The soil acts as a final treatment by removing bacteria, pathogens, contaminants and fine particles from the liquid septic tank effluent. The rate of movement of water and air through a soil, called soil permeability, is a large factor in determining how well an absorption field will function. A shallow depth to groundwater, especially when overlain by sand and gravel, has a severe limitation for septic tanks with absorption Fields. Flood plains, slopes steeper than 15 percent, shallow soils over bedrock 2 to 4 ft. thick and soils with seasonally high water tables also have severe limitations. The amount of sand, silt and clay in the soil influences soil permeability.
Water moves faster through sandy soils than through clay soils.
However, locating an absorption field in a sandy or gravelly soil is not recommended since the septic tank effluent will not be filtered properly, especially if soil is thin and overlies a shallow water table. Similarly, locating an absorption field in a soil having a high clay content is not recommended due to the slow permeability. Also, the clay in most Nebraska soils generally swells when wet, reducing permeability, which limits the effectiveness of the absorption field. The depth to groundwater is an important consideration not only for groundwater protection, but also for insuring efficient operation of systems. In areas that have a seasonal high water table, sewage effluent can easily contaminate the groundwater, especially if the soil above the groundwater is sand or gravel.