WICKLOW COUNTY Council has said it has no reason to believe an unlicensed dump at Russborough on the edge of Blessington Lake is polluting the reservoir.
The council recently took advantage of a dry spell to dig trial holes on the dump, which is frequently submerged due to wet weather.
According to the council, part of the dump is about 25 years old and a Northern Ireland-based contracting firm secured planning permission to use it for construction and demolition waste in the 1990s.
The firm failed, however, to secure an Environmental Protection Agency licence when that became a requirement, and closed the dump more then 10 years ago.
The council subsequently served a Section 55 notice on the firm, requiring it to take steps to protect the area.
It has subsequently been on a list of almost 100 suspect sites that the council drew up in recent years, following revelations of widespread dumping in west Wicklow.
A spokesman for the council said visual inspections at Russborough indicated the waste was from construction and demolition, including builder’s rubble, wood and clay.
Dublin City Council, which owns the reservoir, said it continually tested the water and had no reports of contamination.
The results of the testing are due in the coming weeks, according to the council.