How to take a percolation test

The percolation or “perc” test is used to evaluate the absorption rate of soil, and healthy percolation is necessary for the proper design and function of septic systems. Generally speaking, land that is adequately aerated can effectively absorb water at a faster rate than clay or other dense soil. The location of the natural water table is also a significant factor, because when water already exists close to the surface the soil cannot accommodate absorption as well as it can if the water table is deep within the ground.

By: Tom Kerr
 
Perc tests are an essential tool for evaluating the potential use of land, and are required by regulatory agencies before permits can be issued for such things as septic tank installations. The common method for testing involves digging holes into the ground, filling them with water, and timing how long it takes for the water level to drop. The perc test data can then be extrapolated using mathematical formulas, to determine the optimal size of a leach field or septic system for a particular site. A house with three bathrooms, for example, will require a higher capacity than one with only one bathroom. Whenever equipment or appliances that use water – such as dishwashers, laundry machines, dairy processing facilities, or commercial kitchens, for instance – are added or subtracted, the parameters and corresponding requirements change. If bedrock or seasonal saturation occurs too close to the surface a subsurface sewage treatment system is not possible. A different design, such as a septic mound created above grade, will be necessary as an alternative.

Most perc tests involve drilling a minimum of three test holes at various depths. Depending upon why the test is being done, the holes may range from six inches to four feet in diameter and be drilled to depths of anywhere from three to 20 feet. Testing for installation of a vertical septic pit, for example, demands much deeper holes than those necessary to adequately test a leach line that is closer to the surface. A good test will evaluate the soil texture for every foot of depth drilled into the bore hole, describing it accordingly as “sand”, “loam”, “silt”, “clay” or a blend of those characteristic soil types. Sometimes a tract will have distinctly different sections of soil. For example, one area might be primarily clay while another corner of the property is mostly sand. In that case it is prudent to test each particular soil section separately, to get more precise and accurate site-specific readings.

Other factors come into play when attempting a valid test, and an experienced tester will automatically adjust for these as needed. For instance, if a post hole digger or auger is used to bore a hole into clay, the motion of the tool will smear and compact the clay against the sides of the hole as it is being dug. While this may not seem important, it can skew the perc test results significantly. The sides of the hole become sealed, just as a crevice or gap in a concrete floor is sealed when stucco is spread across it. The absorption rate will be slower and the tester will get a false reading that indicates less percolation than actually exists. So although it is true that simple tests can be done by do-it-yourself landowners, a reliable perc test needs to be conducted by a highly qualified and licensed professional who fully understands the science and methodology involved.

The quality of the soil is the key to excellent percolation, and for that reason a perc test is an important component of a land evaluation or appraisal. Without proper percolation the use of the land and potential improvements to the property may be severely limited, and if percolation is strong the land value is substantially enhanced. For those who invest in land, an initial investment in a perc test is an indispensable tool for identifying the “perks” of the property.

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