The city of Portland announced Friday water bureau customers should boil water they drink or use to brush their teeth. The boil water order is in effect until further notice.
The order affects 670,000 customers in the metro area, primarily in Portland, Gresham, and Tigard.
The city regularly tests for e. coli type bacteria to determine if its drinking water has been contaminated with feces from people or animals.
David Shaff, the Water Bureau Administrator, says the first positive sample came from a reservoir on Mt Tabor.
"We've had 3 positive samples out of what's going to turn out to be 40 or 50 samples over the last couple of days. I think that what that means is, whatever we have is probably very localized and transient," Shaff said.
The bureau will get new water quality test results Saturday morning and will end or update its boil water order then.
Public health officials say the risk to the public is relatively low. But the contaminated water could cause diarrhea in people who drink it.
This story originally appeared on Oregon Public Broadcasting.