Comment surveiller et prévoir la qualité microbiologique des sites de baignade dans les cours d’eau urbains ? Le site d’étude du bassin de la Villette (Paris)

Swimming in urban rivers is becoming increasingly popular. In order to limit the health risks to bathers, a warning system can be used to anticipate episodes of microbiological contamination and thus better manage any bathing closures. This article presents a short-term (24 hours) forecasting system developed at La Villette bathing site in Paris, where a public bathing area was opened in 2017. It is based on the measurement of physico-chemical and hydraulic variables and on the in situ measurement of faecal indicator bacteria (FIB) upstream of the bathing area. The functioning of this early warning system is illustrated during 2021 summer. The data collected from the 1st June to the 31st August 2021 showed a major contamination event following heavy rainfall in mid-July. This event led to the closure of the bathing facility. By integrating these measurements with a hydrodynamic model, Telemac-3D, the transport of bacteria and their spatial distribution were simulated. The observations coupled with the simulations also highlighted the thermal stratification and its impact on the velocity and flow direction, which can significantly alter the transfer time of FIB. The results demonstrate the ability of this approach to anticipate contamination peaks, providing useful information to manage closure and reopening of urban bathing areas.