Pesticides used in pet flea-treatments occur widely in Welsh rivers and were detected in over three quarters of river water samples, finds new research by Cardiff University and Natural Resources Wales.
The study found that two pesticides from flea treatments, imidacloprid and fipronil, exceeded safe levels in almost half of the samples from urban locations.
Professor Steve Ormerod, Cardiff University's Water Research Institute, said: "Wastewater is known to transport veterinary pesticides into rivers after pet owners wash treated animals, their contaminated bedding, or their own hands after handling externally applied insecticides. We wanted to see how these chemicals might be reaching Welsh waters, and at what levels, to assess their potential impacts."
As well as the two pet-flea treatment compounds, the researchers also investigated a veterinary sheep dip chemical, diazinon, in nine Welsh rivers between 2021 and 2023. They analysed 140 samples from rural and urban rivers, and also assessed fish and macroinvertebrate communities in the most contaminated river.
They found imidacloprid in 77% of samples and fipronil in 44%.
The highest levels occurred in a small Cardiff stream, Roath Brook. Concentrations in this stream were over 11-45 times above safe levels and were linked to adverse effects on aquatic insects, including mayflies.
The also team found that both compounds increased downstream in urban areas, where waters received wastewater outfalls and sewer misconnections.
We were able to demonstrate that wastewater release from sewage treatment and misconnected sewers are important routes for flea-treatments into water courses.
"Sewer misconnections are relatively widespread in urban areas – such as in the Roath Brook catchment – but are often overlooked as pollution sources and might affect over 5-10% of properties," added Professor Ormerod.
The team's research is the first study to show how sewer misconnections form a 'down-the-drain' pathway through which flea-treatment chemicals reach rivers.
In contrast, diazinon occurred in local patches, reflecting its use in sheep-dips, and concentrations were at 3-17 times safe levels in some of the rural sheep-rearing areas of the Wye, Tywi and Ely systems.
The study's first author, Molly Hadley from Cardiff University's Water Research Institute, said: "These new data add to growing concern about the presence of veterinary pesticides in rivers. These are chemicals designed to prevent or remove unwanted insects on pets and livestock, but our evidence shows that current uses appear to cause levels in water that are sufficiently toxic to harm river wildlife."
Professor Ormerod added: "For several decades, rivers in Britain were recovering ecologically from the gross pollution problems of the 20th century, but this trend has slowed and might even be reversing. Human and veterinary pharmaceuticals could be partly responsible – and these new data on chemicals used widely to treat livestock or domestic pets align with that view."
It's clear that we're not on top of recognising or controlling some overlooked routes for such chemicals into rivers - in this case misconnected sewers or the disposal of sheep dip.
Anthony Gravell, lead specialist analyst with Natural Resources Wales, said: "Collaborating with Cardiff University has allowed us to focus the specialist analytical expertise and advanced instrumentation within Natural Resources Wales' Analytical Services team to generate important new evidence showing that veterinary pesticides from both urban and rural sources are reaching Welsh rivers at levels that can harm wildlife. This will inform Natural Resources Wales' future monitoring and wider work to reduce river pollution"
The research, Occurrence, patterns and previously overlooked sources of three veterinary ectoparasiticides in rural and urban Welsh rivers , was published in "Environmental Pollution". Working alongside Natural Resources Wales' Analytical Services (Swansea), Cardiff academics and students of all grades – undergraduate, MSc and PhD – contributed to the study.
