For more than a decade, soil scientists in the USA have been consulting with public health officials, geologists, engineers, and others to develop guidelines that could assist in using detailed soil maps for waste disposal. Soil-limitation ratings, based on these guidelines, have now been published (Soil Survey Staff, 1971), and all soils of the USA are being rated for their limitations for waste disposal.
Important soil factors to be considered for septic-tank seepage fields for filtering sewage effluent are outlined in Table 2. Soils with slight limitations are good for seepage fields, and only minimal expenditures are required for safe effluent disposal without environmental hazards. Soils with moderate limitations in Table 2 have some undesirable properties; careful planning and design of seepage fields are needed in these areas to compensate for the limitations imposed by the soils (Figure 13). Soils with severe limitations (Table 2) have problems for seepage fields that are difficult to overcome: slow permeability, shallowness to bedrock, wetness, flooding, slope, stoniness, or some other unfavorable soil property. Costs of acceptable seepage fields in soils with severe limitations may be several times greater than costs in soils with slight limitations. In some cases on-site sewage disposal is not feasible in soils with severe limitations because of the high costs and health and environmental hazards.
In Table 2, the most limiting soil property determines the rating for each soil; thus a soil with slopes greater than 15 percent is rated severe even if all the other properties are good for seepage fields. Of course, seepage fields can be put on steep slopes (Figure 14), but costs and hazards are much greater than on gentler slopes. Factors important in determining limitations of soils for seepage fields include: (1) local experience and records of performance for existing seepage fields, (2) permeability of the subsoil and substrata, (3) depth to bedrock and impermeable layers, (4) flooding, (5) seasonal fluctuating and permanent water tables, and (6) slope (Soil Survey Staff, 1971). Failures of seepage fields are indicated by effluent rising to the soil surface, offensive odors, contamination of ground water and well waters, rank plant growth, and, of course, consequent incidence of disease and sickness in the people affected along with the environmental degradation.
Soils of Kansas are rated for permeability in Table 1; Table 2 enables the permeability classes to be evaluated for each soil for septic-tank seepage fields into categories of slight, moderate, and severe limitations. The appropriate category for each soil in Kansas is listed in Table 6, with major limitations also listed for each soil. Soils with moderate to very rapid permeability are rated as having slight limitations (Soil Survey Staff, 1971). Soils with a permeability at the slower end of the moderate range (about 1.0 to 0.6 in/hr, faster than 60 min/in) are rated as having moderate limitations. Soils with a permeability rate of less than 0.6 in/hr or slower than 60 min/in are rated as having severe limitations. In arid or semiarid areas, soils with moderately slow permeability may have a rating of moderate. Although soils with rapid permeability are considered to have slight limitations, contamination hazards exist where the soils are rapidly permeable and water supplies, streams, ponds, lakes, or water courses are nearby and receive seepage from the septic tank.
Figure 13--Wet soils with slopes and outlets can be considerably improved for subsurface sewage disposal with drainage and good landscape design. Water flowing onto a lot should be intercepted and diverted, and wet soils can be drained for adequate septic-tank seepage field performance. Even if the summer is dry, water in soils may cause problems in cool, wet late winter and early spring. This scene shows a small drainageway on a house lot; landscape design can make this channel an attractive part of the house site.
Table 2--Ratings of limitations of soils for waste disposal in septic-tank seepage fields (Adapted from Soil Survey Staff, 1971
|Item affecting use||Soil-limitation rating|
|Permeability class1||Rapid2, moderately rapid,
and upper end of moderate
|Lower end of moderate||Moderately slow3 and slow|
|> 1.0 in/hr2||1.0-0.6 in/hr||< 0.6 in/hr|
(auger hole method;
|Faster than 45 min/in2||45-60 min/in||Slower than 60 min/in|
|Depth to water table||> 72 in||48-72 in||< 48 in|
|Flooding||None||Rare||Occasional or frequent|
|Depth to hard rock4,
bedrock, or other
|> 72 in||48-72 in||< 48 in|
|Stoniness class5||0 and 1||2||3, 4, and 5|
|Rockiness class5||0||1||2, 3, 4, and 5|
|1. Class limits are the same as those suggested by the Work Planning Conference of the National Cooperative Soil Survey (Soil Survey Staff, 1971 The suitability ratings should be related to the permeability of soil layers at and below the depth of the installed tile lines of the seepage field.
2. Special considerations should be given to places where pollution is a hazard to water supplies.
3. In and or semiarid areas, soils with moderately slow permeability may have a rating of moderate.
4. These depth ratings are based on the assumption that tile in the seepage field is at a depth of about 2 feet.
5. Class definitions are given on pages 216-223 of the Soil Survey Manual (Soil Survey Staff, 1951). Larger numbers indicate more stones and rocks.
Figure 14--Soils at this site have severe limitations for septic-tank seepage fields, due to slopes and slow permeability. Slope limitations have been improved with bulldozer grading, and the tile line for the seepage field has been placed in the looser fill below the house. With good engineering design and proper septic-tank maintenance, this sewage-disposal system could function satisfactorily, but pollution will be a hazard if the volume of soil material is not adequate for filtration.
