Everyone in the Seine this summer? After the Olympics, the challenge of sustainable cleanliness

Parisians and tourists will be able to cool off starting July 5 at three sites set up on the banks of the Parisian river, where Paris Mayor Anne Hidalgo swam a year earlier in front of journalists from around the world.

This historic bath, thirty-five years after the promise of the capital's mayor and future president Jacques Chirac, marked the culmination of years of work to depollute the Seine and the Marne, its main tributary.

A project initiated in the 1990s by the Interdepartmental Syndicate for the Sanitation of the Paris Metropolitan Area (Siaap), which invested more than 9 billion euros. The "bathing plan" was launched in its wake in 2016, for which the State and local authorities injected 1,4 billion euros, and which the Olympic Games helped accelerate.

The objective: to minimize wastewater discharges along the river's route through the capital, which has the unique feature of having a combined network dating back to the Haussmann era. Wastewater and rainwater mix there, and in the event of heavy rain, there is no other solution than to discharge them into the Seine, short of flooding the underground networks.

"No D-Day"

Modernized wastewater treatment plants, construction of retention basins (including the Austerlitz basin with its 50.000 cubic meters of volume), connection of the 255 Parisian boats to the sewerage network... "For the Olympic Games, we had depolluted three-quarters of the Seine. And the water was 100% safe for swimming on dry days," Marc Guillaume, prefect of the Ile-de-France region, explained to AFP.

With the 2.000 new connections to the sewerage networks made since then, the Seine is now 80% decontaminated upstream of the Parisian sites. All that remains is to begin work downstream, with a view to opening new swimming areas outside Paris, where summers will be increasingly hot.

This summer, the prefect predicts "undoubtedly" a popular success. This is based on milder weather than during the Olympic Games, where record rainfall had sent the organizers into a shiver: of the 11 days scheduled for open water competitions and training, only five have been authorized.

"Unlike the Olympic Games, this summer there will be no D-Day" with a competition at stake, Marc Guillaume reassures himself.

Flags will be used to warn of pollution, and "if the water is unfit for swimming, it will be closed, probably the day after heavy rain," predicts Pierre Rabadan, deputy for sports at City Hall.

"We're not just throwing a coin in the air, we're relying on scientific data," the elected official insisted to AFP, recalling that no athlete had been affected after diving.

The water will be analyzed daily by instant probes and culture samples from the Regional Health Agency (ARS) to determine the level of fecal bacteria Escherichia coli (E. coli) and enterococci.

These two markers of bacteriological pollution must not exceed 1.000 and 400 colony-forming units (CFU)/100 ml respectively, thresholds a little higher than for athletes given that the public will linger more in the water.

"Insufficient" markers

But for France Nature Environnement Ile-de-France, these surveys remain "insufficient." The Seine River contains "numerous viruses for which there are no indicators," laments Michel Riottot, honorary president of the association.

Hepatitis, gastroenteritis, skin conditions... These are all pathologies that we risk catching by swallowing too much contaminated water, this former CNRS research engineer, who worked on the intestinal microbiota, worried to AFP.

He also deplores "the lack of measurement of chemical pollution."

"If there is any point pollution upstream, we are informed, so we can take the necessary measures," retorts Pierre Rabadan, also pointing out that the river is now free of most industrial pollutants.

Proof that water quality has improved: there are 36 species of fish in the Seine compared to only 4 in 1970, points out the Siaap.

A step forward that the city of Paris would like to protect by giving the river legal status, as part of a global movement for the recognition of nature that has already granted this right to several sites such as the Whanganui River in New Zealand.