Soils types and their impact on septic systems

However good your septic system is, it depends on the right soil type to complete the process of purifying the wastewater from your home. The soil type in the drainfield area will determine how well the effluent is filtered and if the water that is sent back to the water cycle is good enough. This is why understanding soil composition is very important when putting up a septic system. Soil consists of a number of layers that are grouped into four broad categories namely, surface soil, subsurface soil, subsoil, and substratum.

soil types

Surface soil – this is also referred to as topsoil and it is usually dark in color because it is enriched with organic matter from the decomposing organisms.

Subsurface soil – this is a leached zone that is located beneath the topsoil and it has mobile soil constituents like clay and organic matter removed by the downward percolation of water. This is usually where the drain field is installed. The treated water percolates from here into the subsoil and then back into the water cycle.

Subsoil – this is the layer of soil that is below the subsurface soil and it is made of a mixture of small particles of clay, silt, and sand but it doesn’t have as much organic matter as the surface soil.

Substratum – this is often referred to as a non-soil layer because it is composed of either unconsolidated sediment or bedrock.

Morphological characteristics of soil

The morphology of the soil determines what kind of septic system will be installed and how effective the system will be. There are five important soil morphology characteristics that must be put into account 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 texture of the soil can have an adverse effect on a soil’s ability to treat and safely dispose of wastewater. Texture impacts the porosity, hydraulic conductivity, and structure of the soil. Soils that have heavy texture, like clay soils, have poor draining. As a consequence, water doesn’t move fast enough through them to dispose of the needed amount of wastewater. Predetermining the soil texture is therefore important as it will inform the design of the septic system. For purposes of septic system design, soils are classified into four broad categories according to 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 Group II soils are the most ideal for conventional septic systems. Group III and Group IV soil textures might call for the installation of advanced septic systems.

Soil structure

Soil structure has to do with how the individual soil particles are arranged together to form the larger groupings of the particles that are called aggregates. The soil structure has an impact on the percolation of water, the ability of soil to treat wastewater as well as the amount of air that can be allowed into the soil. Soil structure can be described in five different ways namely;

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

Granular soil structure is ideal for a septic system because it promotes soil separation and internal drainage. On the flip side, Platy, prismatic and massive structure types of soil are not ideal for conventional septic systems. The massive and platy structures restrict aeration as well as internal drainage while prismatic ones allow for a direct flow of untreated wastewater right into the water table.

Clay mineralogy

Clay mineralogy has to do with the amount of clay in the soil and this will also influence the percolation rate of the soil. There are two main types of clays; 2:1 and 1:1. A 2:1 clay is one that expands when wet while a 1:1 clay is one that expands only slightly when wet. Clays that have a 2:1 mineralogy (e.g. montmorillonite) shrink when they get dry and swell when they get wet. As the soil swells, its particles expand into the structural voids and this ultimately reduces its porosity. This means that the soil will have a reduced hydraulic conductivity which reduces the rate at which water percolates. Soils that have a 2:1 clay mineralogy are, therefore, not suitable for the installation of conventional septic systems. Clay soils with 1:1 mineralogy (e.g. kaolinite) do not shrink or swell too much as they get and lose water. They, therefore, do not restrict the flow of water as much as their 2:1 counterparts. They can support the installation of septic systems.

Soil consistency

Soil consistency is determined by measuring how well a given soil can stick to other objects or how well it can form shapes. The consistency of soil can be determined when the soil is dry, moist or even wet. For most soils, the consistency will be determined by firmness, friability, and looseness. Should the soil be very firm when moist, it can be said to contain expansive mineralogy and will, therefore, be classified as not suitable for septic systems. For wet soils, consistence factors that will be checked include plasticity and stickiness. By pressing the soil between the thumb and the forefinger, one can tell how well the soil adheres to other objects. This will give the stickiness of the soil. To test the plasticity, you can roll the soil between the thumb and forefinger. If the soil is very sticky and very plastic when wet, it is classified as unsuitable for septic systems.

