Widespread Pesticide Pollution in Two English River Catchments of Contrasting Land-Use: from Sediments to Fish

Water, sediments, fish and invertebrates were collected along two English rivers (R. Tone, Sommerset and R. Wensum, Norfolk) and analysed for 52 pesticides to assess source to sea spatial distribution and track bioaccumulation within wildlife. Chemical risk assessments, using Toxic Units, Risk Quotients, and Microtox® solid phase tests were applied to understand threats to river health. Widespread pesticide pollution was detected in the water and sediments of both rivers, often forming complex mixtures containing numerous pesticides. Hydrophobic pesticides, such as Fipronil and Propiconazole, were also observed widely bioaccumulating in fish. The veterinary pesticide Fipronil was measured in the highest concentrations, up to 87.7 ng/g in fish muscle and 322 ng/g in invertebrates. Of particular concern were neonicotinoids in water, which frequently exceeded environmental quality standards (detected ranges: Imidacloprid <1.2–97.1 ng/L; Clothianidin <28.7–63.4 ng/L) and presented a significant risk to aquatic invertebrates and overall river health. Chronic sub-lethal risks to fish resulting from pesticide exposure were also identified. In sediments, Fipronil regularly exceeded likely-effect benchmarks by up to 256 % (0–0.355 ng/g OC; 0–12.6 ng/g). The findings highlight the potentially negative impact of pesticide pollution on river health in England, and emphasise the need for stricter regulation of the most high-risk pesticides, particularly those used in veterinary care.