The third Olympic Games to be held in the French capital from 26 July to 8 September (including both the Olympic and Paralympic Games) have suffered major setbacks. This time, the quality of the water in the Seine has become a cause for concern. This river is not only the setting for the impressive opening day, with a 6km ceremony and delegations sailing through the centre of Paris on special barges, but also for the open water swimming events (10km).

Although the issue is not new, it has a long history, including the cancellation of last year's Open Water World Cup due to "the water quality in the Seine being below acceptable levels for the health of the swimmers". These general tests, carried out a year before the start of the Games, did not produce the desired results. The marathon swimming competition between the Pont Alexandre III and the Pont Alma was cancelled because "the quality standards of the International Federation were clearly exceeded".

This longstanding problem persists and, despite expectations of improvement, acceptable levels have not yet been achieved. Although some progress has been made, water quality has not improved at the expected rate. Analyses carried out between 2015 and 2023, sent to AFP by Paris City Hall, show significant fluctuations last summer, with several peaks in the concentration of one of the two bacterial indicators of faecal contamination, Escherichia coli.

According to the 2006 European Bathing Water Directive, none of the 14 water sampling points in Paris reached a sufficient quality level in 2023, mainly from June to September, raising many questions about the Summer Olympics. The concentration of E. coli exceeded the maximum permitted level (900 colony-forming units/100 ml) at all points. The same was true, although to a lesser extent, of Enterococcus faecalis, another organism that lives in the gastrointestinal tract of humans and other mammals and is considered harmful to human health and potentially life-threatening if ingested.

The latter bacteria slightly exceeded the required threshold at the Ivry Bridge (south-east) and the Garigliano Bridge (south-west): 330 CFU / 100 mL, exceeding the promising results of the summer of 2022 (sufficient levels were reached for both bacteria).

Although the results have not yet lived up to expectations, hopes remain high for the world's biggest sporting event. Five projects are underway (to be completed before the start of the Games) to increase the storage capacity of the sewerage network, thereby limiting the use of the Seine as a sewer and the pollution it causes. In addition to the cancellation of the swimming marathon, there were also problems with the triathlon and paratriathlon held in the same area from 16-20 August 2023; two days of competition were cancelled due to water contamination, this time caused by a faulty valve, according to the city council.

There were also problems with sampling, with biochemical tests failing to produce results due to technical problems. Since then, Paris City Hall has announced that the entire analysis process will be duplicated for the summer of 2024 (two samples, two engineers, two laboratories).

In addition to these five projects, the Austerlitz reservoir is under construction to store and treat water from the Seine. The new infrastructure to absorb heavy rainfall, including the 50,000 m3 Austerlitz reservoir, will be ready and would help to reduce undesirable levels of bacteria that can cause serious health problems for competitors and the general public. Significant investment has been made in infrastructure. However, this may not be enough in the event of heavy rainfall in the French capital, which could lead to the suspension, postponement or even cancellation of events on the Seine.

Exceptional rainfall is feared by the prefecture. In the event of heavy rainfall, untreated water - a mixture of rain and sewage - can be discharged into the river, increasing bacteria levels to dangerous levels for the health of competitors and potentially forcing the cancellation of events. What other plans are there for these challenges that are largely dependent on nature? An alternative plan would be to postpone events for a few days without changing locations, but this would affect the swimmers' preparation for what is for many the most important event of their lives.

Waiting until the last moment is not a good plan for athletes who have to add an extra psychological element to their preparation, the psychological burden of preparing for a competition that may or may not take place on the scheduled day (tests will be conducted in the days leading up to and including the competition day, and could become dangerous at any moment due to natural effects). A Plan C could be to find an alternative river or even to hold the competition in the sea (France has the Mediterranean, with very calm waters, and the Atlantic, which is a little more intense), where there is less human and natural interference, thus ensuring that the athletes can compete.

Time and results, as in all aspects of life, will determine whether the original plan, albeit very risky, will ultimately succeed or whether the largest Olympic swimming event will have to be modified on the fly.

What is Escherichia coli (E. coli)?

According to the World Health Organization (WHO), Escherichia coli, or E. coli, is a bacterium commonly found in the distal intestine of warm-blooded organisms. Most strains of E. coli are harmless, but some can cause serious food poisoning. E. coli producing Shiga toxin is a bacterium that can cause serious illness through contaminated food. The main source of outbreaks of Shiga toxin-producing E. coli is raw or undercooked minced meat, raw milk and vegetables contaminated with faeces. Although in most cases the illness resolves spontaneously, it can be life-threatening, for example when it leads to haemolytic uremic syndrome, especially in young children and the elderly.