The percolation test, commonly known as a “perk test,” is a specialized soil evaluation required before installing an on-site wastewater treatment system, such as a septic system. This test is an indispensable step for properties that are not connected to a municipal sewer line. The results determine if the soil can adequately absorb and filter the liquid effluent discharged from a septic tank. This assessment is mandated by local health departments to protect public health and the environment, ensuring the proposed system will function correctly.
The Fundamental Purpose of Soil Percolation Testing
The main objective of the percolation test is to measure the absorption rate of the soil, which is the speed at which water drains into the ground. This measurement is crucial because a conventional septic system relies on the soil to act as the final stage of wastewater treatment, known as the drain field or leach field. The soil must allow the liquid effluent to drain effectively while also providing enough contact time for natural biological and physical filtration to occur.
If the soil’s absorption rate is too slow, often due to dense clay, the system can fail because the wastewater will not drain quickly enough. This slow drainage leads to saturation, causing the effluent to back up into the septic tank and potentially onto the ground surface. Conversely, if the soil is excessively porous, such as very sandy soil, the wastewater may drain too rapidly. This fast rate prevents the soil’s naturally occurring bacteria from properly treating the effluent before it reaches the groundwater, posing a contamination risk. The percolation test provides the data needed to balance these two extremes.
Step-by-Step: Conducting the Percolation Test
Conducting a perk test begins with a certified soil scientist or engineer selecting representative test locations across the proposed drain field area. They typically dig multiple test holes, often six to twelve inches in diameter, to the depth of the planned absorption trenches (usually between 18 and 36 inches below the surface). The sides of the hole are intentionally scratched or roughened to counteract any compaction caused by the digging tools.
The next phase is the critical saturation or presoaking stage, which simulates the working conditions of a septic system over time. The test holes are filled with water and allowed to soak, sometimes for several hours or even overnight, depending on the soil type. This saturation causes the soil particles to swell, providing a more accurate measure of the long-term drainage capacity.
After the presoaking period, the actual measurement begins by refilling the hole with a specific amount of water, often to a depth of six inches above a gravel layer placed at the bottom. The professional records the rate at which the water level drops using timed measurements. The drop in the water level might be measured at regular intervals, such as every 30 minutes, until a consistent rate is observed. This final measured drop is used to calculate the percolation rate, typically expressed in minutes per inch (MPI) of water level drop.
Translating Results into Septic System Requirements
The percolation rate, measured in minutes per inch, directly determines the required size and design of the septic drain field. Local health departments use these results, along with regulatory formulas, to calculate the minimum square footage of the leach field necessary to handle the expected wastewater volume. A slower percolation rate, indicating less permeable soil, requires a significantly larger drain field area to compensate for the reduced absorption capacity and ensure the system does not fail prematurely.
If the percolation rate falls outside the acceptable range, usually between 1 and 60 minutes per inch, the site is considered to have a “failed” perk test for a conventional system. When the soil is deemed unsuitable, property owners must explore alternative wastewater treatment solutions. These solutions include mound systems, sand filters, or aerobic treatment units, which provide a higher level of pre-treatment before the effluent is released. In the most challenging cases, the site may be rendered unbuildable without a connection to a public sewer.