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Soil Evaluation

What is a soil evaluation?

A soil evaluation is required before building a home that is not served by a municipal or centralized sewage treatment facility. A soil evaluation (sometimes referred to as a "perk test") is an assessment performed by a Sanitarian from Washtenaw County Environmental Health. This assessment determines a site's suitability for installing a new or replacement onsite sewage system. Contact your Township office to determine if municipal sewage treatment is available. All building inspection agencies in Washtenaw County require either proof of municipal sewer connection or a sewage permit before building permits can be issued. A soil evaluation must be completed and approved before you can apply for a sewage permit and begin constructing your sewage system.

Why do I need a soil evaluation?

It is necessary to have suitable soil if the drainfield is to function properly. Simply speaking, the most suitable soil would be well-drained sandy soil. However, there is great variation in types of soils within Washtenaw County. It is essential that a careful check be made of soil and drainage conditions before planning the installation and use of onsite sewage systems. The occurrence of saturated soil, or ground water, is an important factor since the sewage systems drainfield must be installed in well drained soil, at least 2 feet above the highest ground water elevation, in order to function properly. In selecting a building site, factors such as soil drainage, permeability, topography and ground water must be considered and are best determined by a soil evaluation.

Is a soil evaluation the same as having a "perk" test performed?

No. A soil evaluation is a more extensive measure that involves the identification of varying soil horizon depths, soil texture, and seasonal water tables. A "perk" test, short for percolation test, uses water to determine the percolation rate in a soil. They are very time consuming and difficult to perform with consistency.

What do I need to do to have a soil evaluation performed?

To have a soil evaluation performed, you will need to complete the following items:

  • Prepare a boundary drawing of the property. The drawing must include property lines, easements and any neighboring wells or sewage systems within 150 feet of the proposed well or sewage system. An aerial photo of the property lines can substitute for the boundary drawing. These can be accessed and printed through the MapWashtenaw tool.
  • Submit a complete soil evaluation application to:
    Washtenaw County Environmental Health
    705 N. Zeeb Rd., P.O. Box 8645
    Ann Arbor, MI 48107-8645
    (734) 222-3800
  • The application must include:
    a.  Completed application form
    b.  10-digit parcel identification number
    c.  Boundary drawing
    d.  Survey, if available
    e.  Legal description of the property
    f.  Fee

  • Contact an excavating contractor and select a few tentative dates for the evaluation. The excavating contractor will dig test holes for the evaluation. Excavating contractors can be found in the yellow pages under "Excavating Contractors" or "Septic Tanks & Systems - Contractors & Dealers." Schedule the soil evaluation with the Sanitarian. Note: Prior to performing any excavation it is your or the excavators responsibility to determine the location of any buried utilities and utility easements on your site. Call MISS DIG at 811 or 1-800-482-7171 for this information. Keep in mind that it usually takes a few days for MISS DIG to mark your utility lines!

How is the soil evaluation scheduled?

Sometimes the soil evaluation can be scheduled when you submit the application at the Environmental Health office. If you are not able to schedule at the time of application, call the Sanitarian a couple of days later to schedule it. Before calling the Sanitarian, get tentative times from your excavating contractor. In many cases, you can even have the excavating contractor schedule the soil evaluation appointment. Appointments can usually be scheduled within 5 business days of receiving a complete application. However, during certain seasonal peak construction times, it may take up to 10 business days to schedule the soil evaluation. Remember, it is your responsibility to coordinate the appointment with the excavating contractor and the Sanitarian, so make sure all parties are aware of the appropriate date, time and location of the soil evaluation.

How is the soil evaluation performed, and what is the Sanitarian looking for during the evaluation?

The evaluation is performed by digging test holes, each generally a minimum of six feet deep, in the area of the proposed sewage system. The Sanitarian will determine how many holes must be dug in order to be assured that the area is acceptable. Typically, this is a minimum 4,000 square foot area (ie., 40' X 100'). If well-drained, sandy soil is not found, then pits should be excavated or borings should be drilled to 20 feet. If sandy soil begins at a depth over 15 feet deep, a well study may be required before approval is granted to make sure area water supplies are protected.

