Traces of heating oil or diesel have been identified in the sewerage network in Ballybofey.
Donegal County Council believe the source to be the southern side of the Donegal Road from Ard Mac Carron to Townview Heights and Blackrock Drive.
Residents in the area are asked to check the condition of their oil tank and boiler as well as all connections and pipework linking the two.
The Council says any detected leaks should be repaired as a matter of urgency.
