© Environment AgencyA tributary of the River Frome has turned bright blue.
The Environment Agency said the drastic colour change affecting a tributary of the River Frome was a "pollution incident".
There are no reports of wildlife in distress or dead, a spokesperson for the agency's southwest contingent said on Friday.
Government experts will continue to monitor the stream over the weekend and samples have been taken for testing.
Residents have previously feared for the state of the river.
In 2018, campaigners worked alongside Frome Town Council on a bylaw to help protect the River Frome from pollution.
The Independent understands an application had been made to the Ministry of Housing, Communities and Local Government (MHCLG) for central government approval.
Campaigners said the MHCLG passed the application over to several other departments until a letter from Nature's Rights lawyer Mumta Ito prompted a pledge that a decision would be made in March. No decision has yet been made.
The MHCLG has been contacted for comment.
The mysterious event creates something of a pattern in Somerset, after thousands of dead fish washed up in the River Sheppey one week earlier.
A pollution spill was blamed for the deaths of around 6,000 trout, bullheads and other species.
The Environment Agency sprayed the river with hydrogen peroxide in order to boost oxygen levels in the water.
Comment: As with the 'mystery chemical incident' at Worthing Pier that sent 2 people to hospital just a few days ago in the UK, the cause behind the incident has yet to be explained by authorities.
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