About 3.5 million gallons of raw sewage spilled into Carroll Creek after a waste water treatment plant lost power for about nine hours, Frederick City officials said.
The spill occurred about 500 feet upstream from the Monocacy River, which supplies the city's drinking water. However, the city said its water supply was not affected because the creek enters the river about a mile downstream from where the water supply is drawn.
The spill also temporarily closed a road near the plant.
The city said power from a generator stopped the sewage spill about 3 a.m. Monday and the cause of the outage is under investigation. County officials say citizens should avoid water contact downstream from the spill.