Hello PA testers! During a recent Virtual Workshop hosted by @coachfaithbroussard, we received the following question: what is a percolation test?
What Is It?
A percolation, or “perc” test, measures the drainage rate of water through soil at a specific location. It is used to assess site suitability for septic and other wastewater management systems.
How Is It Performed?
- After site selection, the engineer determines a series of test locations based on the planned area for the septic or leach field system.
- Holes are dug at each test location, typically measuring 6-12 inches wide and 2-3 feet deep depending on local codes.
- The holes are filled with water to simulate natural rainfall and saturate the soil.
- After the holes have been pre-soaked, they are filled with water to a set level. The drop in water level is measured over time in minutes per inch. Several tests may be repeated in the same location and averaged together for improved accuracy.
- This data is used to calculate the percolation rate of water through the soil, expressed in minutes per inch. For example, a range of 5-30 min/in is an ideal range that indicates favorable water absorption and filtration.
Why Is It Important?
A percolation test protects public health and preserves groundwater quality by ensuring wastewater is properly filtered before re-entering the natural environment. When a site does not have access to a municipal sewer system, local codes may require perc tests before issuing construction permits. A rapid percolation rate risks poor water filtration and groundwater contamination. Conversely, a slow percolation rate could lead to pooling of wastewater and septic system failure. By analyzing site drainage through perc tests, the engineer can determine whether the site is feasible for development, and if so, which design strategies ensure successful wastewater management.
If you have any other questions about perc tests, let us know down in the comments!
Ryan Rendano, Assoc. AIA, LEED GA
Architect 3D Model Designer
Black Spectacles