Soil considerations for onsite wastewater systems
When selecting a new home site, the first two questions should be about water:
- What is the drinking water source?
- Where is the wastewater going to go?
This article focuses on the second question and the most important factors in choosing the location for a new wastewater system, or finding a new site if an existing system fails.
The first step in this process is to contact the local environmental health department. You will need to secure a permit and the staff can fill you in on the local regulations such as the “setbacks” or distances that wastewater systems must be from other features (i.e. property lines, water wells, etc.).
Second, a site and soil evaluation is needed. Ask the local environmental health staff about how this procedure works in their area—some environmental health departments do the soil evaluations themselves, and in some areas this might be performed by a consulting soil scientist.
This comprehensive site and soil evaluation is the key component affecting selection, design, and long-term performance of an onsite wastewater system. A septic tank, followed by a soil dispersal system (often referred to as a lateral field), is the most commonly used onsite wastewater system in Kansas and throughout the nation. Although effluent from a septic tank may be clear, it is still sewage. It is odorous and contains nutrients, disease-causing organisms, and dissolved organic material. The soil must provide the additional treatment required. An effectively operating soil dispersal, or absorption, field treats the septic tank effluent as it enters the soil and percolates downward. The soil is the most important component of the wastewater dispersal field. The soil properties determine appropriate treatment systems, the design loading rate, and the size of the dispersal field. When a building site has difficult or less desirable soils, the cost of the system is usually higher.
The most important soil properties are soil depth, soil texture (sand, silt, and clay content), and the soil structure.
- Soil depth: Features that would limit a soil profile include bedrock or very gravelly layers at a shallow depth, or a high-water table. The state of Kansas requires a minimum of 4 feet of separation distance from the bottom of a dispersal field trench and a water table. That 4 feet of separation is the minimum, and it’s needed so that the soil can act as a natural filter to treat the wastewater.
- Soil texture: An ideal soil for a lateral system would have some sand, some silt, and some clay. If you have too much clay, water will not be able to infiltrate fast enough and could pond to the soil surface. This can be overcome by increasing the size of the absorption field, which requires more space and adds cost. Another solution for clayey soils is a wastewater pond (also called a lagoon) and is a common type of septic system in Kansas. Lagoons require space, a fence, and checking with the local health department, homeowner’s association, etc. to determine if they are allowed in that area. When soils are very sandy, the effluent could move too rapidly into the soil profile. There are some parts of the state, like central Kansas, where there are sandy soils and high water tables. In this scenario, above-ground wastewater mounds are often constructed to increase the separation distance between the bottom of the wastewater absorption field and the water table.
- Soil structure: Lateral fields should be constructed in natural soil profiles. Man-made soils almost never work because they do not contain soil structure (Figure 1) and will compact with time, with the addition of water, and whenever they are trafficked. The stronger the soil structure, the faster wastewater will move into the soil, thus a smaller footprint of the wastewater system.
Figure 1. Examples of two types of soil structure: blocky and prismatic. Other types of soil structure include granular and platy (not shown). Photos provided by DeAnn Presley, K-State Research and Extension.
In summary, soils can provide excellent wastewater treatment and it is one of the ecosystem services that soils provide for humankind. Identifying the most suitable soil on the property will save money and lead to the best long-term onsite wastewater system performance.
For more detailed information, see the KSRE publication “Site and Soil Evaluation for Onsite Wastewater Systems” at https://www.bookstore.ksre.ksu.edu/pubs/MF2645.pdf
DeAnn Presley, Soil Management Specialist