Quantitative Microbial Risk Assessment for Bathers in a Popular Bathing Area in Northern Italy

In this study, we used QMRA based on enterococci counts in order to evaluate the Gastrointestinal Illnesses (GI) risk resulting from recreational activities at a very popular seaside resort in Italy. Three bathing areas were selected, since they are impacted by riverine discharges, that receive polluted waters from point and non-point sources, and have been sometimes closed, owing to enterococci and E.coli levels over the European regulation. The swimmer GI risk from enterococci was estimated applying exponential dose-response model, and the simulation for QMRA model were performed using Vensim software. The ingested enterococci dose was calculated combining ingestion rate, exposure time and enterococci concentrations. In particular, exposure time was fitted to a Weibull distribution and the ingestion rate was drawn from a triangular distribution. Enterococci concentrations were fitted to a lognormal distribution for each monitoring area, basing on 10-years microbial data collected from seawaters at the river mouths. The risk estimation was obtained through the use of a Monte Carlo simulation, with each run consisting of 10 000 trials. The results were compared to acceptable marine recreational illness levels of 10 and 50 GI illnesses per 1 000 exposures (0.01 and 0.05). The results showed a difference in GI risk among the three bathing areas, with a probability of exceeding the illness acceptable levels of 0.6% and 0.01 % (respectively) for No1 area, 0.63% and 0% for No2 area and 1.34% and 0.07% for No3 area. Based on monitoring data collected according to European Bathing Directive, the percentage of values over the limits for beach closures were 3.5% for No1 area, 2.7% for No2 area and 4.9% for No3 area. These results should be further evaluated to better understand why the punctual monitoring based on current legislation seems to overestimate the risk of GI compared to QMRA model outcomes.