Ketchum residents, businesses advised to boil water, part of Warm Springs Road evacuated.
A treated water sample, tested at the same time from the same well, showed no signs of bacteria, a city statement said.
The positive sample is the result of a clerical mistake made by city staff in the process of sending in the sample for testing, the city said.
"On the form sent with the sample, staff should have checked a box indicating that the sample was a ‘special sample’ and that non-treated, raw water was not entering the drinking water system," the city said.
“But I also want to be clear that no untreated water entered the city’s drinking water system.
Boil advisories mean water delivered by the city’s system that will be used for drinking, food preparation, brushing teeth and cleaning dish and glassware should be boiled for at least one minute.
The well has been taken out of service and other wells have been put into service, the city said.
"City staff intends to continue enhanced monitoring of its municipal drinking water system throughout the flood event," KETCHUM — The Blaine County Sheriff’s Office has ordered a mandatory evacuation and hard road closure on Warm Springs Road.
Because of flooding, the road cannot be maintained.
"If you are physically unable to evacuate call 911.