The Percolation Test - AKA 'Perc Test'

An on-site investigation to determine the conditions of the soils on your site

Years ago, many Massachusetts towns had a season for the perc test. The perc test is an essential step for anyone who wanted to build a new house on open land, or for someone who had to replace a failed septic system. The “perc test” is a shorthand for the percolation test, an on-site investigation to determine the conditions of the soils on your site. A perc test determines how porous a soil is, and therefore how many square feet of soil absorption surface are needed for the expected daily sewage flow from a dwelling or business. It also determines where the water table is found, so the new system will be far enough above it, so the sewage gets proper treatment before it falls into a water table where most treatment and pollution reduction nearly stops.

Since the 1995 the Massachusetts Title 5 code changes, perc test can be conducted at any time, since the water table is found by geological indicators, rather than actual dripping water. Some towns have taken a few more years to make the change, but now a perc test can be safely determined any month. But there are some months where is not advisable.

A difficult perc finds the water testing part of the test results in a poorer value than the same soil tested at a better part of the year. This part of the perc test measures how quickly water is absorbed into the existing ground. If the nearby soil is nearly saturated with water, like from a recent rainstorm, or from “mud season” where frost is melting, it hardly can absorb any more water from the test. During a perc test, 12” of water is poured into a shallow hole and the decrease of this water column is timed. A good perc rate may result in the required leach field being one seventh the size of the leach field required in a slow perc rate. So, it is important to do the perc test at the right time.

When is that? A perc test can be conducted in the middle of the winter. Frozen ground usually stops a foot or two down, below which a test can be conducted. Probably inconvenient at that time, but it can be done. The best times to do a perc test is when the weather is warm, it has not rained for a couple of days and the sun is shining. It does usually happen. The only time a perc test is not recommended is during “mud season”. This is in March or even April when the frost in the ground is thawing and everything is wet. How can water soaking into the ground be timed when a hole dug a short distance starts filling up with water. Two weeks later everything will be ready to go.

A perc test often can be done in a morning, except in the difficult soils. This can often be predicted based on the soil maps. Let’s talk it over and get you the best perc test possible.