What is Perc Testing For?

The Percolation Test measures the porosity of the soil which changes if there is more or less clay or sand.

Ahh, Country living: away from nosey neighbors, perhaps, less noise, cleaner air and a more peaceful environment. Those are all worthwhile values that many are working or wishing for. But there are some losses too, one being the unavailability of the public sewer. Out in the country, you have to build and maintain your own sewage treatment system.

An on-site private septic system requires the size and location to be based on the land characteristics. You want to build once and have it last for decades. This means it has to be positioned on your land with respect to your house, any wet areas that could affect its operation, and away from your water well, neighbour’s well, driveways, property lines and other considerations. The soil the system is built to accept the amount of water planned for disposal, and to be built so it does not contaminate the water table for you and your neighbors.

This site evaluation is called the “Perc Test”, a non-spell check word that is short for Percolation Test. The perc test measures the porosity of the soil which changes if there is more or less clay or sand. It also measures the apparent water table, so the system can be built far enough above that water table that the percolating sewage can be cleaned up by soil bacteria. This requires some breathing of the soil, enough air for the bacteria to work. Septic systems do not work if they are drowned below water.

The idea of a water table is the drastic change in the Massachusetts code since 1995. Before that, a septic system design engineer would have a hole dug, maybe in August, and if he did not see water glistening, he declared that there is no water table there. Nowadays, the evidence is the colors of the soil, which can change by subtle or vivid amounts, as the water table rises and falls throughout the seasons. I call it a “bathtub ring in the soil”, the highest color marks made by water leaching and depositing minerals, mostly iron in the soil. That can be several feet higher than the water table under the older method. This is important because the code wants all septic systems to breathe, that is, be in aerated soil, rather than close to a water table at any time.

Leach fields in Massachusetts must be designed so that they are above the water table by a minimum of two feet, and sometimes up to five feet. A five foot separation in soil with a water table only a foot or two down means there will be a big mound in your yard. A leach field is a minimum of a foot thick, and covered over with a foot of topsoil, so that can add up to a seven foot tall system. That system can be a great ski slope in the winter, but otherwise it is intrusive. At Homestead Inc. we make sure that the mounding, if needed, is far reduced. Most of our repair or upgrade systems can be as close as two feet to the water table, greatly reducing any required mound.

A Perc test takes some time for that poured column of water to disappear. Hopefully it is not raining that day and there is some sand in your soil. A fast Perc test takes less than an hour, but a Perc test in a clayey or “tight” soil can go on all day. The slower the Perc test, the larger the leach field is required to be built.

A “Perfect Perc” would have the poured water drop the required 12 inches at a rate around 5 minutes per inch. It would also find the water table at a depth that keeps the leach field from being mounded. This often happens at lower elevations and where there are sandy soils near rivers and the ocean. Only by looking at these soils will you find out. Call us at any time for an evaluation, or even a prediction. Homestead Inc. 413-768-2968. We work throughout Western Massachusetts.