Occurrence and environmental risk assessment of neonicotinoids, fipronil, sulfoxaflor, and their transformation products in European coastal and riverine ecosystems

The widespread use of neonicotinoid insecticides and fipronil has raised concerns regarding their occurrence and environmental impacts in aquatic ecosystems. This study investigates nine neonicotinoids, fipronil, sulfoxaflor and four transformation products (TPs) in surface waters from the Portuguese Mondego River, the German rivers Elbe, Ems, and Weser, and coastal waters of the North Sea, English Channel and Iberian Atlantic Coast. Neonicotinoids, fipronil and their TPs were frequently detected in freshwater (detection frequency 52 - 95%) and seawater (2 - 53%). Median total insecticide concentrations were higher in German rivers (4.1 ng/L) compared to the Mondego River (0.64 ng/L), largely driven by imidacloprid and desnitro-imidacloprid. In seawater, total insecticide concentrations ranged from 0.02 to 3.9 ng/L, with the TPs FIP-desulfinyl, FIP-sulfide, FIP-sulfone, and desnitro-imidacloprid detected for the first time. Thiacloprid (53%) and FIP-desulfinyl (47%) were the most prevalent compounds. An environmental risk assessment revealed potential risks from imidacloprid, fipronil and its TPs to freshwater and marine species, and from clothianidin and thiacloprid to marine organisms. These findings highlight the ecological relevance of fipronil and imidacloprid TPs and support their inclusion in monitoring programs. The widespread occurrence of fipronil-related compounds suggests veterinary ectoparasiticides as an important contamination source in European surface waters.