On holiday weekends there's a lag time in the testing schedule at the St. Cloud water treatment plant. The test for E. coli takes 24 hours to complete. Workers collect water samples on Saturday and Sunday, but the results aren't available until Monday.
So when the weekend tests showed up positive, people had been drinking potentially contaminated water for nearly three days before they got the warning.
Ken Robinson, who has worked at the plant for 28 years, says in all that time, they've never had to issue a warning on fecal coliform bacteria.
"It is the water director's worst nightmare," he said.
Robinson says he and his staff checked the plant from top to bottom to see if something wasn't working right. They couldn't find anything.
"Everything seems to be normal, treatment process is going along just as normal, as is our disinfection," he said.
St. Cloud gets its water from the Mississippi River. "So routinely we have coliform in river, from runoff from agricultural lands in our watershed. So it's very common for us to have coliform in our raw water. The plant's designed to remove them," according to Robinson.
But when two tests showed E. coli, EPA rules require the plant to issue a warning. St. Cloud's public schools closed on Tuesday.
All over St. Cloud and surrounding communities, people started boiling their drinking water.
The presence of E. coli doesn't necessarily mean there are disease-causing organisms in the water, but it indicates there could be. Children and the elderly are especially vulnerable.
Nursing homes mobilized to deal with the problem. At St. Benedict's Senior Community, workers went out Monday night to buy bottled water. Communications director Kelly Ojeda says it's important for elderly people to drink plenty of water.
"We have a large supply of bottled water on hand. And we're also offering a lot of juice to them. The other thing that we're doing is just being sure that our staff has clean hands, so a lot of anti-bacterial gels," Ojeda said.
Restaurants faced similar challenges. Matt Bauer, who cooks the food at Nathan B's Restaurant, says they started boiling water in 40 gallon pots on Monday.
"Oh, we've been boiling it as the day goes on, because we've got to boil the water for the coffee. And, we went out and picked up some ice... the pop machine, it filters with water too, so we had to go pick up some cases of pop for lunch. But, other than that, you know, life goes on," he said.
Meanwhile at the treatment plant, the samples collected Monday morning yielded their results on Tuesday morning, and they showed no E. coli.
And another 20 tests from the pipes that deliver the water to homes were also negative.
Ken Robinson says that's got him convinced the water was never polluted.
"It sure points to that we made a mistake in the plant. Either our sample location was dirty, or we had some coliform in the air that got in our bottle when we were sampling, or something happened internally," he said.
Robinson says the weekend schedule -- allowing two days to go by without conducting a test -- is good enough.
He says they will be taking a hard look at how to prevent future contamination of test equipment.
The Minnesota Department of Health says most drinking-water warnings are issued in smaller communities that use well water and don't routinely use chlorine.