A federal appeals court has upheld Virginia’s water quality certification for the Mountain Valley Pipeline, preserving a major milestone for the controversial project.

In a decision Wednesday, the 4th U.S. Circuit Court of Appeals rejected arguments by the Sierra Club and other environmental conservation groups that the State Water Control Board erred when it found a “reasonable assurance” that streams and wetlands would not be harmed by the natural gas pipeline.

“The construction of the project was exactly that, a large construction project, and the State Agencies very reasonably undertook to protect their waters with the ‘tried and true’ methods developed for just this purpose,” Judge William Traxler wrote for a three-judge panel.

“We see no purpose we would serve by stepping in and second-guessing the analytical methods Virginia deemed appropriate to provide it with reasonable assurance that its water quality would be protected,” the 47-page opinion stated.

The ruling applies to approximately 500 waterbody crossings the pipeline will make on its path through Southwest Virginia.

As such, it is more sweeping than a decision last week by the same three-judge panel, which reversed the U.S. Forest Service’s approval for the 303-mile buried pipeline to burrow through 3.6 miles of the Jefferson National Forest.

The water board’s certification, made in consultation with the Virginia Department of Environmental Quality, was a giant step in a complex regulatory process that lasted nearly four years.

At the time of an emotional, two-day hearing in December, pipeline opponents warned of risks from erosion and sediment caused by cutting trees, blasting bedrock, clearing land and digging trenches for the massive pipeline along steep slopes and through pristine waters.

Critics said they were especially disappointed with Wednesday’s ruling, given that their predictions have been borne out by official warnings the project has since received from state environmental regulators.

“We will continue to closely monitor construction and press DEQ to uphold the promises it made to the court to protect Virginia’s water quality,” said Ben Luckett of Appalachian Mountain Advocates, a nonprofit law firm involved in the case.

“We will also continue to pursue all other available legal avenues to oppose this harmful and unnecessary project,” Luckett said.

A spokeswoman for Mountain Valley, a Pittsburgh-based coalition of five energy companies, said the decision validates its assertion that state waters will not be contaminated.

“Virginia’s water quality certification was the product of months of dedicated work by the VDEQ to evaluate the infrastructure project and ensure the measures to which MVP had committed would protect the Commonwealth’s streams and wetlands,” Natalie Cox wrote in an email.

The director of DEQ — which pipeline opponents have accused of favoring a corporate venture over natural resources — on Wednesday said the agency “has been committed to provide the best environmental protection of our waterways consistent with the science and regulatory power within our authority.”

“The court has agreed with our assessment and upheld our approach,” David Paylor said in a written statement.

Last month, DEQ issued a notice of violation against Mountain Valley, finding that measures to control muddy runoff were inadequate at construction sites in Giles, Craig, Montgomery, Roanoke, Franklin and Pittsylvania counties.

Five similar actions have been taken by regulators in West Virginia, where the pipeline will originate.

Mountain Valley has told West Virginia officials that the problems have been fixed; a Virginia DEQ spokeswoman said Wednesday the agency is still waiting for a meeting with the company to discuss its findings.

Some say it would be unrealistic to expect a perfect record during construction of the $3.7 billion project, which has experienced heavy rainfalls along a 125-foot-wide corridor stripped bare for the 42-inch diameter pipe.

“Certainly, it must be anticipated with large construction projects that unexpected problems will arise, leading at least to minor, short-term issues,” the 4th Circuit wrote.

The opinion also knocked down another assertion made by opponents: that DEQ and the water board improperly limited their analysis to “upland areas,” leaving a stream-by-stream review to the U.S. Army Corps of Engineers.

Virginia’s decision to take a segmented approach, “even if unorthodox, was not arbitrary and capricious,” the court ruled.

Joining the Sierra Club in the challenge were more than a half-dozen other conservation groups, about as many property owners along the pipeline’s route, and Del. Sam Rasoul, D-Roanoke.

Wednesday’s decision marked the third in two weeks to be handed down involving the pipeline by the Richmond-based appeals court.

On July 25, the court upheld the dismissal of a lawsuit that challenged Mountain Valley’s use of eminent domain, the power to condemn private land for a public use.

Landowners had argued that the Federal Energy Regulatory Commission should not have conferred that right to a private company. U.S. District Court Judge Elizabeth Dillon said she lacked jurisdiction to hear the case, a decision affirmed by the appeals court.

Then last Friday, the court struck down a decision by the Forest Service to allow the pipeline to cross through the forest, finding the agency did not sufficiently question Mountain Valley’s promises to control erosion.

The court sent that approval and a related one by the Bureau of Land Management back to the respective agencies for further review. It remained unclear Wednesday how long the process might take.

Any celebration by pipeline opponents last week was cut short by the court’s most recent ruling. According to Cox, Mountain Valley plans to complete the pipeline early next year.