Up to one in five public water supply systems in the country is at high risk of contamination from the cryptosporidium parasite which has caused an outbreak of gastrointestinal illness in Galway city and county.
It has also emerged that the water supply for Galway city was identified in 2005 as being at "very high risk" of contamination from the parasite.
According to figures provided by local authorities to the Environmental Protection Agency in the last two years, 13 per cent of public water supplies surveyed to date have been identified as being at "very high risk" of contamination from cryptosporidium.
A further 8 per cent were identified as being at "high risk".
The information was compiled from risk assessments carried out on public water supplies around the country in the last three years.
A total of 60 per cent of public water supplies have undergone risk assessments, while there is no information relating to private water schemes, which supply water to fewer than 10 per cent of the population.
The EPA, which is now responsible for enforcing drinking water standards, is liaising with the HSE and Galway local authorities in relation to the outbreak.
Successive reports by the agency on water quality have warned about the potential problem of cryptosporidiosis contamination. Up to two people hospitalised with the illness have been in a serious condition, and the 60 reported cases may be a fraction of the total amount, according to medical experts dealing with the problem. Last night, the HSE West said that the reported number of new cases was "continuing to increase".
The locations identified for a "cluster" of cases - Tuam, Athenry, Knocknacarra and Moycullen - are all areas which have experienced recent rapid housing development.
Galway City Council has confirmed that contamination of its city water supply was confirmed in the "old" waterworks at Terryland which supplies about 30 per cent of the urban population. However, the entire city supply is at risk - along with parts of the county - as output from the larger waterworks plant shares the same reservoir.
The city and county councils have stepped up their efforts to trace the precise source of the contamination, believed to be in the proximity of Lough Corrib. It is anticipated that the official advice to boil water could be in place indefinitely.
The Department of the Environment says that there is no specific legislation requiring filtration of public water systems for the cryptosporidium parasite.
However, the department says that local authorities are obliged under EU drinking water regulations to put in place a "monitoring regime" for "certain" drinking water supplies in relation to the parasite, which is transmitted through faeces and can cause serious and fatal illness among people with low immune system.
There has been an angry political reaction to the issue, with a call by two Fianna Fáil councillors, Mike Crowe and Connie Ní Fhatharta, for a joint meeting of the two local authorities. Public representatives were "being left in the dark about what measures are being taken by city council officials to ensure that the contamination is dealt with in the most effective and efficient manner," Cllr Crowe said yesterday.
Labour Party environment spokesman Eamon Gilmore said enforcement of the law and updating of water treatment facilities must be a priority.