Soils, Absorption Fields and Percolation Tests for Home Sewage Treatment
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.
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 Arkansas 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 below the absorption field during the wettest season. Similarly, the depth of soil should be greater than 4 ft 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 100 feet 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.
Why Make a Percolation Test
Percolation tests, a measure of water movement into soils, have been used for many years in sizing absorption fields. If the absorption field is not large enough, the home or facility owner can expect partially treated sewage to appear at ground surface. This failure of the seepage field may not occur immediately after installation, but will most certainly occur within the first two or three years. If this problem continues, about the only thing to do is to replace the old absorption field with one of expanded dimensions and cut back on the effluent entering the field from the septic tank.
Some Pointers in Selecting a Site
Soil varies from one locality to another, even within short distances. To insure that you have a long-term installation that will be trouble free, abide by all local regulations, properly maintain your system and keep the following pointers in mind:
1. Soil permeability should be moderate to rapid. A "perc" test rate should be and the field sized accordingly.
2. The groundwater table or bedrock should be at least 4 ft below the absorption trenches.
3. Sloping areas are pretty, but it is difficult to lay out and construct on slopes steeper than 15 percent.
4. Do not select a site for an absorption field that is within 100 ft of a stream or other body of water. Also, absorption fields should never be constructed in an area subject to flooding.
5. When locating absorption fields, follow health regulation minimum distances from lot lines and drinking water sources.
Minimum Horizontal Distance Requirements
A. Public water supply intake – High water mark . . . . . 300 feet
B. Any source of domestic water supply . . . . . . 100 feet
C. Streams, lakes and ponds – High water mark . . . . . 100 feet
D. Dwellings . . . . . . . . . 10 feet
E. Large trees . . . . . . . . . 10 feet
F. Property lines . . . . . . . . 10 feet
G. Water Lines . . . . . . . . . 10 feet