Earlier this month, Paris Mayor Anne Hidalgo swam in the Seine River, a sign of the city’s success cleaning up its notoriously dirty waterway ahead of the Olympics. The cleanup effort was more than just symbolic: during the Olympic Games, athletes would swim in the river for several events.

But now, at least one of those events has been postponed due to unsafe pollution levels in the Seine. The men’s triathlon race, which was set to take place Tuesday morning, was postponed until July 31, according to the International Olympic Committee and World Triathlon, the governing body for the sport. (Update: On July 31, both the men’s and the women’s triathlon events were able to proceed.) Water tests “revealed water quality levels that did not provide sufficient guarantees to allow the event to be held,” the organizations said; tests showed increased levels of E.coli and other potentially harmful bacteria. 

The men’s triathlon is now scheduled for the same day as the women’s triathlon—but both of those events, the organizations said, are “subject to the forthcoming water tests.” There’s also a contingency day scheduled for August 2, if needed.

How is it that the Seine was safe enough for the mayor to swim in before the Olympics but now contains potentially harmful levels of E.coli? The organizations blamed “meteorological events beyond our control”—namely, the rain that fell on Paris on July 26 and 27. 

Rain, sewage, and water quality 

Paris, like many other large, old cities, has a combined sewer system, which means its wastewater and its stormwater use the same pipes. When rain falls, it can overwhelm those pipes and cause untreated wastewater to flow into the river. Wastewater can spread viruses and bacteria, including E.coli, which normally lives in both human and animal intestines. (Along with waste from humans, rain can also cause animal waste to enter rivers.)

In its attempt to clean up the Seine for the Olympic Games, Paris spent about $1.5 billion to build a giant underground storage bin to catch stormwater when it rains, so that it wouldn’t overwhelm the sewer system and flow into the river.

Even Olympic organizers were aware the issue may not be easily solved; ahead of the Games, they said that if heavy rain affected the Seine and caused high bacterial levels, the swimming portion of the triathlon could be canceled, turning the event into a duathlon. (There has never been a duathlon at the Olympics before.)

The Seine River isn’t alone

Though the problem sounds simple, it’s actually complicated to solve, experts say—and Paris isn’t the only city struggling with overwhelmed sewer systems that cause wastewater to bypass treatment plants and enter local waters. 

New York City also has a combined sewer system that leads raw sewage to stream through water pipes during a storm. “When it rains a tenth-of-an-inch per hour or more, the added volume quickly overwhelms the 150-year-old network,” the Natural Resources Defense Council (NRDC) previously wrote of New York’s system. “When this happens, ‘relief structures’ allow a gruesome mixture of polluted urban runoff and raw sewage to bypass the water treatment plants and discharge directly into local waterways at up to 460 locations throughout the five boroughs.” 

The New York City triathlon has had to deal with this in recent years. In 2021, the swimming portion of the event was canceled because of high bacteria levels in the Hudson River. Sewer upgrades are often large, and thus expensive, projects. In just one example, an effort to protect New York’s Gowanus Canal from sewage overflow—which involves installing two underground storage tanks—is expected to cost the city $1.6 billion.

The United Kingdom’s rivers are also “riddled with sewage pollution,” per a recent BBC article. In 2023, the amount of raw sewage “spilling into England’s rivers and seas” has doubled, with 3.6 million hours of spills compared to 1.75 million hours the year prior.

Cities around the world are working to expand their sewage treatment capacities and improve their sewer systems in order to reduce overflow events. But climate change is making rain more intense and frequent. That’s an even bigger issue for combined sewer systems. When more rain falls in a shorter period of time, pipes are more easily overwhelmed. Scientists say sea-level rise could also make the issue worse, as it could be more difficult for such sewer systems to “discharge into nearby bodies of water.”