Site-specific risk-based threshold (RBT) values for sewage-associated markers in estuarine bathing waters

This study establishes site-specific risk-based thresholds (RBTs) for sewage-associated markers, including Bacteroides HF183 (HF183), Lachnospiraceae Lachno3 (Lachno3), cross-assembly phage (CrAssphage), and pepper mild mottle virus (PMMoV), utilizing quantitative microbial risk assessment (QMRA) for recreational estuarine waters (EW). The QMRA model simulates health risks from ingestion of EW contaminated with untreated sewage. RBTs were estimated considering site-specific decay rates and concentrations of markers and pathogen (human norovirus; HNoV), aiding in the identification of high-risk days during the bathing season. Results indicate varying RBT concentrations for fresh (Day 0) and aged (Days 1 to 10) sewage contamination scenarios over 10 days. HF183 exhibited the highest RBT (26,600 GC/100 mL) initially but decreased rapidly with aging (2570 to 3120 GC/100 mL on Day 10), while Lachno3 and CrAssphage remained relatively stable. PMMoV, despite lower initial concentrations (3920 GC/100 mL), exhibited increased RBT (4700 to 6440 GC/100 mL) with aging due to its slower decay rate. Sensitivity analysis revealed HNoV concentrations as the most influential parameter. Comparison of marker concentrations in estuarine locations with median RBTs showed instances of marker exceedance, suggesting potential health risks. The observed discrepancies between bacterial and viral marker concentrations in EW highlight the need for optimized sample concentration method and simultaneous measurement of multiple markers for enhanced risk predictions. Future research will explore the utility of multiple markers in risk management. Overall, this study contributes to better understanding human health risks in recreational waters, aiding regulators, and water quality managers in effective decision-making for risk prioritization and mitigation strategies.