A percolation test is performed to evaluate the rate at which water and air moves through soil. This is typically to test an area prior to the installation of a water waste treatment system. Soil tests are performed first to determine what type of soil is present in the area and if the presence of shallow bedrock or seasonal saturation is present, making the area unfit for water treatment systems. Once the area is deemed suitable for a treatment system the percolation test can commence.
Dig two or three soil test holes with a shovel in the test area. Make the holes 3 inches in diameter and deep enough to reveal the layer of bedrock or where the soil becomes mottled with gray and red, indicating seasonal saturation.
Examine the soil in the holes to determine its type–such as sandy, sandy-loam, silt or clay. The type of soil may change within the hole, based on depth. Record the soil type(s) on a piece of paper.
Measure the depth of the holes; record on paper.
Dig two or three test holes within the same area using an auger or post-hole digger. Make the holes round and 6 inches in diameter. Make the depth of the holes 3 feet less than the soil test holes.
Scratch the sides of the holes in the bottom 12 inches of the holes with a knife or sharp stick to loosen the soil where the auger or post-hole digger smeared the soil. This allows for proper percolation of the water through the soil walls. Remove any loose soil laying at the bottom of the holes.
Pour 2 inches of 1/4-inch sized gravel into the bottom of each hole. Stand a 12-inch ruler up in the hole with the 1-inch mark towards the gravel.
Place the end of a garden hose into the holes and fill them, slowly, with water to a depth of 12 inches. Maintain a depth of 12 inches of water in the holes for at least four hours; preferably overnight.
Perform the percolation test the next day, unless it is sandy soil, in which case the test will be performed after the four hours.
Begin the test by ensuring there is 6 inches of water in each hole. Add or remove water as needed.
Wait 10 minutes for sandy soil and 30 minutes for all other soils and measure the drop in water level to the nearest 1/16 inch. Record the measurements on paper.
Add water to top up the water level to 6 inches again. Wait another 30 or 10 minutes and record the drop in water level again. Continue to top up the water and measure in 30- or 10-minute intervals until three recordings in a row differ by less than 10 percent.
Calculate the percolation rate based on the measurements. Divide the time of the intervals, 30 minutes or 10 minutes based on soil type, by the average amount of water dropped in the timed interval. The answer is your percolation rate in minutes per inch.