Suitability for Septic Systems

Limitations for Conventional Septic Tank Absorption Fields

Homes with septic tanks require suitable soil absorption fields to treat the effluent. An absorption field is an area in which effluent from the septic tank is distributed into the soil through tiles or perforated pipes that are installed below the ground surface. The purpose of a septic tank absorption field is to treat the organic constituents in the effluent before the effluent is diffused with the water table that occurs below or adjacent to the absorption field.

A conventional septic tank absorption field has the laterals placed at a depth of 24-30 inches below the ground surface. The conventional system is designed in accordance with criteria described by the Environmental Protection Division, Iowa Department of Natural Resources. These criteria require the total lineal footage of laterals to be calculated based on the soil percolation rate and number of bedrooms in the home.

Adequate treatment of effluent in the absorption field is difficult to achieve if the soil is shallow to bedrock, has a seasonal high water table, is subject to periodic flooding, or is slowly permeable. Soils best suited for treatment are those that occur on upland landscapes that are more than 72 inches deep, are well drained, and have good water and air relationships in both the topsoil and subsoil. Because of similar soil requirements, and likely being placed in the same area, septic and basements will be evaluated together on the scorecard.

An estimate of the rate and direction of water movement in the soil profile can be made based on soil texture and structure. Medium and some moderately fine as well as some moderately coarse textured soils with well-developed open structure are most favorable for absorption fields.

Soils with fine texture and well-developed dense structure have slow rates of water movement that result in surface ponding and high water tables. Adequate treatment of the effluent will not occur in these soils. Coarse and some moderately coarse textured soils also will not provide for adequate treatment of effluent because the liquid will move through the soil at a fast rate. Sufficient filtering of organic constituents and chemical exchange will not occur due to low organic matter and clay content in coarse and some moderately coarse textured soils.

Soil properties listed in Table 5 will be used in the contest to determine whether limitations occur for conventional septic tank absorption fields. Evidence of water in the soil profile will be determined by identifying the presence of prominent redox features within the upper 40 inches of the soil profile. A few faint casts, tints, or other variations of color composing less than 5% of the soil matrix are acceptable. The properties listed are not inclusive for making an overall evaluation as to suitability for conventional septic tank absorption fields.