High levels of water pollution in the Seine River caused two swimming events to be cancelled over the weekend, after two others successfully went ahead on Thursday and Friday. A year ahead of the 2024 Paris Olympics, organisers say there is no Plan B for swimming events set to be held in the river.

“I always dive with [an] open mouth. It's not going to be funny if I wake up tomorrow morning with ... whatever," triathlete Kristian Blummenfelt told reporters on Wednesday, August 16, before jumping in Paris’s Seine River.

Athletes were testing out the water on Wednesday to get used to the river currents before four days of triathlon test events ahead of the Paris 2024 Olympics, during which multiple swimming races are set to be held in the river.

The races were a partial success for organisers. The women’s and men’s triathlons went ahead as planned on Thursday and Friday, before the swimming stages were called off for the weekend’s para triathlon and mixed relay races after high levels of E.coli bacteria were detected in the river water.

Results of water quality tests showed “significant discrepancies” in the hours leading up to Saturday’s events, organisers said in a statement.

The water quality did not offer the "necessary guarantees", said the Paris Olympics organising committee and governing body World Triathlon.

The cancellations came just two weeks after races for the 2023 World Aquatics Open Water Swimming World Cup were also cancelled due to the high levels of pollution in the Seine.

Yet the Paris 2024 organisers insist that open water swimming events can – and will ­– go ahead in the river during the games.

"We will remain in this extraordinary location, no matter what happens,'' said Tony Estanguet, head of the Paris 2024 organising committee and a former canoe champion. "We want to preserve this ambition.''

There is one small concession: a contingency plan will allow Olympic swimming events to be postponed for a few days if water quality isn't up to standard.

Bathing in the Seine has been banned since 1923 – although some determined open-water swimmers have continued the practice in the French capital’s waterways.

Promises to restore water quality date back to 1990, when then-Paris mayor Jacques Chirac - later French president - vowed, but failed to make the Seine safe for swimming again.

The plan for Olympic and Paralympic athletes to swim in the Seine is the most high-profile marker of Paris city hall’s recent efforts to clean up the river.

The largest pollution risks now come from heavy rain, which can cause the Parisian sewage system to overflow and be discharged into the Seine, polluting it with faecal bacteria E.coli and Enterococcus.

Swimming events earlier in the summer were disrupted by rain levels that reached four times the usual average, said Christophe Noël du Peyrat, chief of staff of the Paris region authority.

“We still have a lot of work ahead for year 2024 . . . to be able to face exceptional weather like what we've known at the end of July and beginning of August.”

However the cause for high levels of bacteria in the water on Saturday and Sunday is still being investigated. “To date, we have not yet found an explanation," Pierre Rabadan, deputy mayor of Paris in charge of Sport and the Olympics told AFP.

A €1.4 billion investment in waste-water management from the state and local authorities has produced “significant progress” in recent years, and polluted water overflow into the Seine has been reduced, say Olympics organisers.

Additional infrastructure is still a work in progress, including a giant underground reservoir in Paris that will stock excess water during storms so it doesn’t spill untreated into the river and can be treated later.

An extensive water testing system is also in place to ensure athlete satefy including hourly sampling and laboratory tests, according to Paris city hall.

If the Olympic triathlon race can go ahead as planned, the route will showcase some of the highlights of the French capital.

On Thursday and Friday, athletes dived into the river from a floating pontoon overlooked by golden statues on the 19th-century Pont Alexandre III bridge to swim two laps through the heart of the capital, past famous monuments including the Grand Palais and the Place des Invalides.

The cycle and running routes then took them along the Champs-Elysées avenue and past the Orsay Museum on the banks of the Seine.

On Thursday, some competitors were struck by the majesty of their surroundings.

It was a “special place to be in”, said American triathlete Katie Zaferes.

“It’s a magnificent event, everyone is so happy to compete here,” said France's Cassandre Beaugrand, who placed second in the race. The triathlete was unfazed by her dip in the river. "We are used to swim[ming] in much worse waters.”

Briton Beth Potter won Thursday’s event. Asked after her race whether she was concerned about the pollution risk, she said: "It's too early to say. Maybe we'll get sick, you never know,” before adding, “I hope not, but that's the risk you take swimming in open water.”