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1. 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 on-site 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.

2. 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.

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

Find out about how to have a soil evaluation performed on our Soil Evaluation Information page.

4. 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.

5. 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 (i.e., 40 feet by 100 feet). 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.

What Sanitarians Look For

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.

6. 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.

7. 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.

8. 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.

9. What is a soil approval area?

An approval area is an area defined during the soil evaluation for the location of an on-site 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.

10. 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.

11. 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. To find out about what these categories mean visit our Soil Evaluation Categories page.