Microbial contamination at beaches threatens swimmer safety and reduces quality of life for beach-goers. Microbial source tracking (MST) marker concentrations are being measured throughout the world to help identify sources of fecal contamination at beaches, but there is no clear guideline for interpreting these concentrations. The project will determine risk-based thresholds for MST markers based on the risk of swimmer gastro-intestinal illness from exposure to human sewage, wastewater effluent and gull feces. Results will be shared widely to CA beach managers, CA State Water Board, and USEPA through oral presentations and summary tables and figures. Data will be made available as supplemental online information through the peer review publication process or through the Stanford Digital Repository (SDR).