What Is a Percolation (Perc) Test?
A percolation test, or perc test, measures how quickly water soaks into the soil in a test hole. It is one way to judge whether soil can absorb septic effluent. In Florida, though, a septic site evaluation is based mainly on soil profiles, soil texture and the wet-season water table, and a perc test is not usually required.
Explanation
A drain field works because, as the EPA puts it, the soil "accepts, treats, and disperses wastewater as it percolates through the soil". Florida's rule, though, judges a site mainly by other tests.
What Florida's rule requires
- Soil profiles: a construction permit application needs at least 2 soil profile descriptions in the proposed drain field area, dug to at least 6 feet or to refusal. Each one records the soil layers, their Munsell color and their USDA soil texture (Rule 62-6.004(3)(c), F.A.C.).
- Water table: the application records the water table found at the time of the evaluation and the estimated level in the wettest season (Rule 62-6.004(3)(d)). The wet-season water table must be at least 24 inches below the bottom of the drain field (Rule 62-6.006).
- Soil texture sets the size: Rule 62-6.006 groups textures into slightly, moderately and severely limited soils, and Rule 62-6.008 sizes a standard drain field from the estimated sewage flow and the soil texture, using its Table III. The table lists a matching percolation rate next to each texture group.
Where a perc test fits in
Rule 62-6.008 says that where a percolation rate is measured with the falling head percolation test (from a 1980 EPA design manual), the result is used with Table III and weighed with other factors, such as how other systems in the area have performed. So a perc test can add information, but outside the Florida Keys the rules listed below do not require one. In the Florida Keys, Rule 62-6.018 requires at least 1 soil profile and 1 percolation test per application, unless the design engineer uses an injection well for disposal.
Who does the evaluation
Under Rule 62-6.004(3), site investigations and tests are paid for by the owner and done by a Florida-licensed engineer with soils training, DEP staff, a master septic tank contractor, a certified professional soil scientist, or a person certified under s. 381.0101, F.S. A registered (not master) septic tank contractor can do them only for existing systems and repairs, not for new systems.
When to seek help
- You are buying land that will need a new septic system. Ask for a site evaluation before you commit.
- A drain field has failed and needs replacing, which may call for a new evaluation.
- The lot is low, wet or flood-prone, which can mean a raised or mound drain field.
Learn more in new septic system installation costs in Florida, how a septic system works and Florida septic regulations. Find your county permit office, check a contractor with the registration checker, or compare septic installation companies.