Field percolation tests made by local health departments generally are conducted under a wide range of soil and environmental conditions; sometimes even homeowners or contractors conduct the tests. Results must be interpreted with caution. Even nearly impermeable soils on which seepage fields have failed can give high percolation test results after periods of drought (Soil Survey Staff, 1971), especially in soils with a high content of montmorillonite (Table 1). In addition to soil properties that influence percolation rates, changes in the microorganisms in the soil may also help or binder the functioning of the seepage field after it is in operation. Because the methods of measuring percolation and permeability are different, the correlation between the two values is imperfect. The permeability rates listed in Table 1 for soils of Kansas are probably much more reliable than percolation tests in auger holes (see Bouma, 1971).
For septic-tank seepage fields (Table 2), a seasonal water table should be at least four feet below the bottom of the trench at all times for soils with slight limitations; Table 1 lists some of the water-table depths in soils of Kansas. Soils with a water table less than two feet below the bottom of the trench for extended periods have a severe limitation.
Soils that are subject to flooding (Soil Survey Staff, 1971) have severe limitations even if the permeability is satisfactory and the ground-water level is below four feet. Floodwaters interfere with the functioning of the seepage field and carry away unfiltered sewage. Without protection, areas subject to flooding should not be considered for on-site sewage-disposal systems.
Cracked or fractured bedrock without an adequate soil cover permits unfiltered sewage to travel long distances through or into aquifers, as in deeply creviced limestone areas. At least four feet of moderately coarse or finer textured soil material should be between the bottom of the tile trenches and the bedrock.
Soils (Soil Survey Staff, 1971) with slopes of less than eight percent are the best sites for sewage-disposal systems from the standpoints of construction and successful operation of seepage fields. Mechanical problems of layout and construction increase with steepness of slope (Figure 15). Lateral seepage or down-slope flow of effluent is a problem on sloping soils, especially where bedrock or impermeable layers are within four feet of the surface. Large rocks, boulders, and rock outcrops increase construction costs. On slopes, the tile grade is difficult to maintain if the obstacle cannot be removed. Trench lines can be installed and grade maintained around these obstacles on nearly level soils. On sloping sites, detergents in solution can move through some soils and contaminate ground-water and surface water. Sodium salts from sodium chloride water softeners and other sources tend to disperse the clay in the soil and to reduce the effectiveness of the seepage field.
Figure 15--These soils have slight limitations for septic-tank seepage fields due to slope, but severe limitations due to slow percolation rates. Stoniness and rockiness are not problems, but the soils have severe erosion hazards where lawns are newly-planted and where bare surfaces are exposed during landscape design. Each soil has unique characteristics which should be considered for the most efficient and aesthetic development of the site.
An aerobic sewage lagoon (Soil Survey Staff, 1971) or waste-stabilization pond (Kansas State Department of Health, 1971) is a shallow lake (Figure 16) that holds sewage for the time required for bacterial decomposition. Sewage lagoons require considerations of the soils for two functions: (1) as a vessel for the impounded area and (2) as soil material for the enclosing embankment. Enough soil material that is suitable for the structure must be available, and, when the lagoon is properly constructed, it must be capable of holding water with minimum seepage.
Figure 16--View of community sewage lagoon. Large sewage lagoons can be easily constructed in soils with slight limitations, where the soils are nearly impermeable, are nearly level, lack coarse fragments, are not flooded, and have a good engineering classification for embankment construction and compaction. In some cases lagoons are the cheapest method of sewage treatment for communities; sewage lagoons can also be used for individual houses where soils are impermeable and where adequate land area is available.
Table 3 lists soil properties important for determining limitations for sewage lagoons, formulated after years of consultations and research (Soil Survey Staff, 1971). The most important soil-material classification for construction is the unified soil grouping for engineering; this system is explained for laymen in a publication by Olson (1972a). Soils placed in the unified soil classification groups (GC, SC, and SM) are satisfactory for the lagoon bottom; they can be worked easily and have enough fines so that the lagoon bottom will hold liquids. The coarse groups with few fines (GW, GP, SW, and SP) have severe limitations and are poorly suited. The groups consisting of soils high in organic matter (OL, OH, and PT) also have severe limitations and also are poorly suited. Soil material of the other unified classification groups (GM, CL, CH, ML, and MH) are suitable if properly compacted or if used in combination with soil materials classified as GC, SC, and SM. Soil material for sewage lagoons should be free of coarse fragments over 10 inches in diameter that interfere with compaction and earth manipulations.
Soil requirements (Soil Survey Staff, 1971) for basin floors of lagoons are (1) slow rate of seepage, (2) even surface of low gradient and low relief, and (3) little or no organic matter. Official specifications for lagoons state that the depth of liquid should be at least two feet and generally not more than five feet, that the floor should be level or nearly level, and that the materials for the basin floor should be so nearly impervious as to preclude loss of liquid. The relatively impervious soil material should be at least four feet thick; this is especially critical where the local water supply comes from shallow wells that may become contaminated.
