Seine estuary: storms and floods temporarily increase water contamination — results of the TARANIS project (227 pesticides and 92 pharmaceuticals analysed in water)
On 17 September 2026 the French marine research institute Ifremer presented the results of the TARANIS project, carried out with the University of Reims under the Seine-Aval programme. In 10 field campaigns in 2023–2024, samples were taken during storms and floods; intense weather events increase the transfer of contaminants from the catchment to the Seine and its estuary, with measurable effects on water quality and aquatic organisms.
- Several complementary methods were used: passive samplers to capture average contamination levels in the water, zebra mussels (Dreissena polymorpha) as sentinel organisms for bioaccumulation, faecal viral contamination and toxicity biomarkers, and particle traps to analyse suspended matter and its ecotoxicity. Of the 227 pesticides analysed in water, 84 gave quantifiable results, and of the 92 pharmaceutical substances, 60.
- After the first storm studied in the Rouen area in June 2023, the cumulative concentration of trace metals (cadmium, cobalt, copper, iron, manganese, nickel, lead, zinc) reached almost 40 µg/L, nearly double the background level measured a few weeks later. After storms, cumulative concentrations of metabolites, herbicides and substances not approved for agricultural use rose by 36–98%, and at the Petit-Couronne station ketoprofen was up to 45 times and paracetamol up to 32 times higher than in the following campaign.
- In zebra mussels from the Rouen station after the 2023 storm, F-specific RNA bacteriophages — an indicator of faecal pollution — reached up to 200 plaque-forming units per gram, 10 times more than upstream of the main storm overflows. During the floods from November to April 2023–2024, chlortoluron rose 30-fold above background and exceeded its environmental reference value; in suspended matter, 7 metals, 10 PAHs, 5 PCBs and tributyltin (TBT) exceeded reference values.
- The team identified substances for priority monitoring, including fipronil, imidacloprid, metolachlor, chlortoluron, metazachlor, nicosulfuron, metconazole, tebuconazole, tetraconazole, oxazepam, sulfamethoxazole and desethylatrazine; glyphosate and AMPA systematically exceeded their reference value after storms. The authors call for adapting monitoring and the substance lists under the Water Framework Directive to exceptional events; the research report (F. Menet, A. Geffard, 2026) runs to 111 pages.