Irish Water has claimed that the presence of a murky substance and discolouration in the water surrounding Dalkey and Bullock Harbour does not emanate from their waste treatment plants.

South Dublin locals voiced their concern on the quality of the water during the week with images posted online showing a black substance floating to the top of the bay.

In a statement, Irish Water said: “Irish Water has investigated this issue and can confirm that the discolouration at Bullock Harbour is unrelated to the wastewater treatment process.

“All wastewater assets in the area are operating as normal and no incidents have been reported.”

Green Party county councillor Ossian Smyth says that “seawater is tested weekly at nine points along the coast from Shankill to Blackrock.

“There was a major Irish Water pollution discharge on 24 June which made it unsafe to swim. However, the water at Sandycove and other locations has been of excellent quality since the 26 June.”

However, Smyth continues on to say that he has his concerns on the quality of the water surrounding Bullock Harbour and Dun Laoghaire.

“I would not swim in Bullock or Dun Laoghaire harbours in case they are polluted with bilge water or minor fuel spills,” he says.

“As the harbours are now being used more for recreation than commerce, it’s time that they were cleaned up and I want to see them added to the roster of water testing sites and I will be looking at ways that we can make them clean again.”

Irish Water added that discolouration in waters not caused by treatment plant discharge is the responsibility of Dun Laoghaire-Rathdown County Council.

A spokesperson for the council said in a statement that “the lifeguards in Sandycove report no observations of a black murky substance.”

And “it is possible that this is the same algae as was observed last week at Killiney: Phaeocystis – a common marine algae found in coastal waters – is often mistaken for sewage as it forms a brown scum.”