Percolation Testing - Fiskdale, MA

In Massachusetts, the size of a soil absorption system is dependent in large part upon the Design Flow of the facility, and the Long-Term Acceptance Rate (LTAR) of the native soil on site. The Design Flow is the quantity of sanitary sewage, expressed in gallons per day (gpd), that a facility is expected to generate. The Design Flow varies significantly for residential and commercial properties, and is calculated using flow rates provided by the MassDEP in section 310 CMR 15.203 of the State Environmental Code, Title 5. The Long-Term Acceptance Rate (LTAR) is the stable rate at which a given quantity of standard strength settled sanitary sewage can be applied to a specific area of land, expressed in gallons per day per square foot (gpd/sf). The LTAR is a function of the textural class of the native soil on site (relative proportion of sand, silt and clay in the soil as determined by the soil evaluator), and the percolation rate.

The percolation rate is determined by performing a percolation test. A percolation test is a field test which essentially measures the rate at which clear water is transmitted through the soil in a twelve-inch diameter by eighteen-inch deep hole in order to determine the suitability of a native soil beneath the proposed soil absorption system to accept effluent.

A percolation test is conducted by performing the following steps in sequence:

1. Dig a test hole 12” in diameter and 18” deep at the bottom of the proposed soil absorption system;
2. Establish a fixed point at the top of the test hole from which all measurements are taken;
3. Scratch the bottom and sides of the test hole to remove any smeared soils, and protect the bottom of the test hole with coarse sand or stones;
4. Carefully fill the hole with clear water to a minimum depth of 12” and maintain the water level by continuously adding water as necessary for no less than 15 minutes. This process is known as the “Pre-Soak” and is intended to saturate the soils in the test hole;
5. After saturation, let the water level drop from a depth of 12” to 9”, and then measure the length of time it takes for the water level to drop from a depth of 9” to 6”. Dividing this length of time by 3 determines the percolation rate in minutes per inch (mpi). For example, if it takes 30 minutes for the water level to drop from 9” to 6”, the perc rate would be 10 mpi (30min/3”=10mpi);
6. In certain soils, particularly coarse sand, it will be impossible to maintain a minimum depth of 12” during the Pre-Soak. At the discretion of the Soil Evaluator and Approving Authority, the percolation test may be discontinued and a rate of 2mpi or less can be assumed provided that at least 24 gallons of water has been added to the percolation hole with 15 minutes and it is impossible to maintain a liquid depth of 9”.

A soil with a percolation rate of 2mpi will be able to accept significantly more effluent per square foot than a soil with a percolation rate of 60mpi during the same period of time. As a result, a soil absorption system designed with a percolation rate of 2mpi would be significantly smaller (and therefore less expensive) than an equivalent system designed with a percolation rate of 60mpi.

It is important to note that the percolation test does not actually measure percolation: it is not possible to determine how fast water moves through a soil by measuring how fast it disappears from a hole in the soil. The percolation test measures nothing but a gross relative permeability and only provides an approximation of a soil’s percolating capacity. When sizing a soil absorption system the Engineer must carefully consider a number of factors, including but not limited to: the design flow, the percolation rate, the native soil’s textural class, and the anticipated strength of the sanitary sewage.