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Tests have detected coliform in the Bull Run watershed, the source of much of the metro area's drinking water. The bacteria is not dangerous in and of itself, but an indicator of other problems in the water supply.

(Jamie Francis/The Oregonian/2010)

is investigating the cause of bacteria detected in its water supply, and Tigard, a wholesale customer, switched water sources as a temporary precaution.

Portland notified wholesalers that

-- a bacterium commonly found in soil and vegetation -- was detected in the water supply last Wednesday. Total coliform is not necessarily dangerous by itself, but it's an indicator that other pathogens or problems (such as ineffective chlorine) may be present in the system, water officials said.

"The normal water treatment process kills them," said David Shaff, Portland Water Bureau's administrator. "When you have a water sample that comes back with total coliform, there's something out of whack in your system."

To be clear, there are no boil water advisories issued as in 2012 or reasons to believe the drinking water is unsafe, officials say. A detection of total coliform is also distinct from a detection of fecal coliform -- linked to animal and human feces -- and E. coli, both of which have not been detected in Portland.



Bull Run Watershed water is tested hundreds of times a month in Portland. It's not surprising to see total coliform at this time of year, said Shaff, because the water is at its warmest and demand for water is low. Such conditions are conducive to bacteria.

Usually total coliform issues in Portland resolve after a day, but samples have continued to come back positive, Shaff said. Portland has been flushing its water system to get rid of warmer stale water. The response to total coliform in water is governed by the

.

"The water is perfectly safe to drink," said Shaff. "This is what the prudent water system operator does."

Some Portland customers may see discolored water because the system is flushing out materials from the bottom of water mains, Shaff said. Customers may be individually notified if the problem continues.

Tigard, which normally receives 90 percent of its water from the Portland Water Bureau, switched to drawing water from the Clackamas River and its wells on Friday.

"We’ll be on the alternate supply until such time that Portland tells us that water quality issues have been resolved," said Utilities Division Manager John Goodrich, who said the city can remain on the alternate water supply for several weeks.

The Lake Grove Water District in Lake Oswego also switched its supply as a precaution.

Sherwood and Tualatin receive Portland water from an unaffected area of the watershed. Tualatin Valley Water District said most of it district does not receive water from the affected area, except a pocket of 150 homes in the Metzger area and the Valley View Water District.

--Fenit Nirappil: 503-294-4029; fnirappil@oregonian.com