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Wet Weather Percolation and Groundwater Testing


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:

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 April 26, 2017.

AreaTotal Required (inches)Total to Date (inches)GroundwaterPercolation*
Cloverdale20"75.29"CLOSEDOPEN TO 4/28/2017
Guerneville25"61.91"CLOSEDOPEN TO 4/28/2017
Healdsburg20"61.39"CLOSEDOPEN TO 4/28/2017
Petaluma12.5"39.79"CLOSEDOPEN TO 4/28/2017
Santa Rosa15"60.05"CLOSEDOPEN TO 4/28/2017
Sebastopol17.5"52.17"CLOSEDOPEN TO 4/28/2017
Sonoma15"49.74”CLOSEDOPEN TO 4/28/2017

* Note: Wet weather percolation testing has been extended.

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

Phone Operator Hours
Monday – Friday
8:00 AM – 5:00 PM
Public Lobby Hours
Monday – Tuesday,
Thursday – Friday
8:00 AM – 4:00 PM
10:30 AM – 4:00 PM
Permits with plans are not accepted after 3:00 PM.

Holiday Closure Calendar

2550 Ventura Avenue
Santa Rosa, CA 95403
38.465225, -122.723293

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