Paris Olympics 2024: Is the Seine cleaner than the Thames?

After the contamination caused by industrialisation in the 20th century, both the Seine and the Thames, widely regarded as filthy in the past, have both undergone significant transformations in their water quality over recent years, enabling what most Parisians would regard as a sort of miracle: Olympians swimming in the Seine. 

The Seine has seen considerable improvements in water quality in preparation for the 2024 Paris Olympics. A major cleanup project, costing around $1.5 billion, aimed to make the river swimmable again after nearly a century of prohibitions. As part of these efforts, new infrastructure has been developed to reduce pollution from stormwater runoff and combined sewer overflows. These improvements have led to the planned reintroduction of public swimming in the Seine, with multiple designated swimming areas expected to open by 2025.   

Despite these advances, the Seine still faces significant pollution challenges. During events such as heavy rainfall, combined sewer overflows can introduce high levels of contaminants, including E. coli and other harmful bacteria. Recent data showed that while planktonic E. coli levels might decrease temporarily due to rainwater dilution, comprehensive E.coli counts, which include bacteria clumped onto fecal or sediment particles, often remain high. These aggregated bacteria pose a greater health risk and complicate water quality management. Nevertheless, over both Olympics and Paralympics, the triathlon and marathon swimming competitions will be taking place in the Seine, subject to continued monitoring by officials. 

The Thames has had a similar journey, seeing dramatic improvements in water quality since the mid-20th century. Major infrastructure projects like the Thames Tideway Tunnel have been implemented to significantly reduce sewage overflows into the river. These efforts have led to a resurgence in biodiversity, with the Thames now supporting various species of fish, birds, and other wildlife. Like the Seine in 2024, the Thames is generally considered safe for swimming but bathers are made aware that they are potentially exposing themselves, especially after heavy rains. But like the Seine, there is considerable trepidation, and high-profile cases like the Court to Kingston Swim in 2012, after which over 300 participants fell ill, haven’t assuaged anyone’s doubts.  

So, while both the Thames and the Seine have achieved highly significant improvements in water quality and safety, there are lingering questions. Both the French authorities and plenty of Parisians will be crossing their fingers in the hopes that no Olympians fall sick after competing in the Seine. If they’re given the all-clear, Paris will have even more to boast about.