Raw sewage spilled from the only inbound sewer serving the Mattawoman wastewater treatment plant in western Charles County on Tuesday.

The overflow was caused by heavy rain from Hurricane Sandy, according to a county government news release.

The government asked residents to stay away from the spill near the intersection of Hawthorne and Livingston roads, and to stay away from Mattawoman Creek in the La Plata area. Signs will be posted and the warning could last for days, the release stated.

Public Works Director Bill Shreve said his department was still investigating how much sewage spilled and would provide a report to the Maryland Department of the Environment within five days.

He said a fine for the spill was unlikely because events like hurricanes are considered extraordinary and unavoidable.

There was a spill at the same site in September 2011 after Tropical Storm Lee, Shreve said. After that spill, workers bolted down manhole covers. This time, the sewage welled up from the ground, indicating that something underground is leaking.

Jim Long, president of the Mattawoman Watershed Society, said he discovered the spill Tuesday morning and alerted the public works department. Concerned because of previous spills, and interested to see if bolting the manhole covers had worked, Long donned wading boots and ventured into the flooded roadside gully, he said.

“It’s been a chronic problem of sewage overflows there. For example, it happened after [Hurricane] Irene and Lee last fall. I wanted to see if the fact that they now bolted down the manhole covers made a difference. After a large rainfall is when the county’s inflow and infiltration problem is most apparent,” Long said. “And then what I found was that while the manhole covers themselves were only leaking slightly, there was a large flow coming from the fixture itself,” the concrete housing of a vertical sewer pipe.

Long was pleased by the response of public works, which he said sent several workers to wade out and survey the problem, a more prompt response than he said he received after phoning in concerns during last year’s floods.

“I’m still looking into” the spill’s consequences for the health of Mattawoman Creek, which is already in decline, Long said. “But sewage disrupts the pH [of the water]. It, of course, adds bacteria to the waters, and it adds a lot of nitrogen to the waters. Of course, most people are going to be rightfully concerned about the potential for impacting human health with these sewage overflows,” but the environment suffers, as well.

“I would like to see [county government] fix the inflow and infiltration problem. This is probably one of the worst places for it, and they’re talking about expanding the plant again. I don’t know how one can even consider that until you get these chronic problems fixed,” Long said. “I keep coming back to the fact that the Mattawoman’s health is declining. This needs to be addressed in every possible way, and that goes from fixing sewer infrastructure problems to revising the land use plan, the biggest issue facing Mattawoman Creek.”

Mattawoman is one of Maryland’s “enhanced nutrient removal” plants, Shreve said, reducing the harm treated sewage does to water quality.

“They’ve been recognized for that. We actually have engineers coming from all over to look at that plant. It still needs a lot of work. The economy has hit us hard the last couple years. The facility operates fairly well, but it’s difficult to design a wastewater treatment plant to handle a 100-year storm, a 200-year storm or a once-in-a-lifetime storm” like Sandy, Shreve said.

“No final determination has been made” by MDE about the spill, spokesman Jay Apperson wrote in an email, but Shreve is probably correct.

“As a general rule, we do not assess penalties for sewage overflows caused by a 20-year, 24-hour storm or greater. We also look case-by-case at other weather-related to overflows to determine if a ‘force majeure’ condition applies. Sandy, along with Irene and Lee last fall, fall into the preceding description of storms,” Apperson wrote.

The plant is designed to handle up to 20 million gallons of waste a day; during the storm, it received almost 40, Shreve said.

Risk to human health is “minimal” because the spill is about three-quarters rainwater, the cold water inhibits bacterial growth and the waste is flowing rapidly out to the Potomac River on its way to the Chesapeake Bay, Shreve said. But the Charles County Department of Health will test water samples nearby.

In late 2011, Charles County government paid a $33,600 settlement to the Maryland Clean Water fund for 10 other sanitary sewer overflows and two “unauthorized discharges” of treated sewage, according to county and state MDE documents. Those spills occurred between October 2009 and December 2010.

A representative of the Charles County Department of Health could not be reached for comment.

emitrano@somdnews.com