Five expected events at the Paris 2024 Olympics will include men's and women's 10-kilometer marathon swimming races, as well as the 1,500-meter swimming portion of the triathlon.
These events will take place in the Seine River in the heart of Paris. Swimmers, including four Australians, will pass by famous landmarks such as the Musée d'Orsay as they swim through the historic heart of the city.

But the question arises, will it be safe for the swimmers? Rivers flowing through major cities are often polluted, either from stormwater, chemical pollution, or sewage spills.
As marathon swimmers pass by the Paris Sewer Museum, they might wonder if the water is clean.

For over 100 years, swimming in the Seine has been banned due to concerns about the water's impact on human health. Authorities are working on cleaning the water, investing 1.3 billion euros to improve water quality. The goal is to reduce bacterial contamination by 75% before the first swimmer enters the water. These measures are having an impact, but recent heavy rains are causing spikes in bacteria levels.

While officials remain optimistic, there is a plan B in case the Seine is not safe.

Why swim in the Seine?
Urban rivers have a questionable reputation, but this is not the first time the Seine River has been used for Olympic swimming.
At the 1900 Paris Olympics, seven swimming events were held in the river. At that time, the games were the first modern Olympics where women could participate in some sports, but swimming was not one of them.

Australian swimmer Frederick Lane had to swim under the United Kingdom flag because Australia did not yet have its flag until the Federation the following year. He won two gold medals. One was for the 200-meter freestyle race, and the other for a bizarre race that has never been held again: the 200-meter obstacle race, where swimmers had to climb over poles and boats.
These Olympics also saw the first and last underwater race, which was also in the Seine.

At that time, the Seine's water was cleaner because human waste had actual market value as fertilizer, and no one thought of dumping waste into rivers. However, over time, sewer systems developed, and other fertilizers such as guano and mineral fertilizers appeared.
By the early 20th century, most city wastewater ended up in the Seine. In 1923, the swimming ban took effect. A year later, Paris hosted the Olympics for the second time, and swimmers competed in 50-meter pools.

How to clean a river like the Seine?
Cleaning the Seine is a challenge. Paris is home to 11 million people, with a lot of industry. Urban rivers are almost always polluted by waste from the surrounding city.
Sewage system leaks and overflows are the main source of pollution. In places like the United Kingdom, sewage spills into waterways have become a major political issue.

When wastewater reaches rivers, it carries pollutants and dangerous levels of microorganisms that cause diseases, such as Escherichia coli (known as E. coli). Untreated water can contain viruses, bacteria, and protozoa that cause illnesses.
In preparation for the Games, authorities have worked on improving water quality enough to bring some Olympic swimming events back to the Seine. Stormwater - often polluted with dog feces or sewage overflows - is treated before being discharged into the river.

Despite the money and efforts, there are still questions about whether it will be enough to guarantee swimmer safety. Bacteria levels reached risky levels most days in June due to unusually heavy rains, but the water improved in July.

This week, French Sports Minister Amélie Oudéa-Castéra swam a few meters in the Seine to calm concerns.

In contrast, other Olympic swimming events will take place in a newly constructed 50-meter pool, which will have very good water quality. Pool water is filtered and treated with disinfectants like chlorine or bromine. It will be regularly tested to ensure optimal water quality.

At the Tokyo 2020 Olympics, triathletes had to swim in the polluted Tokyo Bay. Similar concerns about illness proved unfounded. The real challenge was the extreme heat.

What's at stake?
The most likely outcome if the races are held when bacteria levels are unsafe would be gastrointestinal infection.
Officials have some control over this. Contamination is worst after heavy rains. Races could be postponed if necessary.

Many swimmers - especially those competing in open waters - are familiar with swimming in water with a certain level of pollution. Some see it as a risk worth taking. Italian double world champion Gregorio Paltrinieri said in January:

"Even if the water is dirty, I'd rather swim in the electric atmosphere in the heart of Paris than in an anonymous stretch of water."

Paris 2024 organizers previously warned that there was no plan B for the 10 km marathon swims in the Seine if water quality testing is inadequate. But that has now changed. If the river is not clean enough, open water swimming will be moved to a rowing venue.

The Olympic triathlon is planned around the swimming stage in the Seine. But triathletes have been informed that the swimming stage could be skipped if the water is unsafe, turning the race into a duathlon of running and cycling.

As the world's attention turns to Paris, many concerned officials behind the scenes will be hoping their efforts to clean the Seine pay off.

Original:
Ian A. Wright
Associate Professor of Environmental Science, Western Sydney University