Olympic officials have given the River Seine the green light to host the men's and women's Olympic triathlon races, despite days of concerns about water quality

The decision to go ahead with the Seine swim for the triathlon competitions is a big win for the city, Olympics organisers and athletes.

Organisers confirmed early Wednesday, local time, the river complied with requisite quality standards.

"The results of the latest water analyses, received at 3:20am, have been assessed as compliant by World Triathlon allowing for the triathlon competitions to take place," a brief statement from Olympic officials read. 

As a result, men's triathlon will now be held at 6:45pm (AEST) on Wednesday, with the women's event scheduled to start two hours' earlier. 

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The men's event was originally scheduled to be held on Tuesday but elevated levels of bacteria forced it to be delayed. 

Preparatory swims, designed to give athletes a chance to familiarise themselves with the course, had already been cancelled due to poor water quality on Sunday and Monday.

Officials undertook an ambitious plan leading into the Games, including 1.4 billion euros ($2.3 billion) in infrastructure improvements, to clean up the long-polluted Seine and have been steadfast in their insistence that the swimming portion of the triathlon and the marathon swimming events next week could safely be held in the river.

The water quality in the Seine is directly affected by rainfall that can lead to wastewater runoff flowing into the river. Heavy rains fell during Friday's Olympic opening ceremony, and rain continued for most of the day Saturday.

Every day since Sunday, even as organisers announced cancellations or postponements, they continued to express confidence that the swims in the Seine would go forward as planned the next day. 

For several days they did not publicly release the data on levels of E. coli and other bacteria that contributed to their decisions.

High levels of E. coli in water can indicate contamination from sewage. Most strains are harmless and some live in the intestines of healthy people and animals. But others can be dangerous. 

Even a mouthful of contaminated water can lead to diarrhoea, and the germ can cause illnesses such as infections in the urinary tract or in the intestines.

Daily water quality tests measure levels of the fecal bacteria E. coli. 

World Triathlon's water safety guidelines and a 2006 European Union directive give a range of E. coli levels from "excellent" to "sufficient." 

Anything beyond 900 colony-forming units per 100 millilitres is not considered safe or "sufficient." But experts stressed that these numbers are simply guidelines used to assess risk.

Plus, Aurélie Merle, the Paris 2024 director of sports, told reporters Tuesday that water samples are taken 21-and-a-half hours before decisions are made about the swim. 

That leaves uncertainty about their accuracy the day of the race.

Merle said Tuesday that testing results showed "we're very close to the threshold of triathlon." 

She noted that one of four test sites along the triathlon course was below the threshold for E. coli. Two other sites were just above the limit and one was more elevated, she said, citing a range of 980 to 1,553.

Organisers have said the marathon swimming races, scheduled for August 8 and 9, could be relocated if needed to the Vaires-sur-Marne Nautical Stadium in the greater Paris region, which already hosts rowing and canoeing competitions and can accommodate up to 15,000 spectators.

Efforts to make the river suitable for swimming included the construction of a giant basin to capture excess rainwater and keep wastewater from flowing into the river, renovating sewer infrastructure and upgrading wastewater treatment plants.

Paris Mayor Anne Hidalgo very publicly took a swim in the river two weeks ago, along with Paris 2024 chief Tony Estanguet; Marc Guillaume, the top government official for the Paris region; and swimmers from local swimming clubs. 

Data released last week show that E. coli levels at the Bras Marie were at 985 units per 100 millilitres that day, slightly above the established threshold.

ABC/AP