Santa Rosa, CA – October 26, 2021 – Heavy rains from Sunday’s potent atmospheric river entered the wastewater collections system in the Sonoma Valley County Sanitation District system causing several wastewater overflows. Another brief overflow occurred in the Penngrove Sanitation Zone. Rainfall measured by Sonoma One Rain gages exceeded 7.5 inches in Sonoma Valley and nearly 9 inches on Sonoma Mountain, causing inflow into the two wastewater collection systems to temporarily exceed capacity.
During heavy rain events wastewater collection systems can become overloaded due to inflow and infiltration of rainwater and groundwater into sewer mains. Wastewater maintenance and work crews were staged in the field in anticipation of the heavy inflows into collection systems and used pump trucks and pumps to divert as much water as possible. Signs were posted at overflow sites, and the State Office of Emergency Services and California Department of Fish & Wildlife were notified.
Crews continue to monitor the collection systems and to evaluate any impacts to public and environmental health. Overflows generally run into storm drains which then drain into nearby creeks. Sunday’s overflows occurred at the following locations:
Sonoma Water, which operates both collection systems, has made significant investments in projects to prevent overflows. 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 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 increase by as much as nine times the average flow.
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.