A key promise in Paris’s bid to host the 2024 Summer Olympics was that its famous river, the Seine, would be cleaned up in time to host open water swimming events: the triathlon, paratriathlon, and marathon swimming. But swimming has been banned in the Seine for a century because the Paris sewer system is designed to dump wastewater into the river during heavy rain when the sewers get overwhelmed by stormwater. When that happens, levels of E. coli, a bacteria associated with fecal matter, spike, making the water too contaminated to swim in.
Source: Can Paris fix its poop problem before the Olympics? – Vox