Soil Evaluation for Home Septic Systems  

AEX- 742
Agriculture and Natural Resources
Date: 
09/11/2019
Brian Slater, Associate Professor Environment and Natural Resources
Karen Mancl, Professor Food, Agricultural and Biological Engineering

The first step in constructing a septic system is to understand and describe the soil.  Soil is an excellent medium for treating wastewater and is the most important part of a septic system.  The system designer uses information about the soil to select the system type and size.

Role of Soil in Wastewater Renovation

In a septic system, the septic tank removes larger solids and grease from the wastewater. The wastewater that flows out of the septic tank still has a number of contaminants that must be removed before the water can safely join surface or groundwater. The septic tank effluent contains microorganisms that can make people sick. Organic matter in the effluent creates bad odors and the nutrients in wastewater (nitrogen and phosphorus) can impact aquatic life.  Fortunately, deep, permeable, unsaturated soil can remove these contaminants through natural processes.

Soil Depth

The depth of soil determines the type of treatment system that can be used on a lot. The deepest soils—greater than 3 feet deep to a limiting layer—are suited for soil treatment systems (also called leach field systems). Most of Ohio’s soils (84%) are shallower than 3 feet, so the designer needs to consider soil depth when selecting the appropriate treatment system (Figure 1).Pie chart with four sections: less than 1 foot 31%, greater than 3 feet 16%, 1 to 1.5 feet 22%, and 1.5 to 3 feet 31%.

A limiting layer is a zone in the soil profile that is unable to treat wastewater. These layers may transmit water too slowly or too rapidly to provide treatment. The depth at which the soil is saturated with water for several weeks each year is also a limiting layer. A limiting layer could be: 

  • Fractured bedrock
  • Sand and gravel layers
  • Hard, solid bedrock
  • Dense, compacted glacial till
  • Dense pans or cemented layers, including fragipans
  • Zones of seasonal, perched, or long-term saturation called water tables

Soil Permeability

The ability of soil to transmit water is described as its permeability. The texture and structure of the soil is used to estimate permeability. Soil texture is the relative amount of sand, silt, and clay in the soil. Sandy soils feel gritty and can allow air and water to move rapidly through the soil. Clay soils are sticky and very dense, restricting the movement of air and water. The soils best suited for wastewater treatment are mixtures of sand, silt, and clays referred to as loamy soils.

The architecture of the soil is also important in estimating its permeability. The soil particles stick together to form structural units. Granular, blocky, platy, and massive are some of the words used to describe soil structure. The soil’s structure creates pathways in the soil profile that allow for the movement of air and water (Figure 2). The soil’s structure changes with depth. The surface layers may be granular while underlying layers may be blocky or massive.Six drawings of soil structure and permeability, single grain, blocky, platy, granular, prismatic and massive.

Soil Saturation

If the soil is saturated with water, the soil is unable to accept the wastewater and remove contaminants. As the water drains away, it takes with it the microbes that can cause disease and pollutants that can contaminate surface and groundwater. A soil evaluator must look for signs of saturation, even if the work is being done during a very dry time of year.

Soil color is used to indicate that the soil is wet for several weeks each year. Once saturated, organic matter accumulates, making the soil appear black. Also, when saturated with water, the minerals that give soil its brown color can dissolve and wash away leaving behind gray colors.

Soil and Site Evaluation

Soil scientists are trained to describe and map soils. Property owners need to hire a soil scientist to collect the soil depth, permeability, and saturation information needed to select and design a wastewater treatment system. This service costs a few hundred dollars depending on the time needed to evaluate the site.

The Association of Ohio Pedologists publishes a list of consulting soil scientists. You can find the list at ohiopedologist.org/consultant-list.html

To find out more about site and soil evaluation, go to the website for the OSU Soil Environment Technology Learning Lab at setll.osu.edu or purchase Bulletin 905 “Soil and Site Evaluation for Onsite Wastewater Treatment Systems” at extensionpubs.osu.edu.