Organic soils

Organic soils can be said to be soils containing 20% or more organic matter to a depth of at least 18 inches. Any soil that fits this description is unsuitable for septic systems. Organic soils typically remain wet all year long because they drain too slowly. Organic soils can also subside and that can damage the septic system.

Soil wetness

Wastewater treatment cannot occur properly in soils that are not well-aerated. When soils are wet, the voids are filled with water which leaves little or no room for air. Lack of air in wet soils means such soils would not support a septic system. The wetness of soil can be determined by the color of the soil. Chroma refers to the relative purity, strength, and saturation of the color of a given soil. The Munsell color chart is used to determine the chroma of colors. For instance, wet soils have a chroma of 2.  Soil wetness can be caused by a number of variables. For instance, a seasonal high-water table might cause the soil to be wetter than usual at certain intervals. Other variables include perched water tables, saturated soils (due to rain or the seasonal movement of groundwater), and tidal water.

Constituents of wastewater and how they react with various soil types

Wastewater has lots of constituents that can have varying behavior on the soil. Let us look at some of these wastewater constituents and how they might behave in different soils.

Organic substances

Concentrations of both synthetic and natural organic compounds in wastewater are measured in terms of Biological Oxygen Demand (BOD), Total Suspended Solids (TSS), and Chemical Oxygen Demand (COD). In an ideal situation, a well-constructed and maintained septic system will remove most of these components through the liquefaction process by the bacteria. However, some organic substances are still passed out of the septic tank into the leach field. This is where the right soil type comes in handy. The soil removes the organic substances through various processes including filtration and decomposition. The effluent organics contribute to the formation of a clogging layer (biomat) which eventually restricts the movement of effluent into the soil. The bacteria from the effluent stores polysaccharides in the form of slime capsules and these cover the soil particles thereby reducing the percolation rate of the soil. Biomat is a double-edged sword – on one hand, it can lead to the premature failure of the septic system while on the other hand, and it can help to filter additional bacteria from the effluent before it is absorbed into the soil. When designing the septic system, proper sizing should be adhered to in order to avoid an overload of effluent in the leach field which could exacerbate the biomat problem.


Septic tank effluent has various forms of nitrogen including ammonia, ammonium, nitrate, nitrite and organic nitrogen. These are usually the by-products of the septic treatment process from the anaerobic bacteria. However, even effluent from aerobic tanks has nitrogen in the form of nitrate. Approximately 10% of the nitrogen is removed via sludge but the rest will be removed by the soil through processes like denitrification, volatilization, plant uptake and adsorption.  Even though some of the nitrogen is removed by the soil before the water reaches groundwater, a good portion of it eventually reaches the groundwater. Nitrate is largely soluble and does not interact well with the soil components in aerobic conditions. It, therefore, travels through the soil unimpeded until it reaches groundwater.


There are two main sources of phosphorus in septic tank effluent – washing detergents and human excrement. Anaerobic bacteria do a pretty good job of converting most of this phosphorus into soluble orthophosphates. Unlike the nitrates, soluble phosphates react with various soil types resulting in the removal of phosphate ions through various processes like adsorption, plant uptake, precipitation as well as biological immobilization. Phosphorous can transport through the soil when the soils are coarse-textured and have a shallow water table.

Détergent surfactants

Generally speaking, surfactants can affect the water retention and water transportation properties of soil. When the surfactant concentrations in the septic system go above 30 mg/l, they can reduce the hydraulic conductivity of the soil, which means the wastewater will not move easily through the soil. The overall effect is that water levels will rise higher than is ideal for the septic tank. As the soil removes detergent surfactants through adsorption, anionic surfactants begin to accumulate in the soil. It is advisable to ensure that wastewater that is applied to the soil has less than 1 mg/L of surfactants. This can easily be achieved by desisting from the use of detergents that have surfactants.

