High levels of sewage and fertiliser residue foundin 40pc of Irish lakes and rivers

High levels of phosphate were found in 41pc of rivers, streams and lakes all over the country, but there were particularly high levels in parts of Wexford and north Dublin.

High levels of nitrogen were found in 18pc of water bodies, with the south-east, plus parts of Cork, north Dublin and Meath, notably elevated.

Both are natural nutrients, but in unnatural concentrations they cause excessive growth of algae, deprive the water of oxygen, and render it uninhabitable for fish and other wildlife.

They build up in water when they leach from soils sprayed with farm fertilisers, or they can escape from poorly performing septic tanks and sewage systems.

“These are things that have to be addressed,” said Professor Fiona Regan, director of the Water Institute at Dublin City University (DCU), which led the research.

“Where you have a finding of nutrient pollution, it’s likely other pollutants are getting into the water too – pharmaceuticals, chemicals, possibly microbes. We only tested for nitrates and phosphates – but if they’re at a high level, it should also be a warning sign for other forms of pollution.”

More than 700 citizen scientists gathered the data in the four-day annual DCU Water Blitz exercise – the highest number since it was first run in 2019. They came from environmental and community groups, kayaking, angling and swimming clubs, schools, Tidy Towns organisations and group water schemes.

The geographic spread was the widest ever, with testers in every county, and sampling carried out in 40 of the 46 catchment areas into which the country’s rivers are divided.

While the overall findings were worrying, samples taken from streams were of even greater concern.

In total, 25pc of streams tested for nitrates and 47pc of those tested for phosphates had high concentrations.

“Streams are under-monitored by state agencies and many are not monitored at all,” Prof Regan said.

“Any pollution in them gets diluted as they enter larger water bodies – so by only testing downstream you’re missing important information that can help target pollution sources.”

Testers were asked to record land uses surrounding the sites where they sampled water – such as whether it was urban, industrial, residential, forestry or agricultural, and, if so, the specific kind of farming taking place.

That data will also be used to develop strategies to work with land users to ease their impact on local water sources.

The Water Blitz findings illustrate the stiff challenge facing Ireland to get water sources up to the standards legally required under the EU Water Framework Directive.

By 2027, all inland and coastal water bodies are supposed to rank of good or high quality. The 41pc of water sources with high phosphate concentrations and 18pc with excess nitrates would fail that test on nutrient levels alone.

Prof Regan said while the majority of water sources tested were of good quality, the overall picture was one of decline.

“We’ve had the Water Framework Directive for more than 20 years – and it’s in that time and under that directive that our water quality has disimproved.”

She said, however, she was heartened by the numbers taking part in the blitz.

“It’s getting bigger and better every year. It’s an incredible achievement and I believe that number is going to increase. I think that we’re at the start of a citizen-led environmental movement.”