Major study identifies ‘increasing urban risk’ of banned insecticide in London waters

An insecticide that was banned in 2018 has been identified as of “highest and increasing urban risk” in a world-leading assessment of contaminants of emerging concern in London’s waterways between 2019 and 2021.

The study, led by researchers at Imperial College London and published in Environment International last week, has revealed how chemical pollutants in London’s rivers changed over the pandemic and identified 98 contaminants of emerging concern (CECs). 

This included three CECs that are deemed to be of ‘high risk’ to aquatic life, one of which was the insecticide imidacloprid, which was banned for agricultural use in the UK in 2018. Imidacloprid, which is designed to mimic nicotine which is toxic for insects, is permitted for use in indoor gardening or greenhouses and as a veterinary parasiticide for pets. The researchers found that between September 2020 and June 2021, quantities increased. 

Two further CECs in the high risk category were identified – these are the antibiotic azithromycin and the anti-inflammatory drug diclofenac, both of which increased during the sampling period.  

Samples were taken not just for the river Thames, but also for 14 other waterways including the rivers Brent, Hogsmill, Lee/Lea, Wandle and the Grand Union Canal. The dataset was heralded by the researchers as the of its kind for any major capital city globally. 

The study identified that in 2020, during the national lockdowns, there was a significant decrease in traces of some types of pollutants, including pharmaceuticals, in the river Thames. 

However, levels of contaminants increased again significantly in 2021, with greater concentrations of antibiotics, anti-anxiety and anti-depressant medications entering the city’s waterways after restrictions were lifted.

Wastewater treatment plants and combined sewer overflows were identified as the main sources of chemical risks overall. 

A wide range of other chemicals including illicit drugs such as cocaine were also identified,  with benzoylecgonine – a cocaine metabolite – being among the top five compounds out of 11 substances that could be analysed for quantity by frequency. 

The researchers also found that smaller rivers feeding into the river Thames were most impacted by wastewater pollution, from both direct release from wastewater treatment plants and combined sewer overflows. 

PhD student and first author of the study, Melanie Egli, said: “This study enabled us to gain insights not only into what chemical contamination was in our rivers, but also provided us with high geographic resolution of where they are coming from. 

“Crucially, we found that some small tributary rivers were particularly impacted by wastewater, highlighting the need for increased monitoring and infrastructure investment for their protection.”

Guy Woodward, professor of Ecology in the Department of Life Sciences, and a co-author of the paper, commented: “This is a comprehensive and detailed study of the huge range of chemicals that we find in our freshwater ecosystems, and it picks up on several that are at potentially harmful concentrations for wildlife, but which have seemingly been overlooked in traditional surveys of our water quality in urban areas at this resolution.”