Limitation classes for slope and relief (Figure 17; Table 3) are determined by the specifications that the liquid body of a sewage lagoon or waste stabilization pond be no less than two feet or generally no more than five feet deep. Slope must be low enough and soil material thick enough over bedrock to make smoothing practical for uniformity of lagoon depth. Greater slope is allowable if soil material is more than six-feet deep, but generally smoothing is impractical where slope is more than seven percent. If the soil is nearly level and requires little or no smoothing, it need not be more than four to six feet deep. Geological investigations are particularly valuable at lagoon sites to determine, with deep borings, exact depths to bedrock and possible contamination hazards.
Figure 17--View of lagoons on sloping soils. Soils with slopes greater than two percent have moderate limitations for sewage lagoons, because more earth must be moved than on slopes less than two percent. On slopes steeper than seven percent, sewage lagoons may not be feasible. Where large volumes of sewage are to be treated, several lagoons are generally built in series to handle wastes by stages because certain holding periods are required for waste stabilization and oxidation.
If floodwaters overtop embankments (Table 3), they interfere with functioning of the lagoons and carry away polluting sewage before sufficient decomposition has taken place. Ordinarily, therefore, soils susceptible to flooding have a severe limitation for sewage lagoons. If, however, floodwaters are slow flowing and never more than about five-feet deep (not deep enough to overtop lagoon embankments) the limitation rating may not be severe because of susceptibility to flooding. General limitations can be evaluated for each soil, but each site has its own specific peculiarities so that careful, expert evaluations are always necessary before construction starts-if errors are to be minimized.
Depths to water tables are disregarded if the lagoon floor consists of soil material at least two-feet thick that is impermeable or nearly so; depths to water tables, however, are critical if the material is permeable or even slowly permeable. A water table that is below a depth of 60 inches at all times permits a rating of slight. If the water table is seasonally between depths of 40 and 60 inches, it imposes a rating of moderate. If free water in the soil is at a depth of less than 40 inches for extended periods, it imposes a rating of severe. The ratings of Table 3 are based on the requirements that (1) the liquid body of a sewage lagoon must be no less than two-feet or generally no more than five-feet deep, (2) the only water in the lagoon, other than from precipitation, must be that of the sewage; and therefore the water table must never rise high enough to contribute water to the lagoon, and (3) there must be at least four feet of slowly permeable material between the bottom of the lagoon and seasonal water table or the cracked and creviced bedrock.
Table 3--Ratings of limitations of soils for waste disposal in sewage lagoons (Adapted from Soil Survey Staff, 1971).
|Depth to permanent or fluctuating water table||> 60 in||40-60 in1||< 40 in1|
|Permeability||< 0.6 in/hr||0.6-2.0 in/hr||> 2.0 in/hr|
|Depth to bedrock||> 60 in||40-60 in||< 40 in|
|Slope||< 2%||2-7%||> 7%|
|Coarse fragments < 10 in diameter, % by volume||< 20%||20-50%||> 50%|
|Percent of soil surface covered by coarse fragments < 10 in diameter||< 3%||3-15%||> 15%|
|Organic matter||< 2%||2-15%||> 15%|
|Flooding2||None||None||Soils subject to flooding|
|Unified soil groups3||GC, SC, CL, and CH||GM, ML, SM, and MH||GP, GW, SW, SP, OL, OH, and PT|
|1. Depth to water table can be disregarded if the floor of the lagoon is to be in nearly impermeable material at least 2 feet thick.
2. Flooding can be disregarded if the flood water has low velocity and a depth of less than 5 feet, if it is not likely to enter or damage the lagoon embankments.
3 The unified soil-engineering classification system has been outlined for laymen in a publication by Olson ( 1972a) available from the Department of Agronomy (Soils) of Cornell University, Ithaca, New York 14850; soil suitability for embankments is also outlined in the publication. The unified soil groups are most relevant in Table 3 to suitability of soil materials for embankments retaining liquids in sewage lagoons.
Soils containing moderate to high amounts of organic matter are unsuitable for the basin floor even if the floor is underlain by suitable soil material. The organic matter promotes growth of aquatic plants which are detrimental to proper functioning of the lagoon.
The third column of Table 6 lists limitation classes of soils of Kansas for sewage lagoons; the numbers and letters list some of the major soil-characteristic limitations. Additional relevant characteristics of soils of Kansas for waste disposal are given in Table 1. Soils are grouped into three classes (Soil Survey Staff, 1971) according to their degree of limitation for use as sites of sewage lagoons. The slight-limitation class includes soils that are effective in functioning as sealed-basin floors and that are low in organic matter. Soils in the moderate-limitation class are those that require special practices or treatment to modify limitations to their use as sites for sewage lagoons. Soils placed in the severe-limitation class are those that are very porous, or that are high in organic matter, or that have other limitations that present or make them very difficult to use as sites for sewage lagoons.
Kansas Geological Survey, Geology
Placed on web Aug. 7, 2009; originally published March 1974.
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