People say "perc test" to mean the whole soil evaluation for a septic system, but in New Jersey that evaluation is really two things: soil logs, where someone digs a deep pit and describes what is in the ground, and permeability testing, which measures how fast water moves through it. Together they decide whether a system can go on your lot, where, and what kind. If you are buying land, building a house or replacing a failed system, this is the step that sets everything else. Here is what happens and who does what.
Why the soil matters so much
A septic tank only settles solids. The real treatment happens in the soil under the disposal field, the part most people call the leach field. The soil has to be deep enough, drain at the right speed, and stay unsaturated long enough to treat the wastewater before it reaches groundwater. Soil that is too tight will not accept the water. Soil that is too coarse passes it through too fast to treat it. A water table or rock that is too close to the surface leaves too little soil to do the job.
New Jersey's rules for this are in N.J.A.C. 7:9A, the Standards for Individual Subsurface Sewage Disposal Systems. The rules are state-wide, but they are administered by your local health department.
What are soil logs?
A soil log is a written description of a soil profile pit. An excavator digs a pit where the disposal field is planned, and the engineer's team describes each layer: its texture, color, structure, how firm it is, how much stone it contains, and where rock or water shows up.
Under N.J.A.C. 7:9A, at least two profile pits are required for each disposal field, located at either end of the field or within 15 feet of it. In some cases, three soil borings may replace the second pit if the soils match. Profile pits are dug, where possible, to at least 10 feet below the surface, or to solid bedrock if it is found first.
The pit is where most problems are found: shallow rock, a layer of dense clay, or signs of seasonal saturation.
What is a perc test?
A percolation test is one way to measure how fast water moves through the soil. A small hole is dug into the layer being tested, soaked, and then the time it takes for the water level to drop is measured. The result is reported in minutes per inch.
It is not the only option. The rule lists several permeability tests, including the tube permeameter test, the soil permeability class rating, the percolation test, the basin flooding test, the pit-bailing test and the piezometer test. The engineer chooses the test based on the purpose and the soil conditions.
For a perc test used to design the disposal field, the rule sets limits at both ends. If any measured rate is slower than 60 minutes per inch or faster than 3 minutes per inch, the application is not approved as tested. Slow soil is considered hydraulically restrictive. Very fast soil is considered excessively coarse.
Who performs and witnesses the tests?
Three roles are involved:
- A licensed professional engineer. All soil evaluation used to design a system must be carried out by, or under the direct supervision of, a New Jersey licensed professional engineer. The engineer, or their field staff, logs the pits and runs the tests.
- The local health department. The administrative authority, which is your local health department, or its agent witnesses the profile pits, borings and permeability testing. It can require up to 15 business days of written notice before testing. It can waive witnessing, and if it does not show up after proper notice, that counts as a waiver.
- The excavator. Someone has to dig the pits with a machine, to the depth and location the engineer calls for, and then backfill them properly afterward. That is where we come in: we dig the test pits.
Seasonal high water table: why the calendar matters
The rule cares about the highest level groundwater reaches during the year, not the level on the day of the test. That is the seasonally high water table.
Usually it is read from the soil itself. Soil that is saturated for part of the year develops mottling, blotches of gray and rust color. Where mottling is present, the seasonally high water table is taken at the highest level it appears, at any time of year.
Where there is no mottling, there are two options. Water levels can be measured directly in the pits or borings during January through April only. At other times of year, the depth can come from the county soil survey, as long as the soil on site is matched to the published soil series. On disturbed ground, direct observation in January through April is the only method allowed.
There is one more seasonal limit. Percolation tests cannot be done in frozen ground, or in holes left open more than three days. So winter can make some tests impossible and others required. If your lot may need a direct water table reading, plan around that January through April window.
What does it mean if the soil fails?
A failed test does not always mean you cannot build or replace a system. It means a standard design will not work at that spot as tested. The options usually include:
- Testing another spot. Soil can change a lot across one lot.
- A different system type. The rule allows soil replacement and mounded disposal fields, which bring in suitable fill to create the treatment zone the natural soil lacks. These cost more and need more room.
- Removing a restrictive layer, in some cases, and replacing it with suitable fill.
- A nonconforming design or other approvals. Where a standard design cannot fit, the local health department has some discretion, and in some cases NJDEP gets involved. Our septic installation walkthrough explains those routes.
In some areas, lot size and septic density matter too. We cover that on septic density in Sussex County.
This is general information, not legal advice. Your engineer and local health department decide what testing and design your property needs.
Next steps after the tests
If the soil passes, the engineer designs the system, the health department reviews it and issues a permit, and then the installation can be scheduled. If you are buying a house with an existing system, that is a different process, a septic inspection, and it is done by someone else.
When you are ready to build, our septic installation page explains what we do.
Need test pits dug?
If your engineer needs test pits dug for a perc test and soil logs, or you want a septic system built from an approved design, call us at 862-268-2510 or send the details through our free estimate form. We dig the test pits to the engineer's layout, work around the health department's witnessing schedule, and backfill the pits properly so they do not create a problem under your future field.
Common questions
What is the difference between a perc test and a soil log? A soil log describes the layers in a deep profile pit dug where the disposal field will go, including texture, color, rock and signs of water. A perc test measures how fast water drains through a specific soil layer, in minutes per inch. New Jersey septic design uses both.
Who does a perc test in New Jersey? Soil evaluation for septic design must be carried out by, or under the direct supervision of, a New Jersey licensed professional engineer. The local health department witnesses the pits and tests unless it waives witnessing, and an excavator digs the pits.
Does JWSR dig test pits for perc tests? Yes. We dig the test pits for perc tests and soil logs to the engineer's layout, and backfill them properly afterward.
What perc rate fails in NJ? Under N.J.A.C. 7:9A, if any perc rate measured for the disposal field is slower than 60 minutes per inch or faster than 3 minutes per inch, the application is not approved as tested.
Can you do a perc test in winter? Not in frozen ground. However, where the soil does not show mottling, the water table can only be measured directly in the pits from January through April, so late winter and early spring are an important testing window.
What happens if my soil fails the perc test? It does not always mean no septic. Options include testing another spot, a soil replacement or mounded system, removing a restrictive layer in some cases, or a nonconforming design reviewed by the local health department.
Still not sure how this applies to your property? That is what a free on-site estimate is for. Call 862-268-2510 or tell us what is going on, and we will give you a straight answer, including when the answer is that you do not need us.
