant trees far enough away that their roots can’t reach the system, and that a septic professional can advise on the distance². New York’s health department explains the risk: roots from trees and long-rooted shrubs can grow into the pipes and clog them⁵.

The verdicts in the table come from Washington State⁴, EPA³ and New York⁵.

Keep extra water away

A drain field can only take so much. EPA explains that if it is overloaded with too much liquid it can flood, sending sewage to the surface or back into toilets and sinks¹. Two kinds of water cause that.

Water that should never go there:

• Roof drains, sump pumps and other rainwater drainage. Keep them away from the field; excess water slows or stops treatment².
• Surface runoff from the driveway and the yard⁴⁶.

Water from the house, all at once:

• Doing all the laundry in one day can flood the field. Spread it over the week².
• A running toilet can add as much as 200 gallons a day². Ohio State lists ignored leaks among the common causes of septic malfunctions⁷.
• Using more water than the system was designed for, for example after adding people or bathrooms, has the same effect⁷.

After heavy rain or flooding the field needs extra care. See what to do with a septic system in heavy rain or a flood.

Keep solids out

The tank exists to keep solids away from the field. When it isn’t pumped, EPA warns that a tank full of solids may eventually clog the drain field³. A garbage disposal sends extra solids the same way and can thicken the layer of buildup in the field³.

Two guides cover this side of it: how often to pump a septic tank and what not to flush with a septic system.

Septic pumping calculator Estimate years between pump-outs from your tank size and household, using extension tables.

Protect the replacement area

Many properties have a second patch of ground set aside for a future drain field. In North Carolina, most sites permitted since the early 1980s are legally required to have one, and NC State Extension says to keep it free of buildings, additions, pools and digging⁶. Washington State gives the same protection to the “drainfield replacement area” as to the field itself⁴.

Check your permit before you plan a garage, a pool or a driveway. Building on the reserve can leave you with nowhere to put a new field when the old one wears out.

Keep an eye on it

• Walk it now and then. Wet or spongy ground, sewage odors or grass that is unusually green in dry weather are warning signs. Our guide to the signs of septic system failure explains what each one means.
• Ask about inspection ports. Ohio State recommends inspection ports and risers so the lines and the tank can be checked each year without digging⁷.
• Know its age. After 25 to 30 years the buildup in the soil can thicken enough to cause ponding or backups, and EPA suggests planning an upgrade before it becomes an emergency³.