In early May, the Paris Mayor, Anne Hidalgo, along with Tony Estanguet, president of the Paris 2024 organizing committee, and other officials, inaugurated a large subterranean chamber located near Austerlitz station in the southeastern part of the French capital.
The chamber, which has a depth of 30m and a width of 50m, features a cathedral-like design supported by 20 massive pillars. Construction took 42 months and cost over €90 million. Unfortunately, it also witnessed the tragic accidental death of a construction worker, Amara Dioumassy.
The Bassin d’Austerlitz is capable of holding 50,000 cubic meters, equating to 20 Olympic-sized swimming pools. It collects stormwater, laden with bacteria, after heavy rainfall which would otherwise flow into the Seine, storing it until the sewage system can process it.
This facility serves as the highlight of an ambitious €1.4 billion project funded by the state and city, aimed at cleaning up the Seine since 2015, with a goal of making the river swimmable by 2025. Its first major challenge was to host the Olympic men’s triathlon swimming event.
As of now, it has not met this challenge: After an inspection at 3:30 AM on Monday (with results taking 18 hours), Games officials announced on Tuesday that the triathlon would be postponed until Wednesday, without confirmation it could proceed then.
“Despite the recent improvements in water quality,” the organizing committee stated, “certain areas of the course registered values above acceptable thresholds.”
The endeavor to guarantee a swimmable Seine for the Games was always going to be a race against time. Experts affirm that while it is feasible in the long run, the Olympic efforts have advanced this timeline by 10 to 15 years.
Swimming has been prohibited in the river throughout central Paris for just over a century, since 1923. However, city authorities indicated last July that it could be permitted again for the public at three supervised locations as early as next summer.
Included in the clean-up initiative are another holding tank constructed north of the city, upgraded treatment and purification facilities along the river, and the connection of 20,000 homes and houseboats to the sewage system.
This effort has yielded positive results: last year, the river water was deemed clean enough for swimming for 70% of July and August, with E coli levels—an indicator of fecal matter—well below the safe limit.
Ten days prior, Hidalgo, Estanguet, and hundreds of others, including journalists, took a swim to showcase safety improvements. The Seine is undeniably much cleaner now: in 1990, it supported 14 fish species; today, that number has risen to 34.
Nonetheless, despite the advancements, the entire complex system fundamentally depends on one critical factor: no extreme rainfall.
Stormwater—combining rainwater and wastewater—from the stressed Paris sewer system is released into the Seine approximately a dozen times a year; engineers now report that the new holding tanks have reduced this number to just two.
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This means that nearly all but the most intense rainfall can now be contained. However, during the opening ceremony on Friday evening and throughout most of Saturday, rain fell in amounts equivalent to half of an average July’s total.
The decision on whether to conduct the race on Wednesday will rely on samples collected at 3:30 AM on Tuesday, which are expected to reflect significantly better water quality after a day of strong sunshine on Monday.
Nevertheless, Météo France is anticipating severe thunderstorms, accompanied by heavy rain, hail, and lightning beginning at 6 PM on Tuesday. If the race needs to be postponed again—this time until Friday—pollution levels would likely be deemed too high.
In addition to pollution, there are concerns regarding the river’s flow rate, which, following the recent heavy rainfall amidst a very wet spring, is now at nearly one meter per second—considered the maximum acceptable limit for athletes.
Back in May, Michel Riottot, a former engineer at France’s National Centre for Scientific Research, cautioned that the swimmability of the river during the Games would largely depend on luck and the law of averages.
“The sewers, tunnels, and tanks like Austerlitz can now hold 1.9 million cubic meters of water,” he explained to AFP. “A small rainfall of around 10mm translates to approximately 1 million cubic meters. With an intense downpour, like 20mm, there will certainly be overflowing everywhere.”
