Is this a concern for the Olympics, or the environment (with a spotlight brought on it due to the Olympics)? Hopefully something is done to correct this for the long term.
It's because of the historical sewage system, which will still occiassionally bleed sewage into the River Seine.
Hoog made a great video about it: https://www.youtube.com/watch?v=S26CHpcD2zk
rfi.fr
Heiß