Open water swimming in urban areas - E. coli distribution with TELEMAC-3D

International audienceSwimming in urban open waters is increasingly popular. In Paris, a bathing area is open during summer in La Villette basin, our study site. To control sanitary risks, the European Bathing Water Directive (2006/7/EC) classifies the water quality according to thresholds of indicators of pathogenicmicroorganisms, the faecal indicator bacteria (FIB), such as E. coli. The reference FIB enumeration methods take more than 24 h.To obtain, in due time, information on the sanitary conditions in the bathing area, in this study, two issues were considered.On the one hand, for assessing more rapidly the microbiological level, a relationship between bacterial activity and fluorescent dissolved organic matter (FDOM) can be used. Here, E. coli wasestimated from a relationship with tryptophan-like fluorescence (TLF) established for La Villette system.On the other hand, the thermal stratification of the water column observed during summer heat waves can impact the spatio-temporal distribution of FIB, in water bodies with low current speed. The three-dimensional hydrodynamic model TELEMAC-3D was run to calculate the space and time evolution of E. coli. To reproduce the thermal stratification in the basin, WAQTEL thermal process was activated. TLF data measured at upstream were used as an input for the simulations.Two periods of ten days were simulated. In the first period (12- 22 September 2021), including a heavy rain event, five field measurements of E. coli were available. The model results were converted from TFL to E. coli and then compared to field data. There was a good agreement between them (RMSE =197 MPN/100mL, r² = 0.74). The regulatory threshold was overpassed during four days and the bathing area would have been closed during this period. The same E. coli input was used for a second simulation run during a heat wave period (13-23 July 2022). The thermal stratification was correctly reproduced. A slight lateral heterogeneity of E. coli distribution between left and right bankof the basin was simulated.These preliminary results indicate the good capability of a warning system combining upstream continuous TLF monitoring with hydrodynamic modelling. It would support a better assessment of sanitary conditions in a bathing area and improve the accuracy on the duration of the regulatorythreshold overpassing