Drinking water in Columbia Falls showed signs of coliform bacteria in three out of five samples tested last week.

It's not a health emergency, but the water surpassed the standard that no more than one sample a month may show coliform.

Coliform bacteria generally are nothing more than an indicator that other potentially harmful microbes may have found their way into the water supply, Public Works Director Lorin Lowry explained in a letter mailed to all city water customers over the weekend.

But he set crews to work flushing the entire water system on Monday and expected to start chlorine treatments today.

"Our first steps were to do repeat sampling to see if it's truly an incident or not," Lowry said Monday. "Now we've determined we do have a problem out there. We're working with the DEQ pretty closely."

The Montana Department of Environmental Quality recommended injecting a chlorine treatment into the system, he said, "pretty much on the same level that all systems, like Whitefish and others, are already treated."

Chlorine will be introduced at a level that leaves a residual level of 0.5 parts per million to kill bacteria. That level dissipates as it reacts with the biology in the system, Lowry said, so they will repeat the treatment for five days. Crews will monitor the chlorine level throughout.

"We'll collect a series of samples near the end of that, and hope for no hits," he said.

He speculated the contamination probably was caused by extensive street construction on Third Avenue East, Second Avenue West and First Avenue East. As streets were torn up, the water lines may have been disturbed enough to let dirt or other contaminants enter.

Lowry said he hopes the systemwide flush and chlorination will take care of those issues.

Lowry expects to resolve the problem in two weeks - by Nov. 20 - and will send out another notice to water customers when the process is finished.

"But we'll continue to treat if our tests keep showing coliform," he said.

He had received just two calls Monday morning. One came from a resident with a specific health condition who wondered how it would affect him, and another from someone asking for clarification in general.

"It's not categorically health threatening, but it's always good to check" if you have health concerns, Lowry said.

Consult your doctor with specific health issues, he recommended in the letter. People with severely compromised immune systems, infants and some seniors may have increased risks.

In general, there's no need to boil drinking water or take other action.

The water department routinely tests city water as part of efforts to maintain safety and quality. Because of its wellhead protection zone and other water treatment regimens, the city is not required to chlorinate its water supply.