Neonicotinoids and fiproles in the floodplain wetlands of the Bure Marshes, Norfolk, UK.

Neonicotinoids and fiproles are systemic insecticides applied in agriculture and as ectoparasiticides in veterinary products, with evidence indicating their application causes a decline in aquatic insect abundance. This study examined these compounds in the Bure Marshes, a shallow lake system within the Broads National Park, UK. Monthly sampling in 2023 at 10 locations produced 113 water samples analysed for neonicotinoids (imidacloprid, clothianidin, thiamethoxam, acetamiprid, thiacloprid) and fiproles (fipronil, fipronil sulfide, fipronil sulfone) using QTOF mass spectrometry, alongside hydrometric and water quality measurements. Acetamiprid, imidacloprid and thiacloprid were detected in over 80% of samples, while clothianidin (36%) and thiamethoxam (49%) occurred less frequently, likely reflecting reduced thiamethoxam use on sugar beet crops in 2023. Maximum values of neonicotinoids ranged from 426 to 1528 ng L-1. Mean concentrations ranged from 25 ng L-1 (clothianidin) to 106 ng L-1 (thiacloprid), near to or exceeding chronic, long-term exposure toxicity limits (37 and 10 ng L-1, respectively). Fipronil and its sulfide and sulfone derivatives were detected in 97%, 93% and 84% of samples, respectively. Maximum concentrations of fiproles ranged from 443 to 2466 ng L-1. Mean concentrations of fipronil (515 ng L-1) and fipronil sulfone (62 ng L-1) were substantially higher than their chronic toxicity limits (0.77 and 0.17 ng L-1, respectively). The persistence of neonicotinoids and fiproles in the floodplain wetlands indicates inputs from historic and current use as crop protection products and veterinary parasiticides. The proximity of the area to a wastewater treatment works suggests that ectoparasiticides may be entering surface waters via household drainage. Overall, the high detection rates and exceedances of toxicity thresholds demonstrate slow flushing and persistent contamination within the marshes. The results emphasise the need for adopting non-ecotoxic alternatives, upgrading wastewater treatment works, improving veterinary practices and establishing formal environmental quality standards as requirements to protect aquatic environments.