The Portland Water Bureau has temporarily stopped delivering Bull Run water to Portland water users after finding the cryptosporidium parasite six times in tests of drinking water from the Bull Run watershed since Jan. 1.

The bureau will instead deliver 100 percent groundwater from the Columbia South Shore Well Field, a decision city officials said is "not required." It could take as many as two weeks for the groundwater to make its way to homes and businesses.

"The recent detections do not pose an increased health risk," Water Bureau director Michael Stuhr said in a statement. "After a series of very low level detections, we are proactively activating our secondary source while we collect more data."

The Portland water bureau has detected the cryptosporidium pathogen in drinking water from Bull Run at least six times this year, raising the risk that the city could have to build a treatment plant that could cost tens of millions of dollars.

The bureau increased its weekly testing of drinking water on Feb. 6 after it detected the parasite four times, following four straight years of finding none. Water officials noted that four positive tests in five weeks was the most they'd seen in more than a decade.

They detected the parasite in water samples two more times that week.

Multnomah County Health Officer Paul Lewis said in an interview that it's unlikely the cryptosporidium found is harmful to humans because it likely originates from the scat of animals living near Bull Run. Humans are prohibited from accessing the protected watershed, and animal-borne pathogens do not often affect humans, he said.

Still, people with compromised immune systems should talk to their doctors about alternative drinking water options, Lewis said.

"This is a cautious approach," Lewis said. "We don't really know if anyone is going to have a problem with cryptosporidium."

Unlike most cities, Portland does not treat its water for cryptosporidium. The Oregon Health Authority gave the city an exemption in 2012 from federal treatment rules, allowing the city to instead monitor for the parasite through regular testing.

The state could revoke Portland's exemption, called a variance, if the water bureau finds more than one oocyst - a hard, microscopic structure found in feces - per 13,300 liters of water in one year. This could force the bureau to build an ultraviolet treatment plant, expected to cost at least $89 million, according to water bureau planning documents, or a filtration system that could cost around $300 million.

Stuhr said in a statement that the water bureau is still in compliance with the variance.

--Jessica Floum

jfloum@oregonian.com

503-221-8306

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