A ruling against the EPA in a Virginia court in January restricts the agency's flexibility in setting the parameters for sediment pollution reductions. But EPA experts say they don't believe the ruling will endanger the Chesapeake Bay's pollution reduction efforts.

The case involves Accotink Creek, a tributary of the Potomac River in Fairfax County. A portion of Accotink has been on the state's impaired waters list since 1996, according to the EPA, while a second, lower part was added in 2008.

In 2011, the EPA established a total maximum daily load for the creek, which has a densely populated and highly impervious 52-square mile watershed. High flows brought tons of sediment into the creek. The TMDL sought to reduce the sediment flowing into the creek by 50 percent, with the aim of improving the quality of benthic life — the organisms that live on the creek's bottom.

But to meet the TMDL, homeowners, business owners and local governments would have had to retrofit their properties to capture and reuse all of their stormwater — at substantial costs. New buildings would also have needed to retain all stormwater.

The cost led the Virginia Department of Transportation to join forces with the Fairfax County Board of Supervisors to sue the EPA in federal court. State officials were concerned about the cost, but what they asked the court to consider was the methodology the EPA used to set up the pollution limits.

EPA scientists had used a surrogate — water — to stand in for sediment, arguing it gave a more accurate picture of how sediment behaved in stormwater. But the Virginia officials argued that the EPA could not use flow to measure a pollutant.

A federal judge in the Eastern District of Virginia agreed with the plaintiffs.

"EPA may not regulate something over which it has no statutorily granted power — annual loads or nonpollutants — as a proxy for something over which it is granted power — daily loads or pollutants," the judge wrote.

The judge noted that the EPA had tried to regulate sediment via looking at flow in only four instances nationwide, so it was not the norm. All four attempts have been challenged in court.

Jon Capacasa, the EPA's director for water in Region 3, said the agency was disappointed in the ruling, but remains resolved to work with the state and local officials to clean up the creek.

He said agency scientists had a good reason for using the surrogate. There was extensive science showing the flow was scouring the stream bed and contributing more sediment.

Capacasa did not want to discuss whether the agency would appeal, but said the ruling is not a huge setback for TMDLs nationwide.

"His ruling was very narrow and spoke to the fact that, if we choose to use a surrogate for the TMDL, then the surrogate needs to be a pollutant under the Clean Water Act," Capacasa said. "That's the extent of the ruling, and it's limited to the Accotink matter. It's not a universal decision."

Jim Edward, deputy director of the EPA's Chesapeake Bay office, agrees with that assessment.

"I don't think the specifics of the case would apply to our (Chesapeake Bay) TMDL," he said. "It's something our attorneys are reviewing."

Kevin Sellner, executive director of the Chesapeake Research Consortium, said he was initially worried about the ruling. But he said he's spoken to experts on the Clean Water Act who have assured him the ruling's scope is narrow.

"When I first heard about it, I wondered, did this mean that the Clean Water Act couldn't be used to force the jurisdictions to reduce their nutrient and sediment loads? To me, that would have been just like, 'what do we do now?' But several people have been bombarding me, saying, 'don't worry.' "

The TMDL for Accotink will need to be redone, as it was set aside. But that doesn't mean doom for the creek, which has been getting a lot of attention in part because of this ruling, said Peggy Sanner, senior attorney for the Chesapeake Bay Foundation in Virginia. Going forward, she said, both flow — which can be its own "causative agent" in polluting waterways — and costs, which are often not as high as initially feared, will be the source of a lot of debate in cleanup discussions.

"The obligation to clean up remains, and it's a pressing obligation," she said. "But the decision does appear at this point to take a tool out of the toolbox."