Around 400 residents in the town of Plymouth received written notices telling them to boil their water.
The city believes it happened after a water line broke last week, but residents didn't know about the contamination until 4:30 Friday afternoon.
“Very shocked that we wouldn't know something was wrong with our water and worried,” said Janet Christensen.
Mayor Hal Mason declined an on-camera interview with ABC4 but said since the contamination he has received reports of a couple people becoming ill including a little girl who complained of diarrhea.
The town is now trying to flush the water system by opening hydrants and adding extra chlorine.
The boil order is expected to continue Sunday.