A "vast" leak of diesel fuel in a Northampton river led to the fire service blocking a waterway while the spillage was cleaned.
Emergency services was alerted to the pollution in the River Nene hours before a community fair on the adjoining Beckets Park on July 30.
A rescue boat was launched to set up a boom across the river, which blocked the flow of water until the source of the diesel could be found.
Nick Stephens, chairman of Buddies of Beckets, said: "On Sunday, one of our members saw lots of oil floating on top of the river.
"The river is an important part of the park and the county and it was dreadful to see so much pollution in it."
The oil spill was later traced back to a company up the river.
A spokesperson for the Environment Agency said: “We can confirm that this incident was referred to us by the Fire and Rescue Service. Our officers traced the diesel back to the source on Sunday. The company responsible for the release of the diesel was unaware of this incident occurring. They have worked to clear the pollution since Sunday and the clean-up process is currently ongoing.
“We are considering our enforcement response at this time.”
The Environment Agency would not name the company involved.
The community Mela event, hosted by Buddies of Beckets, was unaffected by the incident.