The source of oil leaking into the River Severn in Newtown has still not been identified.
Last week Natural Resources Wales issued a plea for local households and businesses to check their oil heating tanks in case they were leaking into the Dolfor Brook.
It followed concerns from local residents who have reported a strong smell of oil and a sheen on the surface of the river close to where the brook enters the river, close to the Scout hut off Park Lane.
NRW has been contacted by Montgomeryshire Member of the Senedd, Russell George, the Mayor, Cllr David Selby and County Councillor Joy Jones about the situation, along with residents.
In response to Mr George, an NRA official said they had contacted a local oil storage company and also checked leak test equipment at the site and no leak had been detected.
"We will continue to investigate the source and take appropriate action.
"To help stop the oil sheen flowing from the tributary into the River Severn we have placed an absorbent boom and pads in the tributary close to the Scout hut.
"This boom is working well to contain the oil behind it and it may be that your constituent is smelling this oil as it is collecting behind the boom. We changed the boom and pads earlier in the week and the fire service have helped us last week by adding more pads in front of the boom."
The official said there was little oil now floating on the river within the boom.