More than a fifth of people who come into contact with the water in Dublin Bay through swimming or water sports become ill, according to a pressure group.
SOS Dublin Bay attributes the problem to increasing discharges of untreated wastewater from Ringsend treatment plant.
The vast volumes of storm water released into Dublin Bay add to what is an unacceptable public health threat, according to the environmental group, which wants more investment and existing plans to tackle the issue greatly accelerated.
The group claims the scale of the problem is indicated by the findings of an online survey of more than 1,200 people in March, where 21.8 per cent of participants declared they had been ill or suffered adverse health effects as a result of recreational activity recently undertaken in Dublin Bay.
Of those reporting illness, 14 per cent said they had diarrhoea, with similar numbers reporting cases of skin rash or gastroenteritis.
A wide range of microbes are potentially found in untreated sewage, of which E. coli bacteria and cryptosporidium are among the most debilitating.
The group called for urgent steps to better inform the public of the extent of the problem, which is “a more significant risk to swimmers than previously thought”.
SOS Dublin Bay says it has analysed data provided by Irish Water and the four local authorities in Dublin that shows between 2017 and 2020, 8.9 million cubic metres of untreated sewage and storm waters built up after periods of high rainfall and were discharged into Dublin Bay – primarily from overflow tanks at the Ringsend Wastewater Treatment Plant.
“The Ringsend discharge equates to 3,550 full-size Olympic 50 metre pools over the four-year period and averages out at 74 Olympic pools full of untreated waste water each month,” it added.
Discharges usually occur during storm periods when the facility reaches maximum capacity and cannot cope with loadings received.
“These types of discharges are not only a serious environmental issue but are a public health hazard,” the organisation warned.
SOS Dublin Bay chairman Gerard Jones said the group was "taken aback by how much wastewater was being illegally dumped into Dublin Bay and at the impact this is having on public health".
Given a major increase in year-round bathing in the bay, people needed to be better informed about periods of poor water quality, he said.
“Dublin Bay is our city’s most treasured public amenity, but it is now heavily polluted and causing illness. There a duty of care to protect public health, and that obligation is certainly not being met,” Mr Jones said.
The group, which acknowledged improvements are being made to the Ringsend plant, said more and accelerated investment is needed in wastewater infrastructure for the greater Dublin region.
Irish Water said its investment plan prioritises the need to support housing and development “with our obligations to protect the environment including Dublin Bay”.
After years of historic underinvestment, a spokesman said, the utility is working to upgrade treatment facilities in greater Dublin and to invest in new infrastructure to meet increased demand of more than 50 per cent by 2050. It is investing over €500 million in the staged upgrading of the Ringsend facility, he said.
“When all the proposed works are complete in 2025, the Ringsend Wastewater Treatment Plant will be able to treat wastewater for up to 2.4 million population equivalent while meeting the required standards,” the spokesman added.