San Luis Rey Wet Weather Bacteria Source Identification Study, Phases I and II

Date / Time:
Wednesday, Sep 16 1:30pm to 2:00pm
Track / Session:
Track: Monitoring and TMDL Implementation / Session 7

According to the United States Environmental Protection Service (EPA), the leading cause of stream and river water quality impairments in the United States is pathogens, indicated by fecal indicator bacteria (FIB). In California alone, there are 441 Clean Water Act section 303(d) waterbody listings for FIB, and over 147 Total Maximum Daily Loads (TMDLs). The sources of FIB are notoriously difficult to identify, particularly non-point sources. As a result of the challenges with FIB source identification and the implementation of bacteria TMDLs, stakeholders in California are working to identify sources of FIB using microbial source tracking (MST) technologies, which are based on DNA sequences instead of the traditional culture methods for FIB. These methods are the future of tracking sources of FIB, and provide the benefit that likely sources of FIB can be identified using specific DNA sequences. Two MST assays, both   referred to as “HF183”are frequently used to identify potential sources of human fecal contamination in Southern California.


One such recent MST study was conducted in the San Luis Rey River (SLR River) in San Diego County during the 2018-2019 and 2019-2020 monitoring seasons. The regulatory driver for the study was a bacteria TMDL, which became effective in 2011. The objective of the study was to assess potential sources of elevated FIB detected during stormwater discharges in the SLR River over three events at 14 stations during Phase I the study. Phase II of the study was focused on identifying additional sources in the SLR main stem and Fallbrook sub-drainage areas. The findings of Phase II are expected to inform follow-up monitoring and actions to abate the sources and assist with TMDL compliance.


Monitoring consisted of the collection of six-hour, flow-weighted composites for FIB, MST markers, and pathogens during Phase I and MST markers during Phase II. Notably, composite samples were analyzed, whereas grab samples are typically analyzed for FIB and MST markers. At main stem stations up to four composite samples were collected during each of the three events, thus achieving up to 24 hours of storm runoff coverage. At tributary locations up to two composites were collected, representing 12 hours coverage.


Results of the 2018-2019 season showed a clear “flushing” effect of both FIB and MST markers during each event, allowing the research team to identify portions of the watershed for further follow-up study. Further, the MST markers were found to be correlated to FIB which is a potential indication of fresh sources. These findings show that composite sampling for FIB and MST is a viable method for sample collection for the purpose of source tracking in other watersheds.

Primary Speaker:
Neil Searing, County of San Diego

In 2017, Neil joined the County of San Diego Watershed Protection Program’s Science and Monitoring team overseeing water quality monitoring, assessment and reporting activities in compliance with MS4 Permit. He also participates in program planning and serves as the County’s representative on SCCWRP’s Commissioner’s Technical Advisory Group, which he is the current Chair. Prior to joining the Watershed Protection Program, he worked for 15 years in the Department of Environmental Health’s Land & Water Quality Division.  Neil has a BS in Biology from San Diego State University and is a CA Registered Environmental Health Specialist.

Supporting Speaker 1:
Alex Schriewer, Weston Solutions, Inc.

Dr. Schriewer is an Environmental Scientist with over 15 years professional experience in the assessment of environmental pollution. His expertise includes the gene-specific detection of a range of bio-targets, particularly pathogens and source identification markers. He has lead studies in support of several bacteria TMDLs in California as well as health impact studies in developing countries. He joined WESTON in 2015 and supports the company as Technical Director and Director of Weston’s Molecular Laboratory. He holds a M.S. in Inorganic Chemistry (2003) and a Ph.D. in Chemistry and Urban Water Management from the Technical University of Munich (2007).