I receive that call every so often. I then spend the next little bit explaining some things.
What is a perc test?
Well, it is actually a percolation test – designed to determine the absorption rate of soils for subsurface disposal systems.
OK, let’s break that down. A subsurface disposal system is a septic system. A means of disposing of sanitary sewage if you do not have a municipal sewer system. This septic system consists of two basic elements. A tank for solid waste and a drain field for disposing of liquid waste. The tank can be a single or a double setup, depending on the topography of the land. If your drain field is above the septic tank, you may have to have a secondary tank with a pump in it.
More important is the drain field. This is where percolation tests actually come into play. There are a multitude of soil types in Tennessee (and around the globe). Each one allows water to seep into in at a different rate. This absorption rate is laid out in TN Rules 0400-48-01-.24 Appendix I. In soils where the rate is 75 minutes per inch (MPI) or greater, a percolation test must be performed.
When is a perc test needed?
Where a percolation test is required to determine the percolation rate for a conventional system, the percolation holes used to determine this rate must be located at the intersection of lines in a grid pattern with maximum perpendicular distances of fifty (50) feet between the lines of the grid. Each hole shall be considered reasonably representative of a square area of two thousand five hundred (2,500) square feet which includes that hole in the approximate center of the square. (TN Rule 1200-1-6-.02(b)2.)
It is important to understand the effects of a failing perc test. As noted above, each test represents a block of land 50’x50’. Should that test fail, that section of land can no longer be utilized for subsurface disposal.
But what is a “failure?”
Septic systems can be installed on lands that meet certain absorption requirements. If the soil type is better than 75 mpi, then there is no required perc test. If the soil does require a perc test, the test can go as high as 106 mpi (Rule 1200-1-6-.06(d)). If, in the course of testing, the rate exceeds 106 mpi, then the hole fails and that section of ground can no longer be used for a subsurface disposal system.
But what is a perc test, physically? This is a hole dug 6 – 12 inches in diameter, about 36 – 42 inches in depth (around my area). There is then 2 inches of gravel placed in the bottom and then the hole is filled (at least 12 inches above the gravel) with water and left to soak for 24-30 hours. Then the “test” is actually done.
Depending on the amount of water left in the hole will govern how the process is supposed to work. Basically, if there is less than 6 inches of water remaining, you bring the water level to 6 inches over the gravel and test the soak rate every 30 minutes (refilling to 6 inches each time) with the final test being the accepted rate. If, however, there is more than 6 inches of water remaining in the hole the following day, you reduce it to 6 inches and then measure the rate of absorption at the end of 30 minutes. I can tell you from experience, if there is much water at all (more than about 2 inches) in the holes after soaking for 24 hours, the holes will more than likely fail.
But what does all this mean?
Essentially, you need a soil map to tell if you need to do percolation testing. A soil scientist, registered in Tennessee, is the only person who can make the map and determine the soil type. If you are in a “modern” subdivision, it is possible that there is already a soil map on record at the county environmental health department. If you are planning to install a septic system, your first stop is the county environmental health department to see what is in place already and what you will need to complete your project.