Marc Valmassoni, in charge of the Water and Health program for the environmental NGO Surfrider Foundation, details the reasons why rain and sun impact the quality of the Seine and can disrupt triathlon events.
Will they swim or not swim? Due to the rains that fell on Friday and Saturday in Paris, which polluted the water of the Seine, the organizers of the Olympic Games had to give up on Monday, July 29, authorizing the triathlon training scheduled for the morning in the river. While the men's event - the first of the Olympic Games to be held in the Seine - is scheduled to begin Tuesday at 8 a.m., the organizers remain "confident" that the competitions will be held on Tuesday and Wednesday, they said. And this "given the weather forecast for the next 36 hours." "As observed in July, with the summer conditions (more sunshine, higher temperatures, prolonged absence of rain), the quality of the water in the Seine has improved significantly," they added.
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For Libération , Marc Valmassoni, in charge of the Water and Health program for the environmental NGO Surfrider Foundation, explains the impact of the weather on the quality of the water in the Seine and looks back at the reasons for its pollution.
Why did the rains that fell last weekend pollute water of the Seine?
These rain events contribute greatly to the poor quality water of the Seine. The heavy rainfall that fell on Paris and the entire watershed, particularly during the Olympic ceremony, could have caused a lot of runoff that washed away sidewalks and roads. The treatment plants collect some of this wastewater. If a lot of water fell on roofs and at house level, some plants could have been saturated. In this case, they can no longer treat the wastewater and discharge untreated water directly into the environment, which can be a contamination factor. If the retention basins overflow, the water is no longer treated either. The question arises for the Austerlitz basin [the centerpiece of the Olympic anti-pollution arsenal, inaugurated in May by Paris Mayor Anne Hidalgo , editor's note] : did it overflow or, on the contrary, did it play its role as a collector of Paris' rainwater? In any case, with these stocks that are not necessarily well contained, a lot of potentially contaminated water, particularly with faecal matter, ends up in the Seine. This water can cause diseases and therefore have an impact on the holding of the events.
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Olympic Games
How could summer conditions improve water quality?
The most important factor is rainfall and the water of the Seine . When conditions are summery, there are no overflows. Water flow rates are then normal and in compliance with regulations, and a lower flow rate indicates a lower bacteriological input. Bathing water is therefore cleaner for the Olympic Games, with a health risk eliminated. There is also a turbidity factor [measures the more or less cloudy appearance of the water, editor's note] : in summer, the water is clearer, less loaded with organic matter, so the sun and UV rays can penetrate deeper and eliminate part of the surface bacterial load of the water. But this "reduction rate" must be put into perspective, because it concerns bacteria on the surface, in the first few centimetres. Regarding high temperature, no study to date stipulates that heat can have an impact on the degradation of the bacterial load.
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Will the water quality be sufficient to start the triathlon tomorrow, Tuesday July 30?
In theory, a return to normal with water presenting a concentration of bacteria below the thresholds recommended by swimming and triathlon federations (1000 npp /100 ml in E. coli and 400 npp /100 ml in enterococci) occurs between 24 and 72 hours after the precipitation. But this is very difficult to predict, and there is no certainty at the moment. We must wait for the sample taken by the Paris city hall on Tuesday morning, which will be taken just before the event, to be sure. All we can ask for is transparency. If the event is maintained, the organizing committee should, in my opinion, publish the results of the samples before it is held. Otherwise, this could fuel rumors.