A percolation test, or “perc test,” evaluates the water absorption rate of soil. Its purpose is to determine how quickly water drains into the ground, which is a foundational measurement for designing a safe and effective septic system. The drain field relies on the soil’s capacity to absorb and treat liquid effluent from the septic tank. A successful test confirms the property can handle the daily wastewater volume without pooling or contaminating groundwater. The results directly influence the feasibility of installing a conventional septic system or dictate the need for an alternative wastewater treatment solution.
Regulatory Compliance and Professional Selection
The process involves navigating the specific requirements of the local jurisdiction, typically managed by the County Health Department or a regional environmental agency. Prior to any physical work, an application must be submitted and an official permit secured to authorize the testing procedure. This ensures the proposed septic system aligns with public health and environmental protection standards.
The percolation test must be performed by a state-licensed professional, such as a certified soil scientist, professional engineer, or authorized health department official. These specialists conduct the test accurately and interpret soil profile characteristics, including restrictive layers like bedrock or seasonal high water tables. Local authorities may also mandate specific site preparation requirements, such as ensuring minimum distances from wells, property lines, or water bodies.
These professionals also observe the soil’s physical makeup in deep observation pits, assessing factors like texture, color, and structure. This visual analysis, performed alongside the percolation measurement, provides a comprehensive picture of the site’s suitability. Testing is often regulated, with some regions restricting it to months when the water table is naturally higher, ensuring the system functions properly during maximum saturation.
Executing the Percolation Test Procedure
The process begins with excavating several test holes in the proposed drain field area, typically at least three, to account for soil variability. These holes are usually six to twelve inches in diameter and dug to the depth of the planned soil absorption trenches, often two to three feet below the surface. The sides and bottom are then scraped to remove smeared soil that could artificially reduce the natural infiltration rate.
Soil saturation, or pre-soaking, simulates the long-term, saturated conditions a functioning drain field will experience. Water is added to the holes and maintained at a specific level, usually overnight, allowing the soil particles to swell completely. This establishes a stabilized absorption rate that reflects the soil’s capacity during the wettest conditions.
The measurement phase begins the following day. The water level is adjusted to a reference point, and the time it takes for the water level to drop one inch is recorded. This measurement is repeated multiple times, typically every 30 minutes, until the rate of drop stabilizes. The final percolation rate is expressed in minutes per inch (MPI) and represents the average time required for the water to be absorbed by the soil.
Interpreting the Results and System Planning
The measured percolation rate determines the property’s suitability for a conventional septic system. Local health codes define acceptable rates, typically between 5 minutes per inch and 60 minutes per inch. A rate faster than the minimum suggests the soil is too permeable, meaning wastewater passes too quickly for proper biological treatment, risking groundwater contamination.
Conversely, a rate slower than the maximum indicates the soil is too dense, such as heavy clay, and cannot absorb the effluent fast enough, which leads to system failure and surfacing sewage. If the test results fall outside the allowable range or if deep pit analysis reveals unsuitable conditions like a high water table or shallow bedrock, the site fails the test for a conventional system.
A successful percolation rate is used to calculate the required drain field size; slower rates necessitate a larger absorption area. If the property fails the conventional test, the data dictates the need for an alternative system, such as a mound system or an aerobic treatment unit. The final report, including the rates and the proposed system design, must be submitted to the regulatory authority for approval before construction commences.