Soils and Septic Systems

Photo credit: www.mfkelly.ie

Much of the treatment in the leaching field is done by soil. As wastewater passes through the leaching field and is slowly released to the soil, the soil filters it, removing nutrients and pathogens. The ability of the soil to act as a filter, however, is dependent on the type of soil particles. Certain types of soil are great filters, and others are not, and for this reason it is important to know what type of soil you have prior to installing a leaching field.

Before a leaching field is approved and installed, a test hole must be dug to expose the soil. At minimum, test holes must be 1 metres wide and 1.5 metres deep. Once the test hole is dug, the types of soil present can be identified in a soil profile. A soil profile may state: 15 cm of sandy topsoil, 30 cm of silty sand, 65 cm of granular sand and clay at 1.5 metres. The types of soil present in the profile will determine whether the site is appropriate for a leaching field or not and whether material will have to be imported.

When installing a leaching field, the designer of the system is responsible for determining the T-time, also known as the percolation rate of the soil. It is the average time (in minutes) required for water to drop one centimeter in the soil. The T-time will tell you how permeable the soil. The soil must be permeable to allow the wastewater to move through it, but not so permeable that the wastewater moves through the soil too quickly and is not properly treated. If the soil is found to drain either too quickly (gravelly or course sand) or too slowly (clays), you may have to import a more appropriate soil for the runs of your leaching field.