"Nature's Guinness", nickname for brown water in some rural areas, could be simple peat stain. Or it could be an indication of one or two virulent pathogens, the Geological Survey of Ireland's latest newsletter suggests.
Listing gastroenteritis as a notifiable disease would help to track this contamination, according to the GSI's groundwater bulletin. Limestone areas, such as those on the west coast, are particularly prone to this type of pollution, Mr David Ball, consulting hydrogeologist, writes.
Contamination by animal faeces can spawn E. coli 0157 and Cryptosporidium, both of which can survive for long periods.
Both are present in livestock, particularly very young animals, and both can contaminate food. While E. coli caused a number of deaths in Scotland last autumn, there is very little awareness among hydrogeologists in Ireland of Cryptosporidium, which is a common cause of gastroenteritis in otherwise healthy people.
Gastroenteritis is lethal to those who are "immune compromised", such as children, the elderly, convalescents and those taking immuno-suppressant drugs.
Increased sheep-stocking rates on shallow soil pastures located over some permeable limestone rocks in the west can increase Cryptosporidium levels in groundwater, he writes. Whereas E. coli dies off over time in a groundwater system, Cryptosporidium is hardy and can persist for longer periods. A very small number of its oocysts can cause an infection, and they are resistant to disinfectants such as chlorine at current levels in treated water supplies.
There are between 5,000 and 10,000 cases of the associated illness, known as Cryptosporidosis, in Britain every year, and it is becoming an increasing occurrence among watersport participants.
In 1995 one child was admitted to Crumlin Children's Hospital in Dublin with the illness, and several others suffered severe diarrhoea after a school visit to a children's farm in Co Kildare.
E. coli 0157 was first detected in the US in 1980 and in this State in 1993. The first outbreak occurred in 1995, and the first fatality occurred here last year when a three-year-old child died in Co Kildare. The pathogen caused the deaths of 18 people in Scotland last winter.
Mr Ball says the infective dose is very low. Whereas it takes 50,000 organisms to make up an infective dose of salmonella, E. coli takes only 10. The pathogen is in the national cattle herd, and death is caused by the bacteria releasing verotoxins that break down the wall of the large intestine in humans, resulting in haemorrhaging.
Recent research in Ireland has found that E. coli 0157 can remain viable on grass for more than 70 days.
It also appears that the amount of E. coli in cattle excreta increases in summer, for as yet unknown reasons. Fortunately, Mr Ball writes, E. coli can be removed by disinfecting water, or by cooking meats right through to a temperature higher than 68 degrees Celsius for 10 minutes.
The author says that many, if not most, of the water supplies for farms and rural households outside village group water schemes are cheaply constructed boreholes, wells and shallow springs. Almost all these supplies are not disinfected, and many are close to, or in, cattle pastures. In effect, he emphasises, Ireland's least protected water supplies are in areas where the bacteria are most prevalent.
Mr Ball says that hydrogeologists needs to shed a "certain complacency" about E. coli. Any groundwater source showing signs of contamination from animal faeces should be disinfected, he says. The rural food and tourism industries are identified as being high-risk sectors.
