Fairfax County has agreed to pay a civil penalty and enter into a consent decree with Virginia’s State Water Control Board because of a series of wastewater spills last year, including the discharge of nearly 600,000 gallons of rust-tinged sludge into Pohick Creek and several smaller sewage spills, officials said.

After a closed session to discuss legal affairs at its regular meeting last Tuesday, the Fairfax Board of Supervisors disclosed that the county would enter the consent decree. The county also will pay more than $15,000 into the Department of Environmental Quality’s Environmental Response Fund, according to an unsigned copy of the decree.

The enforcement action, which is a formal acknowledgment of lapses and of a plan for future compliance, is a rare blemish on the record of a county that prides itself on caring for the environment. It’s also a reminder of the challenges Virginia’s biggest jurisdiction faces in managing a vast, aging water treatment system.

“We take protecting the environment very seriously,” said Michael McGrath, director of the Wastewater Treatment Division of the county’s Department of Public Works and Environmental Services. “We’ve taken responsibility for the spills that occurred.”

The DEQ, which enforces the commonwealth’s environmental protection laws, has cited the county for several violations involving the county’s Noman M. Cole Jr. Pollution Control Plant in Lorton and its wastewater treatment operations since December 2010.

The biggest was the accidental discharge of 580,000 gallons of “tertiary clarifier sludge” from the Lorton plant’s settling tanks into Pohick Creek on Jan. 19, 2011, the consent decree says.

County workers, who reported the spill to the state agency, said the discharge contained mostly water and less than 1 percent rust sediment. They also said the spill caused little or no environmental damage; McGrath said Friday that the water was one stage away from being drinkable.

The county also was cited for a Feb. 24 spill of 1,800 gallons of raw sewage into a tributary of Rabbit Branch; a March 19 discharge of 25,000 gallons of raw sewage from the Waynewood II pumping station into a Potomac River tributary; a May 10 discharge of 5,000 gallons of raw sewage from an overflowing manhole into Pohick Creek; and another manhole overflow on June 6 that sent 50,000 gallons of raw sewage into a South Run tributary, the consent decree says.

The consent decree says most of the discharges were unavoidable and caused by others, such as the June 6 spill, which was attributed to vandalism. But others, such as the large sludge discharge on Jan. 19 and the Waynewood spill on March 19, were caused by mechanical problems compounded by human error, the document says.

Kathleen O’Connell, water enforcement program manager at the state’s environmental agency, said the formal enforcement action is the first against Fairfax County since 2002. At that time, the county had trouble meeting the timetable for plant upgrades to comply with new standards for removing ammonia, largely because of a regionwide shortage of skilled laborers.

McGrath said the county spends approximately $40 million a year to maintain and replace equipment in a system with more than 3,000 miles of underground pipes. County workers also conduct annual inspections on about 250 miles of pipe.

It’s difficult to keep up with such a massive system, McGrath said. “Occasionally, accidents happen,” he said.

Growing maintenance needs will likely require county staff to ask the Board of Supervisors for an increase in sewer rates this year, McGrath said. Merni Fitzgerald, a county spokeswoman, declined to say how large the proposed hike would be until County Executive Anthony H. Griffin unveils his budget plan in the coming weeks.