The failure at Hagerstown's municipal sewage treatment plant occurred gradually between late Friday and noon Saturday, according to Rich McIntire, a spokesman for the Maryland Department of the Environment.
By Saturday afternoon, sewage was being discharged into the river at a rate of 5.7 million gallons per day, McIntire said.
The river is a source of drinking water for the Washington area, but McIntire said there was no threat to the water supply because the polluted water would be diluted by the time it reached the area.
The sewage treatment system was corrupted by an unknown chemical that killed the microbes treating the water, said Rick Thomas, manager of Hagerstown's Water Pollution Control Department.
"We have a biological system here, with microbes that do the work of treating sewage," he said. "The microbes can not withstand the toxins from the chemicals."
Workers were trying to determine the type of chemical that got into the system and the source, Thomas said.
"We've been collecting samples all day and we'll be sending them to a private lab Monday for analysis," he said.
There was no immediate threat to public health, officials said.
The department notified water-intake plants downstream from Hagerstown and they were taking steps to protect their water supplies, McIntire said. He said signs would be posted near the river, warning of the danger.
There was little risk of people having contact with river water in February, due to the cold, Thomas said. Officials said the system would flush itself out and replenish itself.
"At this point there's no choice but to wait and let things run their course," Thomas said.