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How can you tell a drainfield is failing?

The signs of a failing drainfield are sewage odors outdoors, wet or spongy ground over the lines, bright green grass in dry weather, slow drains, gurgling pipes and backups. Thurston County also lists coliform or nitrate in a nearby well. Surfacing sewage carries germs, so keep people and pets away and call a county-certified septic professional. Replacing a drainfield takes a county permit.

Street to Drainfield · updated · sources listed at the end

Parallel stripes of greener grass running across a rural back lawn past a green septic lid
1Parallel stripes of greener grass running across a rural back lawn past a green septic lid AI-generated pictureLook for: a band of grass greener and taller than the rest of the lawn, running in straight lines away from the tank.
Muddy rural ground with a standing puddle beside a wire fence at sunset
2Muddy rural ground with a standing puddle beside a wire fence at sunsetLook for: dark, wet ground or standing liquid with a sewage smell at the low end of the drainfield.

What are the signs of a failing drainfield?

Thurston County's care and maintenance page lists them in one place:

  • odors, surfacing sewage, wet spots or lush growth in the drainfield area;
  • plumbing or tank backups, often a black liquid with a bad smell;
  • slow sinks and tubs, and gurgling in the pipes;
  • coliform bacteria or nitrate in a well test;
  • lush green grass over the field even in dry weather.

The Department of Health adds algae blooms in nearby ponds and lakes. One sign on its own is a reason to look closer. Two or three together, especially a backup plus a wet yard, point hard at the drainfield.

Is a sewage smell always the drainfield?

No. EPA's malfunctions page explains that odor can come from the roof vent that lets the system breathe, or from a vent blocked by leaves or frozen in a long cold spell, or from a downdraft. It also says the cause of an odor cannot be diagnosed remotely and recommends a septic service provider or plumber. A smell that comes with soft, wet ground over the field, especially without recent rain, is the combination EPA treats as a good indication of failure.

If the wet ground only appears after weeks of rain, read a drainfield in a wet winter as well.

Why is surfacing sewage a health risk?

Because it is untreated. The Department of Health's signs-of-failure page says sewage from a failed system carries pathogens and other contaminants that can make people and animals sick, and can reach groundwater, wells, streams and shellfish beds without anyone seeing it. The county's septic questions page says the same about your well and your neighbor's.

Keep children, pets and livestock off the area and do not walk through it. If sewage backs up indoors, EPA advises avoiding contact, contacting the local health department, and having cleanup done by people wearing long rubber gloves and face splash shields, then disinfecting with a mix of 90 percent water and 10 percent household bleach and letting the area dry for at least 24 hours.

Could the trouble be something smaller than the drainfield?

Sometimes. The Department of Health lists other common causes: a clogged pipe between house and tank, a blocked inlet baffle, or a clogged outlet baffle or effluent filter. Those can slow drains or back sewage up in ways that look like drainfield failure, and some are low-cost fixes. Every one of them means opening the tank, so leave opening lids to a county-certified pro, such as a pumper or monitoring specialist; never open one alone or leave one open, and never lean into or enter a tank; the county warns that tank fumes can be fatal. Where the lids are is covered in finding the septic tank lids.

Who do you call in Thurston County?

Start with a county-certified professional. The county asks owners who notice any of the signs to contact a qualified septic professional, and to contact Thurston County Environmental Health if the system fails; the department's numbers are on its septic systems page. Current lists of certified pumpers, installers and monitoring specialists are on the county's hiring a septic professional page.

A permit is required for serious repairs. The county's questions page names septic tank replacement, drainfield replacement and distribution box replacement, and asks owners to call before any repair to find out whether a permit applies.

What happens after a drainfield is found to have failed?

A new field, usually somewhere else. The county says that once a system has failed, pumping the tank will not solve it and a new drainfield has to go in a different location, normally the reserve area set aside on the permit. The Department of Health says a connection to public sewer should be considered where it is possible; in Olympia that can become a requirement, as switching from septic to sewer explains.

The county's 2026 fee schedule lists $1,300 for an on-site sewage system repair review (site application plus design review) and $490 for the construction permit, before a 5 percent technology fee. Design and installation costs are separate and come from your contractor. Loans and grants are covered in paying for septic repair.

Yours to do

  • Keep people, pets and livestock away from wet or smelly ground over the field.
  • Cut water use until a professional has looked.
  • Write down what you see, when, and after how much rain.
  • Get a copy of your record drawing from the county.
  • Ask about a well test if you drink from a private well.

Leave to a licensed or certified pro

  • A county-certified pumper or monitoring specialist opens the tank and checks baffles, filter and levels.
  • A certified designer and installer handle drainfield replacement under a county permit.
  • A plumber clears or replaces a clogged or crushed house-to-tank line.
  • Indoor sewage cleanup by people with protective gear, as EPA describes.

On the record

Common questions

Why is the grass greenest right over the septic lines?

Effluent rising toward the surface feeds it. In dry weather that strip is one of the failure signs the county and the Department of Health list.

Do septic additives bring a failing drainfield back?

The county says additives do not reduce the need for regular maintenance and have not been tested to show that they work. A failed field needs a professional diagnosis.

Can a failing drainfield contaminate my well?

Yes. The county says untreated effluent that reaches groundwater can contaminate your well and your neighbors' wells; ask about testing on the private well testing page.

How old is too old for a drainfield?

There is no fixed age. EPA notes that after 25 to 30 years the biomat in the trenches can thicken and reduce drainage, which can cause ponding or backups.

Does a renter have to report a failing system?

EPA says occupants are responsible for proper use, and the owner for repairs. A tenant who sees the signs should tell the owner promptly.

Sources

  1. Care & Maintenance of Your Septic System, Thurston County — signs of failure, who to contact, new drainfield location, tank fumes warning; read 6 November 2026
  2. Septic Frequently Asked Questions, Thurston County — permits for repairs, well contamination; read 6 November 2026
  3. Hiring a Septic Professional, Thurston County — lists of certified professionals; read 6 November 2026
  4. Environmental Health 2026 Fee Schedule, Thurston County — repair and construction permit fees; read 6 November 2026
  5. Signs of Septic System Failure, Washington State Department of Health — pathogens, smaller causes, sewer connection; read 6 November 2026
  6. Resolving Septic System Malfunctions, US EPA — odors, backups, cleanup and disinfection; read 6 November 2026
  7. Frequent Questions on Septic Systems, US EPA — biomat age, owner and occupant duties; read 6 November 2026

How this page was put together. This page draws on Thurston County Environmental Health's septic pages and its fee schedule in effect from 1 January 2026, the Washington State Department of Health's failure page and EPA's septic guidance, read on 6 November 2026. It does not estimate design or installation prices, because no agency publishes them, and it leaves any work inside a tank to certified professionals.

Next at this stopWhat can you do on top of a drainfield?