Arup partners with County of Sonoma to launch onsite wastewater treatment system information hub

The study, presented on October 8 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 explored the feasibility of forming new wastewater districts or modifying the boundaries of existing districts to serve communities where septic systems are highly concentrated.

Onsite wastewater treatment systems (OWTS), also called septic systems, are useful and necessary for residents at locations that are removed from centralized wastewater treatment systems. When properly sited, designed, operated, and maintained, septic systems treat domestic wastewater to reduce its polluting impact on the environment and protect public health. However, if poorly maintained or improperly sited, septic systems can impact the environment and surrounding communities.

Working together with the County, Arup developed an interactive geospatial tool to help identify locations of septic systems across the county to inform decisions about wastewater infrastructure and about how wastewater may be managed and governed. Well managed and effective wastewater treatment benefits both the environment and communities. As the sole consultant on the project, Arup’s team of geospatial analysts and water engineers provided technical advice, incorporating local community and stakeholder feedback.

To conduct the study, Arup’s geospatial experts mapped the location and relative density of septic systems across the county, identifying over 36,000 known and suspected septic systems. The analyses showed significant concentrations in larger rural communities, and these higher-density parcels were collected into community clusters with similar existing or potential governance structures and similar technical conditions, such as slopes, soil types, and proximity to water bodies. The mapping work forms the basis for identifying and analyzing these clusters and suggests potential alternative wastewater infrastructure and management solutions to positively impact the community and the environment.

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