City staff said Wednesday they were confident the water system was not contaminated with bacteria and that the result was a false positive. But Manitoba's office of drinking water was not convinced.
"We have no evidence at all to suggest there was a false positive at this point," said office director Kim Philip in an interview. "This is an independent, accredited lab and we're confident in the results from the lab, so we are very concerned with the use of the term false positive.
"We certainly don't want the laboratory to be pinned with this as an analytical error when there's nothing that we've identified that would lead one to conclude that."
Hours earlier, Winnipeg's waste and water director Diane Sacher told a city council committee that a false E. coli result, either from a sampling or lab error, was to blame for the citywide boil-water advisory.
"We don't believe there was contamination of our system," she said.
The city is waiting on an independent audit of how water samples are taken and analyzed so as to be sure, she said. A full report on what happened is expected in the next few weeks, Sacher said.
The provincial investigation is a separate "vulnerability assessment" and is due at the end of April.
"We're not lacking confidence in the city's water supply," Philip said. "We're just asking them to look at what might be some potential causes and then how might they take some steps to address those to make sure things like this don't happen again."
For two days in January, Winnipeggers had to boil their water after routine testing found coliform and E. coli at extremely low levels in six of 39 water samples. Businesses and residents were told that tap water was safe for bathing and laundry, but were advised to boil it for at least a minute before drinking it.
Subsequent tests all came back clean and there were no reports of anyone becoming ill from drinking or using tap water.
Officials suspected from the beginning that the initial test results were a false positive, but said they had no choice but to issue the citywide advisory.
The province has asked the city to look at whether it can better isolate neighbourhoods into zones to avoid future blanket advisories that could be unnecessary, Sacher said.
"We're looking into whether that is possible or not," she said. "That's an investigation that needs to happen."
The province has also amended the city's licence so water samples are no longer all collected on the same day, but rather spread over a week, she added. It has also requested the city come up with a better plan to notify potentially vulnerable people rather than relying on the media.
The city is looking at whether people can subscribe to an email notification service that would get advisories out more quickly, Sacher said.