Published · By the Check the Land team
How to check septic suitability in five steps
- Check for sewer. Ask the county or city whether the land is in a sewer service area. A free preview here shows the distance to the nearest mapped one.
- Read the soil map. On the USDA Web Soil Survey, look at the mapped soil's drainage class and its rating for septic tank absorption fields.
- Call the county health department. Ask what test it requires (a perc test, a soil profile evaluation or both), who can do it, and how long it takes to schedule.
- Get the test done on your building site. Do it during your due diligence period, and in some places during the wet season.
- Get the permit answer. Ask what type of system the results would allow and roughly what that system costs locally.
What a perc test measures
A percolation (perc) test measures how fast water soaks into the soil. A tester digs holes where the drain field would go, soaks them, and times how long the water level takes to drop, usually reported in minutes per inch.
Too slow, and wastewater pools on the surface. Too fast, and it reaches groundwater before the soil has treated it. Each state or county sets the acceptable range.
Many health departments now use, or add, a soil profile evaluation: a pit dug a few feet deep where a trained evaluator reads the soil layers, texture and signs of a seasonal high water table. The water table and the depth to rock or a hardpan layer often decide the result as much as the soak rate does.
Why lots fail
The common reasons are:
- Heavy clay soils that drain too slowly
- A seasonal high water table close to the surface
- Shallow bedrock or a dense restrictive layer
- Steep slopes where a drain field can't be laid out
- Not enough usable area once setbacks from wells, streams, wetlands and property lines are applied
Conventional vs alternative systems
A conventional system is a septic tank and a gravity drain field in suitable soil. It is usually the cheapest to install and maintain.
When the soil isn't suitable, many counties allow engineered alternatives such as mound systems (a drain field built up above the natural soil), pressure-dosed or drip systems, or aerobic treatment units. They can work on difficult lots but cost more to build, may need electricity and regular servicing, and some require an ongoing maintenance contract. What's permitted varies by state.
What soil maps tell you first
The USDA soil survey rates each mapped soil for septic absorption fields and lists its drainage class, depth to a restrictive layer and ponding. A soil mapped as poorly drained, or with shallow rock, is an early sign to test before you commit.
The map describes an area that can cover many acres and includes other soils within it, so it can't replace a test on your site. A lot with a poor map rating can still have a good spot, and the reverse. See terrain and slope for how slope and soils fit together.
What it means for your offer
For raw land you plan to build on, a septic suitability contingency is one of the most valuable clauses you can ask for. It lets you walk away if the test fails or only allows a system you can't afford. Ask the seller whether a perc test or septic permit already exists for the lot, and check how long the county treats old results as valid.
From a real sample report
Worked example: poorly drained soil in Story County, Iowa
Our sample screen of Iowa farmland mapped the soil at the pin as Webster clay loam, 0 to 2 percent slopes, rated poorly drained, hydrologic group C/D, with moderate shrink-swell. The report flags it as 'look closer' and says plainly that a perc test is the real answer.
The nearest mapped sewer service area is 3.3 mi (5.3 km) away, so a house here would most likely need on-site septic. Flat, poorly drained farmland like this is often tiled to drain it for crops; a buyer would want to know where the tile lines run, and what system the county health department would allow on this soil, before making a firm offer.
Open the Farmland in central Iowa sample (PDF) · How to read a report
Common questions
What happens if land fails a perc test?
You may still be able to build with an engineered alternative system, such as a mound or aerobic unit, at a higher cost. Sometimes no system can be permitted, which makes the lot unbuildable for a house that needs septic.
Who does a perc test?
Usually a licensed soil evaluator, engineer or septic designer, with the county or state health department reviewing the results. Ask the health department who is approved locally.
Should I get a perc test before buying land?
If you plan to build and there's no sewer, yes, or at least make the purchase contingent on one. It's one of the few tests that can decide whether a house is possible at all.
Questions to follow up
- Ask the county health department what septic test it requires and how long scheduling takes.
- Make the purchase contingent on a passing test, or on a permit for a system you can afford.
- Ask the seller for any past perc test, soil evaluation or septic permit for the lot.
What a Check the Land screen checks
- Soil map unit, drainage class, hydrologic group, ponding and depth to a restrictive layer (USDA SSURGO)
- Modelled depth to bedrock and the slope at the pin
- Distance to the nearest mapped sewer and public water service areas
- Mapped wetlands and streams near the point
What it can't tell you
- Whether the soil will pass a perc test or soil evaluation
- Which septic system the county would permit, or what it would cost
- The seasonal high water table at the building site
Sources
This guide is general information, not legal, engineering or insurance advice. Rules vary by state and county.