Septic 101: Septic Systems and the Soil

Every septic system type is based on the soil conditions, size of the home, and the amount of land available for the system. If you were present for the installation process of your septic system you might remember what was called a soil evaluation, perc test, or having test pits dug. This process tests the soil to see how it filters and takes in liquids to help determine what type of septic system is needed. While this topic covers a lot of ground, we will teach the basics of how the soil in your yard can play a big role in determining what kind of system is chosen. This may also help you understand what you can do maintain your system well or make better choices about what goes down your drain.

To understand why soil is so important to a septic system you must first understand why soil is the key to clean water. Soil acts as a living filter for effluent as it exits the drainfield and enters the soil. Your soil is made of billions of organisms that work together with air, minerals, water, and other living material to trap and destroy disease-causing organisms, much like your own immune system. In perfect conditions, the wastewater is purified by the healthy aerated soil as it percolates into the ground and then returns, purified, to the groundwater or is evaporated by the surrounding plants. In a worst case scenario, you, your family, and your groundwater can be exposed to some serious diseases that a well-maintained system and good soil would otherwise filter out, like tapeworms, hepatitis A, salmonellosis, and giardia. Though this is rare, it’s best to be aware of this when having a system installed or planning how to best care for your system.

Soil texture can play a large role in the type of system or performance of a system installed in your yard. The size of the soil particles and its ability to absorb the wastewater make a huge difference in the treatment process. Coarse soil, like coarse sand, allows wastewater to move more rapidly through the soil than fine soil, like clay. However, coarse soil must be deep for it to filter well. On the other end, fine soil can become oversaturated fast as fine soil filters slower. The best type of soil for a septic system is a balanced mix of coarse and fine soil.

In Oregon, the first 2 feet of soil is considered to be the best performing layer of soil so most drainfields in Oregon are installed in this shallow layer. In areas with sandy soil some local or state governments may prohibit drainfields due to filtration issues. Instead they will require an elevated drainfield with imported soil. Each area is different and with centuries of septic systems and onsite wastewater treatment advances, there are good solutions for any type of soil.

Maintaining the soil around your system is easy. Here’s our rule of thumb: don’t do anything. Don’t drive over your septic system with any type of motorized vehicle, especially the drainfield, as it can compact the soil and it’s ability to purify the effluent. Don’t let your livestock stand on your system and don’t plant anything above it but grass. Finally, don’t add more than 1 inch of soil over your drainfield, if any. This easy to follow rule will keep your drainfield and soil working together, undisturbed.

Knowing the basics of how soil works for you and your system can be very helpful in how you maintain your septic system.