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Soils and Soil Testing

Soil is any mixture of particles that come from disintegrated rock and can support the growth of vascular plants, which are those that can internally transport water and food. Soil particles range in size from sand, which is the largest particle, to silt which is of medium size or smaller, to clay which is the very smallest size and most easily compacted. The inherent properties of soil and the depth of it are highly significant because it influences the movement of water and the decomposition of organic material. The function of the soil is to move the water through the soil pores away from the area and to allow microorganisms to operate in the aerated soil to renovate the wastewater.

Soil varies greatly from property to property in a given area and may vary within the individual property from its natural deposition to the removal of the soil when preparing the land for the building of a structure to the depositing of material in order to build up the slope of the given piece of land. This fill material may be of various compositions. Climate and the presence of organic matter can cause the soil to be altered.

The soil profile of the given area provides information on the ability of the soil to aid in the removal of certain chemicals and minerals, and the decomposition of the organic material within the effluent from the septic tank as the liquid flows into the ground. The soil profile depending on the depth of the soil at any given point may contain clay, a mixture of sand and clay or silt and clay, a mixture with loam, or be predominantly silt or sand. The soil profile whether it be the original ground or fill can vary enormously from place to place and therefore soil that appeared to be good for drainage at any given point where a test is conducted may be just a few feet away from soil that may be poor for drainage. When the soil is examined and mottling is encountered, it indicates that there is a seasonally high groundwater table problem and this area may be unfit for an on-site sewage disposal system.

Soil drainage is the ability of the soil pores to avoid saturation and the speed at which the water will move through the soil. Soil texture is related to the various types of soils mentioned in the soil profile. The soil best able to deal with effluent is sand. However, along with the quick flow of the liquid is the problem of microorganisms and chemicals moving into the groundwater supply or, depending on the slope of the land, into surface bodies of water. The problem with silt is that it can flow like water into spaces within the soil and cause clogging. The problem with clay is that the clay will swell and thereby reduce the ability for the effluent to flow away from the septic tank and reduce the organic load. Rarely do you find simply one type of soil.

The color of the soil can indicate if the soil is saturated or unsaturated. When the soil is saturated, it is gray instead of brown from iron oxide minerals. Iron oxides are a major pigment in the soil. Soils with a uniform bright red, brown, or yellow color may be able to drain normal rainfall but have problems with sewage effluent.

The texture of the soil has a significant influence on the percolation rate of effluent from the septic tank. With sand, water percolates at less than 10 minutes per inch. With sandy loam, it percolates at 3-30 minutes per inch. With loam, water percolates at 10-45 minutes per inch. With silt loam, water percolates at 30-90 minutes per inch. With clay loam, water percolates at over 45 minutes per inch. And with clay, water percolates at over 60 minutes per inch. It is the rate of percolation which may help determine the size and nature of the septic tank system.

The slope of the land is very important because if it is greater than 10% then there are severe limitations to subsurface disposal fields for the effluent. Also erosion may remove some of the better soils that could have been used for the disposal of the septic tank effluent.

Soil in a well-designed well-aerated properly functioning subsurface disposal system of proper depth which is not overloaded can reduce the sewage effluents significantly and allow the remaining liquid to disperse appropriately. Part of the dispersion is through evapotranspiration in climatic conditions which allow this to occur.

Soils in the subsurface trenches and beds may become clogged because of the physical, biological, and chemical agents present in the effluent from the septic tanks. Silt, as has been previously mentioned, can flow like water and contribute to this clogging. This may lead to a drastic reduction in the percolation rate of fluid into the ground.

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