A drainfield takes the clarified wastewater leaving the septic tank and disperses it slowly into the soil, usually through a network of perforated pipe in gravel or chamber trenches. The soil itself does the final treatment: as effluent moves through it, naturally occurring soil organisms and physical filtration remove pathogens and break down remaining organic material before it reaches groundwater.
The tank only handles settling; the drainfield is where the actual treatment finishes. Effluent leaving the tank still contains dissolved and suspended material and pathogens the tank cannot remove — that work happens as the effluent percolates through unsaturated soil, which is why the soil’s characteristics at a given site (not just its ability to drain water quickly) determine whether a conventional field will work there at all.
A field is sized from two things: the design flow the house will generate (from bedroom count) and the soil’s measured application rate (from a percolation test or a soil evaluation). More flow or slower-absorbing soil both mean a larger field footprint is required.
Because the field is doing biological and physical work, not just draining water, it can be damaged by things that would not obviously seem harmful — compaction from vehicles or heavy structures, root intrusion, or solids reaching it from a neglected tank. None of those show up immediately; field failure tends to be gradual until it suddenly is not.
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It disperses clarified wastewater from the tank into the soil, where soil organisms and filtration complete the treatment before the water reaches groundwater.
No, that's the tank's clarified output going in; the field's soil is doing real biological treatment as the water percolates through it, not just absorbing it.
The house's design flow (from bedroom count) and the soil's measured application rate (from a percolation test or soil evaluation) together set the required field size.
Guide last reviewed 2026-08-31.