Percolation Test (Perc Test) in Western MA

A percolation test — commonly called a perc test — measures how quickly your soil absorbs water. This rate, expressed in minutes per inch, is one of the key inputs used to design a septic system. Before any new septic system can be designed or permitted in Massachusetts, the soil on your property must be evaluated to determine what kind of system will work and how large the leach field needs to be. Without a passing perc test, you cannot get a septic permit or begin construction.

Massachusetts Title 5 regulations (310 CMR 15.104) require soil evaluations to be conducted by a licensed site evaluator — typically a professional engineer or registered sanitarian. Importantly, a representative from your town’s Board of Health must be present during the test. This means scheduling the test requires coordinating with the BOH in advance, and test dates may be limited by inspector availability, time of year, and soil conditions. Saturated or frozen soils cannot be properly tested, so testing windows matter.

During the test, the evaluator digs test holes (called soil test pits) at the proposed leach field location and examines the soil profile at different depths. This reveals the soil type, depth to groundwater, depth to bedrock, and any seasonal high water table indicators. One or more perc holes are then dug and filled with water repeatedly; the rate at which the water drains is measured and recorded. The combination of the soil profile and the perc rate determines what type of leaching system the site can support.

If your soil percs slowly — which is common in parts of Franklin County where soils are heavy, glacially deposited, or have a high clay content — you may be limited to an innovative or alternative (I/A) system, a mound system, or a pressure-dosed system. Some sites may not support a conventional gravity system at all, which has implications for both design and cost. A licensed engineer will interpret the results and incorporate them into the system design.

Costs typically run $500–$1,500, depending on the complexity of the site, the number of test pits required, and whether the soil evaluation is part of a larger design engagement. In some cases the perc test fee is folded into the overall design fee. The permit required to conduct testing is typically obtained by the site evaluator and is separate from the construction permit issued later. Check with your specific town’s Board of Health — requirements for scheduling, witnessing, and submitting test results vary by municipality.