Donegal County Council carried out two inspections week after reports that traces of home heating oil were found at Rossnowlagh Beach.
A spokesperson says that both inspections did not identify any evidence of home heating oil and that the issue appears to occur in a periodic fashion.
This has led investigators to believe that a possible cause is a local oil tank that leaks when filled beyond a particular threshold.
Donegal County Council is now appealing to anyone in the Rossnowlagh area, who filled their oil tank in the last week or so, to check for signs that oil is being lost through their tank, valves or pipework.
They add that older brands of plastic oil tanks can perish in sunlit areas over time and small cracks can develop.
In addition, it is known that there are some underground metal tanks in the area and these may be susceptible to corrosion.
The council are continuing to investigate and would be grateful for assistance to see the matter resolved as quickly as possible.
