Human enteroviruses have been found in water from a chalk well with history of excellent bacteriological and organoleptic quality. A proportion of these viruses survived disinfection treatment which consisted of a nominal 1 mg/ℓ of free chlorine maintained for a minimum contact period of 15 minutes at a pH value of 7.2. It is concluded that viruses may occur in underground sources which have consistently proved to be satisfactory in routine monitoring tests for bacterial indicators of faecal pollution. Possible mechanisms which these circumstances may arise are discussed.

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