Despite an investment of 1.4 billion euros ($1.5 billion) to prevent sewage leaks into the waterway, the Seine has been causing suspense in the run-up to the opening of the Paris Games on July 26 after repeatedly failing water quality tests.
But since the beginning of July, with heavy rains finally giving way to sunnier weather, samples have shown the river to be ready for the open-water swimming and triathlon -- and for 65-year-old Hidalgo.
"On the eve of the Games, when the Seine will play a key role, this event represents the demonstration of the efforts made by the city and the state to improve the quality of the Seine's waters and the ecological state of the river," Hidalgo's office said in a statement on Tuesday.
She had originally planned to swim last month, but had to delay because bacteria indicating the presence of faecal matter were found to be sometimes 10 times higher than authorised limits.
Her long-awaited dip has sparked jokes and memes on social media, with one viral AI-generated image showing her looking like the wrinkled Dobby character from the Harry Potter films as she emerges from the water.
A short-lived protest group posting under the hashtag #jechiedanslaSeine (#IshitintheSeine) had also encouraged people to empty their bowels in the river upstream to protest against the Socialist.
She is set to be accompanied on Wednesday by chief Paris Games organiser Tony Estanguet, who is a former canoeist, and the top security official for the greater Paris region, Marc Guillaume.
President Emmanuel Macron, who had promised to join the Seine bathers, will be a notable absentee, with the head of state occupied by a political crisis caused by his decision to call snap parliamentary elections last month.
The Seine is set to be used for the swimming leg of the Olympics triathlon on July 30-31 and August 5, as well as the open-water swimming on August 8-9.
Strong currents
The locations chosen for open-water swimming have caused difficulties at past Olympics, notably ahead of the 2016 Rio de Janeiro Games and those in Tokyo in 2021.
"It's been raining all over France. Summer has been very late to arrive and so have the good results," said Marc Valmassoni from clean-water campaign group Surfrider which has been conducting weekly tests on the Seine since last year.
"They're not excellent, they're not terrible, they're average. But at this time the water is swimmable."
Cleaning up the Seine has been promoted as one of the key legacy achievements of Paris 2024, with Hidalgo intending to create three public bathing areas for the city's residents next year, a century after swimming was banned.
"We're not doing it for three days of competition in the Seine," chief organiser Estanguet told AFP during an interview last week. "We're going it above all for environmental reasons... I'm proud that we've served as an accelerator."
French authorities have invested in new water treatment and storage facilities in and around Paris, as well as ensuring that thousands of homes and canal boats without waste water connections are linked up to the sewerage system.
Major storms still overwhelm the Paris underground waste-water network, however, some of which dates back to the 19th century.
The heavy rains of May and June have led to regular discharges of untreated effluent into the Seine and have also increased the flow and height of the river, causing different problems for Paris 2024 organisers.
Rehearsals for the Seine-based opening ceremony, during which thousands of athletes are set sail down the river, have had to be repeatedly postponed because of the strength of the currents.
The Seine's flow is currently around 400 m3/second -- many times above the usual level for this time of year of around 100-150 m3/s.
Sports Minister Amelie Oudea-Castera became the first public figure to take to the Seine on Saturday, with videos of her slipping on a walkway into the water while wearing a full wet-suit going viral.
