PORTLAND -- The two Mt. Tabor water reservoirs that tested positive for a type of E. coli bacteria last week have been drained and will be scrubbed with chlorine Tuesday and Wednesday.

Water Bureau officials said Monday two out of the three water samples that prompted the boil alert came back negative for a nasty strain of E. coli that causes severe food poisoning symptoms.

The third sample's results should come back Tuesday.

Background: Portland boil water alert lifted, tap water now safe

The Water Bureau says the type of E. coli in the water doesn't affect the decision to issue a boil water notice. If any trace over the regulated amount is found, the alert goes out. Water boil notices have been issued three times since 2009, when sample testing standards were tightened.

Commissioner Nick Fish, in charge of the Water Bureau, said he would make the same decision again about the timing of the alert.

Think about the flip side if we get a false positive before it's been tested, and we go out and tell the public there's a problem with the water, Fish told KGW. Think of the enormous inconvenience we would cause grocery stores, hospitals, restaurants. So it's a delicate balance. We don't want to scare people and cause disruption, but at the same time we want to make sure no one gets sick.

The most important outcome, he said, was that no one was reported seriously ill, he added.

Despite draining the reservoirs, crews still haven't found the source of the contamination. Originally they thought it could have come from a dead animal that somehow got into the water, but so far, nothing has been found. Water bureau officials said the actual cause may never be known.

Reservoir 1 will be cleaned Tuesday and back online Wednesday. Reservoir 5 will finish draining Tuesday and be refilled either Thursday or Friday.

More: Why the boil water alert was slow to reach Portlanders

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