There bathing in the Seine It is one of the most sensitive issues of the 2024 Paris Olympics due to theuncertainty and unpredictability on pollution levels in the Parisian river that hosts the Olympic ceremonies and some competitions including the triathlon. For example, on July 17, a few days before the opening ceremony of the Olympic Games, the mayor of Paris Anne Hidalgo she dived and swam in the river waters; subsequently an increase in pollution levels led to the suspension of triathlon training, the races scheduled for yesterday 30 July have been postponed but today the organizing committee and the World Triathlon Federation they gave the green light to let athletes dive into the Seine. In short, it seems that the pollution of the Seine and its consequent suitability for swimming are changing from day to day. The main reason is that River pollution depends significantly on weather conditionswhich vary on short time scales and in a not entirely predictable way (and Paris is known for its “dancing” weather). In this sense the most important factor is the rainwhich if too intense can cause the waste water to overflow into the river.
Let’s start from the beginning. When we talk about “pollution of the Seine” we are actually referring to the concentrations of E. colia bacterium found in the intestinal tracts of mammals (including humans) and birds. Too high levels in the waters make them unsuitable for swimming since E. coli can cause gastrointestinal symptoms. In Europe the legal limit is 900 units of bacterial colony formation per 100 milliliters of water, a quantity considered safe even by the World Triathlon Federation. Measurements are taken daily at four points on the Seine (Bercy, Bras Marie, the Olympic site at Pont Alexandre III and Bras de Grenelle) and analyzed by laboratories Eau de Paris in accordance with the 2006 European directive on bathing water quality, assessing the microbiological risk through measurements of concentrations of E. coli and intestinal enterococci.
E. coli ends up in river waters through sewer systems following torrential rains: heavy rainfall can in fact cause sewers to overflow, causing waste water to flow into rivers. Essentially, therefore, The Seine is more polluted after the rainsIt is no coincidence that last summer in Paris swimming competitions on the Seine were cancelled precisely following a period of heavy rain. The levels of E. Coli therefore depend on intrinsically unpredictable conditions such as weather ones. Generally the concentrations of E. coli they go up with sudden peaks that then they return just as briefly because the bacteria are naturally carried by the river waters.
In the past few days, the rains have worsened the quality of the water in the past few days, leading to the suspension of the triathlon races, but once the rains stopped the bacteria went downstream and today the waters were deemed suitable for bathing.
The rains also directly impact the river flowwhich in turn has a significant effect on bacterial concentrations: when the water flow is modest, bacteria spread more slowly and therefore pollution can disappear before it has spread to bathing areas.
It must also be said that the quality of the water does not depend only on the rainfall but on other weather conditions, in particular thesun exposure: this is both because ultraviolet rays can degrade bacteria and because high temperatures favor their death. However, there are many factors that affect the microbiological risk and pollution of the waters of a river, especially if it interacts with a large metropolis that is hosting the Olympic Games. In short, it may be a little annoying but there’s nothing strange about it and we have to adapt: Pollution in the Seine can vary suddenly from day to daymaking the swimming competition calendar uncertain until the last minute.
