Even though the quality of the Seine has been gradually improving since the 1970s, pollution episodes continue to occur, particularly during floods. Indeed, these can carry and deposit significant quantities of contaminated sediments (metals) on the Seine's quaysides throughout Paris.
The 2019 fire at Notre-Dame Cathedral also rekindled public fears about the impact of this event on lead contamination in Paris and the river.
Unsplash illustration image
To verify the reality of this contamination, a team of scientists led by CNRS Terre & Univers studied the lead contamination in sediments carried by the Seine, identified its sources, and analyzed the spatial and temporal evolution of the observed concentrations.
Analysis of contaminants during Seine floods between 2016 and 2024
To this end, sediments deposited on the quays during the river's most recent floods were sampled at several points along the Seine in Paris. The events studied include the major 2016 flood (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.
Various physico-chemical properties (content of radionuclides, organic matter, metals, etc.) were analyzed in these 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 2016 flood, the sediments came mainly from soils in the upstream Seine basin, which explains why they showed relatively low and very homogeneous levels of metal contamination (including lead).
High lead concentrations not linked to the fire
In contrast, the 2020 and 2021 floods mainly deposited older materials, probably resuspended from the riverbed or eroded from the banks, which explains why the sediments deposited by these floods showed higher and more heterogeneous levels of metal contamination.
Among the metals, lead showed the highest enrichment factors (i.e., the ratio between the measured concentration and the natural concentration in Seine basin soils), with values close to 8 around the Île de la Cité and reaching 18 in sections of the river located further downstream. Particularly high lead concentrations were also observed near Notre-Dame Cathedral.
However, the isotopic signatures of lead, which vary depending on its origin, suggest that while some samples taken in 2020 showed 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 after the fire appears limited.
The lead contamination instead shows a signature corresponding to so-called "urban" lead sources found throughout the city (fountains, pipes, roofs, paints, leaded gasoline additives, etc.), which constitute the main origin of the detected lead.
In addition to the metal analyses carried out during the 2016, 2020, 2021, and 2024 floods, other - organic - contaminants such as drugs, medicines, antibiotics, and pesticides were also analyzed in sediments collected after the March 2024 flood, before the Olympic Games.
The results show contrasting trends, with an increase in the content of drugs, medicines, and antibiotics from upstream to downstream of the Seine and, conversely, a decrease in pesticide content along the same transect across the city.
Several substances such as lead, authorized herbicides (diflufenican, pendimethalin) or even banned ones (atrazine) were detected at levels exceeding the so-called "Predicted No Effect Concentration" (PNEC) for aquatic organisms.
Overall, these two studies show that the quality of the Seine remains fragile and sensitive to floods. With the summer reopening of swimming areas in the river since summer 2025, these results highlight the need to continuously monitor a broader range of contaminants (such as metals, pesticides, antibiotics, etc.) beyond just microbiological indicators like E. coli levels, the analysis of which is mandated by the European Bathing Water Directive.