Chatsworth Water Works manager Steve Smith says someone changed the settings for how chlorine and fluoride feed into the system.
He says about 10 to 15 residents near the Carters Lake Water Treatment Plant in Murray County have no water at all.
The FBI, Georgia Bureau of Investigation and a sheriff's department are investigating the break-in.
Smith says the water company's tests show none of the changed levels of chemicals ever made it out of the plant, which was shut down as a precaution.
Smith says he hopes the plant will be back in operation by the end of the week.