It is thought the contamination took place on Wednesday. Authorities say residents in the Luolavuori, Ispoinen, Uittamo, Vähä-Heikkilä, Koivula, Haritu, Korppoolaismäki, Hirvensalo, Satava and Kakskerra districts should boil tap water before use as a precaution. In all, about one-third of the city's residents are affected.
Preliminary tests on water from the tower will be ready on Friday afternoon. It has been emptied and disinfected and will not be operational again until the contamination threat has passed. Fresh drinking water is being provided from containers stationed in the affected parts of the city starting on Thursday evening.