(CNN) -- About 670,000 residents in Oregon were advised Friday to boil drinking water after several tests showed E. coli in some water samples, the Portland Water Bureau said.
"While we believe at this time that the potential health risk is relatively small, we take any contamination seriously and are taking every precaution to protect public health," bureau Administrator David Shaff said in a statement.
The boil water advisory was issued to about 670,000 customers in the Portland Water Bureau and the Burlington, city of Gresham (north of Interstate 84), Lake Grove, Lorna Portland Water, Palatine Hill, Rockwood, Tigard Water Service Area (including Durham, King City and Bull Mountain), Valley View and West Slope Water Districts.
All tap water in those areas should be boiled at a full rolling boil for at least one minute, officials said. Ice or beverages prepared with tap water on or after May 20 should be discarded, officials said.
The presence of E. coli bacteria indicates human or animal fecal contamination, and authorities are investigating how it entered the water supply, officials said.
The bureau has taken two water reservoirs out of service and is now conducting additional sampling throughout the affected area, officials said. One water system has already been flushed.
The tainted water supply was discovered during routine sampling beginning Tuesday.
A sample collected Thursday at the outlet of Reservoir 5 on Mount Tabor was tested and returned Friday as positive for total coliform and E. coli, authorities said. Additional routine samples from the outlet of Reservoir 1 at Mount Tabor on Tuesday and at a second outlet on Wednesday were also positive for total coliform and E. coli, officials said.
However, follow-up samples collected from Reservoir 1 and the second outlet have since turned up negative for total coliform and E. coli, officials said.
In the past, the Portland water bureau issued a boil water alert to 135,000 area households in 2012 and to 50,000 residents in 2007, both because of E. coli contamination, the Portland Oregonian reported.