Occurrence, patterns and previously overlooked sources of three veterinary ectoparasiticides in rural and urban Welsh rivers

Chemicals from pet flea-treatments or sheep-dips sometimes exceed no-effect concentrations in rivers. We investigated three such compounds - imidacloprid, fipronil and diazinon - in nine Welsh rivers during 2021-2023. We analysed 140 grab samples using ultra high-performance liquid chromatography with quadrupole-time-of-flight mass spectrometry (LC/Q-TOF-MS) to assess how concentrations varied i) within and among rural and urban rivers in relation to wastewater inputs; ii) with an indicator of wastewater contamination, caffeine, and iii) with flow. We assessed fish and macroinvertebrate communities along a concentration gradient in the most contaminated stream. Imidacloprid (0-76 ng/L) occurred in 77 % of samples and fipronil (0-35 ng/L) in 44 %. Odds of detection were 26X and 8X greater in urban than rural sites for imidacloprid and fipronil, respectively, exceeding predicted no-effect concentrations (PNECs) in 38 % and 44 % of urban samples. Both compounds increased downstream in urban reaches i) receiving wastewater outfalls and ii) where sewer misconnections apparently impacted invertebrate communities. Significant correlations with caffeine confirmed links with wastewater. Imidacloprid, fipronil and caffeine were modelled effectively from Wastewater Treatment (WWTP) discharge, but model residuals were consistent with additional effects from misconnected sewers. In contrast, diazinon occurred patchily linked to livestock farming in the Wye (174 ng/L), Tywi (29 ng/L) and Ely (94 ng/L). Flow effects on all concentrations were weak. These data provide important support for the role of 'down the drain' routes through which compounds used as pet flea-treatments reach British rivers, for the first time revealing that misconnected sewers might increase imidacloprid concentrations sufficient for observable biological effects.