Weeks before the Olympic Games are set to begin in Paris, the River Seine is still not safe for swimming on most days due to high levels of E. Coli bacteria in the water, according to official data.

Official testing by the city of Paris shows E. Coli levels at Alexandre III Bridge, where the triathlon events are planned to start later this month, have been above acceptable levels on the majority of days 鈥� 22 out of 30 鈥� between June 3 and July 2, potentially exposing athletes to serious health risks.

However, authorities say there has been a general improvement in the Seine鈥檚 water quality as weather conditions have improved in recent weeks and are 鈥渃onfident for the Olympics now.鈥�

鈥淲e are having weather that is more consistent with the season,鈥� Antoine Guillou, Deputy Mayor of Paris, told CNN Wednesday in a telephone interview. 鈥淭he quality of the Seine is good.鈥�

The mayor of Paris told French radio Wednesday that she would swim in the city鈥檚 River Seine next week, in a bid to show its suitability for the Olympics despite ongoing cleanliness concerns.

Mayor Anne Hidalgo told France Inter radio that she would 鈥渄ive in next week鈥� and the water would be 鈥渄epolluted, that鈥檚 for sure.鈥�

Official data shows one-off rainfall events can quickly cause E. Coli levels to spike again.

As recently as June 30, following rain the previous day, E. Coli levels increased to around 2000 CFU/100mL at Alexandra III Bridge. That鈥檚 double the level needed for 鈥済ood鈥� water quality, as per World Triathlon standards.

If E. Coli levels are above 1000 CFU/100mL, the swimming leg of the triathlon will have to be cancelled, according to World Triathlon competition rules, unless the organization鈥檚 medical committee decides the race can go ahead.

Earlier this week, Paris 2024 told CNN that it has 鈥渇ull confidence鈥� in the work undertaken by the City of Paris. 鈥淣evertheless, the potential for unseasonal weather events requires us to be cautious,鈥� they added.

The opening ceremony of the Paris Olympics is set to take place on the Seine on July 26 if currents are not too strong. A recent rehearsal on June 24 was cancelled due to strong water flows caused by heavy rainfall.

10x acceptable level

Swimming in the Seine has been illegal for more than 100 years due to the river鈥檚 pollution levels, though some swimming has been permitted recently as part of triathlon test events.

The individual triathlon races will begin with a 1.5-kilometre swim, followed by a 40K bike race 鈥� passing by the Le Grand Palais and Champs-Elysées 鈥� before finishing with a 10K run through the heart of the city.

A mixed team relay race involving four athletes 鈥� two men and two women 鈥� will start with each team member completing a 300-meter swim, followed by a 5.8K bike ride and 1.8K run.

Authorities have spent at least 鈧�1.4 billion (US$1.55 billion) to clean up the Seine. One of the key infrastructure projects designed to help tackle the issue of pollution has been a new rainwater storage basin - called the Austerlitz basin - which can hold 20 Olympic swimming pools worth of water, according to a statement from Paris 2024.

The basin is now operational. Following rainy weather on June 17 and 18, the basin filled to 80% capacity, successfully preventing 40,000 m3 of wastewater and rainwater from being discharged into the Seine, according to the city of Paris.

But E. Coli levels near Alexandra III Bridge still climbed to around 10000 CFU/100mL on the second day of rain. That鈥檚 around 10 times higher than World Triathlon鈥檚 鈥済ood鈥� water quality level, according to CNN鈥檚 calculation.

鈥淚f there is a storm on one day, yes, you can see a deterioration of the quality due to that storm. But then it is possible to wait until the swimming conditions are back,鈥� Guillou said.

Cautious optimism

Concerns about the Seine鈥檚 poor water quality have been bubbling away for a while now. 鈥淲e鈥檝e had lots of rain in June, much more than usual. That explains why the quality of the Seine was above acceptable levels until late June,鈥� Guillou said.

But some say the tide could be turning with the arrival of sunnier and drier weather conditions and new technology being deployed by the city of Paris.

Technology company Fluidion, founded in Paris in 2012, has been testing the Seine鈥檚 water near Alexandre III Bridge multiple times a week since early April. Longer term trends show there has been a 鈥渟ignificant improvement鈥� in water quality, according to Dan Angelescu, the company鈥檚 CEO.

The Seine serves as Fluidion鈥檚 primary research site, according to the company, hence why they routinely collect water samples for internal research efforts. CNN has not independently verified the organization鈥檚 findings.

Earlier this year, between early April and late May, E. Coli levels were just under 3400 MPN/100mL (an equivalent unit to CFU/100mL), according to Fluidion鈥檚 testing. That鈥檚 more than three times higher than the level needed for 鈥済ood鈥� water quality, according to CNN鈥檚 calculation.

In comparison, between June 24 and July 5, average E. Coli levels dropped to around 880 MPN/100mL, according to Fluidion.

But Angelescu said standard testing methods cannot accurately detect 鈥渁ggregate-bound鈥� E. Coli bacteria that are clumped together in the water, potentially posing an 鈥渦nderreported health risk.鈥�

Testing carried out using Fluidion鈥檚 ALERT technology, which the company says can measure aggregate-bound E. Coli bacteria, shows E. Coli levels have decreased in recent weeks, but much less dramatically than standard testing indicates, Angelescu said.

鈥淭his discrepancy raises important questions about the true safety of the water for swimming events and highlights the need for current regulations to evolve,鈥� he added.

Paris 2024 shares the view that sunshine and higher temperatures have brought about a significant improvement in water quality, but they add that caution is still needed.

鈥淭he potential for unseasonal weather events requires us to be cautious, which is why we have put in place a number of contingency plans including options for postponing the events,鈥� a Paris 2024 spokesperson said in a statement, adding that the 鈥渇inal resort鈥� would be to downgrade the triathlon events to duathlons with no swimming.

For the marathon swimming events, the Vaires-sur-Marne Nautical Stadium in Seine-et-Marne, where the canoe-kayak and rowing events are being held, could act as an alternative venue, they said.

鈥淎s an organizing committee, our first priority and responsibility is to allow athletes who have been training for these Games for years, to complete their preparation for the competition with full peace of mind,鈥� the spokesperson said.

Guillou added that a new 10-kilometre-long basin, located between Essonne and Val-de-Marne, is going to be made operational in the coming days, which like the Austerlitz basin will help to reduce pollution. 鈥淲e are confident that with normal weather conditions there will not be any problem,鈥� he said.

CNN鈥檚 Julen Chavin in Paris contributed to this report.