Understanding Soil Types: A Crucial Factor in Septic System Design

Understanding Soil Types

Regardless of how good your septic system is, it depends on the right soil type to complete the purification process of wastewater in your home. Therefore septic soil testing is pivotal.

The soil type in the drain field area determines how efficiently the effluent is filtered and if the water sent back to the water cycle is good enough. Therefore, understanding soil composition is paramount when installing a septic system.

Soil consists of several layers that are grouped into four broad categories – surface soil, subsurface soil, subsoil, and substratum.

This article explains the intriguing relationship between septic systems and soil types, and how different soil compositions can severely impact your septic system’s performance.

Understanding the Different Soil Types

  • Surface Soil – Also referred to as topsoil, surface soil is usually dark in colour as it is enriched with organic matter from decomposing organisms.
  • Subsurface soil – This is a leached zone that is located beneath the surface soil. It has mobile soil components like organic matter and clay removed by the downward percolation of water. 
  • Subsoil – This layer of soil is below the subsurface soil. It is made of small particles of silt, clay, and sand but does not possess as much organic matter as the surface soil. 
  • Substratum – This is generally considered a non-soil layer as it is composed of bedrock or unconsolidated sediment. 

Morphological Characteristics of Soil

The morphology of the soil determines the type of septic system that will be installed and its effectiveness. There are five prominent soil morphology characteristics that should be considered when designing a septic system. These are:

Soil Texture

Soil texture refers to the relative proportions of the various soil particles in the soil. The soil’s texture can have an adverse effect on its ability to treat and dispose of the wastewater. Texture heavily impacts the hydraulic conductivity, porosity, and structure of the soil.

Soils with heavy texture have poor draining. Therefore, water does not move fast enough to dispose of the required amount of water.

Soil predetermination is significant, as it will define the design of the septic system. For septic system design, soils are classified into four categories based on their texture. 

  • Group I – Sandy Textured Soils
  • Group II – Coarse Loamy Textured Soils
  • Group III- Fine Loamy Textured Soils
  • Group IV – Clayey textured soils

Group I and II are most suitable for conventional septic tanks. Groups III and IV might demand the installation of advanced septic systems.

Soil Structure 

Soil structure refers to how the individual particles of soil are arranged to form the larger groupings of the particles known as aggregates. The structure has an impact on the ability of the soil to treat wastewater, percolation of water, as well as the amount of air that can be allowed into the soil.

Soil structure is described in five different ways, namely:

  • Crumb and granular
  • Block-like
  • Platy
  • Prismatic
  • Absence of structure (e.g. single grain or massive).

A granular soil structure is ideal for septic systems as it promotes internal drainage and soil separation. On the other hand, prismatic, platy and massive soil structures are not suitable for conventional septic systems.

Clay Mineralogy 

The amount of clay in the soil affects its mineralogy, which in turn affects how quickly the soil percolates. Clays come in two primary varieties: 2:1 and 1:1.

Clay with a 2:1 ratio expands when wet, but clay with a 1:1 ratio just slightly expands. Clays with a 2:1 mineralogy, such as montmorillonite, expand when wet and contract when dry.

The soil’s porosity eventually decreases as a result of the particles’ expansion into the structural spaces during swelling. This implies that the rate at which water percolates will be slowed down due to the soil’s decreased hydraulic conductivity.

When they absorb and lose water, clay soils with a 1:1 mineralogy (such as kaolinite) do not significantly shrink or swell. As a result, they do not significantly impede the flow of water as their 2:1 counterparts do.

Thus, they are easily able to facilitate the installation of septic tanks. 

Soil Consistency

The ability of a particular soil to adhere to other objects or to take on shapes is a measure of its consistency. It is possible to assess the soil’s consistency; whether it is dry, damp, or even wet.

Firmness, friability, and looseness will mostly dictate a soil’s consistency. If the soil becomes extremely hard after being wet, it is said to have expansive mineralogy and is therefore not recommended for use with septic tanks.

Plasticity and stickiness are consistency parameters that are associated with moist soils.

You can test the soil’s ability to stick to other items by pushing it between your thumb and forefinger. This will contribute to the soil’s stickiness.

You can roll the dirt between your thumb and forefinger to see how malleable it is. Septic systems should not use soil that is excessively sticky or plastic when it becomes wet.

Organic Soils

Soils that contain 20% or more organic matter to a minimum depth of 18 inches are considered organic soils. This type of soil is not appropriate for use with septic systems.

Soil Wetness

Poorly aerated soils are not suitable for the effective treatment of wastewater. There is little to no space for air in moist soils because the spaces are filled with water.

Wet soils are devoid of air, which implies a septic system cannot be supported in them.

The hue of the soil can be used to gauge its level of moisture. The relative strength, saturation, and purity of a soil’s colour are referred to as its chroma.

Colour chroma can be ascertained using the Munsell colour chart.

For example, the chroma of wet soils is 2. Numerous factors might contribute to the moisture of the soil.

For example, the soil may be wetter than usual at specific intervals due to a seasonal high-water table. Tidal water, perched water tables, and wet soils from rain or seasonal groundwater flow are additional variables.

Ready to Take Control of Your Septic System’s Health?

Certain soils are not the best candidates for conventional septic systems. Installing septic systems on them without following the correct precautions can lead to a variety of problems, including contaminated water.

Because clay soil is so compacted, there is no space for wastewater to soak through. Therefore, leach field backups may occur from clay soils.

The ideal soil type for a septic system is intermediate between clay and gravel. It is just the right amount of looseness and density.

The ideal circumstances exist in this soil for both allowing wastewater to seep through and filtering it. Thus, before you start designing a septic system for your property, it is advisable to do a soil percolation test.

The ideal circumstances exist in this soil for both allowing wastewater to seep through and filtering it. Thus, before you start designing a septic system for your property, it is advisable to do a soil percolation test.

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