French authorities are in a race against time to clean up the Seine, which will host the swimming leg of the triathlon at the end of July and open water swimming in August. According to AFP, the water charity Surfrider has been carrying out regular tests to measure levels of two key bacteria that indicate the presence of faecal matter. Their latest results showed levels of E.Coli and Enterococci above the acceptable levels set by sporting bodies and European bathing standards.

One reading for E.Coli at the Alexandre III bridge was more than three times the maximum level allowed by triathlon and open water swimming federations. Heavy rainfall in May is likely to have contributed to what Surfrider called "poor" water quality. It noted that there had been heavy rain 36 hours before the tests, but only light rain in the 12 hours before. Heavy rainfall is known to overwhelm Paris's century-old sewage system, resulting in untreated sewage being dumped directly into the river.

Paris Olympics organisers have postponed the rehearsal of the opening ceremony on the Seine for the second time due to high water levels. The river, a central element and highlight of this year's Games, has been affected by a stormy May, leading to the cancellation of the 27 May rehearsal after an earlier one on 8 April. "It will take place when the weather conditions allow for it," officials told AFP.

Organisers are praying for good weather during the Olympic Games, which run from 26 July to 11 August, and have been open about the possibility of having to delay or even cancel the Seine swim in the event of storms. Cleaning up the Seine was "probably the most difficult (Olympic) project to organise", Paris deputy mayor Emmanuel Gregoire told reporters in April.

Although a new water treatment plant was inaugurated upstream of Paris at Champigny-sur-Marne at the end of April, another major Olympics-related water infrastructure project has yet to come on stream. A huge new underground stormwater facility near Austerlitz station in eastern Paris - which will store water to prevent it being discharged into the river - is due to come on stream in early June.

Around 1.4 billion euros ($1.5 billion) has been spent by French authorities on upgrading sewage treatment and stormwater facilities around Paris to reduce the amount of untreated faecal matter flowing into the river and its main tributary, the Marne.

The clean-up of the Seine has been promoted as one of the key legacies of the Paris 2024 Games, with Mayor Anne Hidalgo planning to create three public bathing areas in the river next year. Both the mayor and President Emmanuel Macron have promised to take a dip before the Games to show it is safe, with sources saying Hidalgo has pencilled in 23 June for her swim.

Olympic open-water swimming has been hit by pollution concerns in the past, most notably at Rio 2016 and Tokyo 2021.