New study reveals widespread use of septic systems in Sonoma County, identifies opportunities and challenges

The County of Sonoma today unveiled a new study and interactive map of more than 36,000 local parcels with septic systems, highlighting opportunities and challenges relating to wastewater treatment in vast areas of the county that are not currently served by a sewer system. 

The study, presented today to the Sonoma County Board of Supervisors, examined the locations of septic systems and identified areas where existing or future wastewater management entities could serve rural property owners. It explores the feasibility of forming new wastewater districts or modifying the boundaries of existing districts to serve communities where septic systems are highly concentrated. 

“The state is tightening requirements for septic systems to ensure the water in our rivers, creeks, lakes and underground aquifers is clean and safe,” said Supervisor David Rabbitt, chair of the Board of Supervisors. “This study, and its accompanying web hub, provides government and property owners with the data we need to find solutions that work for everyone.” 

The study found 12,504 parcels with onsite wastewater treatment systems, also known as septic systems, that have been permitted since 1991. It identified another 23,742 parcels believed to have septic systems that were either permitted before 1991 or installed without a permit. More than 41 percent of local septic systems serve single-family homes on parcels of less than 1.25 acres, a usage considered high-density by the study. 

The study also identified 47 community clusters that could potentially benefit from shared septic systems, connecting to adjacent sewer systems, or the creation or modification of a shared wastewater management entity. Of those, 39 are in the Russian River watershed, four in the Petaluma River watershed and four in the Sonoma Creek watershed. All are located in areas subject to the more stringent septic standards resulting from existing or foreseeable state requirements that limit discharge of bacteria and pathogens. 

An interactive map displaying the location of local septic systems and other data is available at gis-owts.sonomacounty.ca.gov. The site includes resources for property owners who rely upon septic systems. 

The Board of Supervisors commissioned the study in 2021. It was conducted by Arup, a global engineering firm that worked with the County Administrators Office, Permit Sonoma and the County’s Information Systems Department to create the Mapping Study Report, GIS mapping tool and septic system resource hub. 

The mapping study will help advance several efforts by the County and its partners to manage wastewater and increase climate resilience in our water systems. Next year, the County will launch a grant-funded project that will evaluate sources of pathogens in the Russian River and engage with communities along the river to develop potential solutions that could improve water quality. 

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Media Contact: 
Ted Appel, Communications Specialist 
publicaffairs@sonoma-county.org 
(707) 565-3040 
575 Administration Drive, Suite 104A 
Santa Rosa, CA 95403