A mainline water break has the city of St. Maries potentially exposed to contaminated drinking water.

But city officials assure residents the risk is minimal.

"There's always the possibility (of contamination), but it's very unlikely to happen," said Mark Reynolds, St. Maries public works director.

An older valve on the city water system broke around 4 a.m. on Sunday, Reynolds said, resulting in pressure loss in the system.

This left the system open to dirty water through either back pressure or back siphoning, he said.

"I would guess maybe only 10 percent (depressurized)," he said, adding that the risk is equally small.

Still, that meant potential risks for all 1,600 water users who rely on the system.

The city has been broadcasting warnings to all residents through radio and television announcements to boil their water to kill off any bacteria.

Even though the main was repaired and back online Monday afternoon, replaced with a part that meets current engineering standards, the city must still taking precautions in case the water is tainted, Reynolds said.

This includes flushing increased amounts disinfectant through the entire system, and then taking bacteriological sampling to ensure no contamination remains.

"It takes 24 hours to culture a test, and then getting the system up to the required level of chlorine will take another day and a half, so it could be up to three days," Reynolds said of when folks can trust their taps again.

School was canceled on Tuesday for St. Maries Idaho Joint School District #41, due to the lack of drinking water at the buildings.

Students were expected to be back in class today, however, with schools providing potable water on site. Class will continue through the week, said Karen Robinson, district board clerk.

The Idaho Department of Environmental Quality has been working with the city every day since the mainline break, said Suzanne Scheidt, regional drinking water manager for IDEQ.