An Integrated Monitoring Framework for Assessing Sanitary Wastewater Pressure from Recreational Boating in Coastal Anchorages

Sanitary wastewater discharges from recreational vessels are a difficult-to-quantify source of pressure in coastal anchorages because discharge events are intermittent, mobile and strongly dependent on user behaviour. We developed an integrated screening framework that jointly interprets environmental state (faecal indicator bacteria; FIB), pressure proxies (instantaneous vessel abundance and size structure), and behavioural/infrastructure context (a survey of wastewater systems and disposal practices). Ninety-six seawater samples from three stations were analysed for Escherichia coli and intestinal enterococci during 32 morning/afternoon campaigns. All individual samples were below the Croatian single-sample thresholds for excellent sea-bathing quality (E. coli < 100 and intestinal enterococci < 60 CFU/100 mL), although intestinal enterococci and E. coli were detected in 32% and 15% of the samples, respectively. The microbiological signal was descriptively stronger in the morning even though vessel abundance was substantially higher in the afternoon (121 vs. 266 vessels), and the highest FIB concentrations occurred at the southern low-local-pressure reference station, which had the fewest observed vessels (44). These differences were not statistically significant. Survey data showed that 82% of the respondents used vessels equipped with black-water-holding tanks, while marina pump-out facilities were used infrequently and disposal during navigation was reported more often. The integration therefore lies in triangulating non-equivalent lines of evidence: microbiological signals, observed boating pressure, and user/infrastructure conditions. This approach is intended for first-line screening and targeted monitoring, not for direct source attribution.