Soil consistence is used to determine clay mineralogy in the field and, from that, to judge water flow through the soil, because soil consistence is very dependent on the clay mineralogy. The type of clay in the soil affects both the consistence and the permeability of the soil.
Soil consistence is important for on-site system site evaluation because it indicates how much the soil will shrink and swell upon wetting and drying. If the soil swells when wetted by the effluent from a septic tank system, the structural voids will be filled by the expanding clay particles. The rate at which wastewater moves through the soil is greatly reduced by this loss of structural porosity, and it cim cause a septic tank system to fail.
Soil consistence and moisture content.
The consistence of a soil changes with moisture content and consistence can be described when the soil is dry, moist, or wet. In North Carolina, only two moisture conditions are important, moist and wet, because dry soils are rare in North Carolina. Soil consistence is listed and the field evaluation procedure is described for these two moisture conditions in Table 4.4.9.
When the soil is wet, its consistence can be described by two characteristics, plasticity and stickiness. Plasticity, how easily the soil can be shaped by pressing or molding it, is a good indicator of the type of clay in the soil. The more plastic the soil, the higher the content of clay and the less suitable the soil is for an on-site system.
Soil Consistency Descriptions and Abbreviations
Clay Mineralogy
Clay mineralogy describes the type of clay in a soil and how the soil behaves when wet and dry. Understanding the behavior of wet soil is important in siting on-site systems.
Oays are particles of soil less than 2 microns in diameter. Clays formed from aluminosilicate minerals in the southeastern part of the country are known as temperate region clays or silicate clays.
Silicate clays are composed of a definite crystalline structure derived from the original rock minerals. Silicate clays have two basic structural units: a tetrahedron of four oxygen atoms surrounding a central cation, which is usually silicon (Si4+l; or an octahedron of six oxygen atoms surrounding a cation, usually aluminum (AJ3+). See Figure 4.4.17.
The tetrahedra or octahedra are linked together to form microscopic silica and alumina sheets, respectively (as shown in Figure 4.4.18). These alumina and silica sheets combine to form lamellae, a two- or three-layer stack of aluminum and silica sheets. Stacks of lamellae form clay particles.
The arrangement of the alumina and silica sheets determines the clay type. The lamella of a 1:1 clay mineral, such as kaolinite, is a silica sheet attached to an alumina sheet. Shared oxygen atoms link the two sheets. See Figure 4.4.19. These two-layer silica-alumina lamellae are then stacked together in a repeating fashion. Hydrogen bonds between the lamellae hold the overall crystal lattice together in a rigid fashion as shown in Figure 4.4.20.
The lamella of 2:1 clay minerals, such as montmorillonite and bentonite, is an alumina sheet sandwiched between a silica sheet on either side and held by a shared oxygen atom, as can be seen in Figure 4.4.21. The three-layer lamellae of montmorillonite are held together loosely. This loose arrangement allows water and cations to move in and out of the clay mineral structure by moving between the lamellae. Figure 4.4.22 shows the loose lamellae of montmorillonite.
Mixed mineralogy clays have a mixture of both 2:1 and 1:1 clays.
Soils containing the 1:1 clay types swell less upon wetting than 2:1 clays and therefore do not impede water flow when wetted as much as do the 2:1 or mixed mineralogy clays. If the percentage of clay is not too large, soils that have 1: 1 clay mineralogy, such as kaolinite, have excellent potential to absorb and treat wastewater, if the drainfield is large enough. These soils generally have good structure and the increased surface area of the clay particles provides additional sites for wastewater treatment to occur.
Because ions and water can move in and out between the lamellae or a layered stack made of alumina and silica, 2: 1 clays shrink when dried and swell when wetted. This shrinking and swelling makes these soils UNSUITABLE for on-site
Repeating three layer lamellae
systems. When these clays are wet and swollen, little wastewater can flow through the soil. When the clay is dry, shrinkage cracks form and the effluent receives no treatment because it moves through the open cracks too quickly.
Formation of 1:1 and 2:1 clays is primarily determined by the parent material. For example, 1:1 clays form from felsic parent material, such as granite, whereas 2:1 clays form from mafic parent material, such as diabase.
Soil particles, particularly clay, have negative charges. These negative charges attract and hold nutrients and other positively charged ions or molecules.
Surface area, and thus total negative charges, differs between clay types. The surface area of a 2:1 clay is 40 times greater than the 1:1 clay.
Table 4.4.10 summarizes the suitability of various clays for on-site wastewater systems
Influence of Clay Type on Siting of On-Site Systems
From the North Carolina Onsite Guidance Manual