Researchers have identified over 100 distinct chemical pollutants in two UK harbours, raising alarm over their potential impact on marine life.
The study, conducted in Langstone and Chichester harbours—both designated Sites of Special Scientific Interest—revealed a dramatic surge in pollution levels, increasing more than 100-fold following sewage discharges.
Scientists from Brunel University London, the University of Portsmouth, and Imperial College London collaborated with citizen scientists from the Clean Harbours Partnership to collect 339 samples from 21 locations during spring and autumn 2022. The analysis uncovered 105 unique contaminants, including pharmaceuticals, pesticides, and illegal drugs.
Contaminants are often present year-round due to limitations in wastewater treatment. However, the autumn sampling period coincided with heavy rainfall, which triggered combined sewer overflows (CSOs), allowing untreated sewage to enter surface waters.
“CSO discharges were an obvious source of chemical pollution in these waters,” said Dr Thomas Miller from Brunel University of London, who led the study.
“Our study found that some chemicals increased by over 100-fold in water after these discharges, making them a high risk to aquatic wildlife.”
Of particular concern was the insecticide imidacloprid—commonly used in flea and tick treatments for pets—which posed the highest risk based on surface water concentrations.
“Out of so many chemicals detected, the presence of pet tick and flea treatments in the harbours was especially concerning,” said Dr Leon Barron, from Imperial College London.
“We urgently need to reassess the practice of blanket preventative treatment of our dogs and cats to help bring the risk down.”
The findings come just after Environment Agency data revealed raw sewage was discharged into England’s waterways for a record 3.6 million hours in 2024. The full study was published in the journal Environment International.
Prof Alex Ford, from the University of Portsmouth, added: “The impact of these contaminants we are yet to fully understand, but it’s clear from our data that they increase substantially under storm events which discharge untreated sewage.”
A Defra spokesperson said: “This government is committed to tackling all sources of pollution to clean up our rivers, lakes and seas.
“That’s why we have announced the first action plan on pesticides in a decade and banned the use of harmful neonic pesticides to protect the environment and human health.
“We’re also carrying out a full review of the water sector to shape further legislation that will transform how our water system works and speed up infrastructure delivery.”
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