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, while a women's event was postponed.

Organisers said the women's 10 kilometre race originally planned for Saturday - a qualifying event for marathon swimming at the Paris 2024 Olympics - had been postponed to 6 August.

International federation World Aquatics did not immediately respond to a request for comment about further backup plans for the races if the Seine's water quality does not improve this weekend.

"If the results are bad again ... obviously, to ensure the swimmers' safety, we'll have to continue with the same restrictions," said engineer Aurelie Lemaire, as she was taking samples from the Seine.

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.

"(Water) quality varies greatly depending on the weather," said engineer Maelys Hemon.

Construction on an €80-billion underground overflow basin designed to prevent such contamination is expected to be completed before summer 2024.

Cancelling the Seine swimming events 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.

Meanwhile, tourists wondered whether cleaning up the Seine would be possible at all.

"We were wondering when we came and we did take a cruise on the Seine, how they could possible get the water clean enough and safe enough for swimmers in a short period of time also with all the visitors still coming," said US tourist Lisa Larson.

"It's a worthwhile undertaking but I don't know if financially and technologically that could be done," said tourist Glen Kundert.