A percolation test measures how quickly the soil at a site accepts water, and the result determines whether a conventional septic system can be built and how large its drainfield must be.
How it is done
Holes are dug to the proposed depth of the drainfield trenches, in the area where the field would go.
They are presoaked — filled with water and allowed to stand, usually overnight — so the soil is at the condition it would be in during use rather than dry.
Water is then added and the drop in level is timed, over repeated fillings, until the rate stabilises.
The result is expressed as minutes per inch (MPI) — how long the water takes to fall one inch.
Reading the number
Under about 5 MPI — very fast. Sandy or gravelly soil. This can fail in many jurisdictions, because effluent passes through too quickly for the soil to treat it before reaching groundwater.
5 to 30 MPI — good. Conventional trenches, moderate field size.
30 to 60 MPI — acceptable in most jurisdictions, with a larger field.
Over 60 MPI — generally a fail for a conventional system. Clay, or compacted soil.
Exact thresholds vary by state and county, and the local health department's rules are the ones that apply.
Why the field size follows from it
Slower soil accepts less effluent per square foot per day, so more square feet are needed for the same daily flow.
Regulations express this as an application rate in gallons per square foot per day, tabulated against the perc rate or the soil texture. The design flow divided by that rate gives the required absorption area.
That is why a slow site needs a large field and a very slow site needs an alternative system entirely.
The soil profile matters as much
Many jurisdictions now rely on a soil evaluation by a qualified professional as well as, or instead of, a perc test.
That looks at texture, structure, and — critically — restrictive layers: bedrock, hardpan, or the depth to seasonal high groundwater, shown by mottling in the soil profile.
A site can perc acceptably and still fail because there is not enough vertical separation between the trench bottom and groundwater or rock. That separation, commonly a few feet, is what actually treats the effluent.
When and where it is done
In the area the field will occupy, not a convenient corner of the lot.
At the season the rules require, which in many places means during the wet season, when groundwater is highest — a test done in August can give a result that does not hold in April.
Before buying land, if the intention is to build. A failed perc test on an unimproved lot is the difference between a building site and a field.
If it fails
Test elsewhere on the lot. Soils vary over short distances.
An alternative system — a mound, a sand filter, an aerobic treatment unit, or drip dispersal — which can work where a conventional field cannot, at a substantially higher cost.
A holding tank, in some jurisdictions, pumped periodically. Expensive to operate and usually a last resort.
Preserving the site
The area intended for the drainfield, and the reserve area that most jurisdictions require to be held for a future replacement, must not be driven on, built on, or compacted.
Compaction destroys soil structure and can turn a passing site into a failing one. Protecting that area during construction is free and it is frequently not done.
Who performs it
Requirements vary: some jurisdictions require a licensed soil scientist or engineer, others accept a certified installer, and most require the health department to witness or to review the results.
A test performed without the required notification or witness is frequently not accepted, which is worth confirming before anyone digs.