Finland is known as a country with thousands of unpolluted lakes and pristine groundwaters. For this reason treatment processes applied in the drinking-water industry are quite simple. These kinds of water supplies, as well as private wells, are vulnerable for accidental pollution, leading to twenty-four reported waterborne epidemics outbreaks in Finland in 1980-1992. About 40 per cent of these outbreaks were due to contaminated water from community drinking-water supplies. The number of people affected in these outbreaks was around 7 700. Contaminated groundwater was a more common cause than surface water. The majority of Finnish groundwater supplies distribute water without any treatment or only with alkalization. In most outbreaks leakage and blockage of a sewage pipe in the vicinity of a groundwater well resulted in the contamination of drinking-water. The largest of these outbreaks affected some 5 000 people. The etiologic agents in these epidemics were most probably viruses; faecal indicator bacteria and enteric viruses were detected in water samples during the epidemics. Contamination of water distribution networks due to cross-connection caused two restricted epidemics. Inadequate disinfection of surface water was the reason for three outbreaks. Two of these were caused by the same water supply in subsequent years. The raw water source for this supply was of quite a high quality, for which reason the treatment consisted only of rapid sand filtration and chlorination. For fear of the chlorinated organic compounds formed during disinfection the amount of chlorine in water treatment was reduced to a level where disinfection was inadequate, and some hundred people became ill. The etiologic agent in these outbreaks remained unknown.

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