Fairfax County and the state of Virginia have accused the U.S. Environmental Protection Agency of “massive” and expensive regulatory overreach in its attempts to control sediment buildup in the Accotink Creek watershed, according to a lawsuit filed Thursday.

The lawsuit — which was filed by the county and by Virginia Attorney General Ken Cuccinelli’s (R) office in federal court in Alexandria on behalf of the Virginia Department of Transportation — says the federal agency has gone too far by requiring the county to control the flow of water itself as a way of managing sediment discharges.

If the county were to comply with the EPA, the financial impact on homeowners and property owners would also be significant, county officials said. Building any new impervious surface, for example, including a home addition or new residential development, would require taking steps to retain all storm water runoff from the expanded area.

Although county officials have talked about increasing tensions for some time with the EPA over managing the Accotink Creek watershed, the Democratic-led Board of Supervisors wrestled with taking legal action against the federal agency or teaming with arch-conservative Cuccinelli, particularly in an election year when Virginia is a swing state and the EPA has been a periodic campaign issue. But board members, meeting in closed session during the board’s regular meeting Tuesday, said they thought that the county had to take legal action, and felt that joining with the state would strengthens the board’s case, officials said.

“A political body could make a decision based on politics and how things look, or they could do what’s right for Fairfax County. That’s what we’re doing here,” Board of Supervisors Chairman Sharon S. Bulova (D) said. “We happen to be intertwined with the the Commonwealth of Virginia on this. It would be very difficult for us to challenge the EPA on something we just think is wrong, all by ourselves.”

The Accotink Creek watershed is Fairfax’s second largest, covering 52 square miles. The principal stream winds 23 miles before entering Accotink Bay on the Potomac River. The county has been in talks for several years with the EPA and had warned that conflicts over sediment management could lead to legal action.

In asking for declaratory judgment and an injunction, the state and county’s lawsuit argues that the EPA’s proposed restrictions on water flow exceed its authority under the Clean Water Act and would divert public funds that could be spent more effectively on restoring Accotink Creek and other waterways. They said the EPA would require the county to cut the flow of water by half at a cost that could reach as much as $500 million.

“EPA is literally treating water itself — the very substance the Clean Water Act was created to protect — as a pollutant,” the lawsuit says.

The state and county say that although the EPA has the power to regulate the discharge of pollutants, including sediment, the agency last year issued a restoration plan — known as Total Maximum Daily Load (TMDL) — that would require the county to control the quantity or flow of water itself in Accotink Creek to improve aquatic life there. Similar TMDL plans were issued in four areas of the United States, including waterways in Missouri, and all have been challenged in federal court, according to state and county officials.

An EPA spokeswoman said the agency is currently reviewing the lawsuit.

“The Agency established this science-based TMDL to ensure the health of the Accotink Creek,” the spokeswoman said. “The TMDL is supported by sound science and well within our authority under the Clean Water Act.”

State and Fairfax officials said they decided to challenge the EPA’s proposed restrictions because, they said, the cost of implementation is higher than restoration plans that target the pollutant itself, and because they believe the EPA’s plan “will deliver a worse environmental result.”

“When people talk about federal agencies running amok, this is exactly what this looks like,” Supervisor John C. Cook said. “The EPA’s overreach is so extreme that the Democrats on the board realized that, even in an election year, they had to do this for the county.”

State and local officials said the EPA’s mandated reduction in water flow could not be achieved by retrofitting existing storm water management structures and would instead require VDOT to acquire significant amounts of private property for new storm-water facilities. They also argue that the EPA would effectively require the county and state to regulate runoff from private property beyond their control.

“It’s about putting aside politics and doing the right thing for Fairfax County,” said Supervisor Pat S. Herrity (R-Springfield). “Any kind of expansion in the Accotink watershed would have become prohibitively expensive because of what you would have to do with storm-water management.”

This story has been updated to clarify that the county and the state have jointly filed the lawsuit.