The river Ouseburn in Newcastle has been foaming since April, with the agency deeming the matter a serious pollution incident and urging people to stay away. Warning signs have been installed, asking people to keep out of the river.
According to the BBC, the agency said it was likely that the pollutant had entered the river via the surface water drainage system and that it had finally isolated the unknown substance.
Urban Green, the charity which runs Newcastle's parks, said the Environment Agency was working with Northumbrian Water to clean the pipes, reported the BBC.
The Ouseburn suffered a similar pollution incident in 2011, when huge volumes of foam were spotted on the river. Northumbrian Water and Environment Agency officers traced the suspected detergent upstream and found that pollution had entered the stream via a surface water drain, but were unable to trace it further.