Is it safe to swim in the Seine in Paris?
Although the quality of the Seine has been gradually improving since the 1970s, episodes of pollution continue to occur, particularly during floods. These floods can carry and deposit significant amounts of contaminated sediment along the banks of the Seine throughout Paris. Furthermore, the fire at Notre-Dame Cathedral in 2019 has also reignited public concerns about the impact of this event on lead contamination in the city and its river. The findings of the analysis of contaminants during the Seine floods that occurred between 2016 and 2024 have just been published.
Scientists studied lead contamination in sediments carried by the Seine, identified the sources of this metal, and described the spatial and temporal patterns of its concentrations and sources. To do this, sediments deposited on the riverbanks during the river’s most recent floods were collected at several points along the Seine throughout Paris. The events targeted were the major flood of 2016 (which occurred before the Notre-Dame fire), the more moderate winter floods of 2020 and 2021 (after the fire), and the spring flood of March 2024, which occurred a few months before the Olympic Games.
Flooding of the Seine, Paris 2021. © O. Evrard, LSCE
Various physicochemical properties (radionuclide content, organic matter, metals, etc.) were analyzed in the collected sediments. The results show that the magnitude of the flood has a major impact on the source of the transported sediments and their contamination. Thus, during the major flood of 2016, the sediments originated mainly from soils upstream of the Seine basin, which explains why they exhibited relatively low and very homogeneous levels of metal contamination (including lead). In contrast, the floods of 2020 and 2021 mainly deposited older materials, likely resuspended from the riverbed or eroded from the banks, which explains why the sediments deposited by these floods exhibited higher and more heterogeneous levels of metal contamination.
Among the metals, lead had the highest enrichment factors (which correspond to the ratio between the lead levels measured in the samples and the naturally occurring lead levels in the soils of the Seine basin), with values close to 8 around the Île de la Cité and as high as 18 in the sections of the river further downstream. Particularly high concentrations of lead were also observed near Notre-Dame Cathedral.
On the other hand, the isotopic signatures of lead (which vary depending on the source of the mobilized lead) suggest that, while some samples collected in 2020 exhibited a signature similar to that attributed to the dust generated during the Notre-Dame fire, the overall contribution of this event to lead contamination in Seine sediments following the fire appears to be limited. Instead, the lead contamination exhibits a signature corresponding to so-called “urban” lead sources found throughout the city (fountains, pipes, roofs, paint, additives in leaded gasoline, etc.) and which therefore largely dominate the lead contamination observed across the city.
In addition to the metal analyses conducted during the floods of 2016, 2020, 2021, and 2024, other organic contaminants—such as drugs, medications, antibiotics, and pesticides—were also analyzed in sediments collected after the March 2024 flood, prior to the Olympic Games. The results show contrasting trends, with an increase in the levels of drugs, medications, and antibiotics from upstream to downstream along the Seine and, conversely, a decrease in pesticide levels along the same transect through the city. Several substances, such as lead, authorized herbicides (diflufenican, pendimethalin), and even banned herbicides (atrazine), were found at levels exceeding the so-called “Predicted No Effect Concentration” (PNEC) for aquatic organisms.
Swimming at Bercy, Paris, summer 2025. © O. Evrard, LSCE
Overall, these two studies show that the quality of the Seine remains fragile and sensitive to events such as flooding. With the reopening of swimming areas in the river during the summer since 2025, the results suggest the need for continuous monitoring of a broader range of contaminants (such as metals, pesticides, antibiotics, etc.) in addition to the microbiological indicators (E. coli levels) whose analysis is mandated by the European Bathing Water Quality Directive.
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References
– Evrard, O., Bizeul, R., Xu-Yang, Y., Huon, S., Le Callonnec, L., Foucher, A., Ayrault, S. (2026). Did the 2019 Notre-Dame Cathedral Fire impact the lead contamination of sediment in the Seine River in Paris, France? Journal of Hazardous Materials Advances 21, 101072. https://doi.org/10.1016/j.hazadv.2026.101072
– Evrard, O., Xu-Yang, Y., Thiébault, T., Foucher, A., Bizeul, R., Asselin, C., Baudin, F., Thomas, D., Ayrault, S. (in press). Physico-chemical properties and contamination of flood sediment deposits collected along the Seine River in Paris during the March 2024 flood before Paris Olympics. Bulletin of Environmental Contamination and Toxicology.
