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Wet Weather Percolation & Groundwater Testing

Wet Weather Percolation and Groundwater Testing is CLOSED for the 2016-2017 season.


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 Closure Dates for 2016-2017 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 have been updated on May 1, 2017.

AreaTotal Required (inches)Total to Date (inches)GroundwaterPercolation
Santa Rosa15"60.07"CLOSEDCLOSED

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.

Inspection requests for wet weather percolation/groundwater tests must be made by calling the County’s automated inspection request system, SELECTRON, at (707) 565-3551.

Contact Information

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

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