How to Begin a Site Evaluation

How to Begin a Site Evaluation

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A good site evaluation is key to the design process and provides sufficient information to select a suitable, cost-effective treatment system. To this end, a site evaluation should be a systematic process that provides information with enough detail to be useful for the design. This evaluation must collect relevant data to allow a designer to determine whether or not a lot contains a sufficiently large area with suitable soil to serve the proposed uses of the lot. The person conducting the site evaluation may be a county/state sanitarian, an engineer, soil scientist or designer. Regardless of the required professional, they must understand the key aspects of site evaluation that must be gathered to design the system. The landowner/client should be aware that a soil and site evaluation does not guarantee that the lot is suitable for a soil-based septic system. Many sites present severe limitations and may require advanced pretreatment or imported soil.

The site evaluation considers placement of the septic system in relation to setbacks and topography. It also determines the proposed elevations of the system, performs soil descriptions including the limiting condition, and determines the sizing of the SSTS by accurate soil texture, structure, and consistence description

A good site evaluation consists of three parts: a desktop evaluation, a field evaluation, and site evaluation reporting. These three key parts will be discussed in this series of articles, starting with the desktop evaluation.

Desktop Evaluation

While there is certainly no substitute for a field evaluation, a desktop evaluation provides useful information about the site and allows the designer to work more efficiently in the field. Here is a summary of the information to be gathered prior to visiting the site. Consider developing a client checklist to assure the property owner is serious and part of the process.

Client provided: The legal location and structure/house specifications (location, number of bedrooms, size water-using devices). The needs and wants of the property owner are important. New construction versus existing properties will have different limitations and it is important at this stage to evaluate the current and likely amenities of the dwelling (bedrooms, water-using devices, etc.) to estimate flow amounts.

State/local regulatory authority: This information should include easements, utilities, setbacks, legal description, house specifications and flooding information. There are numerous reasons that a setback can exist on a site. It is the job of the designer to determine what setbacks exist, determine the extent of their impact on the parcel, and identify suitable areas for the septic system given these limitations. It is important to note that setbacks can differ between the sewage tank and the soil treatment area in many rules/codes.

Soil and Water Conservations Districts: This information focuses on site topography: slope, surface drainage topographic maps, flooding information. Vegetative and topographic information is available on U.S. Geological Survey 7.5-minute quadrangle maps and indicate areas of excessively steep slopes, depressions and surface drainage characteristics.

The slope information can also be assessed by investigating the soil survey report online.

NRCS soil survey information: The soil map unit(s) including drainage class, water table, bedrock, flooding, colors, textures, structures, slope, etc. Soil surveys across the United States are now found online at sdmdataaccess.nrcs.usda. gov/

Since soil treatment areas are a small portion of the parcel, a soil survey does not provide enough accuracy to determine the suitability of the soils and site but does provide general information that is useful for system design.

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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