Participants of this weekend's Open Water Swimming World Cup were barred from a Friday morning training session in Paris' river Seine as heavy rainfall caused water quality to dip below health standards, the French Swimming Federation (FFN) said.
The women's 10 kilometre race on Saturday - a qualifying event for marathon swimming at the Paris 2024 Olympics - is still scheduled to go ahead, pending a new river water quality test on Friday evening.
"It won't be at another site, but if swimming is not possible on Saturday then both (men's and women's) races will have to take place on Sunday," said an FFN spokesperson.
"The water quality tests are done regularly, and they are progressively improving," the spokesperson added.
International federation World Aquatics did not immediately respond to a request for comment about further backup plans for the races if Seine water quality does not improve this weekend.
Last year's open-water swim competitions took place at the French capital's Parc de la Villette in northern Paris.
The city has been working on clean-up efforts to make the Seine swimmable again, as it was during the 1900 Paris Olympics more than a century ago.
But heavy rain causes the Parisian sewage system to overflow and be discharged into the river, polluting it with fecal bacteria E.coli and Enterococcus.
As a result, bathing in the Seine has been banned since 1923, with promises to restore water quality going back to 1990, when Paris mayor Jacques Chirac - later French president - vowed to make the Seine swimmable again.
Construction on an 80 billion euro ($88 billion) underground overflow basin designed to prevent such contamination is expected to be completed before summer 2024.
Cancelling the Seine swim meets would be bad optics for Paris ahead of the Olympics -- this year's first three qualifying races were swum in idyllic locales including the Red Sea's Soma Bay in Egypt and Sardinia's Golfo Arancini in Italy.
