Civil Engineering and Land Surveying firm with expertise in residential and commercial industries

Percolation Test

What is a Percolation Test?

A percolation test – or perc test – is performed to determine your soils application rate, this is the amount of gallons of wastewater per square foot per day your site can absorb. The application rate is then used as the basis for sizing your system. The higher the absorption rate, the smaller the sewage disposal field.

Perc tests are performed by digging holes to the soil depth of the horizon wastewater will be discharged within, adding water to the holes, and measuring the depth at which it drops. Once the data has been gathered for each hole, the numbers are averaged, and the percolation rate is determined. The soil application rate is based on the County’s perc rate chart.

There are two types of perc tests, dry weather perc tests and wet weather perc tests.  Which type of test your parcel requires is determined based upon the percentage of clay within your soil and whether the clay is expansive. The wet weather test period begins after half of the seasonal precipitation has fallen. If your project requires any wet weather testing, you must wait until the wet weather period begins before moving forward with your septic design – typically January-March.

Adobe Associates’ wastewater department has years of experience performing percolation tests in Sonoma, Napa, and Marin County which has led to a great working relationship with the Counties staff. If you are interested in learning more about this service, and the ways Adobe Associates can help, please contact our office, and ask for someone in the wastewater department.