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Heavy Rains Cause Sonoma Valley Wastewater Overflows

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Heavy Rains Cause Sonoma Valley Wastewater Overflows

Heavy rainfall from a large atmospheric river resulted in several wastewater overflows within the Sonoma Valley County Sanitation District (District). Nearly 2 inches of rain was measured during a 24-hour period ending Tuesday at 9 a.m. at the Sonoma Valley wastewater treatment plant. Other areas of the valley reported 3 inches of rain overnight.

Wastewater maintenance and work crews, including biologists, responded to nine separate sanitary sewer overflows (SSOs) starting at 2:30 a.m. Tuesday. Sanitation District staff are working to minimize the flows as much as possible, evaluate any impacts to public and environmental health, and will continue to monitor the collection systems.  The State Office of Emergency Services and California Department of Fish & Wildlife were notified. 

The majority of the SSOs occurred in the Boyes Hot Springs and Fetters Hot Springs areas, with two overflows in Eldridge. Several of the overflows resulted in wastewater flowing into Sonoma Creek. One reported SSO at the Sonoma Charter School and Flowery Elementary School campus was determined to be a private overflow resulting from a failure of the collection system on school district property. Total volume of the overflows will be calculated at the end of the storm event.

During heavy rain events the District’s wastewater collection system can become overloaded due to inflow and infiltration of rainwater and groundwater into sewer mains. The District is currently implementing a multi-year sewer main replacement project. Another contributing factor to SSOs, which is being addressed by the District, is leaky private sewer laterals and illegal connections of roof downspouts, yard drains, sump pumps and other non-sewer discharges into the sewer system that can greatly increase the amount of wastewater entering the system during storms. The SVSCD Board recently approved an ordinance that will require older laterals to be inspected and repaired, if necessary. 

The Sonoma Valley wastewater treatment plant is capable of treating up to 12 million gallons per day (mgd) and storing up to 35 million gallons of untreated wastewater, but during heavy rain events inflow into the plant can more than triple. Prior to the storm on Monday and early Tuesday, inflow to the treatment plant was about 4 mgd. During the peak of the storm the plant was taking in between 16 and 17 mgd.

The wastewater collection system treats wastewater from approximately 17,000 equivalent single-family dwellings within the City of Sonoma and the unincorporated areas of Agua Caliente, Boyes Hot Springs, Eldridge, Fetters Hot Springs, Glen Ellen, Schellville, Temelec, and Vineburg.



The Sonoma County Water Agency is working to secure our future by investing in our water resources, community and environment.  The Water Agency provides water supply, flood protection and sanitation services for portions of Sonoma and Marin counties. Visit us on the Web at