Is there a difference between a Site Evaluation and Pre-Percolation Test?
- In Sonoma County it is called a Soils Profile Evaluation aka Pre-percolation test.
- In Napa and Mendocino Counties, it is called a Site Evaluation.
For either term, the process involves digging test pits using a backhoe or hand auger in order to identify soil types, their depths, and any the limiting factors such as elevated seasonal groundwater, bedrock, and/or massive clay.
It is important to the functionality of the system that we keep the leachfield within suitable soils and have sufficient separation to those limiting factors noted above when designing the septic leachfield.
Looking at the soil profile, we can see the types of soil that make up what you are standing on. While there is some research that can be done from the office relating to existing soil conditions, we will not know for certain until we dig a hole and actually study the soils on site.
The soil type and depth indicate the type of leachfield septic system allowed as well as the approximate percolation rate – the rate at which the soil is able to accept and disperse wastewater (aka whether there is a good chance you will pass a perc test).
This type of evaluation will also give us an idea on the amount of clay present in the soils. For soils with very high clay content a Wet-Weather Percolation Test may be needed.
This type of test requires enough rainfall in the season, December to March, typically, for the soils to be sufficiently saturated to have the county open the wet weather testing window.
We can only perform this test during that time, if we do not get enough rainfall, you may have to wait up to 10 months for the next opening for formal answers.
During extreme droughts, we can go for several years before another wet weather testing season opens. There are other measures that may be offered (such as noting/recording the presence of soil mottling which indicates seasonal elevated groundwater).
In most cases, we can tell you if your septic project is feasible with a soil evaluation. It can mean the difference between a vacant parcel being potentially developable or not.
With an accurate site evaluation plan, we can approximate how many bedrooms are possible for your project vision.
More details can be found at Permit Sonoma and County of Napa.
https://www.countyofnapa.org/1917/Well-and-Onsite-Wastewater-Treatment