Natural Resources Wales has launched an investigation after what could be turpentine has polluted a river that flows into Llyn Padarn, near Llanberis.
The pollution was reported by Dŵr Cymru Welsh Water earlier today (12 June, 2014) after the substance had entered their sewage treatment works through the surface water system.
This has now entered the Afon y Bala, which feeds Llyn Padarn, causing a strong smell and visual impact on the river.
Officers have attended site and worked with Dŵr Cymru Welsh Water to trace the source.
This has been identified and officers are now investigating the cause of the spill and assessing any potential impact on wildlife and the environment.
Gwynedd Council will be putting up signs advising bathers not to swim in Afon y Bala.
The investigation continues.