Tips for Conducting a Thorough Field Evaluation

Tips for Conducting a Thorough Field Evaluation

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After completing the desktop evaluation, a field evaluation is the next step in the design process. The site evaluation is the only way to accurately determine the actual conditions present on the site. Be sure to request utility locates as required in your state and have a discussion with the property owner about any utilities or underground structures that may have been installed by the owner. After lot boundaries have been established, the process of selecting locations for the various septic system components can begin. Key setbacks to determine include wells, bodies of water, structures and property lines. 

The first observations on the site should rule out areas that are obviously unsuitable. A check of the vegetation and topography will help rule out some areas of wet soil, bedrock outcropping, steep slopes and drainage ways. The designer should evaluate topography, landforms, vegetation, drainage ways and recent construction activities that may have disturbed or removed the topsoil. Whenever possible, the soil treatment area should be located in original, naturally occurring soil. The system designer should locate the likely best spot for the STA based on setbacks, past uses, general topography and owner preferences.

Key Aspects of a Field Evaluation

Soil observations: Once the likely area is identified, soil observations must be performed with moist soils with adequate light within the STA boundaries. The total number of soil observations required is based on the judgment of the designer and/or the permitting authority. An auger is commonly used as long as it provides an unmixed noncompacted sample, but a soil pit provides a much broader perspective. The designer will record the various horizons in the soil. The two key soil properties for STA design are:  

Redoximorphic features
Redoximorphic features

1. The depth to limiting condition: periodically saturated soils, bedrock, standing water and other limiting factors identified in local codes. Periodically saturated soil is identified by redoximorphic features that consist of color patterns in the soil, formed by oxidation and reduction of iron or manganese in saturated soil. The iron or manganese is removed, translocated, or accumulates, which results in the loss (depletion) or gain (concentration) of mineral compounds compared to the base/matrix color.

2. The soil textures, structure and consistence information to determine the soil-loading rate. In some regulations, a percolation test may be used instead of, or along with, a soil texture, structure and consistence determination. It is always advisable to confirm that these characteristics match the soil percolation rate.

Slope: The slope percent and shape must be identified as they influence the retention and movement of water; rate and amount of runoff; ease with which machinery can be used; and the soil moisture. The slope also is used to determine the contours or equal lines of elevation. The STA must be laid out along the contour. 

Flooding: If the site is subject to flooding from a nearby body of water, this should be identified and the area avoided if possible. If not, provisions should be included in the design to deal with times of flooding.

Vegetation: The native or cultivated vegetation should be identified as it can indicate the typical soil moisture, current and past use, and compaction.

After the site characteristics are confirmed and the soil treatment area determined, the area should be protected prior to installation.

About the author: Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation, and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to


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