More than one in 10 water supply systems in Ireland in 1996 were found to contain coliform bacteria, according to the Environmental Protection Agency.
The annual report on Ireland's drinking water says the presence or absence of the bacteria, usually arising from human or animal sewage, is the most important factor in determining water quality.
Water supplies are obliged to have a zero coliform content. The finding has most implications for those who receive their water from group or private schemes, most of which are in rural areas, rather than from local authorities providing public supplies. The highest number of samples above the legal limit, known as the maximum admissible concentration, was in group supplies, where the number of supply systems was taken into account.
Out of 14,064 water samples, 1,707 (12 per cent) were found to exceed the maximum admissible concentration for coliforms. Coliforms, which often include E.coli bacteria, are considered indicators of faecal pollution, although the EPA stresses that non-faecal varieties were found in many cases.
Despite the level of coliform contamination, the EPA report says "public drinking water supplies are of satisfactory overall quality". It cites continuing evidence of efforts to improve supply leading to better water quality.
Where fluctuation in quality occurred, it was mostly because of "natural or seasonal variations in raw water sources or because of `one-off' contamination incidents".
The report evaluates 1,730 individual supplies and 95,000 water analysis tests. Based on coliform analysis, it concludes that 92 per cent of public supplies were acceptable, but only 58 per cent of group schemes were considered acceptable. There are more than 5,000 schemes in the Republic.
The EPA warns those involved in group schemes and small supplies to be constantly vigilant on public health grounds. Contamination of such supplies is often caused by run-off from septic tanks or agricultural slurry pits.
In many rural areas supplied by group supply schemes or private supplies from wells, the practice of chlorination or effective disinfection "may be largely or wholly unknown". They are also more at risk of contamination, being especially vulnerable to pollution.
While the highest quality is found in public water supplies, there were many instances of coliform presence, even if a lot were non-faecal in origin. These were due largely to a combination of treatment anomalies or contamination in the distribution network "rather than the omission of the vital disinfection phase".
Where limits based mainly on EU directives were exceeded, the EPA found many were marginal and reflected naturally occurring local conditions. Excessive colour in water occurs in peatland areas, such as in the midlands, while excessive iron and/or manganese is found is mineral-rich areas.
Often limits were only marginally exceeded but "significant numbers of drinking waters are still bacteriologically unfit for human consumption". Equally, the presence of non-faecal coliforms indicates cleaning-out and maintenance of distribution networks is overdue.
The agency found most sanitary authorities were making pro gress in addressing the significant demands of drinking water regulations. "Nonetheless, there is still a clear need for improvements and rationalisation both in the production of potable water supplies and in the assessment of their quality."
The EPA report reflects an "ab ysmal level of funding for the group water scheme sector over many years", according to a National Federation of Group Water Schemes spokesman, Mr Bernard Keeley. "Under the present capital grant allocation arrangements, it will take over 100 years to upgrade schemes in Co Galway alone."
The Government would have to ensure the hazards were spelt out and introduce preventive measures urgently to eliminate sources of water contamination, he added.
