More than 100,000 people are at risk of cryptosporidium infection from drinking water supplied by treatment plants that are failing to eliminate the parasite.
Cryptosporidium was found in 25 public water supplies last year, up from 17 cases in 2017 and 12 in 2016.
Dr Tom Ryan, head of enforcement at the Environmental Protection Agency (EPA), said the increase was a concern, given the serious risk the parasite posed to public health.
“Of particular concern are supplies which have inadequate processes to treat or remove cryptosporidium and those where there is no treatment in place at all,” he said.
The parasite can cause serious gastro-intestinal illness, yet in seven of the contaminated supplies the water-treatment plant had no processes in place to deal with it.
Eight plants had inadequate processes and three had the ability to deal with the problem but were failing to do so.
Currently, there are 16 treatment plants serving more than 110,000 people that fall into one of those three categories. They include small plants in Cork, Kerry, Donegal, Mayo, Sligo, Laois, Tipperary, Kilkenny and Limerick but also one serving 49,000 people in Mullingar and one used by 27,000 people in Letterkenny.
Dr Ryan said it was vital that Irish Water fix the problem. “Irish Water must make certain that water-treatment plants are properly and effectively operated to protect public health,” he said.
“Those plants without appropriate treatment for cryptosporidium need to be prioritised for investment.”
Ireland has 804 public water supplies, serving 1.3 million households and while 99pc of them passed the key quality tests last year, with reductions recorded in the cases of E.coli and breaches of pesticide and chemical limits, other problems worsened.
“Boil water” notices were issued 44 times during the year, affecting more that 97,000 people in 14 counties – four times the number of people affected in 2017.
Blame for the increase was attributed to Storm Emma and one major failure at the Vartry Reservoir in Co Wicklow.
Two-thirds of the notices were lifted within a month, which is categorised as short term, but five were in place for more than a year. Latest figures show 14 notices are currently in place.
Water-restriction notices, which advise against drinking the water completely, were issued 15 times, a substantial increase on the four restrictions imposed in 2017.
The number of water-treatment plants the EPA says are in need of upgrade or replacement to safeguard public health, and the number of people getting their drinking water from such plants, had been falling in recent years but the trend reversed in the past year.
Currently, 64 public water supplies serving more than 568,000 people are on the EPA’s remedial action list, requiring investment running into many millions of euro.
Andy Fanning, of the EPA, said the increase was disappointing. “There are still significant problems at many of Ireland’s water-treatment plants, with the potential to harm people’s health.
“The EPA is particularly concerned about supplies where we have seen poor operational practices at water-treatment plants.
“Consumers must have confidence that their water supply is not just safe to drink today but will also be safe in the long term,” he said.
Irish Water said new cryptosporidium monitoring procedures were devised during 2018 to enable risks to be identified earlier and better responses provided.