Health officials say they have discovered E. coli bacteria in several private wells south of Nampa.
Southwest District Health says six out of 11 households tested positive for the bacteria. There have been no reports of illness connected to the outbreak.
The agency is working the Idaho Department of Environmental Quality to determine the source of the contamination.
Residents within 1,000 feet of the contaminated wells have been notified of health risks.
Health officials are advising private well users to get their water tested.