Paris made an Olympic-sized effort to clean up the Seine—did they succeed?

After an ambitious $1.5 billion clean-up project, the Seine River will play a leading role in both the 2024 Summer Olympic and Paralympic Games. Paris’s famous urban river will serve as the stage for the Olympic opening ceremony and, if all goes to plan, as a venue for three swimming events.

What was once a portal to pleasure—Parisians sunbathing and splashing in the river, modeling bikinis in the Seine-fed Deligny pool—has been banned for more than a century because of river traffic and pollution. But that’s set to change.

To demonstrate the river’s safety, Paris Mayor Anne Hidalgo swam in the river on July 17, ahead of the games. A plunge at the end of June was rescheduled to accommodate elections, Hidalgo said in a press conference. And tests later that month had shown that the river still contained unsafe levels of bacteria, making the mayor’s swim a potential health risk. But organizers remain hopeful the river will stay safe enough for the Olympics to debut a new era for the historic river.

“Our objective is an Olympic legacy,” says Pierre Rabadan, the deputy mayor in charge of sport, the Olympic and Paralympic Games, and the Seine. “That you and me or whoever happens to be in Paris can swim in the Seine.”

For centuries, the Seine has been a dumping ground for laundry suds, human waste, and animal parts tossed by medieval butchers.  

In the 19th century, factory and human wastewater was often discharged directly into the Seine. A revolutionary new Paris sewer system, developed during city planner Baron Haussmann’s seismic urban renewal project throughout the second half of the 19th century was an engineering triumph for Paris, yet toxic for the Seine’s health.

Today, new feats of engineering are set to restore the river. By summer 2025, the city plans to open three public swimming spots along the river, transforming a sewer into the sublime.

River restoration decades in the making

The tide began to turn for the Seine’s water quality in 1991, when the European Union passed legislation addressing a main source of water pollution: urban wastewater. The Greater Paris Sanitation Authority took significant strides to modernize sanitation networks, including major infrastructure investments at the Seine Aval treatment plant responsible for three-quarters of the area’s wastewater. Later, in 2015, the city launched its plan baignade, or swimming plan, with concrete measures to clean the Seine and Marne, a tributary, and make the Seine swimmable by the 2024 Olympics—a centerpiece of its successful bid to host the games. 

The plan would connect more than 23,000 residences, as well as houseboats, to the municipal sewer systems, which previously had dumped untreated wastewater into the rivers.

“The Olympics acted as an accelerator,” says Rabadan. “Without the games, [the project] would probably have taken 10 more years.”

The effects of the cleanup are felt downstream in some of France’s most urbanized watershed areas.

“There are a lot of people in Paris who don’t know that the Seine is in good health right now. I’m not saying it’s clean, but it’s healthy for aquatic life,” says Sandrine Armirail, director of the Maison de la Pêche et de la Nature, an environmental education center. “We look at water quality in terms of what’s living in it. The more species you have, the healthier the environment.”

When she was a child growing up outside Paris, only four fish species, all pollution resistant, could survive. In fact, by the 1970s the Seine down stream of Paris was nearly biologically dead. Today there are 36 different fish species. 

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“This means that water quality has improved quite significantly,” Armirail says. 

The maison’s aquariums show off many of the species that inhabit the Seine today, including the emblematic brochet (pike)—a predator with 700 teeth that Armirail calls the “shark of the river.” At the water’s edge, the maison is restoring the flooded prairies the fish needs as spawning habitat. Outside the building under the quay nests a pair of Eurasian kingfishers, just one example of the increasing numbers of nesting bird species lured back to the Seine.

A technical marvel

Hidden underground near Paris’s Austerlitz train station is a stormwater cistern that holds the equivalent of 20 Olympic-size swimming pools. Métro passengers witnessed the colossal construction project for three and a half years as their trains passed over it. Soon to be covered by green space, the concrete tank is supported by columns that reach 260 feet into the ground to secure it. The Bassin d’Austerlitz is a cornerstone of the plan to keep the Seine safe for swimming. 

Paris’s sanitation system is largely a legacy of 19th-century engineer Eugène Belgrand. Both rainwater and waste ­water are channeled into a vast underground labyrinth of sewers and carried gravitationally to treatment plants outside Paris. But in times of heavy downpours, sewer valves were opened into the Seine to prevent an overflow into the streets. Tanks like this one will help avoid that scenario. Developed by a team of 40 engineers, the Bassin d’Austerlitz is a technical marvel.

Paris is an extremely well organized city, whose bowels are crisscrossed by layers of old quarries, Métro tunnels, sewers, gas pipes, and electricity cables. Within this densely urban milieu, a tunnel was bored underground and piped under the Seine, allowing a flow of stormwater. Eventually, the water held by the bassin releases slowly into the sewers, then funnels to water treatment plants, before returning to the river

A river for the people

There is still work to be done. Officials were forced to cancel some of the swimming test events last August after rain spiked pollution levels—­ and a particularly severe storm could still disrupt Olympic events. But already last summer, scientific readings showed the Seine swimmable an average of seven days out of 10, and three additional collectors and stormwater tanks have opened since then.

The ultimate result—as with the pedestrianized Parisian quays that used to be highways—is returning the river to the people. 

Soon, the Seine will be on full display, for the Olympics and at the new public swimming spots that will help the people living on its banks cool off during summer heat waves—just one of the extreme weather events the city is seeing as a result of climate change. Teens will loaf at the water’s edge. Athletes will race in lap lanes. Kids will jump with abandon. Together they will follow in the rippling wake of the swimmers who competed in the first Paris Olympics, in 1900. And long before that, a tradition of 17th-century hedonists with a thing for skinny-dipping. And earlier still, even before a great city stood at this bend in the river, countless generations who lived their lives along, and in, the Seine’s waters.