Explainer: The race against time to clean the Seine

The Paris Games have put a spotlight on the challenges facing Olympic cities -- ike using a polluted river for major swimming events. Darcy McCartin examines the issue.

It was the biggest gamble at the biggest sporting event on Earth.

The River Seine has faced water pollution problems primarily due to sewage discharge, industrial waste, and urban runoff. For years, untreated or poorly treated wastewater from Paris and surrounding areas has flowed into the river, leading to high pollution levels. Industrial activities along the river have also contributed to contamination. In heavy rainfall, the city's combined sewer system overflows which releases untreated sewage and waste into the Seine. 

Dr Scott Rayburg has a PHD in geomorphology (a geomorphologist being someone who studies how the earth's surface is formed and changed by rivers, mountains, oceans, air and ice) from University of New York, Buffalo.

He says that ‘pollution in urban environments comes from two main sources, which is non-point source and point source pollutants.’ These two forms both align with the known pollution sources for the Seine, where point sources are factories discharging waste directly, and non-point source pollution being everything in urban landscape that, when it rains, gets washed in.

These pollution forms are of a particular problem for Paris, as an older city ‘the stormwater system and the sewage system are the same system, says Dr Rayburg.

If there is no rain, the water can be relatively clean because the sewage is running to the sewage treatment plants, but when it rains, rainfall and sewage occurs at the same time, causing the system to overflow. ":The sewage has nowhere to go but being pushed into the river," says Dr Rayburg.

The city of Paris worked tirelessly in the lead up to the Olympics to get the Seine into a position where athletes could safely swim. The river clean involved constructing large rainwater catch basins and tunnels to prevent untreated sewage from overflowing into the river when it rained. Treatment plants were also modernised for water purification.

However, this $1.4 billion project could only do so much, as the bacteria levels continued to fluctuate in the lead up to the games.

There were many periods in the days leading up to the Games where the levels of E.Coli and enterococci bacteria (bacteria commonly found in the intestines of humans and animals) were above levels required for safe swimming.

The Paris Olympics officially announced the postponement of the men’s triathlon due to water contamination, just hours before start on August 10.

“Despite the improvement of water quality levels over the last hours, the readings at some points of the swim course are still above the acceptable limits,” organisers said.

Despite postponement, the triathlon did go ahead, as did the other events held in the river with tests completed every 12 hours.

Eau de Paris' data showed high levels of E.coli were detected in the river at least once each day since the start of the Olympics, between July 26 and Sunday, with two exceptions: July 30 and July 31, the day of the men's and women's triathlon. It was also later revealed that "the only point in which the enterococci levels were slightly above our limits was outside and downstream of the swim course of the Mixed Relay," said Paris 2024 spokeswoman Anne Deschamps.

Many athletes were confirmed to be sick post-event after swimming in the River Seine, as the worst fears of Paris 2024’s committee eventuated.

Belgium was forced to pull out of the mixed triathlon relay after one of their athletes, Claire Michel was hospitalised with an E. coli infection after her initial triathlon event in the river. Swiss triathlete Adrien Briffod also fell seriously ill following the men’s individual event.

More casualties built up, with three German and three Portuguese open water swimmers falling ill.

The ramifications of Paris allowing athletes to compete in water that had previously been highly polluted are unknown. Despite bacteria levels being given the clear for the events to be held, the fallout of sick athletes is likely to continue. The aim to have a key landmark of Paris, the famous River Seine be utilised so heavily for competition seems good in theory, but athletes’ welfare should be the top priority.