WHEN OLYMPIC TRIATHLETES plunged into the Seine river close to the Alexandre III bridge in Paris on July 31, they were making history—and not just by going for gold.

The event, which also saw the competitors cycle and run along the Champs-Elysées and past the Grand Palais, was far more than an eye-catching sporting spectacle. It also marked a significant milestone in the ambitious €1.4 billion ($1.5 billion), multiyear plan led by Paris and the broader Île de France region to clean up the Seine.
There were real concerns that the triathletes might not make it into the river. After failed water-quality tests, the swimming leg of the triathlon seemed like it might be canceled. It eventually went ahead with a delay. But the intense scrutiny on the Seine ahead of the triathlon has distracted from the true goal of the project: to make the river suitable for public swimming for the long term—and to better protect the river environment and boost biodiversity along the way.

Seine swimming had been banned since 1923, the year before Paris last hosted the Olympics, due to the health risk posed by a river contaminated by a city’s wastewater. French authorities want swimmers to be able to enjoy the river once again from next summer, at three permanent dedicated sites in Paris, with 20 or so more sites planned (so far) outside the city limits.

Teams have been working hard on the swimming plan since 2016, with the Olympic and Paralympic Games in focus as a high-profile opportunity to show off their achievements on the way to opening up the river to the public. Work has included an overhaul of outdated sewer systems and the construction of giant rainwater retention basins—such as the 50,000-cubic-meter Bassin d’Austerlitz—designed to stock excess rainwater during heavy storms and reduce the amount of untreated water flowing into the Seine.

The operation was hailed as a success on July 17, when Paris mayor Anne Hidalgo and Paris 2024 president Tony Estanguet took a high-profile and long-promised dip in the river. They swam on a day when the water passed its safety tests, as it did on an average of 70 percent of days across Paris’ designated bathing sites during the swimming season last summer. When the conditions are right, the systems put in place keep the river clean enough for bathing.

It was heavy rain in the early days of the Olympics that almost pulled the plug on the Seine triathlon plans. Samples of river water showed that levels of potentially harmful bacteria were too high. Authorities finally gave the go-ahead in the early hours of the morning of July 31, saying analysis of water samples had shown a return to safe levels for the athletes.
Those last-minute doubts underscore that maintaining the river’s water quality could become more challenging as the planet warms and episodes of extreme weather become more frequent. “Rising global average temperatures increase the likelihood of short, intense downpours,” says Jamie Wilks, a doctoral researcher at the Institute of Sports Business at Loughborough University London. “Paris has experienced heavy rainfall during June and July in five of the last eight years.”
Despite the creation of multiple stormwater reservoirs, like the Bassin d’Austerlitz, which collect stormwater and slowly release it after the bad weather has passed, if enough rain is concentrated into a small enough time, not everything that falls from the sky can be captured. In such a situation, runoff water has to be released into the river, driving up bacterial levels.

“[Weather] variability due to climate change is a major issue, and this will only make things more difficult,” says Dan Angelescu, CEO of water-monitoring start-up Fluidion at a July 31 press conference at the company’s office in Alfortville, just outside Paris. The company makes remote water-sampling devices that beam their readings back to a central base, and it has been working with Paris authorities since 2016 providing water analysis at the Bassin de la Villette reservoir, a separate swimming site in the north of Paris that is already open for public swimming.

“If new projects to collect waterway runoff are not carried out in the coming years, it is highly likely that the swimmability of the Seine and the opening of recreational and sports areas will depend on weather events, with swimming bans following rainy days,” says Loïs Mougin, a doctoral researcher in exercise and environmental physiology at the School of Sport, Exercise, and Health Science at Loughborough University.

Even without a rise in extreme weather, keeping the Seine clean enough to swim in in the face of normal weather events—such as regular summer rain—is a big challenge, says Jean-Marie Mouchel, professor of hydrology at the Sorbonne University. “There are also plenty of non-exceptional weather events that have an impact on the water quality. We need to make the system more efficient at improving water quality in the face of these.” Water-quality data from last summer backs up this point. The Seine was unswimmable roughly 30 percent of the time—but Paris wasn’t enduring extreme rainfall a third of the time.

Experts argue that how water-monitoring is done, and what information is shared with the public and when, also has to improve. “It is crucial that bacteriological data be published daily, along with information on the associated risks,” Mougin says. These include the potential for gastrointestinal issues and eye and skin infections.

“Monitoring is going to become critical,” Angelescu says. “Having technology that’s able to monitor the right risk, measure the actual risk coming from all the bacteria, and provide results fast is going to be extremely important.” Conventional monitoring methods, which were used to make decisions for the triathlon (and didn’t involve Fluidon), involve taking samples from the river and sending them to a lab—a process that is far slower than the real-time monitoring.

So separately, Fluidon has been trialing its technology at the triathlon site near the Alexandre III bridge throughout the Games, focusing on levels of the E. coli bacteria, to show how a quicker system that involves on-site processing might perform in the river. It has been publishing its results in near real time on an open data site, and says its technology provides a more accurate and up-to-date picture of water conditions.

Public bodies are aware that the cleanup and monitoring program is collectively a huge and ongoing challenge—but that the benefits are broad too. “We haven’t invested more than a billion euros just for swimming—we’ve invested it to improve the Seine’s environment in the years to come,” says Frédéric Muller, who is responsible for efforts to make the Seine safe for swimming at the Eau Seine Normandie water agency.

There will be more public investments in improving the public water treatment system, and authorities will also be working with the private sector to ensure existing and new homes and buildings meet the right standards. A big part of the clean-up operation so far has involved upgrading plumbing networks to stop moored boats and some older houses and other buildings discharging their waste directly into the river.

“A lot of progress has been made, but some work still remains to be done,” Muller says. “The plans were already being taken very seriously, but the Olympics has perhaps forced them to go even further.”

“Of course, we’re going to continue our work,” says Christelle Monteagudo, a spokesperson for the Île de France prefecture, the state administration for the region which has cofinanced the infrastructure projects.

Mouchel is optimistic that getting citizens swimming in their river could help raise awareness and support for environmental efforts to take better care of the Seine and its ecosystems—the improvements are already visible: 34 different types of fish now make the Seine their home, up from 14 in 1990. “The idea that the city is a kind of hermetic zone that has nothing to do with its water is disappearing—this is positive,” he says. “If people are bathing in the Seine, then they are becoming more aware of the problem. It could be a catalyst for more progress.”

In the coming years, the prefecture will continue rationalizing the sewer and water treatment system, making sure that buildings are properly connected to the network and not to the river directly, and that the network has sufficient capacity to avoid being overwhelmed. It hopes the progress made so far will inspire more clean-up activities and river-swimming projects along the Seine and further afield. Around 20 local authorities have already set out plans to open permanent public bathing sites along the Seine, in addition to the three planned in Paris.

“We would like to inspire others. We’re working with the French embassy in the US—we are putting together some specifications,” says Monteagudo. “They are very interested in knowing how we made our river swimmable.”

“The success will be complete if politicians and scientists keep up their efforts for the next few years,” Mouchel says. “Swimming in the Seine is becoming a more and more realistic objective. We will never be able to swim in the Seine every day, but the number of days on which it is possible to swim in the Seine is increasing.”

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