A percolation test, often called a “perc” test, is a standardized soil evaluation required before installing a conventional septic system and its drain field. The test measures the soil’s water absorption rate, determining how quickly water dissipates into the subsoil of a proposed septic area. This measurement ensures the soil can safely and effectively treat wastewater effluent, which is mandatory for public health and environmental protection before local authorities issue a septic permit.
Preparing for the Percolation Test
Securing a percolation test begins with administrative steps driven by local regulations, which vary significantly by jurisdiction. Property owners must first apply for the necessary permits or site evaluation applications, typically through the local health department or environmental agency overseeing onsite wastewater management. This initial contact clarifies the specific professional and procedural requirements for the region.
The physical test must be conducted by a licensed professional, such as a soil scientist, professional engineer, or certified septic system installer, depending on local public health codes. Engaging a qualified expert ensures the test follows strict regulatory protocols and that the subsequent septic system design meets compliance standards. The professional coordinates the testing schedule with a local governing inspector, who is often required to witness the procedure and validate the results.
A preliminary site evaluation precedes the actual test, where the professional assesses the proposed drain field location for soil characteristics and potential limiting factors. Variables like a high water table, excessive rock, or unsuitable soil types such as dense clay can render the site inappropriate for a conventional system, necessitating an alternative approach. This preparation ensures the test is conducted in the most suitable area of the property.
Execution of the Percolation Test
The physical methodology involves a precise, multi-step process conducted on-site by the licensed professional. The procedure starts with digging multiple test holes, typically three to five, spaced across the proposed drain field area. These holes are generally six to twelve inches in diameter and dug to the depth of the proposed absorption trenches, often two to six feet below the surface.
The next step is the presoaking or saturation phase, which simulates the long-term, saturated conditions of a working drain field. The holes are filled with water and allowed to seep away, often requiring the water level to be maintained for at least four hours or sometimes overnight. For soils with high clay content, this saturation period may be extended to allow the soil to fully swell, which is necessary to achieve an accurate measurement of permeability.
The measurement phase begins once the soil is saturated, ensuring a stable rate of water absorption is recorded. The professional refills the hole to a specific level and measures the rate at which the water level drops over timed intervals. These intervals can be as short as ten minutes in fast-draining, sandy soils or thirty minutes in slower soils. The measured drop in water level during the final interval calculates the soil’s percolation rate, reported in minutes per inch (MPI). This measurement confirms whether the soil drains too quickly, potentially leading to groundwater contamination, or too slowly, which could result in surface pooling and system failure.
Understanding the Results and System Design
The outcome of the test is the “perk rate,” which is the average time in minutes required for the water level to drop one inch. This rate is the foundation for all subsequent septic system design decisions. A passing rate for a conventional system typically falls between six minutes per inch and sixty minutes per inch, though the exact limits are set by local health codes.
If the soil drains too quickly (faster than approximately five minutes per inch), it is unsuitable because wastewater passes through too fast for proper treatment before reaching groundwater. Conversely, if the rate is too slow (exceeding sixty minutes per inch), the soil cannot absorb the daily volume of wastewater, leading to system backup or surface discharge. When the test yields a passing rate, the professional uses this exact minutes-per-inch figure to calculate the necessary size and configuration of the drain field.
The design uses the perk rate in a regulatory formula to determine the required absorption area, ensuring the system handles the anticipated wastewater load for the property. If the test results fall outside the acceptable range, indicating failure, the property owner must explore alternative septic systems. These alternatives may include complex engineered solutions like mound systems, aerobic treatment units, or pressure distribution systems, which often involve greater construction complexity and higher costs. The final report, including raw data and the proposed system design, is submitted to the local health authority for official review and final septic permit approval before construction can commence.