Toxic organic compounds

Toxic organic compounds like trichloroethylene (TCE), chlorinated hydrocarbons (MC), methyl chloroform, etc. are commonly found in chemical septic additives and cleaners. MC and TCE are denser than water and if they reach the saturated zone, they could easily sink to the bottom of the water phase. Because they are not biodegradable, some of these organic compounds remain in the sludge while some end up in the drain field and end up polluting the groundwater. Excessive accumulation of toxic chemical compounds in the sludge also makes the treatment process deficient. For this reason, these substances should not be used by septic system owners. If you need to clean your septic system, go for biological additives that are made from bacteria and Enzymes like the products from Bio-Sol.


Bacteria are single-celled organisms and it is not uncommon for them to be trapped in the pore spaces of soil particles. This is, in fact, an important mechanism because it helps to remove enteric bacteria from the effluent in the leach field. This process also results in the formation of biomat, which helps in the trapping of bacteria. Attenuation of bacteria helps to prevent the pollution of groundwater with disease-causing microbes. This attenuation happens in the biomat in an interface in between the native soil and the drain field media. The attenuation of bacteria is also influenced by the numbers of bacteria in the effluent, the texture of the soil, the loading rate, type of bacteria, soil wetness and the temperature. The soil type beneath the drain field needs to allow for unsaturated flow and slow travel to allow for maximum adsorption of bacteria to the particles of the soil and their eventual death before the water seeps through.


Viruses are not only smaller than bacteria but they also behave differently in soil. The process of virus inactivation or removal from the soil happens through precipitation, adsorption, filtration, natural die-off, and enzyme attack. Most of the factors that affect the adsorption of bacteria by the soil also affect the adsorption of viruses.  Some important soil conditions that affect virus adsorption include mineralogy, pH, texture, and temperature.

How the soil type and its percolation impacts the performance of the septic system

The behavior of effluent not only depends on the particular constituent but also on the condition and nature of the soil. The degree of wetness is determined by various factors including how far the water table is from the surface. The depth of the water table might fluctuate depending on the rainfall patterns and human activities like irrigation and stormwater management.

When designing a septic system it is important to ensure that the vertical separation between the water table and the bottom of the drain field is big enough. This will help to ensure that unsaturated conditions are maintained even when there is a lot of rain. When soil is unsaturated, the water will travel slower than it would travel through the same soil if it was more saturated. The idea is to keep effluent in the unsaturated soil for as long as possible to allow for maximum cleanup of the wastewater before it seeps through the soil.

The depth of the water table during the wet season is an important factor to consider when designing the septic system. Since it might not be feasible to wait until the rainy season kicks in in order to do the percolation test, the engineer typically guestimates by studying the color patterns of the soil, the vegetation, and other factors like the water table fluctuations in the local soil landscape. Soils that have impermeable horizons usually develop perched water tables when the wet season kicks in. a perched water table may lead to a saturated flow of wastewater or the transportation of the wastewater to the surface of the soil.

During the site investigation, it is important to take note of some important soil characteristics like the texture of the soil, cemented layers, the aggregation of the soil particles as well as the water table level in the wet seasons. These characteristics are important because they can be used to determine the performance of the system as well as determine the specifications of the septic system that should be built for the site. For instance, alternative systems like mounds might have to be installed to increase the distance between the wet season water table and the bottom of the system.  The same may be needed in cemented soil, clay soil or in the event of some other unsatisfactory conditions that might be noticed during the site inspection.


Some soils are not the most ideal for conventional septic systems and building septic systems on them without taking the necessary precautionary measures can result in a number of issues including water contamination. Clay soil is very compact and does not give room for the effluent to seep through. Clay soils can, therefore, result in backups in the leach field.  The best soil for a septic system is a soil that lies somewhere in between gravel and clay. It is neither too dense and neither is it too loose. This soil has the perfect conditions for filtering effluent while at the same time allowing it to continue to seep through.  It is therefore advisable to do a soil percolation test before you embark on designing a septic system for your property.


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