Officers from Natural Resources Wales (NRW) are investigating after oil entered the Afon Alaw on Anglesey.
Officers attending the scene have identified the source of the pollution and placed booms in the river in an effort to stop it heading further downstream.
It is not currently known how much oil has spilled into the river and surrounding area.
Although the source of the pollution has been blocked, officers are working to prevent oil which has already leaked into the ground from reaching the river.
Local people may notice a smell of oil in the area, however there are currently no reports of any damage to fish or other wildlife in or near the river.
Richard Berry, Environment Manager at Natural Resources Wales said:
“We are doing all we can to stop the pollution and prevent it from reaching the reservoir as well as reducing its impact on fish and other wildlife which depend on the river.
“Our team have put booms in place in the Afon Alaw to soak up as much of the pollution as possible.
“Officers are also currently conducting a full assessment on site to help us determine the impact on the wildlife in the river.”
Environmental Health Officers from the Isle of Anglesey County Council and officers from Dwr Cymru Welsh Water have also attended the scene.
A contractor for NRW will start work tomorrow to reduce the threat of pollution from the contaminated soil on site.