Short definition
A drainfield (also leach field, soil absorption system, or SSAS in Washington terminology) is the network of perforated lateral pipes laid in gravel-filled trenches that disperses partially-treated septic effluent into the surrounding soil for final treatment. It’s the disposal half of a standard anaerobic septic system, and in Washington it’s where most expensive septic failures happen.
What it is
After effluent leaves the septic tank’s outlet baffle and passes through the distribution box, it flows by gravity into 4 to 6 perforated laterals laid in gravel-filled trenches at controlled depth — typically 18 to 36 inches below grade. Effluent seeps out through the perforations into the gravel and the surrounding soil; aerobic bacteria in the top 2 feet of soil complete the biological treatment.
The drainfield is sized by soil percolation test results, expected daily flow (gallons per day per bedroom), and the reserve area required by WAC 246-272A. Setbacks are strict: typically 100 feet from a well, 30 feet from surface water, 5 feet from a property line — though shellfish-protection counties have stricter numbers.
Why it matters to a homeowner
Drainfield failure is the expensive end of septic ownership. New install: $5,000–$15,000+ depending on size and soil. Replacement after failure: $10,000–$30,000+ — and only viable if the reserve area is intact. If solids overflowed from the tank into the laterals (the result of deferred pumping), the damage is usually permanent.
The two practical things to know:
- Don’t drive over, park on, or pave the drainfield. Soil compaction crushes pipes and damages the biomat layer that does the treatment.
- Don’t plant trees or vegetable gardens over the field. Roots invade laterals; root vegetables in effluent are a food-safety problem.
When you’ll encounter this term
- Buying rural WA property: ask for drainfield location, age, last inspection date, and whether the reserve area is documented. Walk both areas before closing.
- Drainfield “breakout” — wet or unusually green patches in winter — is an immediate concern; redirect downspouts and schedule an inspection.
- Building a deck, addition, or ADU: the project cannot encroach on the drainfield or the reserve area.
Common variants / not the same as
- Drainfield vs. leach field. Synonymous in US usage.
- Drainfield (septic) vs. drainfield (rainwater drywell or soakaway). Rainwater systems use similar gravel-and-perforated-pipe geometry but for clean stormwater. Different code chapter.
- Standard drainfield vs. mound system. A mound is engineered above-grade for high-water-table or shallow-soil sites. A drainfield is in-ground.
- Drainfield vs. sand filter. A sand filter sits between the tank and the drainfield, providing additional treatment for poor soils.
Common failure modes
- Saturation during heavy WA winter rain → effluent surfaces in the yard (“breakout”).
- Solids overflow from the tank (deferred pumping) → laterals clog → entire field fails. Often permanent.
- Driving or parking over the field → soil compaction; pipe crushing; biomat damage.
- Tree roots invading laterals over years.
- Biomat overgrowth at the soil-effluent interface → reduced infiltration; slow failure.
Washington note
About 1.1 million on-site sewage systems statewide — the drainfield is WA’s default disposal mechanism. Concentrated in Mason, Kitsap, Jefferson, Clallam, San Juan, Skagit, Island, and rural Snohomish counties — outside the Puget Sound urban core. WAC 246-272A requires every new system to designate a separate reserve area of equal or greater size for future replacement; that reserve must remain undisturbed and meet the same setback requirements as the active drainfield. The reserve-area rule is one of the WA-strongest septic permit points and binds the property for its life.