A breakdown in the sewage treatment process, believed to be caused by an as yet unidentified chemical, prompted the shutdown of the plant Saturday.
The public is being advised not to go swimming, fishing or boating on the creek, because the contaminated water could cause infection.
Washington County and Maryland Department of the Environment officials say the pollution has not harmed wildlife or drinking water downstream, and isn't expected to.
Washington County Health Officer William Christoffel said the sewage is going into the creek - an above-ground water source more than 100-feet above the aquifers tapped by wells.
"There should be no concern for public drinking water," MDE spokesman Richard McIntire said.
The closest downstream public water plant is in Brunswick, Md., in Frederick County. Water that may have gone past the Hagerstown sewer plant has been greatly diluted by the creek and the Potomac River by the time it reaches Brunswick, McIntire said.
The chlorine used to disinfect drinking water would kill any bacteria, he said.
The City of Hagerstown's water treatment plant is up the Potomac River at Williamsport.
According to city tests of the waste water going into Antietam Creek, the levels of suspended solids, which are small particles in the water, and fecal coliform, a bacteria that comes from feces, have both increased dramatically.
The amount of suspended solids went from 3 milligrams per liter (mg/l) of wastewater on Thursday, to 6.8 mg/l Friday, to a high of 41 mg/l Sunday. MDE allows the plant to produce suspended solids levels averaging 45 mg/l a day for seven days or 30 mg/l a day for a month.
The levels of fecal coliform sky-rocketed above MDE approved limits after the plant was shut down and the disinfection process was stopped.
On Wednesday, a 100 milliliter sample of wastewater had 4 fecal coliform bacteria. On Saturday, that count jumped to 23,000 per 100 milliliters, and Sunday there was 230,000 fecal coliform bacteria in 100 milliliters of waste water.
Chlorine will be used to disinfect the waste water beginning today. That should bring down the level of fecal coliform bacteria, although probably not to allowable levels, Barton said.
MDE allows city waste water to have 200 fecal coliform bacteria in 100 milliliters of waste water, he said.
Since late Friday or early Saturday, an unusual amount of white foam has been floating down Antietam Creek from the plant.
MDE officials have inspected insects, fish and birds downstream from the plant and found no ill effects from the discharge, McIntire said. MDE will continue to monitor the situation.
In the city's sewage treatment process, first the incoming raw sewage passes through screens and then sits in large holding tanks where heavy solids sink and are pumped out. Some of the waste goes to the landfill, some is turned into fertilizer at the city pellet plant.
Those first two steps of the process, during which material such as toilet paper and sludge are removed, continued after the rest of the process was shut down.
After the larger solid material is removed, the watery sewage is moved into holding tanks where bacteria "turns raw sewage into a more treatable sewage," Barton said.
It was here that the system broke down.
"Some chemical, we think, came into the system" and killed the bacteria used in this step of the treatment process, Barton said. As of Monday night, Barton didn't know what that chemical was, where it came from, or how much of it came into the system.
The first indication a problem existed may have come Thursday, when there was an odor, like paint thinner or kerosene, around the plant.
But Barton said, "It's not unusual to get those smells sometimes."
On Friday, the water in the tanks that hold the water after it passes through the bacteria treatment were cloudy. Normally that water is clear. So, Barton called MDE at around 4:30 p.m. "to report the possibility of a problem."
"It was not a concern, we were just covering our bases," he said.