Crews found the bacteria Friday in the water, which affects 324 homes and about 870 people, the state Health Department said.
The agency said it’s working with the operator of the system to find the source of contamination.
Officials advise residents to boil tap water used for drinking, food preparation, dish washing, brushing teeth, or making ice.
Water boiled for one minute will kill germs.
Anyone with questions can call 253-884-2088.