A closed beach after a thunderstorm in Perpignan; swimming sites in the seine inaccessible after rains in Paris; A purple flag, a sign of pollution, hoisted in Marseille after showers … The swimming areas are often closed after bad weather. And even if they remain accessible and the sun is back, 20 Minutes Explain to you why it is advisable to wait before throwing yourself in sea, lake or river waters after a downpour.
Bacteria and trash
“What generates water contamination?” In addition to the direct rejection of wastewater or punctual pollution linked to an accident, it is rain. It can cause waste of wastewater and a runoff, therefore leaching of roads which pours up to bathing water, “deciphers Raphaël Grisel, director of the Gipreb, mixed union of the Berre pond, in the Bouches-du-Rhône, which has 15 beaches.
Intense rain can indeed cause a phenomenon of soil leaching, that is to say the transport of elements (pesticides, animal excrement or various other dirt). Part of this water infiltrates the ground while the other runs towards the rivers, carrying invisible bacteria and waste, before reaching the sea and the oceans.
And this is how we find in water bacteria linked to fecal material, such as intestinal or E. coli enterococci. But pollution can also be visual: in Marseille, the beach located at the mouth of the Huveaune, a river which takes its source in the Sainte-Baume massif, is for example nicknamed “Beach peelings” because there are all kinds of trash after the rainy days.
“Giant toilets”
But it is not only on the urbanized coast that the rain, raising the dirty sidewalks, can cause contamination. The link between rain and development of bacteria in water, Fabrice Hamon has also established it for a few years in Landunvez, in Finistère. For several years, he regularly has checked the quality of the water from the Foul, a river flowing in his garden before going to reach the sea at the level of the castle beach. “We see the stream swelling or deflating according to the rains,” says the local resident. I saw colors not very friendly, even smells… ”
By dint of samples, the one that founded its application and its website for real -time monitoring, Bacterio Météo, notes that the level of bacteria soar in spring and summer, before falling in the fall and improving in winter. He then makes the link with the spreading of slurry in the neighboring fields. “The rain pollutes when there is something to wash,” he explains. It is like open -air toilets waiting for the flushing. Enough to discourage the most motivated swimmers.
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To avoid contamination, some adopt a precautionary principle. “In the Berre pond, our procedure is to operate a preventive closure of 24 hours, without samples, as soon as the rain exceeds 10 mm accumulated,” explains Raphaël Grisel, from the Gipreb. On a hot floor, it is from this quantity that there is runoff ”. To establish this period, the teams have taken samples after thunderstorms in order to observe bacterial decrease. They noticed that in 95 % of the cases, 24 hours after the rain, the waters found an “excellent” sanitary quality.
Last year, the Regional Health Agency (ARS) PACA, quoted by BFMTV, recalled that he was not recommended to bathe “within 72 hours after each strong bad weather”.