The smooth running of the free swimming and triathlon events at the Paris Games depends, in large part, on this new wastewater storage basin, located in Austerlitz, in the 13e district of Paris. Inaugurated Thursday May 2, this pool, nicknamed the “underground cathedral” 50,000 m3, the equivalent of 20 Olympic swimming pools, must limit pollution of the Seine. During the inauguration, a plaque was unveiled in tribute to Amara Dioumassy, a 51-year-old team leader of Malian origin, who died on the site in June 2023.
When you are in front of the pool, it looks like a concrete slab, except that there is a hole 50 meters in diameter which has been dug and which goes up to 30 meters underground. A large tunnel is connected to it, explains Pierre Rabadan, deputy in charge of the Seine at the city of Paris, “which passes under the Seine, under line 10, under the RER C, to collect the water from the left bank and the right bank, which connects to this storage basin.”
The water collected in this tunnel is that which falls during very large storms. Until now, they caused the sewers to overflow and therefore all the wastewater flowed into the Seine, including all the fecal matter and excrement, the Source of the main pollution of the river. From now on, this water will be stored in this basin, then very gradually, over 24-48 hours, redirected to the wastewater treatment plants.
This will be particularly useful during the Olympics, since the triathlon and open water swimming events must be held in the Seine, but only if the water quality is good. So will it be in the event of a big storm, like during the 1er May (the Île-de-France region was placed on orange alert for storms)? Yes, assures Tony Estanguet, the boss of Paris 2024. “A big storm like this could have been absorbed by this 50,000 cubic meter retention basin, before, potentially, the sewers flowed back into the Seine.”
But there can be even more severe storms, even if it only happens two or three times a year. If this were to happen during the Games, the events would be postponed by a day or two. The other interest of this basin is therefore to quickly find healthy water, with, to verify this, reinforced measures, thanks to eight control points in June and around thirty this summer.
For Ile-de-France residents who want to swim after the Games, it remains the same. In the vast majority of cases, the brand new Austerlitz basin will be able to retain even heavy rainfall. But zero risk does not exist, recalls Pierre Rabadan. “We must take the example of the Villette basin, which has been open for seven years every summer, and in which we take samples every day, to know if the quality of the water is swimmable,” he explains. “When the water is good, we authorize swimming, when it is not, for reason poor quality water, in these cases, swimming will not be authorized.” Swimming, for Ile-de-France residents, should be authorized in the summer of 2025.
