The Environmental Protection Agency (EPA) is investigating a pollution incident on the River Tolka at Blanchardstown after the discovery of dead fish for the second time in a year.

It is believed that a sewer in Damastown became blocked by branches from a falling tree, resulting in raw sewage overflowing from a manhole into a field and the river.

Inland Fisheries Ireland reported a fish kill which extended over a 5km stretch of the river before officials from Irish Water and Fingal County Council managed to stem the overflow.

Cllr Matt Waine (Sol) said it was the second significant fish kill in 12 months and he called on Irish Water to take preventative action to protect the waterway.

He said: “It was a miracle that there were fish in the river at all recently, given what happened last year as it can take many years for rivers to recover from such incidents.

“Responsibility for ensuring that sewers are well maintained and that blockages are detected and remedied before causing such environmental destruction lies with Irish Water.

“I now have major reservations over the 9C sewer duplication works that are due to commence shortly through the Tolka Valley.

“If Irish Water cannot protect the Tolka from discharges and blockages on the existing sewer line, what are the chances of a more serious incident while these works are being carried out?”

A spokesperson for the EPA said: “Our priority is to ensure that Irish Water completes the corrective actions needed to bring this fully under control, protect the River Tolka and prevent a reoccurrence of this type of incident.”

Green Party Cllr Roderic O’Gorman expressed his anger after learning that a large number of dead fish were found in the river below the Mulhuddart Bridge.

He said: “It is so frustrating to see this happening again. Local clubs like the Tolka River Environmental Alliance have done so much to re-stock and rejuvenate the river.

“All this work can be wiped away in a matter of hours by effluent entering the river. There must be improved monitoring measures to ensure this does not happen again.”

In a statement to Dublin Gazette, Irish Water said the sewer became blocked by a branch and last year’s blockage was caused by a tyre thrown by vandals.

A spokesperson added: “The sewer in question runs under the M3 to an area of land which is very difficult to access. Works to clear the blockage began immediately.

“A large tree branch was removed from the sewer during the process. It is likely that this was the primary cause of the blockage.

“This obstruction in the sewer had gathered more material, mainly grease blocks, which resulted in the overflow.”