Last August, the marathon swimming test events were cancelled because the water was too dirty, as were the swimming legs on two of the four days of the triathlon and para-triathlon trials. The City of Paris insisted that "there is no plan B". 

The course for the men's and women's 10km events starts at the iconic Beaux-Arts Pont Alexandre-III and, with the Invalides and the Eiffel Tower in the background, heads 1km down the river past other famous landmarks, including the Musee d'Orsay, and the Grand Palais. 

Perhaps fittingly, it passes the newly renovated Paris Sewerage  Museum and the more minimalist Pont d'Alma before looping back. The triathlon swims are shorter and return earlier. The route was chosen to showcase the beauty of Paris. 

It is also politically symbolic: swimming in the Seine has been banned since 1923, but successive Paris mayors have vowed to open it up.

In 1990, when he was mayor before becoming French president, Jacques Chirac promised that the river would "soon" be clean enough to swim in and that he would celebrate by taking a dip. He never did. 

The current mayor, Anne Hidalgo, a strong proponent of green initiatives, has also promised to take the plunge before the Olympics start and that the public will be able to swim in three places by 2025. 

She has yet to get her feet wet. City officials argue that the quality of the water has improved, but none of the samples taken between June and September 2023 met European standards for minimum water quality for swimming. The big problem is faecal matter. 

Bacteria in the water spike when heavy rains sweep debris and untreated sewage from the banks and overflowing drains and sewers into the river. The city tests the water at 14 points. In 2022, water quality was rated 'sufficient' at three of them, but had deteriorated by last summer.

Last August's open-water swimming event was cancelled after heavy run-ups sent E.coli levels up to six times the target level set by World Aquatics. The city of Paris insisted it had "learned" from the sampling problems at the test events. National and local authorities are also investing 1.4 billion euros (more than $1.5 billion) in five projects to clean up the water. 

According to a Paris city official, the failed tests for the team triathlon and, two weeks later, the para triathlon swimming were not caused by rain but by a "valve malfunction" in the Paris sewerage system. The weather remains the "main risk", according to Paris City Hall, which fears "exceptional rainfall". 

The only contingency plan for the swimming events is to postpone them for a few days. "There is no solution to move the event, the triathlon and open water swimming will take place in the Seine next year," said Tony Estanguet, director of the Paris 2024 organising committee, after the cancellations last August. For the athletes these are the Olympic Games and dirty water is a constant risk in open water competitions.

At the end of the trials for the 2019 Tokyo Olympics, swimmers protested about the quality of the water in Tokyo Bay. Polluted Guanabara Bay hit the headlines ahead of the 2016 Rio Olympics. "A glittering backdrop," Italian double world champion Gregorio Paltrinieri told Italian media in January. 

"Even if the water is dirty, I would rather swim in an electric atmosphere in the centre of Paris than in an anonymous stretch of water." France's Marc-Antoine Olivier, who won silver at the World Championships in Qatar in early February, said he was excited about the venue. 

"People might be afraid of what's in the water, but swimming in a historic place will be incredible," he said. "Of course, a lot of people will try to make a bit of a fuss about the conditions we'll have in the water, but if we can swim then there's no problem. 

"They are not going to take the risk of us swimming and someone catching something. The triathlons could become duathlons like some did last summer. It would be a shame, but we have adapted to a duathlon," said Briton Beth Potter, who won the individual test event.