"Swimming in the Seine has been banned for over a century. Since 2015, organizers have invested $1.5 billion to prepare the Seine for the Olympics and to ensure Parisians have a cleaner river after the Games. The plan included constructing a giant underground water storage basin in central Paris, renovating sewer infrastructure, and upgrading wastewater treatment plants."

.https://apnews.com/article/paris-olympics-river-seine-swim-clean-pollution-b1e7330d778babc070bc7348a8c998f3

Pretty interesting WSJ video on the project

https://www.youtube.com/watch?v=iZIGhdxplWk