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Wet Weather Percolation & Groundwater Testing

Rainfall Totals and Testing Open/Closure Dates – 2018-2019 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 and closure information were updated on March 25, 2019.

AreaTotal Required (inches)Total to Date (inches)GroundwaterPercolation
Cloverdale20"62.32”Open To 4/2/2019Open To 4/16/2019
Guerneville25"44.43”Open To 4/3/2019Open To 4/17/2019
Healdsburg20"40.55”CLOSEDOpen To 4/3/19
Santa Rosa15"40.11”Open To 4/2/2019Open To 4/16/2019
Sebastopol17.5"34.82”Open To 4/3/2019Open To 4/17/2019
Sonoma15"36.24”Open To 4/3/2019Open To 4/17/2019

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

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

Inspection Codes

Wet weather percolation test: 384
Wet weather groundwater test: 386
View inspection codes for the Well & Septic section


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).


Contact Information

Contact Well & Septic by Phone
2550 Ventura Avenue
Santa Rosa, CA 95403
38.465074, -122.723705

Well & Septic Districts Map

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Well and Septic Districts Map

Map of well and septic environmental health districts within Sonoma County.