Cross-sector agreement reached to study exfiltration

Leaders from Southern California’s stormwater and wastewater management communities have agreed to jointly pursue research aimed at assessing the potential for raw wastewater to exfiltrate from public sewer systems and reach storm drain systems – a landmark cross-sector collaboration that paves the way for improved understanding of this emerging management concern.

During a day-long workshop in February facilitated by SCCWRP and the California Association of Sanitation Agencies (CASA), attendees from the wastewater, stormwater, regulatory, and academic sectors identified three priority exfiltration-related projects that the wastewater and stormwater communities could work on together. CASA and the Southern California Stormwater Monitoring Coalition (SMC) have agreed to collaboratively pursue one or more of these projects.

The agreement to work together follows publication of SCCWRP-led study in 2025 that attempted to measure sewer exfiltration in the urbanized lower San Diego River watershed during wet weather. Although multiple lines of evidence pointed to the possibility of sewer exfiltration during wet weather, the technology that underlies two of the three lines of evidence used in the study were relatively novel. Multiple questions about exfiltration remain, including how applicable the San Diego study’s findings are to other Southern California watersheds, and how fecal contamination might be transported from the sewer pipe it has exfiltrated from to a water body downstream.

Preventing fecal contamination from reaching water bodies where people swim and surf has long been a top priority of both stormwater and wastewater managers.

Sanitary sewer collection system managers use robust surveillance systems for detecting spills and leaks, including routine camera-based inspections for detecting structural problems with underground sewer pipes. Meanwhile, stormwater managers routinely trace fecal contamination signals to their upstream origin points, enabling them to eliminate common fecal sources such as illicit sewer connections and illegal discharges.

Collectively, these investments by the stormwater and wastewater management communities have been highly effective at reducing human fecal contamination during dry weather, with more than 97% of Southern California beaches meeting water-quality objectives for fecal indicator bacteria. During wet weather, however, water quality rapidly declines and about two-thirds of these Southern California beaches fail to meet the water-quality objectives.

To understand the potential role that sewer exfiltration might be playing in contributing to water-quality degradation, the SMC and CASA agreed to collaboratively pursue up to three of the following priority projects:

Workshop participants are in the process of collaboratively scoping out these projects so both the SMC and CASA can further evaluate each project’s feasibility to implement. The SMC and CASA are targeting summer 2026 to launch at least one project.

During the day-long workshop, participants initially identified 30 candidate research projects related to exfiltration that span five thematic areas: improving existing tools to increase confidence in results; characterizing exfiltration prevalence and magnitude; investigating subsurface transport; investigating other potential sources of human fecal contamination; and developing guidance for stormwater and wastewater agencies.

Both the SMC and CASA have agreed they should co-manage and co-fund the projects, ensuring both sectors have an opportunity to directly shape the projects, review all findings, and coordinate on management actions they might take following the projects.

A technical report summarizing the workshop discussions and outcomes, including summaries of the three priority projects, is expected to be published in the coming months.

For more information, contact Dr. John Griffith and Dr. Joshua Steele.