Biological Systems Engineering



Soils, Absorption Fields and Percolation Tests for Home Sewage Treatment

Date of this Version



WASTE MANAGEMENT D-4, Home Waste Systems Issued November 1980, 15,000


The most common home sewage treatment system for farm and country homes is a septic tank and absorption field. In fact, about 36 percent of all American homes have such systems. The success of a septic tank and absorption field depends largely upon soil characteristics, design, and management of the system. The soil acts as a final treatment by removing bacteria, pathogens, contaminants and fine particles from the liquid septic tank effluent.

Soil Factors that Affect Absorption Fields

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. Depth, seasonal high water table and bedrock, slope and proximity to streams or lakes are other factors that need to be considered in the layout of a septic tank and absorption field (Figure 1).

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 over-lies 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. In other areas there may be a seasonal high water table due to a clay layer which inhibits downward flow. In this case, adding septic effluent to the soil will bring the water table even closer to the surface during the wet season. Effluent in this perched water can cause odor and result in the spread of disease.

Generally speaking, a groundwater table should be at least 4 ft (1.2 m) below the absorption field during the wettest season. Similarly, the depth of soil should be greater than 4 ft (1.2 m) from the bottom of the absorption trench to coarse sands and gravels or to bedrock. This thickness is needed for adequate filtration and purification. In sandy or gravelly soils, additional depth to the water table will help prevent contamination.

Soil slopes of less than 15 percent usually do not create a serious problem in laying out or maintaining an absorption field. However, where slope exceeds 5 to 6 percent, extra caution should be taken to place absorption trenches on the contour. On steeper slopes, laying out and maintaining absorption fields is more difficult, especially where the downward flow of effluent is intercepted by a horizontal layer of clay or rock. Interception of these flows will cause effluent to move horizontally and to seep to the soil surface.

Septic systems should be located at least 50 feet (15m) from streams or lakes. This is important to insure proper filtration and the removal of disease organisms before septic effluent reaches surface waters. Never locate an absorption field in a flood-prone area. Occasional flooding reduces the efficiency of the system, while frequent flooding could destroy its effectiveness as well as contaminate the flowing stream.

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