More than 100 different pesticides and 21 veterinary drugs have been found in samples taken from small rivers and canals across the European Union.
Greenpeace researchers based at Exeter University tested samples from 29 water courses across 10 European countries, including two rivers in the UK.
All 29 of the rivers and canals selected tested positive for multiple pesticides, the researchers found.
The highest level was in a Belgian canal where 70 different pesticides were identified. In 13 waterways concentrations of at least one chemical exceeded European safety limits.
The neonicotinoid insecticides imidacloprid and clothianidin were the most likely to be found above the safety limits. Among the pesticides found, 24 were no longer licensed for use.
However, researchers pointed out that the presence of unlicensed pesticides did not necessarily mean they were still being used illegally.
They could have been used before the bans came into force, the research team suggested.
Of the 103 pesticides identified, half were herbicides while the remainder were fungicides or insecticides.
Most of the veterinary drugs found were antimicrobials with the β-lactam antibiotic dicloxacillin – a type of penicillin – present in two thirds of the analysed samples.
Report co-author Paul Johnston, stressed that the work was not “a case of us versus farmers or water companies”.
“Farmers don’t want to pollute rivers, and water companies don’t want to have to remove all that pollution again downstream.”
Bayer spokesman Julian Little said Greenpeace’s study was a demonstration of how it was possible to use sophisticated, modern tests to find tiny amounts of pesticide in water.
But he added that the study should not be seen as reflecting a worsening picture.
Working with farmers and environment groups through the Voluntary Initiative and Catchment Sensitive Farming, the industry is helping to cut the level of pesticides and vet medicines that are reaching waterways, he said.
“We are winning the battle to reduce the amount of pesticide in water courses.
“Top predators like otters and osprey are returning to rivers, indicating there is a more abundant aquatic wildlife to sustain them,” Dr Little said.
However, he added that the improvements should not encourage complacency.
“We must continue to work hard to reduce the amount of pesticide in water still further.”
The study took samples from waterways in Austria, Belgium, Germany, Denmark, France, Italy, the Netherlands, Poland, Spain and the UK.
The UK sampling was carried out close to Exeter University in the rivers Otter and Tale, both in Devon.
The paper, published in the journal Science of the Total Environment, is entitled: “Screening of pesticides and veterinary drugs in small streams in the European Union by liquid chromatography high resolution mass spectrometry.”