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Wet Weather Groundwater Determinations & Percolation Testing

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Rainfall Totals and Testing Open/Closure Dates

2024 Wet Weather Testing Season – Closed for the Season

Groundwater determinations and percolation testing have closed for the season in all testing areas.

The information in the table below is updated when open dates change or when new rainfall totals data are collected. Testing is open up to and including the date displayed. The latest updates to the data are displayed in bold below. Data and open/closure information are valid as of April 24, 2024.

Test Area Total Required (inches) Total to Date (inches) Groundwater Determinations Percolation Testing
Cloverdale 20” 41.32” Closed Closed
Guerneville 25” 43.56” Closed Closed
Healdsburg 20” 46.73” Closed Closed
Petaluma 12.5” 26.89” Closed Closed
Santa Rosa 15” 36.38” Closed Closed
Sebastopol 17.5” 30.31” Closed Closed
Sonoma 15” 31.26” Closed Closed

Permit Sonoma allows wet weather groundwater determinations and percolation testing between January 1 and March 1, and after having received 50% of the average rainfall. However, time extensions may be authorized by Permit Sonoma based on extended periods of rainfall before January 1 and/or after March 1.

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.

Permit Sonoma has tentatively proposed in pending OWTS Manual revisions assigning observation holes an effective radius of 50 feet and soil percolation test holes an effective radius of 25 feet to establish appropriate sites for primary dispersal areas and reserve replacement areas. Applicants for wet weather ground water determinations and soil percolation tests are strongly encouraged to consider the proposed revisions, or similar concepts, to create sufficient observation and/or percolation holes for the anticipated septic system types for primary dispersal areas and reserve replacement areas. Applicants must also provide clear and accurate site plans identifying the location of observation and/or percolation holes and how to access them.

Inspection Request

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