How does Paris test the Seine's water quality? (2024)

To make swimming in the Seine possible, the water quality analysis system was strengthened at the end of 2015. In addition to the numerous improvements on infrastructure, it will ensure safe bathing for Olympic athletes and Parisians alike.

A European directive on bathing water sets microbiological standards based on the concentration of certain bacteria in water, notably Escherichia coli (E. coli) and intestinal enterococci. This bacteria mainly comes from pollution (wastewater, agricultural or urban runoff, etc.) emanating from over saturated sewer systems, following heavy rainfall for example.

To keep it from overflowing onto the streets, the overflow is discharged into the Seine. That's why storage basins have been built, like the 50,000-cubic-meter Austerlitz. Their aim is to retain and store surplus water, which is then treated.

These discharges can also come from barges and floating structures that are not connected to the Paris sewerage system and discharge their wastewater into the Seine. This will be a thing of the past by early summer 2024, when 100% of barges will be connected to the mains.

Bacteria-killing sunlight

Bacteria in the water is broken down, in part, by the sun's UV rays. This is much less the case between October and April! The river's flow is at its highest during the winter and rainfall is at its heaviest, factors that are not conducive to the quality of the Seine. The river is thus naturally cleaner in the summer.

To determine the quality of the water at the Olympic site, samples are taken daily by staff and sent to laboratories. These measurements are standardized, and the results take at least twenty-four hours to come in.

It is therefore impossible to know the quality of the Seine in real time. That's why this highly reliable but slow method is supplemented by analysis of water upstream of bathing sites, to detect potentiel contamination likely to float into the area in the next few hours.

This detection method makes it possible to monitor changes in water quality in near-real time (although this does not make it possible to define precise water quality, as required by the European directive). Thanks to fluorescence spectroscopy, which can be used as an indirect indicator of bacteriological contamination, the City of Paris, supported by Eau de Paris, uses sampling and analysis equipment capable of delivering a result in twenty minutes (compared to up to ten hours for other instruments).

This highly sophisticated type of sampling requires a power supply and must be installed in a closed room. Currently, they can be found on the floating gardens of the Niki-de-Saint-Phalle Seine banks archipelago (^7th arrondissem*nt) or on the CROUS barge (^13th arrondissem*nt).

Between now and the Paris 2024 Games, they will be moved upstream of the capital to detect pollution as early as possible, before it reaches the Olympic site. In addition to this system, 39 different spots upstream of Paris will be monitored via standardized measurements, in order to detect any contamination.

In the months leading up to the Paris 2024 Games, a systematic inspection of the wastewater network will be carried out to ensure that there are no faulty valves allowing wastewater to be discharged. Furthermore, boat trips will be made to detect any undetected wastewater discharges. If a problem is detected, a specialist team will be able to intervene immediately.

During the entire period of the 2024 Olympic and Paralympic Games, representatives of Paris 2024, the City of Paris, State services and Île-de-France wastewater network managers will meet every day at 5 pm. This will be an opportunity to consult the latest available results and trends, thanks to the monitoring of the 39 spots upstream.

Sewage system managers will be able to share their short-term discharge forecasts: in the event of bad weather, these will give an indication of the quantity of water that can be stored in the various basins, and quantify the risk of wastewater being discharged into the Seine. All this information provides an estimate of the quality of water arriving at bathing sited the following day.

A second meeting will be held every night at 3:30 am. It will bring together Paris 2024, the City of Paris, government departments, the International Olympic Committee (IOC) and the various international sports federations. At this point, they will be able to check the analysis that was carried out 24 hours earlier at the laboratory. On the basis of these results, together with the previous day's predictions and the involvement of medical and meteorological experts, the federations will give the go-ahead (or not) to events held in the Seine.

A river full of life

In the 1970s, there were only three different species of fish living in the Seine. Today, thanks to major water purification efforts, 32 species live in the river. More info

Thanks to the Games, in just 7 years we've improved conditions enough to be able to swim in the Seine!

Paul Kennouche

IN CHARGE OF THE WATER CYCLE AND QUALITY AT THE CITY OF PARIS