Perc Tests and Soil Evaluation

The test that decides everything

Before a system can be designed, someone has to establish how fast the soil at the field location accepts water and how far it is to groundwater or rock. That answer sets the application rate, which sets the absorption area, which — along with what the site cannot support — sets the system type and most of the cost.

Percolation test

Holes are bored to trench depth, pre-soaked so the soil is at field capacity, then filled with water. The drop is timed and reported in minutes per inch. Fast rates mean sandy, permeable soil; slow rates mean tight soil that needs a much larger field.

Perc rate	Reading	Implication
Under 5 min/inch	Very fast	May need extra separation to groundwater
5 – 30 min/inch	Good	Conventional gravity field
31 – 60 min/inch	Moderate	Larger field, still conventional
61 – 90 min/inch	Slow	Very large field or an alternative system
Over 90 min/inch	Failing	Mound or aerobic, usually

Soil morphology evaluation

Many states have moved away from perc tests toward a soil profile evaluation. A licensed evaluator digs a pit, reads the horizons, and records texture, structure, and the redoximorphic features that mark the seasonal high water table. It is more reliable, because a single hole full of water tells you very little about what the soil does across a wet spring.

Getting a useful result

Test where the field will actually go, not where it is convenient to dig.
Test in the wet season if your state allows it — that is the condition that governs.
Have the reserve area evaluated at the same time. You will need it on the permit.
Do this before you buy. A failed evaluation on a lot you already own is an expensive surprise.

If the site fails

A failure is rarely absolute. The usual paths forward are a mound system, an aerobic treatment unit with a reduced field, pressure distribution to spread the load, or relocating the field to a better part of the parcel. All cost more. None are impossible.

Size a field from your perc rate →