Federal regulators have approved Alpha Natural Resources’ plan to expand one of the nation’s biggest coal slurry impoundments to a height taller than the Hoover Dam.

The plan will also increase the volume of waste it holds to 8.5 billion gallons.

The Mine Safety and Health Administration confirmed the approval this week and provided The Associated Press a copy of a letter giving Virginia-based Alpha permission to expand the Brushy Fork impoundment near Whitesville in southern West Virginia.

Alpha calls MSHA’s approval an incremental development in what has long been part of the construction plan. The impoundment was built in the 1990s by Marfork Coal Co., which was a subsidiary of Massey Energy Co. before Alpha bought Massey in 2011.

Since 2009, Brushy Fork has held 6.5 billion gallons of coal waste that Alpha says is mostly solid, not liquid.

Though the coal slurry impoundment has never failed, some citizen-activists question whether it was properly built. Retired miner Joe Stanley doubts the years-old waste inside has ever properly compacted and dried, meaning Alpha could be building on an unstable base.

“What about the additional weight placed on top of that?” he said. “I don’t think this is going to fail. I truly believe it will fail. It’s just a matter of time.”

If it does, emergency plans suggest a 100-foot wave of sludge would reach Sherman High School in 17 minutes.

“This thing could go all the way to Charleston,” Stanley said, “depending on what it takes out and which way it goes.”

But Alpha says past state and federal reviews found no deficiencies and that the company is “committed to designing, building and operating facilities safely.”

“We make the safety of our impoundments among the highest priorities in our company because we recognize the responsibility that comes with impoundment ownership,” Alpha spokeswoman Samantha Davidson said. “If safety is ever in doubt, we won’t hesitate to stop working and shut down a mining operation.”

In central Appalachia, coal companies use impoundments to dispose of both “coarse refuse,” or larger pieces of rock separated from coal during the cleaning process, and “fine refuse,” or clay, silt and sand-size particles. Fine refuse is pumped in from the processing plant to the reservoir behind the coarse refuse. Over time, the “fines” are supposed to settle to the bottom, compressing and solidifying.

At Brushy Fork, the water that remains is pumped out.

When fully expanded, the impoundment will stretch 910 feet from the toe of the embankment to the crest, but Alpha says the vertical dimension from the natural surface to the top of the dam is 740 feet.

The vertical height of the Hoover Dam is 726 feet.

Once the impoundment reaches capacity, the remaining water will be drained, and it will be filled with coarse material, then graded and capped with soil and vegetation.

The West Virginia Department of Environmental Protection has previously ruled there was no reason to believe the impoundment was improperly built or that its contents could liquefy, triggering a failure. But the state also said it would require continued testing.

DEP spokeswoman Kathy Cosco said the agency is still reviewing Alpha’s plan for the final stages, but the state needs MSHA approval before it can act. DEP has returned Marfork’s application with some comments, so the timeline for state approval is uncertain.

The federal Office of Surface Mining Reclamation and Enforcement upheld the state’s decision about the stability of Brushy Fork. But field director Roger Calhoun also said his agency had hired consultants to study it and six other impoundments, and the results could cause him to reopen the matter.

Rob Goodwin, who monitors slurry impoundments for Coal River Mountain Watch, sought the results of those studies under a Freedom of Information Act request.

But on March 15, the same day MSHA sent its letter to Alpha, the Office of Surface Mining denied Goodwin the documents. It cited two legal exemptions, calling the reports “interagency or intra-agency memoranda.” It also cited an exemption for records “compiled for law enforcement purposes.”

Companies have been doing their own impoundment tests for years, Goodwin said, “and all the regulators have just been taking their word for it.”

Goodwin said citizens worry about Brushy Fork because the engineer long responsible for the impoundment was also involved in illegal ventilation plans at Massey’s Upper Big Branch mine, where an April 2010 explosion killed 29 men.

MSHA later discovered Massey maintained two sets of safety records, one sanitized to throw off federal inspectors.

The engineering firm that Massey used on Brushy Fork, Goodwin said, was also linked to the 2000 failure of a Massey impoundment in Martin County, Ky.

Slurry burst through the bottom of a 68-acre holding pond, sending black goo through an underground mine and into 100 miles of waterways. The spill polluted the water supply of more than a dozen communities and killed aquatic life before reaching the Ohio River.  

West Virginia has more slurry impoundments than any other state with 114, but it hasn’t had a major failure since 1972, when the earthen dam at Buffalo Creek collapsed after heavy rain.

The flood killed 125 people, injured 1,100 and left 4,000 homeless.