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Wet Weather Percolation & Groundwater Testing


In order to protect the County's groundwater from pollutants from septic systems, the County of Sonoma Onsite Wastewater Treatment Systems (OWTS) Regulations and Technical Standards requires that wet weather groundwater table determinations be conducted when each of the following situations are true for a property:

  • Installation of new or replacement septic systems.
  • Located in a basin area with slopes of 0% to 5%.
  • Its defined geographic area has received 50% of seasonal rainfall.

Testing must be conducted within 10 days of receipt of 0.8 inch or more of rainfall within a 48 hour period (as reported by the officially recognized reporting stations as published in the Press Democrat).


Rainfall Totals and Testing Open/Closure Dates for 2017-2018 Season

The information in the table below is updated when open dates change or when new rainfall totals data are collected. The latest updates to the data are displayed in bold below. Data were updated on March 19, 2018.

Area Total Required (inches) Total to Date (inches) Groundwater*Percolation
Cloverdale20"23.45"Open to 03/27/2018 Open to 04/10/2018
Santa Rosa15"17.80"Open to 03/26/2018 Open to 04/09/2018

The Memorandum of Understanding with the State of California Regional Water Quality Control Board allows for wet weather percolation and groundwater testing between January 1 and March 1, and after having received 50% of the average rainfall.

Depth to groundwater determinations can be conducted within ten (10) calendar days following a rainfall of 0.8 inches in a 48 hour period. Sonoma County is divided into 7 geographic areas for this purpose. The amount of rainfall will be posted for each area.

*Due to lack of rainfall this winter, Permit Sonoma has extended the wet weather testing season beyond March 1, 2018. Geographic areas will be opened for wet weather percolation testing when each geographic area receives the total inches required of rainfall indicated in the table above. Geographic areas will be opened for wet weather groundwater testing when each geographic area receives the total inches required of rainfall indicated in the table above and receives 0.8 inches of rain in a 48 hour period.

Inspection Requests

Visit our Inspection Scheduling page to make inspection requests for wet weather percolation/groundwater tests.
View inspection codes.

Contact Information

Contact Well & Septic by Phone
2550 Ventura Avenue
Santa Rosa, CA 95403
38.465225, -122.723293