The Environment Agency has confirmed that dead fish have been found in a river in Norfolk where a large area of white foam has appeared.
Pictures published by the agency on Saturday showed foam covering an area of the Thet River.
The Environment Agency said its officers “saw dead and fry fish”. It said it was investigating the incident and that “the contaminant is an unknown substance.”
Specialized teams were sent to collect samples from the river, which flows through Thetford, on Saturday with the aim of finding the source of the pollution and preventing further discharge.
The source of the foam, which remained visible, was detected and further discharge was stopped.
Firefighters also attended and Norfolk Fire Service warned people not to enter the water or allow their pets to swim in or drink from the river.
Anglian Water confirmed there was a “third party” involved and it was not from any of its “equipment or assets in the area”.
An Environment Agency spokesman said more foaming could occur, but said it would have a “minimal environmental impact when it dissipates”.
They said: “Environment Agency officers are continuing to investigate why a large amount of foam was found in the River Thet in central Thetford yesterday.
“Our specialist teams collected samples, identified the potential source, and prevented any further discharge. There will be an ongoing investigation to try to ensure this does not happen again.
“It is possible that we will see a second foam plume today and the foam will continue to move down the river. It is still visible but will have a minimal impact on the environment as it dissipates.”
The agency said Sunday it would wait until the contaminant disappeared to evaluate long-term effects.
They added that its officers attended a foam-related incident on the Thet River on 23 December 2024 but were unable to identify the source on that occasion.
In 2023, an investigation by The Guardian and Watershed found that toxic substances called Pfas, which include firefighting foams, had polluted the protected River Wyre in Lancashire at “extremely high levels”.