Posted by: TerraSaver | December 3, 2019

What is a ‘Pre-Perc Test for a Septic System?

County of Sonoma PRMD required soil testing for septic development for an onsite septic disposal system.

​A Pre-Perc Site Evaluation is the formal documentation of the onsite soil conditions located on a subject parcel. This is commonly mistaken for a percolation test, or perc test. Although both are testing procedures for gathering data related to soil for use in a septic design, one is used to determine septic system type, while the other is used to determine the overall size of the proposed septic system.

​Well, which one is which?

  • A Percolation Test is performed by digging many test holes across a subject parcel and over the course of two days adding water and monitoring the drop or percolation rate of the soil. Typically, these tests are only required by the County of Sonoma on undeveloped properties or during the process of sub-dividing a parcel. The average percolation rate that is calculated after the test has been performed will be the major factor in the overall size of the proposed septic system.

  • A Pre-Perc Site Evaluation is the process of documenting the soil depth and composition for use in a onsite sewage disposal system. This process must be performed by a Qualified Consultant and a member of County of Sonoma Well and Septic Department. During the site evaluation, a backhoe or mini excavator will also be required to be onsite to dig profile holes in the area of the proposed septic system location and required reserve area.

After each profile hole is dug, the soil depths and compositions are logged throughout the total depth of the hole. The Qualified Consultant will be looking for any changes in the soil throughout the depth of the hole, these changes are called “horizons”. Each soil horizon will be measured in depth and documented with the specific characteristics of the soil observed within the horizon. These characteristics and depths associated with them determine whether a site has septic potential, and if so, what type of system the soil can support.

The subject parcel must have at least 24” of septic suitable soil located from grade to have septic potential. If we only find the minimum soil depth required, the property would only have the potential for an above ground septic system like a mound, or an at-grade mound with pre-treatment. The deeper we can find septic suitable soils the better, as it will allow for other septic system types to be designed.

If you’re looking to expand an existing septic system to allow for additional bedrooms or an accessory dwelling unit, you will have to perform a Pre-Perc Site Evaluation if one hasn’t been performed on the property previously.  The process of expanding an older existing gravity trench based septic system can be more challenging then designing a new system, as the soils present in the location of the existing system will need to meet the County’s requirements for depth below trench bottom. Gravity septic systems typically requires 66” – 72” of acceptable soil type to meet the County requirements to allow for the septic system to be expanded.

A large portion of properties across Sonoma County located in basin areas will also require a Wet-Weather Groundwater Determination Test be performed if looking to expand or develop a septic system on a subject parcel; please see our blog Groundwater Testing for Septic Development in Sonoma County for additional information.

Addressing County Requirements for performing a Pre-Perc Site Evaluation for Your Project

BC Engineering Group has performed Pre-Perc Site Evaluations on hundreds of properties across Sonoma County, we’re able to handle all aspects of the project from developing site exhibits, submittal to the County, scheduling of the backhoe and performing the onsite soil work.

As a leading civil engineering design firm, feel free to reach out to us with any questions you may have regarding the process of documenting the soil conditions on the subject parcel.

For more information on Pre-Perc Site Evaluation’s and other septic planning and design services, please contact us at 707-542-4321. You can also stay up to date with civil engineering and land planning issues by subscribing to our blog.


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