Insecticides, antibiotics and illicit drugs that threaten aquatic life are present throughout UK waterways, research by Imperial and Earthwatch finds.
Imperial scientists and Earthwatch have collected and are analysing chemical residues in more than a thousand samples collected by the public, some of which pose high risks to aquatic life.
The insecticide imidacloprid, banned for agricultural use but still applied in spot-on flea and tick treatments for pets, posed the highest risk to aquatic invertebrates, according to the scientists. The researchers also found human and veterinary antibiotics at concentrations high enough to suggest that antimicrobial resistance should be studied further in UK freshwaters.
The scientists found chemical pollutants from every single sample tested in the UK-wide water testing project. Illicit drugs including ketamine and benzoylecgonine (a cocaine metabolite) were also prevalent, although at concentrations posing limited risk to aquatic life.
The team, led by Professor Leon Barron at the School of Public Health, required only a thimble full of water to rapidly identify and measure more than 100 everyday-use chemicals, such as pharmaceuticals, illicit drugs and pesticides.
In the report, published today, Earthwatch enrolled 5000 citizen scientists to test the UK’s waterways as part of the Great UK WaterBlitz, which ran between 19-22 September 2025. Over a thousand of these people were also given sampling kits to collect water to be analysed by the Imperial team.
Pet treatments for ticks and fleas pose the highest risk
Every single water sample tested contained additional chemical pollutants including those from wastewater discharges, as well as agricultural and surface runoff. The level of pollutants in several samples posed risks to aquatic life despite being present at ultra-low concentrations in water, at the equivalent of a sugar cube in roughly 500 Olympic sized swimming pools.
Of the chemicals tested, the insecticide imidacloprid, used in flea and tick treatments for pets, posed the highest risk to aquatic invertebrates. Despite its ban for outdoor agricultural use since 2018, a market shift towards its use in flea and tick treatments for household pets means the chemical enters waterways via wastewater after washing off from contaminated fabrics, surfaces and owners’ hands for up to 28 days after application.
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It’s not always something obvious that poses the highest risk. We’ve been tracking a rise in pet parasiticide treatments such as imidacloprid in waterways for some time, particularly in urban areas. Combining the local knowledge of so many people allows us to create a massive snapshot of how big the issue is. Thankfully, there are solutions outlined in the report that can benefit both pet and environmental health.
