The problem was discovered when the heating system was shut down for the installation of a district heating system, the Swedish news agency TT reported.
It turned out that the coffee machine was hooked up to the heating unit, a closed system which pumps water between the radiators, instead of the drinking water. The radiators simply refilled automatically as staff made coffee with heating system water.
"As the coffee is black and the radiator water is a bit brown, then we haven't noticed the difference," said Capt. Catharina Bergsell, information officer at the F 17 squadron.
It was unclear why the machine was connected to the heating system and the matter will be investigated. Had the radiator not been switched off, the problem would most likely not have come to light.
"No one had a clue about this until the water was turned off. Had that not happened, I reckon it would not have been noticed until there had been a change of supplier or some such," Bergsell said.
Staff at the base will undergo tests for potential contamination, but there have been no reports of personnel falling ill.
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