The Sanitarian observes the excavation looking for these items:

  • A consistent layer, at least three feet thick, of naturally occurring permeable soil (sand). The formation must be less than twenty feet below original ground surface and may not be acceptable if water is present.
  • Evidence of a seasonal high water table.
  • Isolation distances to area wells, surface water, structures, easements, and property lines.
  • Topography, vegetation and drainage patterns.
  • Other site characteristics may be considered at the Sanitarian's discretion.

Who determines the location for the test holes/sewage system?

You should have an idea of where you would like to have the sewage system placed prior to the soil evaluation. However, the Sanitarian or excavating contractor may suggest a different area if the original area selected appears unsuitable. Keep in mind that the Sanitarian's role on the site is to provide expertise and guidance to assist the homeowner or builder in making these decisions.

Who needs to be present at the soil evaluation?

The Sanitarian, excavating contractor, and property owner or his/her designated representative must be on site during the evaluation. Important decisions will need to be made and it is a good idea for the property owner to be present. At a minimum, you are encouraged to select your desired home location and drainfield area.

How long will a soil evaluation take?

The time it takes to complete a thorough evaluation depends on the depth, location, and availability of an approvable soil formation, as well as the equipment and expertise of the excavating contractor. Your application gives the Sanitarian approximately two hours of field time to complete the evaluation. If a considerable amount of extra time is needed, a new application and fee may be required.

What is a soil approval area?

An approval area is an area defined during the soil evaluation for the location of an onsite sewage system. The approval area includes room for the current drainfield and future expansion or replacement. It is important that this area be preserved and it is highly recommended that this area be staked or flagged if you do not intend to build right away.

How long is a soil evaluation valid?

Typically, a soil evaluation is good indefinitely as long as no major changes are made to the property and the test pit locations can be identified. Changes made to the property line location, parcel size, grading changes, drainage changes, and soil mining can void the soil evaluation approval.

What happens after the soil evaluation is completed?

Results of the soil evaluation generally fall into one of three categories: Approval, Denial, and Further Evaluation Needed.

  • Approval: The site is approved for construction of a sewage system. The Sanitarian will either provide you with written notification on site indicating the property is approved, or you will be mailed written notification within a few days. Once your site is approved, you may apply for your permits.
  • Tank-First Areas: For sites where the water table or topography are of concern, it may be necessary to have the septic tanks installed before you can be issued a sewage permit. This process will ensure that the sewage system is installed in a manner that will minimize soil grading costs, as well as eliminate the need for a sewage pump.
  • Well First Areas: Some areas of the county have experienced difficulty in finding safe and dependable well water supplies. In these areas, you must receive a well permit and have the well installed and approved before your sewage permit will be issued. The purpose is to ensure that an acceptable water supply can be obtained before developing a property.
  • Parcel Splits: In instances where the property is to be split, surveying test pit locations may be required prior to receiving written approval. When five or more parcels are proposed, and each is less than five acres, additional requirements apply. In this case, it is advisable to contact your area Sanitarian prior to starting your project.
  • Denial: The site is not approved for an onsite sewage system. If your site is not approved, the Sanitarian will discuss with you what options exist for your site. You will also receive a written copy of a denial letter in the mail within a few days. The denial letter will list your options based on your site conditions. Please review the denial letter and keep it for further reference.
  • Further Evaluation Needed: An approval area could not be located within the time constraints of the evaluation. Contact the Environmental Health office at (734) 222-3800 to make a follow-up appointment. Depending on the circumstances, an additional application and fee may be required.

For more information, please contact:

  • Your Area Sanitarian (based on your Township)
  • Washtenaw County Environmental Health Division - (734) 222-3800


Please note that it is the responsibility of the contractor or owner to contact the Miss Dig 
notification system at 811 or 800-482-7171 and comply with all requirements of the
Miss Dig Underground Facility Damage Prevention and Safety Act before starting any excavation work.
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