ARLINGTON — Arlington residents are drinking their own water again.

Even if people hadn’t noticed it, the city replaced its primary water supply with water from the Snohomish County Public Utility District for more than a month, at a cost that could run into the six figures.

The reason: fear that contamination from the Oso mudslide would leech into the city’s well water.

Haller Well, about 60 feet from the Stillaguamish River in Haller Park, supplies about 89 percent of the city’s drinking water, according to the city’s annual water quality report.

When the slide blocked the North Fork Stillaguamish on March 22 and the river began to back up behind the mudslide debris field, the fear was that a sudden failure of the dam would send a flash flood downstream and damage the well.

“We didn’t know what would be coming down that river,” Mayor Barbara Tolbert said.

The decision was made to turn off the pumps and draw water from the county.

The city maintains a long-term contract with the Snohomish County Public Utility District to buy up to 1,000 gallons of water per day.

Arlington Public Works director James Kelly said that PUD water normally serves only customers who live outside the area of the well’s pressure zone, or is used to handle peak demand on hot summer days.

In this case, PUD water replaced the city’s supply entirely and remained that way for more a month because of fear of contamination from slide debris.

While the water in the well is groundwater, the well is close enough to the river that sediment or contaminants in the river can get into the well water, Kelly said.

Testing of the river in the days after the slide showed a high degree of turbidity, or lack of clarity, as well as levels of antimony, arsenic, chromium and lead that were above state and federal limits for drinking water.

That could have contaminated the city’s drinking water had the well been active, Kelly said.

“A lot of this we did for the safety of our customers,” Kelly said.

It wasn’t until April 22, a month after the slide, that the levels of contaminants in the river had dropped to within acceptable ranges and the city began pumping water from the well again.

The well pumping increased gradually and, by May 1, was up to full production.

How much this will cost the city is not known, but Tolbert guessed Arlington could be on the hook for between $100,000 and $200,000 for the PUD’s water.

She said the city would work with the Federal Emergency Management Agency to see how much can be reimbursed.

Municipalities can be compensated for some emergency expenses under major disaster declarations, said Donald Jacks, assistant external affairs officer for FEMA.

FEMA has closed the three Disaster Recovery Centers in Washington it had operated after the mudslide but is still operating out of the Snohomish County Family Resource Center in Darrington.

The process for a city like Arlington involves working with Snohomish County and the state to identify which expenses FEMA might cover. The process is the same that has been used in other major disasters. The disaster declaration covers impacts to government and tribal agencies as well as individuals, as in the case of Hurricane Katrina and Super Storm Sandy, Jacks said.

Chris Winters: 415-374-4165 or cwinters@heraldnet.com.